Immune System Problems

Things That Can Go Wrong With the Immune System

Disorders of the immune system can be broken down into four main categories:

1. immunodeficiency disorders (primary or acquired)
2. autoimmune disorders (in which the body's own immune system attacks its own tissue as foreign matter)
3. allergic disorders (in which the immune system overreacts in response to an antigen)
4. cancers of the immune system

Immunodeficiency Disorders

Immunodeficiencies occur when a part of the immune system is not present or is not working properly. Sometimes a person is born with an immunodeficiency — these are called primary immunodeficiencies. (Although primary immunodeficiencies are conditions that a person is born with, symptoms of the disorder sometimes may not show up until later in life.) Immunodeficiencies can also be acquired through infection or produced by drugs. These are sometimes called secondary immunodeficiencies.

Immunodeficiencies can affect B lymphocytes, T lymphocytes, or phagocytes. Some examples of primary immunodeficiencies that can affect kids and teens are:

• IgA deficiency is the most common immunodeficiency disorder. IgA is an immunoglobulin that is found primarily in the saliva and other body fluids that help guard the entrances to the body. IgA deficiency is a disorder in which the body doesn't produce enough of the antibody IgA. People with IgA deficiency tend to have allergies or get more colds and other respiratory infections, but the condition is usually not severe.
• Severe combined immunodeficiency (SCID) is also known as the "bubble boy disease" after a Texas boy with SCID who lived in a germ-free plastic bubble. SCID is a serious immune system disorder that occurs because of a lack of both B and T lymphocytes, which makes it almost impossible to fight infections.
• DiGeorge syndrome (thymic dysplasia), a birth defect in which children are born without a thymus gland, is an example of a primary T-lymphocyte disease. The thymus gland is where T lymphocytes normally mature.
• Chediak-Higashi syndrome and chronic granulomatous disease both involve the inability of the neutrophils to function normally as phagocytes.

Acquired immunodeficiencies usually develop after a person has a disease, although they can also be the result of malnutrition, burns, or other medical problems. Certain medicines also can cause problems with the functioning of the immune system. Secondary immunodeficiencies include:

• HIV (human immunodeficiency virus) infection/AIDS (acquired immunodeficiency syndrome) is a disease that slowly and steadily destroys the immune system. It is caused by HIV, a virus which wipes out certain types of lymphocytes called T-helper cells. Without T-helper cells, the immune system is unable to defend the body against normally harmless organisms, which can cause life-threatening infections in people who have AIDS. Newborns can get HIV infection from their mothers while in the uterus, during the birth process, or during breastfeeding. People can get HIV infection by having unprotected sexual intercourse with an infected person or from sharing contaminated needles for drugs, steroids, or tattoos.
• Immunodeficiencies caused by medications. Some medicines suppress the immune system. One of the drawbacks of chemotherapy treatment for cancer, for example, is that it not only attacks cancer cells, but other fast-growing, healthy cells, including those found in the bone marrow and other parts of the immune system. In addition, people with autoimmune disorders or who have had organ transplants may need to take immunosuppressant medications. These medicines can also reduce the immune system's ability to fight infections and can cause secondary immunodeficiency.

Autoimmune Disorders

In autoimmune disorders, the immune system mistakenly attacks the body's healthy organs and tissues as though they were foreign invaders. Autoimmune diseases include:

• Lupus is a chronic disease marked by muscle and joint pain and inflammation. The abnormal immune response may also involve attacks on the kidneys and other organs.
• Juvenile rheumatoid arthritis is a disease in which the body's immune system acts as though certain body parts such as the joints of the knee, hand, and foot are foreign tissue and attacks them.
• Scleroderma is a chronic autoimmune disease that can lead to inflammation and damage of the skin, joints, and internal organs.
• Ankylosing spondylitis is a disease that involves inflammation of the spine and joints, causing stiffness and pain.
• Juvenile dermatomyositis is a disorder marked by inflammation and damage of the skin and muscles.

Allergic Disorders

Allergic disorders occur when the immune system overreacts to exposure to antigens in the environment. The substances that provoke such attacks are called allergens. The immune response can cause symptoms such as swelling, watery eyes, and sneezing, and even a life-threatening reaction called anaphylaxis. Taking medications called antihistamines can relieve most symptoms. Allergic disorders include:

• Asthma, a respiratory disorder that can cause breathing problems, frequently involves an allergic response by the lungs. If the lungs are oversensitive to certain allergens (like pollen, molds, animal dander, or dust mites), it can trigger breathing tubes in the lungs to become narrowed, leading to reduced airflow and making it hard for a person to breathe.
• Eczema is an itchy rash also known as atopic dermatitis. Although atopic dermatitis is not necessarily caused by an allergic reaction, it more often occurs in kids and teens who have allergies, hay fever, or asthma or who have a family history of these conditions.
• Allergies of several types can occur in kids and teens. Environmental allergies (to dust mites, for example), seasonal allergies (such as hay fever), drug allergies (reactions to specific medications or drugs), food allergies (such as to nuts), and allergies to toxins (bee stings, for example) are the common conditions people usually refer to as allergies.

Cancers of the Immune System

Cancer occurs when cells grow out of control. This can also happen with the cells of the immune system. Lymphoma involves the lymphoid tissues and is one of the more common childhood cancers. Leukemia, which involves abnormal overgrowth of leukocytes, is the most common childhood cancer. With current medications most cases of both types of cancer in kids and teens are curable.

Although immune system disorders usually can't be prevented, you can help your child's immune system stay stronger and fight illnesses by staying informed about your child's condition and working closely with your doctor.

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Heart and Circulatory System Problems

Problems of the Heart and Circulatory System

Problems with the cardiovascular system are common — more than 64 million Americans have some type of cardiac problem. But cardiovascular problems don't just affect older people — many heart and circulatory system problems affect children and teens, too.

Heart and circulatory problems are grouped into two categories: congenital, which means the problems were present at birth, and acquired, which means that the problems developed some time during infancy, childhood, adolescence, or adulthood.

Congenital heart defects. Congenital heart defects are abnormalities in the heart's structure that are present at birth. Approximately 8 out of every 1,000 newborns have congenital heart defects ranging from mild to severe. These defects occur while the fetus is developing in the mother's uterus and it's not usually known why they occur. Some congenital heart defects are caused by genetic disorders, but most are not. What all congenital heart defects have in common, however, is that they involve abnormal or incomplete development of the heart.

A common sign of a congenital heart defect is a heart murmur — an abnormal sound (like a blowing or whooshing sound) that's heard when listening to the heart. Usually a heart murmur is detected by a doctor who's listening to the heart with a stethoscope during a routine exam. Murmurs are very common in children and can be caused by congenital heart defects or other heart conditions.

Arrhythmia. Cardiac arrhythmias, also called dysrhythmias or rhythm disorders, are abnormalities in the heart's rhythm. They may be caused by a congenital heart defect or they may be acquired later. An arrhythmia may cause the heart's rhythm to be irregular, abnormally fast, or abnormally slow. Arrhythmias can occur at any age and may be discovered during a routine physical examination. Depending on the type of rhythm disorder, an arrhythmia may be treated with medication, surgery, or pacemakers.

Cardiomyopathy. Cardiomyopathy is a chronic disease that causes the heart muscle (the myocardium) to become weakened. Usually, the disease first affects the lower chambers of the heart, the ventricles, and then progresses and damages the muscle cells and even the tissues surrounding the heart. In its most severe forms, this condition may lead to heart failure and even death. Cardiomyopathy is the #1 reason for heart transplants in children.

Coronary artery disease. The most common heart disorder in adults, coronary artery disease is caused by atherosclerosis. Deposits of fat, calcium, and dead cells, called atherosclerotic plaques, form on the inner walls of the coronary arteries (the blood vessels that supply the heart) and interfere with the smooth flow of blood. Blood flow to the heart muscle may even stop if a thrombus, or clot, forms in a coronary vessel, which may cause a heart attack. In a heart attack (or myocardial infarction), the heart muscle becomes damaged by lack of oxygen, and unless blood flow returns within minutes, muscle damage increases and the heart's ability to pump blood is compromised. If the clot can be dissolved within a few hours, damage to the heart can be reduced. Heart attacks are rare in children and teens.

Hyperlipidemia/hypercholesterolemia (high cholesterol). Cholesterol is a waxy substance that's found in the body's cells, in the blood, and in some of the foods we eat. Having too much cholesterol in the blood, also known as hypercholesterolemia or hyperlipidemia, is a major risk factor for heart disease and can lead to a heart attack.

Cholesterol is carried in the bloodstream by lipoproteins. Two kinds — low-density lipoproteins (LDL) and high-density lipoproteins (HDL) — are the most important. High levels of LDL cholesterol (the bad cholesterol) increase a person's risk for heart disease and stroke, whereas high levels of HDL cholesterol (the good cholesterol) can protect against these.

