An abdominal ultrasound uses reflected sound waves to produce a picture of the organs and other structures in the upper abdomen. Sometimes a special ultrasound is done for a closer look at a specific organ, such as a kidney ultrasound.

An ultrasound does not use X-rays or other types of radiation. During an abdominal ultrasound, a small handheld unit called a transducer is gently moved over the belly. A computer converts the reflected sound waves into a picture shown on a TV screen.

An abdominal ultrasound may help find problems of the liver, gallbladder, spleen, pancreas, abdominal aorta, and kidneys.

An abnormal heartbeat (arrhythmia) is any variation in the normal heartbeat. Abnormal heartbeats occur when the heart has an irregular heart rhythm, beats too fast (tachycardia), or beats too slow (bradycardia).

The electrical system of the heart creates signals that trigger the heart to pump. These electrical signals control the heart rate and rhythm. Normally, the heart beats in a regular rhythm and at a rate that is appropriate for the work the body is doing. An arrhythmia results from a problem in the electrical system of the heart. Things that can cause the heart to beat abnormally include heavy smoking, alcohol use, excess caffeine or other stimulants, stress, thyroid disease, and fever.

Many arrhythmias are minor, causing only occasional abnormal heartbeats and requiring no treatment. Others, such as atrial fibrillation, can be life-threatening because they increase the risk of blood clots and strokes. Arrhythmias are of special concern in people who also have heart disease or heart failure. Some arrhythmias can be treated with medicine. Others may require an electrical shock (cardioversion), surgery, or a pacemaker.

An abscess is a pocket of pus that forms at the site of infected tissue. An abscess can form on the skin or on tissues within the body and cause pain, swelling, and tenderness.

Bacteria are a common cause of the infections that form abscesses.

Depending on the size and location of the abscess, your doctor may treat the abscess by:

• Using a needle to drain it.
• Cutting open the abscess to remove the pus and infectious material.
• Prescribing antibiotics (pills or a shot). This may be adequate treatment if the abscess is small and treatment is not delayed.

Acanthosis nigricans is a skin condition that results in darkened skin and velvety growths that look like warts. These skin changes can occur under the arm, on the back of the neck, over finger and toe joints, or in the groin and genital area.

In adults, acanthosis nigricans may be a sign of disorders of the hormone system (endocrine system). It can occur in people who are very overweight (obese). It may be a sign of lung cancer or of a cancer of the digestive system. It can occur in people who have type 2 diabetes and in people who are resistant to insulin but have not yet developed type 2 diabetes. Acanthosis nigricans can occur in women who have polycystic ovary syndrome (PCOS).

Acute coronary syndrome (ACS) occurs when a plaque inside a coronary artery suddenly ruptures. A blood clot forms and blocks blood flow to the heart muscle, depriving the heart of oxygen-rich blood and causing unstable angina or a heart attack. The amount of time blood flow is blocked and the extent of heart damage determine which type of ACS occurs, but it is always considered a life-threatening condition.

Acute renal failure is the sudden loss of kidney function. When acute renal failure occurs, the kidneys are unable to remove waste products and excess fluids, which then build up in the body and upset the body's normal chemical balance.

The most common causes of acute renal failure are dehydration, blood loss from major surgery or injury, or medicines such as nonsteroidal anti-inflammatory drugs (NSAIDs), antibiotics, or the dyes (contrast agents) used in X-ray tests.

Symptoms depend on the cause of the problem and can include:

• Little or no urine output.
• Dizziness upon standing.
• Swelling, especially of the legs and feet.
• Loss of appetite, nausea, and vomiting.
• Feeling confused, anxious and restless, or sleepy.
• Pain in the flank, which is felt just below the rib cage and above the waist on one or both sides of the back.

The treatment of acute renal failure includes correcting the cause and supporting the kidneys with dialysis until proper functioning is restored. Most people who develop acute renal failure are already in the hospital.

Acyanotic heart defects are heart problems that develop before or at birth but do not normally interfere with the amount of oxygen or blood that reaches the body's tissues. Acyanotic heart defects include ventricular septal defect (VSD), atrial septal defect (ASD), patent ductus arteriosus (PDA), pulmonary valve stenosis, aortic valve stenosis, and coarctation of the aorta.

Acyanotic heart defects do not usually cause cyanosis—a bluish tint to the skin, lips, and nail beds due to reduced oxygen flow. If cyanosis develops in a person with acyanotic heart defects, it is typically a result of increased activity (such as crying and feeding) during which more oxygen is needed.

Addison's disease is a rare condition that develops when the adrenal glands, located above the kidneys, are not able to produce enough of the hormones cortisol and aldosterone.

The adrenal glands release cortisol to help the body cope with stress from illness, injury, surgery, childbirth, or other reasons. Aldosterone helps the body retain salt and maintain blood pressure. Adrenal gland failure can be caused by a problem with the body's immune system (autoimmune disease) or by infection, tumor, bleeding, or injury.

Symptoms of Addison's disease include weakness, weight loss, nausea, vomiting, diarrhea, darkening of the skin (called hyperpigmentation), emotional distress, and a decreased ability to tolerate cold.

People who have Addison's disease need to take medicine for the rest of their lives to replace cortisol and aldosterone. Treatment relieves symptoms and usually reverses any darkening of the skin. After proper treatment is started, most people with Addison's disease can lead healthy lives. If Addison's disease is not treated, an adrenal crisis may occur that can lead to death because of a severe drop in blood pressure.

The adrenal glands are located above the kidneys. The hormones produced by the adrenal glands affect nearly every organ in the body.

The inner layer of the adrenal gland releases substances (hormones) such as adrenaline that:

• Help control blood pressure.
• Help the body react to stress by increasing heart rate, opening the airways, and shifting blood flow to the large muscles.

The outer layer of the adrenal gland releases hormones such as cortisol that:

• Help the body deal with stress.
• Help control the breakdown of fats and proteins to make sugars in the liver.
• Control blood pressure and the blood levels of some substances (such as potassium and sodium).
• Affect sexual development and reproduction.

An advance directive is a legal form you fill out to say what kinds of medical care you want if you aren't able to make decisions for yourself. It tells your family and your doctor what to do if you're badly hurt or have a serious illness that keeps you from saying what you want.

There are two main types of advance directives: a living will and a medical power of attorney.

Some people continue to drink excessive amounts of alcohol even though it causes problems in their lives. These people have an alcohol use problem.

• Early problems usually develop at home (such as arguments about drinking), at work (such as absences or decreased work performance), or with the law (such as arrests for driving under the influence of alcohol).
• Later, health problems (such as high blood pressure, liver problems, or digestive problems) usually develop.

Most people refer to people who drink too much alcohol as having a drinking problem. Health professionals distinguish between people who have problem drinking (alcohol abuse) and people who are dependent on alcohol (alcohol dependence or alcoholism). The causes of alcohol use problems and alcoholism are complex and involve physical, social, and emotional aspects.

People with alcoholism may not drink alcohol every day, but they usually drink heavily when they drink and have withdrawal symptoms if they stop drinking.

Alcoholism is a lifelong disease that often causes health, emotional, behavioral, and social problems. If untreated, it can even lead to death from the effects of alcohol on the heart, liver, and other parts of the body. Alcoholism cannot be cured, but it can be successfully treated.

Beer, wine, and liquor (distilled spirits) are alcoholic beverages. The following common alcoholic beverages contain equal amounts of alcohol:

• One 12 fl oz (354.9 mL) bottle of beer or wine cooler
• One 5 fl oz (147.9 mL) glass of wine
• One mixed drink containing 1.5 fl oz (44.4 mL) of 80-proof liquor, such as gin, whiskey, or rum

Proof is the percentage of alcohol in an alcoholic drink. The percentage of pure alcohol is usually one-half the proof (for example, 100-proof liquor is about 50% pure alcohol). The higher the proof, the more pure alcohol the drink contains.

An allergic reaction is an overreaction of the immune system to a substance called an allergen. Allergens include chemicals, foods, medicines, mold, plants, and pollen.