A blood test can indicate if someone's cholesterol is too high. A child's cholesterol level is borderline if it's 170 to 199 mg/dL, and it's considered high if it's above 200 mg/dL.

About 10% of teens between 12 and 19 have high cholesterol levels that put them at increased risk of cardiovascular disease.

High blood pressure (hypertension). Over time, high blood pressure can cause damage to the heart and arteries and other body organs. The symptoms of hypertension can include headache, nosebleeds, dizziness, and lightheadedness. Infants, kids, and teens can have high blood pressure, which may be caused by genetic factors, excess body weight, diet, lack of exercise, and diseases such as heart disease or kidney disease.

Kawasaki disease. Kawasaki disease (also known as mucocutaneous lymph node syndrome) affects the mucous membranes (the lining of the mouth and breathing passages), the skin, and the lymph nodes (part of the immune system). It can also lead to vasculitis, an inflammation of the blood vessels. This can affect all major arteries in the body — including the coronary arteries. It can also cause inflammation of the heart muscle, called myocarditis. When coronary arteries become inflamed, a child can develop aneurysms, which are weakened and bulging spots on the walls of arteries. This increases the risk of a blood clot forming in this weakened area, which can block the artery, possibly leading to a heart attack. In addition to the coronary arteries, the heart muscle, lining, valves, or the outer membrane that surrounds the heart can become inflamed. Arrhythmias or abnormal functioning of some heart valves can occur. Kawasaki disease has surpassed rheumatic fever as the leading cause of acquired heart disease in children in the United States.

Rheumatic heart disease. Usually the complication of an untreated strep throat infection, rheumatic fever can lead to permanent heart damage and even death. Most common in children between 5 and 15 years of age, it begins when antibodies the body produces to fight the strep infection begin to attack other parts of the body. They react to tissues in the heart valves as though they were the strep bacteria and cause the heart valves to thicken and scar. Inflammation and weakening of the heart muscle may also occur. Usually, when strep throat infections are promptly treated with antibiotics, this condition can be avoided.

Stroke. Strokes occur when the blood supply to the brain is cut off or when a blood vessel in the brain bursts and spills blood into an area of the brain, causing damage to brain cells. Children or infants who have experienced stroke may be suddenly numb or weak, especially on one side of the body, and they may experience a sudden severe headache, nausea or vomiting, and difficulty seeing, speaking, walking, or moving. During childhood, strokes are rare.

Getting plenty of exercise, eating a nutritious diet, maintaining a healthy weight, and getting regular medical checkups are the best ways to help keep the heart healthy and avoid long-term problems like high blood pressure, high cholesterol, and heart disease.

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Vision Problems

Some of the most common eye problems are refractive errors. These are the problems that eye doctors check for routinely in a vision test. Refraction means bending of light rays to focus the light coming from an image. Refractive errors are problems with the focusing of the eye, because of the way the eye is shaped, which causes the image you see to be blurred.

Refractive errors include:

• Astigmatism. In astigmatism, there's a problem with the curve of the cornea. This causes part of the eye's image to be blurry. Corrective lenses such as contact lenses or glasses can usually correct vision in people with astigmatism.
• Nearsightedness. Also called myopia or shortsightedness, nearsightedness occurs when the eye focuses the image of an object in front of the retina instead of directly on it. In most cases, people can't see well far away, but can see objects clearly close up. The condition tends to get somewhat worse through childhood and adolescence, but stabilizes in adulthood. People with this condition may need to wear glasses or contacts to correct their vision. Laser eye surgery is now commonly used in adults to correct nearsightedness permanently by changing the shape of the cornea. Laser surgery is not used in kids and teens because the eye may still be growing and the refractive error changing.
• Farsightedness. Also called hyperopia or longsightedness, farsightedness occurs when the incoming image is not focused on the retina, but behind it. This makes it difficult to see close objects clearly, even though far-off objects can be seen clearly. Glasses or contact lenses can correct this problem in kids and teens. Most adults develop a form of farsightedness called presbyopia as they get older.

Some of the other eye problems that can affect children and teens include:

Blindness. This is the loss of useful sight. Blindness can be temporary or permanent and it has many causes. Birth defects or damage to any portion of the eye, the optic nerve, or the area of the brain responsible for vision can lead to blindness. The visual impairment cannot be corrected with surgery or corrective lenses, and the condition can make it difficult to perform everyday activities. Diabetes, problems with the macula, and eye changes because of aging account for most blindness in the United States.

Cataracts. Some children are born with cataracts, a cloudiness of the eye's lenses that prevent images from being seen clearly or at all. But cataracts are much more prevalent in the elderly among whom surgery to remove cataracts and correct vision is common. In infants and young children, cataracts must be treated to prevent permanent problems with development of vision.

Color blindness. Color blindness is caused by problems in the pigments of the cones in the retina. Most people who are color-blind can see some colors. In most cases, someone who is color-blind confuses some colors with others, usually red and green. People can be born color-blind or they may develop the condition over time. The most common form of color blindness is an inherited condition that affects boys much more often than girls. Color blindness cannot be corrected.

Conjunctivitis. Conjunctivitis refers to inflammation (redness, pain, and swelling) of the conjunctiva. One type of conjunctivitis is called pinkeye, a common contagious infection in which the eyes become pinkish red and watery, and pus may form. Pinkeye is usually treated with eyedrops. Conjunctivitis may also occur due to allergies or from a scratch on the surface of the eye.

Dacryostenosis. Dacryostenosis is a blockage of the tear drainage system of the eye occurring in the lower eyelid. This is a fairly common problem in infants, but often improves with time.

Eye injuries. Injuries to the eye are one of the most common preventable causes of blindness. Eye injuries may be caused by irritants such as sand, dirt, or other foreign bodies on the eye's surface. Chemicals or foreign bodies that become embedded in the eye can also cause pain and loss of vision. Forceful blows to the eye can cause bleeding inside the eye and damage to the cornea, retina, and other important eye structures.

Glaucoma. One of the leading causes of blindness in the United States, glaucoma is a disorder that causes fluid pressure to build up inside the eye, potentially causing damage to the optic nerve. Although it can occur in infants and children, it is much more common among older adults. Unless detected early by routine screening tests, it usually goes undiagnosed until some vision is lost.

Macular degeneration is an eye disease that mainly affects older people. But sometimes infants and children can develop it, too. When someone has macular degeneration, the middle area of the retina becomes scarred. This causes eyesight to deteriorate over many years. The cause of macular degeneration is unknown in most cases, but it tends to run in families, which indicates that genes and heredity are involved.

Retinoblastoma. This is a cancerous tumor in the eye that usually appears in the first 5 years of life. There may be permanent vision loss, and the affected eye(s) may have to be removed.

Retinopathy of prematurity. Premature babies sometimes have damage to the blood vessels in the eye's retina, a condition known as retinopathy of prematurity, which can lead to severe vision loss in some cases.

Strabismus. A condition in which the eyes appear crossed, strabismus is usually caused by a defect in the balance of the pull exerted by the muscles that help the eyes stay straight and move together properly. Surgery on the eye muscles can help realign the eyes. Permanent vision loss in the less-used eye (amblyopia) can occur if the condition is not treated in early childhood.

Sty. A sty is an infection of one of the follicles from which the eyelashes grow. Sties are treated by applying warm compresses and using antibiotics.

Proper care of your child's eyes includes having periodic eye exams at routine office checkups and by an eye specialist according to the schedule recommended by your doctor. Signs that a very young child may be having vision trouble include constant eye rubbing, tearing or redness, and trouble following an object. Older kids may squint or sit too close to the TV. Kids who are having trouble seeing might also complain of headaches at the end of the day.

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Endocrine System Problems

Problems With the Endocrine System

Too much or too little of any hormone can be harmful to the body. For example, if the pituitary gland produces too much growth hormone, a child may grow excessively tall. If it produces too little, a child may be abnormally short.

Controlling the production of or replacing specific hormones can treat many endocrine disorders in children and adolescents, some of which include:

Adrenal insufficiency. This condition is characterized by decreased function of the adrenal cortex and the consequent underproduction of adrenal corticosteroid hormones. The symptoms of adrenal insufficiency may include weakness, fatigue, abdominal pain, nausea, dehydration, and skin changes. Doctors treat adrenal insufficiency by giving replacement corticosteroid hormones.

Cushing syndrome. Excessive amounts of glucocorticoid hormones in the body can lead to Cushing syndrome. In children, it most often results when a child takes large doses of synthetic corticosteroid drugs (such as prednisone) to treat autoimmune diseases such as lupus. If the condition is due to a tumor in the pituitary gland that produces excessive amounts of corticotropin and stimulates the adrenals to overproduce corticosteroids, it's known as Cushing disease. Symptoms may take years to develop and include obesity, growth failure, muscle weakness, easy bruising of the skin, acne, high blood pressure, and psychological changes. Depending on the specific cause, doctors may treat this condition with surgery, radiation therapy, chemotherapy, or drugs that block the production of hormones.

Type 1 diabetes. When the pancreas fails to produce enough insulin, type 1 diabetes (previously known as juvenile diabetes) occurs. Symptoms include excessive thirst, hunger, urination, and weight loss. In children and teens, the condition is usually an autoimmune disorder in which specific immune system cells and antibodies produced by the immune system attack and destroy the cells of the pancreas that produce insulin. The disease can cause long-term complications including kidney problems, nerve damage, blindness, and early coronary heart disease and stroke. To control their blood sugar levels and reduce the risk of developing diabetes complications, kids with this condition need regular injections of insulin.

Type 2 diabetes. Unlike type 1 diabetes, in which the body can't produce normal amounts of insulin, in type 2 diabetes the body is unable to respond to insulin normally. Children and teens with the condition tend to be overweight, and it is believed that excess body fat plays a role in the insulin resistance that characterizes the disease. In fact, the rising prevalence of this type of diabetes in kids has paralleled the dramatically increasing rates of obesity among kids in recent years. The symptoms and possible complications of type 2 diabetes are basically the same as those of type 1. Some kids and teens can control their blood sugar level with dietary changes, exercise, and oral medications, but many will need to take insulin injections like patients with type 1 diabetes.

Growth hormone problems. Too much growth hormone in children who are still growing will make their bones and other body parts grow excessively, resulting in gigantism. This rare condition is usually caused by a pituitary tumor and can be treated by removing the tumor. In contrast, when the pituitary gland fails to produce adequate amounts of growth hormone, a child's growth in height is impaired. Hypoglycemia (low blood sugar) may also occur in kids with growth hormone deficiency, particularly in infants and young children with the condition.

Hyperthyroidism. Hyperthyroidism is a condition in which the levels of thyroid hormones in the blood are excessively high. Symptoms may include weight loss, nervousness, tremors, excessive sweating, increased heart rate and blood pressure, protruding eyes, and a swelling in the neck from an enlarged thyroid gland (goiter). In kids the condition is usually caused by Graves' disease, an autoimmune disorder in which specific antibodies produced by the immune system stimulate the thyroid gland to become overactive. The disease may be controlled with medications or by removal or destruction of the thyroid gland through surgery or radiation treatments.

Hypothyroidism. Hypothyroidism is a condition in which the levels of thyroid hormones in the blood are abnormally low. Thyroid hormone deficiency slows body processes and may lead to fatigue, a slow heart rate, dry skin, weight gain, constipation, and, in kids, slowing of growth and delayed puberty. Hashimoto's thyroiditis, which results from an autoimmune process that damages the thyroid and blocks thyroid hormone production, is the most common cause of hypothyroidism in kids. Infants can also be born with an absent or underdeveloped thyroid gland, resulting in hypothyroidism. It can be treated with oral thyroid hormone replacement.

Precocious puberty. Body changes associated with puberty may occur at an abnormally young age in some kids if the pituitary hormones that stimulate the gonads to produce sex hormones rise prematurely. An injectable medication is available that can suppress the secretion of these pituitary hormones (known as gonadotropins) and arrest the progression of sexual development in most of these children.

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Digestion Problems

Digestive System Problems

Nearly everyone has a digestive problem at one time or another. Some conditions, such as indigestion or mild diarrhea, are common; they result in mild discomfort and get better on their own or are easy to treat. Others, such as inflammatory bowel disease, can be long lasting or troublesome. A doctor who specializes in the digestive system is called a GI specialist or gastroenterologist.

Problems With the Esophagus

Problems affecting the esophagus may be congenital (present at birth) or noncongenital (developed after birth). Examples include:

• Congenital conditions. Tracheoesophageal fistula is a connection between the esophagus and the trachea (windpipe) where there shouldn't be one. In babies with esophageal atresia, the esophagus comes to a dead end instead of connecting to the stomach. Both conditions are usually detected soon after a baby is born — sometimes even before — and require surgery to repair.
• Noncongenital conditions. Esophagitis (inflammation of the esophagus) can be caused by infection, certain medications, or gastroesophageal reflux disease (GERD). With GERD, the esophageal sphincter (the valve that connects the esophagus with the stomach) doesn't work well and allows the acidic contents of the stomach to move backward up into the esophagus. GERD often can be corrected through lifestyle changes, such as dietary adjustments. Sometimes, though, it requires treatment with medication.

Problems With the Stomach and Intestines

Almost everyone has experienced diarrhea or constipation. With diarrhea, muscle contractions move the contents of the intestines along too quickly and there isn't enough time for water to be absorbed before the feces are pushed out of the body. Constipation is the opposite: The contents of the large intestines do not move along fast enough and waste materials stay in the large intestine so long that too much water is removed and the feces become hard.

Other common stomach and intestinal disorders include:

• Gastrointestinal infections can be caused by viruses, by bacteria (such as Salmonella, Shigella, Campylobacter, or E. coli), or by intestinal parasites (such as amebiasis and giardiasis). Abdominal pain or cramps, diarrhea, and sometimes vomiting are the common symptoms of gastrointestinal infections. These usually go away on their own without medicines or other treatment.
• Appendicitis, an inflammation of the appendix, most often affects kids and teens between 11 and 20 years old, and requires surgery to correct. The classic symptoms of appendicitis are abdominal pain, fever, loss of appetite, and vomiting.
• Gastritis and peptic ulcers arise when a bacterium, Helicobacter pylori, or the chronic use of drugs or certain medications weakens the protective mucous coating of the stomach and duodenum, allowing acid to get through to the sensitive lining beneath. This can irritate and inflame the lining of the stomach (gastritis) or cause peptic ulcers, which are sores or holes in the lining of the stomach or the duodenum that cause pain or bleeding. Medications usually successfully treat these conditions.
• Inflammatory bowel disease (IBD) is chronic inflammation of the intestines that affects older kids, teens, and adults. There are two major types: ulcerative colitis, which usually affects just the rectum and the large intestine; and Crohn's disease, which can affect the whole gastrointestinal tract from the mouth to the anus as well as other parts of the body. They are treated with medications and, if necessary, intravenous (IV) feedings to provide nutrition. In some cases, surgery may be necessary to remove inflamed or damaged areas of the intestine.
• Celiac disease is a disorder in which the digestive system is damaged by the response of the immune system to a protein called gluten, which is found in wheat, rye, and barley and a wide range of foods, from breakfast cereal to pizza crust. People with celiac disease have difficulty digesting the nutrients from their food and may experience diarrhea, abdominal pain, bloating, exhaustion, and depression when they eat foods with gluten. Symptoms can be managed by following a gluten-free diet. Celiac disease runs in families and can become active after some sort of stress, such as surgery or a viral infection. A doctor can diagnose celiac disease with a blood test and by taking a biopsy of the small intestine.
• Irritable bowel syndrome (IBS), a common intestinal disorder, affects the colon and may cause recurrent abdominal cramps, bloating, constipation, and diarrhea. There is no cure, but IBS symptoms may be treated by changing eating habits, reducing stress, and making lifestyle changes. A doctor may also prescribe medications to relieve diarrhea or constipation. No one test is used to diagnose IBS, but a doctor may identify it based on symptoms, medical history, and a physical exam.

Problems With the Pancreas, Liver, and Gallbladder

Conditions affecting the pancreas, liver, and gallbladder often affect the ability of these organs to produce enzymes and other substances that aid in digestion. Examples include:

• Cystic fibrosis is a chronic, inherited illness where the production of abnormally thick mucus blocks the ducts or passageways in the pancreas and prevents its digestive juices from entering the intestines, making it difficult to properly digest proteins and fats. This causes important nutrients to pass out of the body unused. To help manage their digestive problems, people with cystic fibrosis can take digestive enzymes and nutritional supplements.
• Hepatitis, a condition with many different causes, is when the liver becomes inflamed and may lose its ability to function. Viral hepatitis, such as hepatitis A, B, or C, is highly contagious. Mild cases of hepatitis A can be treated at home; however, serious cases involving liver damage may require hospitalization.
• The gallbladder can develop gallstones and become inflamed — a condition called cholecystitis. Although gallbladder conditions are uncommon in kids and teens, they can occur in those who have sickle cell anemia or are being treated with certain long-term medications.

Keeping Digestion on Track

The kinds and amounts of food a person eats and how the digestive system processes that food play key roles in maintaining good health. Eating a healthy diet is the best way to prevent common digestive problems.

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Brain Problems

Things That Can Go Wrong With the Brain

Because the brain controls just about everything, when something goes wrong with it, it's often serious and can affect many different parts of the body. Inherited diseases, brain disorders associated with mental illness, and head injuries can all affect the way the brain works and upset the daily activities of the rest of the body.