Symptoms of an allergic reaction can range from mild and annoying to severe and life-threatening.

• Allergens can affect different tissues in the body, such as the airways, eyes, gastrointestinal tract, nose, lungs, and skin.
• Some allergic reactions, such as hives or itching around an insect bite or where a plant or chemical touched the skin, affect only one area of the body.
• Other allergic reactions may affect the whole body, causing itching, swelling, or difficulty breathing.
• A severe allergic reaction (called anaphylaxis) can lead to shock and even death.

Allergic reactions do not occur the first time a person is exposed to an allergen. A person may become more sensitive to the allergen with each exposure.

An allergic reaction to a medicine is an overreaction by the body's immune system to a substance (allergen) in a medicine that a person has taken. An allergic reaction to a medicine may cause symptoms that range from a minor rash to severe anaphylactic shock, depending on the person and the type and dose of the medicine.

A medicine allergy is different from an adverse medicine reaction, such as a medicine side effect or a reaction when taking more than one medicine. Because symptoms and treatments vary, a doctor should determine whether a person has a medicine allergy or an adverse reaction. A severe medicine allergy can be life-threatening. An adverse reaction usually is not.

A person who has been diagnosed with a medicine allergy should wear a medical alert bracelet or other identification and should not take that medicine again.

Allergic rhinitis means allergies or hay fever. When you are exposed to particles in the air that you're allergic to, like pollen, dust, or cat dander, your immune system attacks the particles and causes symptoms such as sneezing and a runny nose.

You may have symptoms often during the year, or just at certain times.

Alzheimer's disease damages the brain and causes a steady loss of memory and of how well you can speak, think, and do your daily activities. It gets worse over time, but how quickly this happens varies. There are medicines that may slow down the symptoms for a while and make the disease easier to live with.

Amino acids are the chemicals that make up proteins. The human body needs 20 different amino acids to function properly.

Some amino acids are made by the body, while others, called essential amino acids, can be obtained only from foods.

Amniotic fluid is the liquid that surrounds a developing baby (fetus) in the uterus throughout pregnancy. The amniotic fluid protects the fetus from temperature extremes and from being bumped or hurt as the mother moves around.

Amniotic fluid allows the fetus to move before birth and is important for lung development. It is produced by the fetus and the placenta and contains cells and other substances that have been shed by the fetus.

Amniocentesis is a test done to collect a sample of amniotic fluid.

Amphetamines are drugs that stimulate the nervous system. They may be used in the treatment of attention deficit hyperactivity disorder (ADHD), narcolepsy (a sleeping disorder), and other conditions.

Amphetamines produce increased alertness and a feeling of euphoria. Common amphetamine medicines include dextroamphetamine (Dexedrine) and methamphetamine (Desoxyn).

Amphetamines can be abused and are sometimes sold illegally with names such as speed, ice, and lid poppers.

Amyloidosis refers to a group of conditions in which proteins (amyloids) accumulate in the body's organs and tissues in abnormal amounts, disrupting normal functions. Amyloidosis may be caused by a bone marrow cell disorder or an inherited protein variation. Or it may be associated with other conditions, including multiple myeloma, tuberculosis, and familial Mediterranean fever.

Organs most often affected by amyloidosis include the heart, liver, kidney, skin, and certain nerves. But any organ system may be involved. Symptoms of amyloidosis depend upon the organ system affected and may not help determine what condition is causing the protein accumulation. Amyloidosis can be disabling and even life-threatening.

Treatment of amyloidosis depends upon the condition that is causing the protein accumulation. In some cases chemotherapy, corticosteroid therapy, or stem cell transplant may be used.

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a progressive wasting away of the nerve cells in the brain and spinal column that control the muscles that allow movement. Over a period of months or years, ALS causes increasing muscle weakness, inability to control movement, and problems with speaking, swallowing, and breathing.

The cause of ALS is unknown, and there is no cure. Treatment focuses on helping you keep your strength and independence for as long as possible. Treatment includes medicines to slow the disease and help with symptoms, physical therapy, occupational therapy, speech therapy, and supportive devices to help with daily tasks.