Problems that can affect the brain include:

Brain tumors. A tumor is a swelling caused by overgrown tissue. A tumor in the brain may grow slowly and produce few symptoms until it becomes large, or it can grow and spread rapidly, causing severe and quickly worsening symptoms. Brain tumors in children can be benign or malignant. Benign tumors usually grow in one place and may be curable through surgery if they're located in a place where they can be removed without damaging the normal tissue near the tumor. A malignant tumor is cancerous and more likely to grow rapidly and spread.

Cerebral palsy. Cerebral palsy is the result of a developmental defect or damage to the brain before or during birth. It affects the motor areas of the brain. A person with cerebral palsy may have average intelligence or can have severe developmental delays or mental retardation. Cerebral palsy can affect body movement in many different ways. In mild cases of cerebral palsy, there may be minor muscle weakness of the arms and legs. In other cases, there may be more severe motor impairment — a child may have trouble talking and performing basic movements like walking.

Epilepsy. This condition is made up of a wide variety of seizure disorders. Partial seizures involve specific areas of the brain, and symptoms vary depending on the location of the seizure activity. Other seizures, called generalized seizures, involve a larger portion of the brain and usually cause uncontrolled movements of the entire body and loss of consciousness when they occur. Although the specific cause is unknown in many cases, epilepsy can be related to brain injury, tumors, or infections. The tendency to develop epilepsy may be inherited in families.

Headaches. Of the many different types of headaches, the most frequently occurring include tension headache (the most common type), caused by muscle tension in the head, neck, and shoulders; migraine, an intense, recurring headache with an unclear cause; and cluster headache, considered by some to be a form of migraine. Migraines occur with or without warning and may last for several hours or days. There seems to be an inherited predisposition to migraines as well as certain triggers that can lead to them. People with migraines may experience dizziness, numbness, sensitivity to light, and nausea, and may see flashing zigzag lines before their eyes.

Meningitis and encephalitis. These are infections of the brain and spinal cord that are usually caused by bacteria or viruses. Meningitis is an inflammation of the coverings of the brain and spinal cord, and encephalitis is an inflammation of the brain tissue. Both conditions may result in permanent injury to the brain.

Mental illness. Mental illnesses are psychological and behavioral in nature and involve a wide range of problems in thought and function. Certain mental illnesses are now known to be linked to structural abnormalities or chemical dysfunction of the brain. Some mental illnesses are inherited, but often the cause is unknown. Injuries to the brain and chronic drug or alcohol abuse also can trigger some mental illnesses. Signs of chronic mental illnesses such as bipolar disorder or schizophrenia may first show up in childhood. Mental illnesses that can be seen in younger people include depression, eating disorders such as bulimia or anorexia nervosa, obsessive-compulsive disorder (OCD), and phobias.

Head injuries. Head injuries fit into two categories: external (usually scalp) injuries and internal head injuries. Internal injuries may involve the skull, the blood vessels within the skull, or the brain. Fortunately, most childhood falls or blows to the head result in injury to the scalp only, which is usually more frightening than threatening. An internal head injury could have more serious implications because the skull serves as the protective helmet for the delicate brain.

Concussions are also a type of internal head injury. A concussion is the temporary loss of normal brain function as a result of an injury. Repeated concussions can result in permanent injury to the brain. One of the most common reasons kids get concussions is through sports, so it's important to make sure they wear appropriate protective gear and don't continue to play if they've had a head injury.

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Bones, Muscles, and Joints Problems

Problems With the Bones, Muscles, and Joints

As strong as bones are, they can break. Muscles can weaken, and joints (as well as tendons, ligaments, and cartilage) can be damaged by injury or disease.

Problems that can affect the bones, muscles, and joints include:

• Arthritis. Arthritis is the inflammation of a joint, and people who have it experience swelling, warmth, pain, and often have trouble moving. Although we often think of arthritis as a condition that affects only older people, arthritis can also occur in children and teens. Health problems that involve arthritis in kids and teens include juvenile rheumatoid arthritis (JRA), lupus, Lyme disease, and septic arthritis (a bacterial infection of a joint).
• Fracture. A fracture occurs when a bone breaks; it may crack, snap, or shatter. After a fracture, new bone cells fill the gap and repair the break. Applying a strong plaster cast, which keeps the bone in the correct position until it heals, is the usual treatment. If the fracture is complicated, metal pins and plates can be placed to better stabilize it while the bone heals.
• Muscular dystrophy. Muscular dystrophy is an inherited group of diseases that affect the muscles, causing them to weaken and break down over time. The most common form in childhood is called Duchenne muscular dystrophy, and it most often affects boys.
• Osgood-Schlatter disease(OSD). Osgood-Schlatter disease is an inflammation (pain and swelling) of the bone, cartilage, and/or tendon at the top of the shinbone, where the tendon from the kneecap attaches. OSD usually strikes active teens around the beginning of their growth spurts, the approximately 2-year period during which they grow most rapidly.
• Osteomyelitis. Osteomyelitis is a bone infection often caused by Staphylococcus aureus bacteria, though other types of bacteria can cause it, too. In kids and teens, osteomyelitis usually affects the long bones of the arms and legs. Osteomyelitis often develops after an injury or trauma.
• Osteoporosis. In osteoporosis, bone tissue becomes brittle, thin, and spongy. Bones break easily, and the spine sometimes begins to crumble and collapse. Although the condition usually affects older people, kids and teens with eating disorders can get the condition, as can girls with female athlete triad — a combination of three conditions that some girls who exercise or play sports may be at risk for: disordered eating, amenorrhea (loss of a girl's period), and osteoporosis. Participation in sports where a thin appearance is valued can put a girl at risk for female athlete triad.
• Repetitive stress injuries (RSIs). RSIs are a group of injuries that happen when too much stress is placed on a part of the body, resulting in inflammation (pain and swelling), muscle strain, or tissue damage. This stress generally occurs from repeating the same movements over and over again. RSIs are becoming more common in kids and teens because they spend more time than ever using computers. Playing sports like tennis that involve repetitive motions can also lead to RSIs. Kids and teens who spend a lot of time playing musical instruments or video games are also at risk for RSIs.
• Scoliosis. Every person's spine curves a little bit; a certain amount of curvature is necessary for people to move and walk properly. But 3–5 people out of 1,000 have scoliosis, which causes the spine to curve too much. It can be hereditary, so someone who has scoliosis often has family members who have it.
• Strains and sprains. Strains occur when muscles or tendons are overstretched. Sprains are an overstretching or a partial tearing of the ligaments. Strains usually happen when a person takes part in a strenuous activity when the muscles haven't properly warmed up or the muscle is not used to the activity (such as a new sport or playing a familiar sport after a long break). Sprains, on the other hand, are usually the result of an injury, such as twisting an ankle or knee. A common sprain injury is a torn Achilles tendon, which connects the calf muscles to the heel. This tendon can snap, but it usually can be repaired by surgery. Both strains and sprains are common in kids and teens because they're active and still growing.
• Tendinitis. This common sports injury that usually happens after overexercising a muscle. The tendon and tendon sheath become inflamed, which can be painful. Resting the muscles and taking anti-inflammatory medication can bring relief.

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Blood Problems

Things That Can Go Wrong With Blood

Most of the time, blood functions without problems, but sometimes, blood disorders or diseases can cause illness. Diseases of the blood that commonly affect kids can involve any or all of the three types of blood cells. Other types of blood diseases affect the proteins and chemicals in the plasma that are responsible for clotting.

Diseases of the Red Blood Cells

The most common condition affecting RBCs is anemia, a lower-than-normal number of red cells in the blood. Anemia is accompanied by a decrease in the amount of hemoglobin. The symptoms of anemia — such as pale skin, weakness, a fast heart rate, and poor growth in infants and children — happen because of the blood's reduced capacity for carrying oxygen.

Anemia typically is caused by either inadequate RBC production or unusually rapid RBC destruction. In severe cases of chronic anemia, or when a large amount of blood is lost, someone may need a transfusion of RBCs or whole blood.

Anemia resulting from inadequate RBC production. Conditions that can cause a reduced production of red blood cells include:

• Iron deficiency anemia. The most common type of anemia, it affects kids and teens of any age who have a diet low in iron or who've lost a lot of RBCs (and the iron they contain) through bleeding. Premature babies, infants with poor nutrition, menstruating teenage girls, and those with ongoing blood loss due to illnesses such as inflammatory bowel disease are especially likely to have iron deficiency anemia.
• Lead poisoning. When lead enters the body, most of it goes into RBCs where it can interfere with the production of hemoglobin. This can result in anemia. Lead poisoning can also affect — and sometimes permanently damage — other body tissues, including the brain and nervous system. Although lead poisoning is much less common now, it still is a problem in many larger cities, especially where young children might ingest paint chips or the dust that comes from lead-containing paints peeling off the walls in older buildings.
• Anemia due to chronic disease. Kids with chronic diseases (such as cancer or human immunodeficiency virus infection) often develop anemia as a complication of their illness.
• Anemia due to kidney disease. The kidneys produce erythropoietin, a hormone that stimulates production of red cells in the bone marrow. Kidney disease can interfere with the production of this hormone.

Anemia resulting from unusually rapid red blood cell destruction. When RBCs are destroyed more quickly than normal by disease (a process called hemolysis), the bone marrow will make up for it by increasing production of new red cells to take their place. But if RBCs are destroyed faster than they can be replaced, a person will develop anemia.

Several causes of increased red blood cell destruction can affect kids:

• G6PD deficiency. G6PD is an enzyme that helps to protect red blood cells from the destructive effects of certain chemicals found in foods and medications. When the enzyme is deficient, these chemicals can cause red cells to hemolyze, or burst. G6PD deficiency is a common hereditary disease among people of African, Mediterranean, and Southeast Asian descent.
• Hereditary spherocytosis is an inherited condition in which RBCs are misshapen (like tiny spheres, instead of disks) and especially fragile because of a genetic problem with a protein in the structure of the red blood cell. This fragility causes the cells to be easily destroyed.
• Autoimmune hemolytic anemia. Sometimes — because of disease or for no known reason — the body's immune system mistakenly attacks and destroys RBCs.
• Sickle cell anemia, most common in people of African descent, is a hereditary disease that results in the production of abnormal hemoglobin. The RBCs become sickle shaped, they cannot carry oxygen adequately, and they are easily destroyed. The sickle-shaped blood cells also tend to abnormally stick together, causing obstruction of blood vessels. This blockage in the blood vessels can seriously damage organs and cause bouts of severe pain.

Diseases of the White Blood Cells

• Neutropenia occurs when there aren't enough of a certain type of white blood cell to protect the body against bacterial infections. People who take certain chemotherapy drugs to treat cancer may develop neutropenia.
• Human immunodeficiency virus (HIV) is a virus that attacks certain types of WBCs (lymphocytes) that work to fight infection. Infection with the virus can result in AIDS (acquired immunodeficiency syndrome), leaving the body prone to infections and certain other diseases. Newborns can become infected with the virus from their infected mothers while in the uterus, during birth, or from breastfeeding, although HIV infection of the fetus and newborn is usually preventable with proper medical treatment of the mother during pregnancy and delivery. Teens and adults can get HIV from sex with an infected person or from sharing contaminated needles used for injecting drugs or tattoo ink.
• Leukemias are cancers of the cells that produce WBCs. These cancers include acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphocytic leukemia (ALL), and chronic lymphocytic leukemia (CLL). The most common types of leukemia affecting kids are ALL and AML. In the past 25 years, scientists have made great advances in treating several types of childhood leukemia, most notably certain types of ALL.

Diseases of Platelets

• Thrombocytopenia, or a lower than normal number of platelets, is usually diagnosed because a person has abnormal bruising or bleeding. Thrombocytopenia can happen when someone takes certain drugs or develops infections or leukemia or when the body uses up too many platelets. Idiopathic thrombocytopenic purpura (ITP) is a condition in which the immune system attacks and destroys platelets.

Diseases of the Clotting System

The body's clotting system depends on platelets as well as many clotting factors and other blood components. If a hereditary defect affects any of these components, a person can have a bleeding disorder.

Common bleeding disorders include:

• Hemophilia, an inherited condition that almost exclusively affects boys, involves a lack of particular clotting factors in the blood. People with severe hemophilia are at risk for excessive bleeding and bruising after dental work, surgery, and trauma. They may experience episodes of life-threatening internal bleeding, even if they haven't been injured.
• Von Willebrand disease, the most common hereditary bleeding disorder, also involves a clotting-factor deficiency. It affects both males and females.

Other causes of clotting problems include chronic liver disease (clotting factors are produced in the liver) and vitamin K deficiency (the vitamin is necessary for the production of certain clotting factors).

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Male Reproductive System

All living things reproduce. Reproduction — the process by which organisms make more organisms like themselves — is one of the things that sets living things apart from nonliving things. But even though the reproductive system is essential to keeping a species alive, unlike other body systems it's not essential to keeping an individual alive.

In the human reproductive process, two kinds of sex cells, or gametes, are involved. The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system to create a new individual. Both the male and female reproductive systems are essential for reproduction.

Humans, like other organisms, pass certain characteristics of themselves to the next generation through their genes, the special carriers of human traits. The genes parents pass along to their offspring are what make kids similar to others in their family, but they're also what make each child unique. These genes come from the father's sperm and the mother's egg, which are produced by the male and female reproductive systems.

Understanding the male reproductive system, what it does, and the problems that can affect it can help you better understand your son's reproductive health.

About the Male Reproductive System

Most species have two sexes: male and female. Each sex has its own unique reproductive system. They are different in shape and structure, but both are specifically designed to produce, nourish, and transport either the egg or sperm.

Unlike the female, whose sex organs are located entirely within the pelvis, the male has reproductive organs, or genitals, that are both inside and outside the pelvis. The male genitals include:

• the testicles
• the duct system, which is made up of the epididymis and the vas deferens
• the accessory glands, which include the seminal vesicles and prostate gland
• the penis

In a guy who has reached sexual maturity, the two testicles, or testes, produce and store millions of tiny sperm cells. The testicles are oval-shaped and grow to be about 2 inches (5 centimeters) in length and 1 inch (3 centimeters) in diameter. The testicles are also part of the endocrine system because they produce hormones, including testosterone. Testosterone is a major part of puberty in boys, and as a guy makes his way through puberty, his testicles produce more and more of it. Testosterone is the hormone that causes boys to develop deeper voices, bigger muscles, and body and facial hair, and it also stimulates the production of sperm.

Alongside the testicles are the epididymis and the vas deferens, which make up the duct system of the male reproductive organs. The vas deferens is a muscular tube that passes upward alongside the testicles and transports the sperm-containing fluid called semen. The epididymis is a set of coiled tubes (one for each testicle) that connects to the vas deferens.

The epididymis and the testicles hang in a pouch-like structure outside the pelvis called the scrotum. This bag of skin helps to regulate the temperature of testicles, which need to be kept cooler than body temperature to produce sperm. The scrotum changes size to maintain the right temperature. When the body is cold, the scrotum shrinks and becomes tighter to hold in body heat. When it's warm, the scrotum becomes larger and more floppy to get rid of extra heat. This happens without a guy ever having to think about it. The brain and the nervous system give the scrotum the cue to change size.

The accessory glands, including the seminal vesicles and the prostate gland, provide fluids that lubricate the duct system and nourish the sperm. The seminal vesicles are sac-like structures attached to the vas deferens to the side of the bladder. The prostate gland, which produces some of the parts of semen, surrounds the ejaculatory ducts at the base of the urethra, just below the bladder. The urethra is the channel that carries the semen to the outside of the body through the penis. The urethra is also part of the urinary system because it is also the channel through which urine passes as it leaves the bladder and exits the body.

The penis is actually made up of two parts: the shaft and the glans. The shaft is the main part of the penis and the glans is the tip (sometimes called the head). At the end of the glans is a small slit or opening, which is where semen and urine exit the body through the urethra. The inside of the penis is made of a spongy tissue that can expand and contract.

All boys are born with a foreskin, a fold of skin at the end of the penis covering the glans. Some boys are circumcised, which means that a doctor or clergy member cuts away the foreskin. Circumcision is usually performed during a baby boy's first few days of life. Although circumcision is not medically necessary, parents who choose to have their children circumcised often do so based on religious beliefs, concerns about hygiene, or cultural or social reasons. Boys who have circumcised penises and those who don't are no different: All penises work and feel the same, regardless of whether the foreskin has been removed.

What the Male Reproductive System Does

The male sex organs work together to produce and release semen into the reproductive system of the female during sexual intercourse. The male reproductive system also produces sex hormones, which help a boy develop into a sexually mature man during puberty.

When a baby boy is born, he has all the parts of his reproductive system in place, but it isn't until puberty that he is able to reproduce. When puberty begins, usually between the ages of 10 and 14, the pituitary gland — which is located near the brain — secretes hormones that stimulate the testicles to produce testosterone. The production of testosterone brings about many physical changes. Although the timing of these changes is different for every guy, the stages of puberty generally follow a set sequence.

• During the first stage of male puberty, the scrotum and testes grow larger.
• Next, the penis becomes longer, and the seminal vesicles and prostate gland grow.
• Hair begins to appear in the pubic area and later it grows on the face and underarms. During this time, a male's voice also deepens.
• Boys also undergo a growth spurt during puberty as they reach their adult height and weight.

A male who has reached puberty will produce millions of sperm cells every day. Each sperm is extremely small: only 1/600 of an inch (0.05 millimeters long). Sperm develop in the testicles within a system of tiny tubes called the seminiferous tubules. At birth, these tubules contain simple round cells, but during puberty, testosterone and other hormones cause these cells to transform into sperm cells. The cells divide and change until they have a head and short tail, like tadpoles. The head contains genetic material (genes). The sperm use their tails to push themselves into the epididymis, where they complete their development. It takes sperm about 4 to 6 weeks to travel through the epididymis.