Anaphylaxis is a severe allergic reaction that affects the entire body (systemic). It can occur within a few seconds or minutes after a person is exposed to a substance (allergen or antigen).

Symptoms and signs of a severe allergic reaction may include:

• Itching.
• Raised, red bumps on the skin (hives or wheals).
• Wheezing or difficulty breathing.
• Swelling, either in one area or over the entire body. Swelling is most serious when it involves the lips, tongue, mouth, or throat and interferes with breathing.
• Abdominal cramps.
• Low blood pressure, shock, and unconsciousness.

The sooner symptoms occur after exposure to the substance, the more severe the anaphylactic reaction is likely to be. An anaphylactic reaction may occur with the first exposure to an allergen, with every exposure, or after several exposures. An anaphylactic reaction can be life-threatening and is a medical emergency. Emergency care is always needed for an anaphylactic reaction.

Androgens, such as testosterone, are male sex hormones that produce male sexual characteristics; androgens are also present at low levels in women. These hormones are important to the healthy functioning of many parts of the body.

Androgens are used to treat men whose hormone levels have decreased.

Anemia is a decrease in the amount of oxygen-carrying substance (hemoglobin) found in red blood cells. Anemia causes weakness, pale skin, and general tiredness (fatigue).

Anemia can be caused by blood loss or bleeding, an increase in the destruction of red blood cells, or a decrease in the production of red blood cells. Types of anemia include iron deficiency anemia, folic acid deficiency anemia, and vitamin B12 deficiency anemia, among others. Each type of anemia is treated differently.

Anesthesiologists are medical doctors who specialize in anesthesiology, which is the use of pain-blocking techniques or medicines (anesthetics) during surgery and other medical procedures.

An anesthesiologist may administer medicine that numbs the area of the body where a procedure is being performed (local or regional anesthesia), inject medicine into the spinal canal to numb an area of the body (spinal or epidural anesthesia), or make sure a person is unconscious and pain-free during a procedure (general anesthesia) while also monitoring heart rate, breathing, and blood pressure. An anesthesiologist may also supervise a nurse anesthetist. Anesthesiologists can further specialize in critical care medicine, pain management, pediatrics, or obstetrics.

Anesthesiologists can be board-certified through the Board of Anesthesiology, which is recognized by the American Board of Medical Specialties.

An aneurysm is a bulging section in the wall of a blood vessel that has become stretched out and thin. As the wall of the blood vessel bulges out, it becomes weaker and may burst or rupture, causing bleeding.

If an aneurysm in the brain bursts, it may cause a stroke. An aneurysm in a vessel that carries a lot of blood, such as the aorta, is often fatal if it bursts.

Angina is a symptom of heart disease. Angina happens when there is not enough blood flow to the heart muscle. This is often a result of narrowed blood vessels, usually caused by hardening of the arteries (atherosclerosis).

The most common symptom of angina is chest pain or pressure, or a strange feeling in the chest. Some people feel pain, pressure, or a strange feeling in the back, neck, jaw, or upper belly, or in one or both shoulders or arms.

Other symptoms of angina include shortness of breath, nausea or vomiting, lightheadedness or sudden weakness, or a fast or irregular heartbeat.

Angina can be stable or unstable. Stable angina happens at fairly predictable times, usually with activity or exertion. It also may occur during exposure to cold or times of emotional stress. Stable angina can be relieved by rest or nitroglycerin. Unstable angina is a change in your usual pattern of stable angina. Unstable angina is a warning sign that a heart attack may soon occur.

If you have angina, pay attention to your symptoms, know what is typical for you, learn how to control it, and understand when you need to get treatment.

An angiogram is an X-ray test that provides pictures of the blood flow in a blood vessel. During an angiogram, a material that shows up on an X-ray (contrast material) is put into a blood vessel through a thin tube (catheter).

An angiogram may be done to look for problems such as a tear in a blood vessel that can cause blockage or bleeding, aneurysms (which are weaknesses in the blood vessel wall), and blood clots or the buildup of fatty deposits.

Sometimes a problem can be treated during angiography. For example, a catheter can be used to open a blocked blood vessel, deliver medicine to a tumor, or stop bleeding in the intestines.