The sperm then move to the vas deferens, or sperm duct. The seminal vesicles and prostate gland produce a whitish fluid called seminal fluid, which mixes with sperm to form semen when a male is sexually stimulated. The penis, which usually hangs limp, becomes hard when a male is sexually excited. Tissues in the penis fill with blood and it becomes stiff and erect (an erection). The rigidity of the erect penis makes it easier to insert into the female's vagina during sexual intercourse. When the erect penis is stimulated, muscles around the reproductive organs contract and force the semen through the duct system and urethra. Semen is pushed out of the male's body through his urethra — this process is called ejaculation. Each time a guy ejaculates, it can contain up to 500 million sperm.

When the male ejaculates during intercourse, semen is deposited into the female's vagina. From the vagina the sperm make their way up through the cervix and move through the uterus with help from uterine contractions. If a mature egg is in one of the female's fallopian tubes, a single sperm may penetrate it, and fertilization, or conception, occurs.

This fertilized egg is now called a zygote and contains 46 chromosomes — half from the egg and half from the sperm. The genetic material from the male and female has combined so that a new individual can be created. The zygote divides again and again as it grows in the female's uterus, maturing over the course of the pregnancy into an embryo, a fetus, and finally a newborn baby.

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Female Reproductive System

All living things reproduce. Reproduction — the process by which organisms make more organisms like themselves — is one of the things that sets living things apart from nonliving matter. But even though the reproductive system is essential to keeping a species alive, unlike other body systems, it's not essential to keeping an individual alive.

In the human reproductive process, 2 kinds of sex cells, or gametes, are involved. The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system to create a new individual.

Both the male and female reproductive systems are essential for reproduction. The female needs a male to fertilize her egg, even though it is she who carries offspring through pregnancy and childbirth.

Humans, like other organisms, pass certain characteristics of themselves to the next generation through their genes, the special carriers of human traits. The genes that parents pass along to their children are what make children similar to others in their family, but they are also what make each child unique. These genes come from the male's sperm and the female's egg, which are produced by the male and female reproductive systems.

About the Female Reproductive System

Most species have two sexes: male and female. Each sex has its own unique reproductive system. They are different in shape and structure, but both are specifically designed to produce, nourish, and transport either the egg or sperm.

Unlike the male, the human female has a reproductive system located entirely in the pelvis. The external part of the female reproductive organs is called the vulva, which means covering. Located between the legs, the vulva covers the opening to the vagina and other reproductive organs located inside the body.

The fleshy area located just above the top of the vaginal opening is called the mons pubis. Two pairs of skin flaps called the labia (which means lips) surround the vaginal opening. The clitoris, a small sensory organ, is located toward the front of the vulva where the folds of the labia join. Between the labia are openings to the urethra (the canal that carries urine from the bladder to the outside of the body) and vagina. Once girls become sexually mature, the outer labia and the mons pubis are covered by pubic hair.

A female's internal reproductive organs are the vagina, uterus, fallopian tubes, and ovaries.

The vagina is a muscular, hollow tube that extends from the vaginal opening to the uterus. The vagina is about 3 to 5 inches (8 to 12 centimeters) long in a grown woman. Because it has muscular walls, it can expand and contract. This ability to become wider or narrower allows the vagina to accommodate something as slim as a tampon and as wide as a baby. The vagina's muscular walls are lined with mucous membranes, which keep it protected and moist.

The vagina serves three purposes:

1. It's where the penis is inserted during sexual intercourse.
2. It's the pathway that a baby takes out of a woman's body during childbirth, called the birth canal.
3. It provides the route for the menstrual blood (the period) to leave the body from the uterus.

A thin sheet of tissue with one or more holes in it called the hymen partially covers the opening of the vagina. Hymens are often different from female to female. Most women find their hymens have stretched or torn after their first sexual experience, and the hymen may bleed a little (this usually causes little, if any, pain). Some women who have had sex don't have much of a change in their hymens, though.

The vagina connects with the uterus, or womb, at the cervix (which means neck). The cervix has strong, thick walls. The opening of the cervix is very small (no wider than a straw), which is why a tampon can never get lost inside a girl's body. During childbirth, the cervix can expand to allow a baby to pass.

The uterus is shaped like an upside-down pear, with a thick lining and muscular walls — in fact, the uterus contains some of the strongest muscles in the female body. These muscles are able to expand and contract to accommodate a growing fetus and then help push the baby out during labor. When a woman isn't pregnant, the uterus is only about 3 inches (7.5 centimeters) long and 2 inches (5 centimeters) wide.

At the upper corners of the uterus, the fallopian tubes connect the uterus to the ovaries. The ovaries are two oval-shaped organs that lie to the upper right and left of the uterus. They produce, store, and release eggs into the fallopian tubes in the process called ovulation. Each ovary measures about 1½ to 2 inches (4 to 5 centimeters) in a grown woman.

There are two fallopian tubes, each attached to a side of the uterus. The fallopian tubes are about 4 inches (10 centimeters) long and about as wide as a piece of spaghetti. Within each tube is a tiny passageway no wider than a sewing needle. At the other end of each fallopian tube is a fringed area that looks like a funnel. This fringed area wraps around the ovary but doesn't completely attach to it. When an egg pops out of an ovary, it enters the fallopian tube. Once the egg is in the fallopian tube, tiny hairs in the tube's lining help push it down the narrow passageway toward the uterus.

The ovaries are also part of the endocrine system because they produce female sex hormones such as estrogen and progesterone.
What the Female Reproductive System Does

The female reproductive system enables a woman to:

* produce eggs (ova)
* have sexual intercourse
* protect and nourish the fertilized egg until it is fully developed
* give birth

Sexual reproduction couldn't happen without the sexual organs called the gonads. Although most people think of the gonads as the male testicles, both sexes actually have gonads: In females the gonads are the ovaries. The female gonads produce female gametes (eggs); the male gonads produce male gametes (sperm). After an egg is fertilized by the sperm, the fertilized egg is called the zygote.

When a baby girl is born, her ovaries contain hundreds of thousands of eggs, which remain inactive until puberty begins. At puberty, the pituitary gland, located in the central part of the brain, starts making hormones that stimulate the ovaries to produce female sex hormones, including estrogen. The secretion of these hormones causes a girl to develop into a sexually mature woman.

Toward the end of puberty, girls begin to release eggs as part of a monthly period called the menstrual cycle. Approximately once a month, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes.

Unless the egg is fertilized by a sperm while in the fallopian tube, the egg dries up and leaves the body about 2 weeks later through the uterus — this is menstruation. Blood and tissues from the inner lining of the uterus combine to form the menstrual flow, which in most girls lasts from 3 to 5 days. A girl's first period is called menarche.

It's common for women and girls to experience some discomfort in the days leading to their periods. Premenstrual syndrome (PMS) includes both physical and emotional symptoms that many girls and women get right before their periods, such as acne, bloating, fatigue, backaches, sore breasts, headaches, constipation, diarrhea, food cravings, depression, irritability, or difficulty concentrating or handling stress. PMS is usually at its worst during the 7 days before a girl's period starts and disappears once it begins.

Many girls also experience abdominal cramps during the first few days of their periods caused by prostaglandins, chemicals in the body that make the smooth muscle in the uterus contract. These involuntary contractions can be either dull or sharp and intense.

It can take up to 2 years from menarche for a girl's body to develop a regular menstrual cycle. During that time, her body is adjusting to the hormones puberty brings. On average, the monthly cycle for an adult woman is 28 days, but the range is from 23 to 35 days.

If a female and male have sex within several days of the female's ovulation, fertilization can occur. When the male ejaculates (when semen leaves a male's penis), between 0.05 and 0.2 fluid ounces (1.5 to 6.0 milliliters) of semen is deposited into the vagina. Between 75 and 900 million sperm are in this small amount of semen, and they "swim" up from the vagina through the cervix and uterus to meet the egg in the fallopian tube. It takes only one sperm to fertilize the egg.

About a week after the sperm fertilizes the egg, the fertilized egg (zygote) has become a multicelled blastocyst. A blastocyst is about the size of a pinhead, and it's a hollow ball of cells with fluid inside. The blastocyst burrows itself into the lining of the uterus, called the endometrium. The hormone estrogen causes the endometrium to become thick and rich with blood. Progesterone, another hormone released by the ovaries, keeps the endometrium thick with blood so that the blastocyst can attach to the uterus and absorb nutrients from it. This process is called implantation.

As cells from the blastocyst take in nourishment, another stage of development, the embryonic stage, begins. The inner cells form a flattened circular shape called the embryonic disk, which will develop into a baby. The outer cells become thin membranes that form around the baby. The cells multiply thousands of times and move to new positions to eventually become the embryo.

After approximately 8 weeks, the embryo is about the size of an adult's thumb, but almost all of its parts — the brain and nerves, the heart and blood, the stomach and intestines, and the muscles and skin — have formed.

During the fetal stage, which lasts from 9 weeks after fertilization to birth, development continues as cells multiply, move, and change. The fetus floats in amniotic fluid inside the amniotic sac. The fetus receives oxygen and nourishment from the mother's blood via the placenta, a disk-like structure that sticks to the inner lining of the uterus and connects to the fetus via the umbilical cord. The amniotic fluid and membrane cushion the fetus against bumps and jolts to the mother's body.

Pregnancy lasts an average of 280 days — about 9 months. When the baby is ready for birth, its head presses on the cervix, which begins to relax and widen to get ready for the baby to pass into and through the vagina. The mucus that has formed a plug in the cervix loosens, and with amniotic fluid, comes out through the vagina when the mother's water breaks.

When the contractions of labor begin, the walls of the uterus contract as they are stimulated by the pituitary hormone oxytocin. The contractions cause the cervix to widen and begin to open. After several hours of this widening, the cervix is dilated (opened) enough for the baby to come through. The baby is pushed out of the uterus, through the cervix, and along the birth canal. The baby's head usually comes first; the umbilical cord comes out with the baby and is cut after the baby is delivered.

The last stage of the birth process involves the delivery of the placenta, which is now called the afterbirth. After it has separated from the inner lining of the uterus, contractions of the uterus push it out, along with its membranes and fluids.

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Spleen and Lymphatic System

The lymphatic system is an extensive drainage network that helps keep bodily fluid levels in balance and defends the body against infections. It is made up of a network of lymphatic vessels that carry lymph — a clear, watery fluid that contains protein molecules, salts, glucose, urea, and other substances — throughout the body.

The spleen, which is located in the upper left part of the abdomen under the ribcage, works as part of the lymphatic system to protect the body, clearing worn out red blood cells and other foreign bodies from the bloodstream to help fight off infection.

About the Spleen and Lymphatic System
One of the lymphatic system's major jobs to collect extra lymph fluid from body tissues and return it to the blood. This process is crucial because water, proteins, and other substances are continuously leaking out of tiny blood capillaries into the surrounding body tissues. If the lymphatic system didn't drain the excess fluid from the tissues, the lymph fluid would build up in the body's tissues, and they would swell.

The lymphatic system also helps defend the body against germs like viruses, bacteria, and fungi that can cause illnesses. Those germs are filtered out in the lymph nodes, small masses of tissue located along the network of lymph vessels. The nodes house lymphocytes, a type of white blood cell. Some of those lymphocytes make antibodies, special proteins that fight off germs and stop infections from spreading by trapping disease-causing germs and destroying them.

The spleen also helps the body fight infection. The spleen contains lymphocytes and another kind of white blood cell called macrophages, which engulf and destroy bacteria, dead tissue, and foreign matter and remove them from the blood passing through the spleen.

Basic Anatomy
The lymphatic system is a network of very small tubes (or vessels) that drain lymph fluid from all over the body. The major parts of the lymph tissue are located in the bone marrow, spleen, thymus gland, lymph nodes, and the tonsils. The heart, lungs, intestines, liver, and skin also contain lymphatic tissue.

One of the major lymphatic vessels is the thoracic duct, which begins near the lower part of the spine and collects lymph from the pelvis, abdomen, and lower chest. The thoracic duct runs up through the chest and empties into the blood through a large vein near the left side of the neck. The right lymphatic duct is the other major lymphatic vessel and collects lymph from the right side of the neck, chest, and arm, and empties into a large vein near the right side of the neck.

Lymph nodes are round or kidney-shaped, and can be up to 1 inch in diameter. Most of the lymph nodes are found in clusters in the neck, armpit, and groin area. Nodes are also located along the lymphatic pathways in the chest, abdomen, and pelvis, where they filter the blood. Inside the lymph nodes, lymphocytes called T-cells and B-cells help the body fight infection. Lymphatic tissue is also scattered throughout the body in different major organs and in and around the gastrointestinal tract.

The spleen helps control the amount of blood and blood cells that circulate through the body and helps destroy damaged cells.

How A Healthy Lymph System Typically Works

Carrying Away Waste
Lymph fluid drains into lymph capillaries, which are tiny vessels. The fluid is then pushed along when a person breathes or the muscles contract. The lymph capillaries are very thin, and they have many tiny openings that allow gases, water, and nutrients to pass through to the surrounding cells, nourishing them and taking away waste products. When lymph fluid leaks through in this way it is called interstitial fluid.

Lymph vessels collect the interstitial fluid and then return it to the bloodstream by emptying it into large veins in the upper chest, near the neck.

Fighting Infection
Lymph fluid enters the lymph nodes, where macrophages fight off foreign bodies like bacteria, removing them from the bloodstream. After these substances have been filtered out, the lymph fluid leaves the lymph nodes and returns to the veins, where it re-enters the bloodstream.

When a person has an infection, germs collect in the lymph nodes. If the throat is infected, for example, the lymph nodes of the neck may swell. That's why doctors check for swollen lymph glands in the neck when your throat is infected.

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Skin, Hair, and Nails

Skin Basics
Skin is our largest organ. If the skin of a typical 150-pound (68-kilogram) adult male were stretched out flat, it would cover about 2 square yards (1.7 square meters) and weigh about 9 pounds (4 kilograms). Our skin protects the network of muscles, bones, nerves, blood vessels, and everything else inside our bodies. Our eyelids have the thinnest skin, the soles of our feet the thickest. Skin is essential in many ways. It forms a barrier that prevents harmful substances and microorganisms from entering the body. It protects body tissues against injury. It also controls the loss of life-sustaining fluids like blood and water, helps regulate body temperature through perspiration, and protects from the sun's damaging ultraviolet rays.

Without the nerve cells in skin, people couldn't feel warmth, cold, or other sensations. For instance, goosebumps form when the erector pili muscles contract to make hairs on the skin stand up straight when someone is cold or frightened — the blood vessels keep the body from losing heat by narrowing as much as possible and keeping the warm blood away from the skin's surface, offering insulation and protection.

Every square inch of skin contains thousands of cells and hundreds of sweat glands, oil glands, nerve endings, and blood vessels. Skin is made up of three layers: the epidermis, dermis, and the subcutaneous tissue.

The upper layer of our skin, the epidermis, is the tough, protective outer layer. It's about as thick as a sheet of paper over most parts of the body. The epidermis has four layers of cells that are constantly flaking off and being renewed. In these four layers are three special types of cells:

Melanocytes produce melanin, the pigment that gives skin its color. All people have roughly the same number of melanocytes; those of dark-skinned people produce more melanin. Exposure to sunlight increases the production of melanin, which is why people get suntanned or freckled.
Keratinocytes produce keratin, a type of protein that is a basic component of hair, skin, nails, and helps create an intact barrier.
Langerhans cells help protect the body against infection.

Because the cells in the epidermis are completely replaced about every 28 days, cuts and scrapes heal quickly.

Below the epidermis is the next layer of our skin, the dermis, which is made up of blood vessels, nerve endings, and connective tissue. The dermis nourishes the epidermis. Two types of fibers in the dermis — collagen and elastin — help the skin stretch when we bend and reposition itself when we straighten up. Collagen is strong and hard to stretch, and elastin, as its name suggests, is elastic. In older people, some of the elastin-containing fibers degenerate, which is one reason why the skin looks wrinkled.

The dermis also contains a person's sebaceous glands. These glands, which surround and empty into hair follicles and pores, produce the oil sebum that lubricates the skin and hair. Sebaceous glands are found mostly in the skin on the face, upper back, shoulders, and chest.

Most of the time, the sebaceous glands make the right amount of sebum. As a person's body begins to mature and develop during the teenage years, though, hormones stimulate the sebaceous glands to make more sebum. When pores become clogged by too much sebum and too many dead skin cells, this contributes to acne. Later in life, these glands produce less sebum, which contributes to dry skin as people age.

The bottom layer of our skin, the subcutaneous tissue, is made up of connective tissue, sweat glands, blood vessels, and cells that store fat. This layer helps protect the body from blows and other injuries and helps it hold in body heat.

There are two types of sweat glands. The eccrine glands are found everywhere, although they're mostly in the forehead, palms, and soles of the feet. By producing sweat, these glands help regulate body temperature, and waste products are excreted through them.

The apocrine glands develop at puberty and are concentrated in the armpits and pubic region. The sweat from the apocrine glands is thicker than that produced by the eccrine glands. Although this sweat doesn't smell, when it mixes with bacteria on the skin's surface, it can cause body odor. A normal, healthy adult secretes about 1 pint (about half a liter) of sweat daily, but this may be increased by physical activity, fever, or a hot environment.


Hair Basics
Hair is actually a modified type of skin. Hair grows everywhere on the human body except the palms of the hands, soles of the feet, eyelids, and lips. Hair grows more quickly in summer than winter, and more slowly at night than during the day. The hair on our heads isn't just there for looks. It keeps us warm by preserving heat. The hair in the nose, ears, and around the eyes protects these sensitive areas from dust and other small particles. Eyebrows and eyelashes protect eyes by decreasing the amount of light and particles that go into them. The fine hair that covers the body provides warmth and protects the skin. Hair also cushions the body against injury.

Human hair consists of the hair shaft, which projects from the skin's surface, and the root, a soft thickened bulb at the base of the hair embedded in the skin. The root ends in the hair bulb, which sits in a sac-like pit in the skin called the follicle, from which the hair grows.

At the bottom of the follicle is the papilla, where hair growth actually takes place. The papilla contains an artery that nourishes the root of the hair. As cells multiply and produce keratin to harden the structure, they're pushed up the follicle and through the skin's surface as a shaft of hair. Each hair has three layers: the medulla at the center, which is soft; the cortex, which surrounds the medulla and is the main part of the hair; and the cuticle, the hard outer layer that protects the shaft.

Hair grows by forming new cells at the base of the root. These cells multiply to form a rod of tissue in the skin. The rods of cells move upward through the skin as new cells form beneath them. As they move up, they're cut off from their supply of nourishment and start to form a hard protein called keratin in a process called keratinization. As this process occurs, the hair cells die. The dead cells and keratin form the shaft of the hair.

Each hair grows about ¼ inch (about 6 millimeters) every month and keeps on growing for up to 6 years. The hair then falls out and another grows in its place. The length of a person's hair depends on the length of the growing phase of the follicle. Follicles are active for 2 to 6 years; they rest for about 3 months after that. A person becomes bald if the scalp follicles become inactive and no longer produce new hair. Thick hair grows out of large follicles; narrow follicles produce thin hair.

The color of a person's hair is determined by the amount and distribution of melanin in the cortex of each hair (the same melanin that's found in the epidermis). Hair also contains a yellow-red pigment; people who have blonde or red hair have only a small amount of melanin in their hair. Hair becomes gray when people age because pigment no longer forms.


Nail Basics
Like hair, nails are a type of modified skin. Nails protect the sensitive tips of fingers and toes. Human nails aren't necessary for living, but they do provide support for the tips of the fingers and toes, protect them from injury, and aid in picking up small objects. Without them, we'd have a hard time scratching an itch or untying a knot. Nails can be an indicator of a person's general health, and illness often affects their growth. Nails grow out of deep folds in the skin of the fingers and toes. As epidermal cells below the nail root move up to the surface of the skin, they increase in number, and those closest to the nail root become flattened and pressed tightly together. Each cell is transformed into a thin plate; these plates are piled in layers to form the nail. As with hair, nails are formed by keratinization. When the nail cells accumulate, the nail is pushed forward.

The skin below the nail is called the matrix. The larger part of the nail, the nail plate, looks pink because of the network of tiny blood vessels in the underlying dermis. The whitish crescent-shaped area at the base of the nail is called the lunula.

Fingernails grow about three or four times as quickly as toenails. Like hair, nails grow more rapidly in summer than in winter. If a nail is torn off, it will regrow if the matrix isn't severely injured. White spots on the nail are sometimes due to temporary changes in growth rate.

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Mouth and Teeth

Here's how each aspect of the mouth and teeth plays an important role in our daily lives.

Basic Anatomy of the Mouth and Teeth
The entrance to the digestive tract, the mouth is lined with mucous membranes. The membrane-covered roof of the mouth is called the palate. The front part consists of a bony portion called the hard palate, with a fleshy rear part called the soft palate. The hard palate divides the mouth and the nasal passages above. The soft palate forms a curtain between the mouth and the throat, or pharynx, to the rear. The soft palate contains the uvula, the dangling flesh at the back of the mouth. The tonsils are located on either side of the uvula and look like twin pillars holding up the opening to the pharynx.

A bundle of muscles extends from the floor of the mouth to form the tongue. The upper surface of the tongue is covered with tiny bumps called papillae. These contain tiny pores that are our taste buds. Four kinds of taste buds are grouped together on certain areas of the tongue — those that sense sweet, salty, sour, and bitter tastes. Three pairs of salivary glands secrete saliva, which contains a digestive enzyme called amylase that starts the breakdown of carbohydrates even before food enters the stomach.

The lips are covered with skin on the outside and with slippery mucous membranes on the inside of the mouth. The major lip muscle, called the orbicularis oris, allows for the lips' mobility. The reddish tint of the lips comes from underlying blood vessels. The inside portion of both lips is connected to the gums.

There are several types of teeth. Incisors are the squarish, sharp-edged teeth in the front of the mouth. There are four on the bottom and four on the top. On either side of the incisors are the sharp canines. The upper canines are sometimes called eyeteeth. Behind the canines are the premolars, or bicuspids. There are two sets, or four premolars, in each jaw.

The molars, situated behind the premolars, have points and grooves. There are 12 molars — three sets in each jaw called the first, second, and third molars. The third molars are the wisdom teeth, thought by some to have evolved thousands of years ago when human diets consisted of mostly raw foods that required extra chewing power. But because they can crowd out the other teeth, sometimes a dentist will need to remove them.

Human teeth are made up of four different types of tissue: pulp, dentin, enamel, and cementum. The pulp is the innermost portion of the tooth and consists of connective tissue, nerves, and blood vessels, which nourish the tooth. The pulp has two parts — the pulp chamber, which lies in the crown, and the root canal, which is in the root of the tooth. Blood vessels and nerves enter the root through a small hole in its tip and extend through the canal into the pulp chamber.

Dentin surrounds the pulp. A hard yellow substance consisting mostly of mineral salts and water, it makes up most of the tooth and is as hard as bone. It's the dentin that gives teeth their yellowish tint. Enamel, the hardest tissue in the body, covers the dentin and forms the outermost layer of the crown. It enables the tooth to withstand the pressure of chewing and protects it from harmful bacteria and changes in temperature from hot and cold foods. Both the dentin and pulp extend into the root. A bony layer of cementum covers the outside of the root, under the gum line, and holds the tooth in place within the jawbone. Cementum is also as hard as bone.

Normal Development of the Mouth and Teeth
Humans are diphyodont, meaning that they develop two sets of teeth. The first set of 20 deciduous teeth are also called the milk, primary, temporary, falling-off, or baby teeth. They begin to develop before birth and begin to fall out when a child is around 6 years old. They're replaced by a set of 32 permanent teeth, which are also called secondary or adult teeth.

Around the 8th week after conception, oval-shaped tooth buds consisting of cells form in the embryo. These buds begin to harden about the 16th week. Although teeth aren't visible at birth, both the primary and permanent teeth are forming below the gums. The crown, or the hard enamel-covered part that's visible in the mouth, develops first. When the crown is fully grown, the root begins to develop.

Between the ages of 6 months and 1 year, the deciduous teeth begin to push through the gums. This process is called eruption or teething. At this point, the crown is complete and the root is almost fully formed. By the time a child is 3 years old, he or she has a set of 20 deciduous teeth, 10 in the lower and 10 in the upper jaw. Each jaw has four incisors, two canines, and four molars. The molars' purpose is to grind food, and the incisors and canine teeth are used to bite into and tear food.

The primary teeth help the permanent teeth erupt in their normal positions; most of the permanent teeth form close to the roots of the primary teeth. When a primary tooth is preparing to fall out, its root begins to dissolve. This root has completely dissolved by the time the permanent tooth below it is ready to erupt.

Kids start to lose their primary teeth, or baby teeth, at about 6 years old. This begins a phase of permanent tooth development that lasts over the next 15 years, as the jaw steadily grows into its adult form. From ages 6 to 9, the incisors and first molars start to come in. Between ages 10 and 12, the first and second premolars, as well as the canines, erupt. From 12 to 13, the second molars come in. The wisdom teeth (third molars) erupt between the ages of 17 and 21.

Sometimes there isn't room in a person's mouth for all the permanent teeth. If this happens, the wisdom teeth may not come through at all. Overcrowding of the teeth is one of the reasons kids get braces.
What the Mouth and Teeth Do

The first step of digestion involves the mouth and teeth. Food enters the mouth and is immediately broken down into smaller pieces by our teeth. Each type of tooth serves a different function in the chewing process. Incisors cut foods when you bite into them. The sharper and longer canines tear food. The premolars, which are flatter than the canines, grind and mash food. Molars, with their points and grooves, are responsible for the most vigorous chewing. All the while, the tongue helps to push the food up against our teeth.

During chewing salivary glands in the walls and floor of the mouth secrete saliva, which moistens the food and helps break it down even more. Saliva makes it easier to chew and swallow foods (especially dry foods), and it contains enzymes that aid in the digestion of carbohydrates.

Once food has been converted into a soft, moist mass, it's pushed into the throat (or pharynx) at the back of the mouth and is swallowed. When we swallow, the soft palate closes off the nasal passages from the throat to prevent food from entering the nose.

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