Angioplasty is a procedure done to open a partially blocked blood vessel so that blood can flow through it more easily. It is most often done on arteries that deliver blood to the heart (coronary arteries) when they are narrowed by plaque buildup (atherosclerosis) or blood clots.

During the angioplasty procedure, a thin flexible tube (catheter) is inserted through an artery in the groin, arm, or wrist and is carefully guided into the artery that is narrowed. After the tube reaches the narrowed artery, a small balloon at the end of the tube is inflated. The balloon may remain inflated for a short time. The pressure from the inflated balloon presses fat and calcium deposits (plaque) against the wall of the artery to improve blood flow.

After the fat and calcium buildup is compressed, a small, expandable tube called a stent is sometimes inserted into the artery to hold it open. The artery is less likely to get narrow again (restenosis) after angioplasty with stenting than after angioplasty alone.

An anterior cruciate ligament (ACL) injury is a tear in one of the knee ligaments that join the upper leg bone to the lower leg bone. The ACL helps keep your knee stable.

When you twist, bend, or over-straighten your knee, you can injure your ACL. This can cause knee pain, swelling, and weakness. Rest and physical rehabilitation (rehab)—and sometimes surgery—are important to prevent a long-lasting knee problem.

ACL injuries range from mild, such as a small tear, to severe, such as when the ligament tears completely or when the ligament and part of the bone separate from the rest of the bone.

Antiarrhythmic medicines may be used to return an irregular heartbeat (arrhythmia) to its normal rhythm, prevent an arrhythmia, or control the heartbeat during an arrhythmia. These drugs work mostly by slowing the heart rate or by helping the heart muscle tissue become stable.

Many antiarrhythmic medicines have side effects, and many interact with other medicines. A person taking any of these medicines should talk often with his or her doctor.

Antibiotics are medicines used to kill bacteria that cause infections.

A doctor will choose an antibiotic to treat an illness based on:

• Whether taking an antibiotic will reduce the length or severity of the illness.
• Whether the person is allergic to any antibiotics.
• How likely it is that a certain antibiotic will kill the bacteria believed to be causing the symptoms.
• The symptoms and the severity of the illness.
• Other medical problems that the person has.
• The person's age. (For example, some antibiotics are not safe for children.)
• Whether a woman is pregnant.

Anticoagulant medicines, often called blood thinners, do not actually thin the blood but increase the time it takes a blood clot to form. Anticoagulants help prevent existing blood clots from becoming larger and may be used to prevent deep vein blood clots or to treat certain blood vessel, heart, or lung conditions.

Antihypertensives are medicines that help keep blood pressure within normal limits.

Examples of antihypertensive medicines are diuretics, beta-blockers, angiotensin-converting enzyme (ACE) inhibitors, calcium channel blockers, and angiotensin II receptor blockers (ARBs).

Antioxidants protect the body from damage caused by harmful molecules called oxygen free radicals. Most antioxidants are vitamins (such as beta-carotene and vitamins E and C) that are naturally found in many fruits and vegetables or are available as supplements.

Oxygen free radicals are normal by-products of cell metabolism. But at high levels they can cause damage to cells. Antioxidants may protect the body from cell damage.

Anxiety is an uncomfortable feeling of fear, uneasiness, or concern that something bad may happen. Anxiety can cause physical symptoms such as trembling, shaking, muscle aches, restlessness, insomnia, rapid heartbeat, sweating, and clammy hands.

If anxiety interferes with your daily activities, you may need treatment with medicines (such as antidepressants or antianxiety medicines) and/or professional counseling.

The aorta is a large blood vessel that carries oxygen-rich blood from the heart to the rest of the body. It leaves the heart, arches upward, then travels down through the chest and into the abdomen.

The aorta has two main portions:

• The thoracic portion of the aorta travels through the chest and supplies blood to the upper body.
• The abdominal portion of the aorta travels through the abdomen and supplies blood to the lower body.

An aortic aneurysm (say "a-OR-tik AN-yuh-rih-zum") is a bulge in part of the aorta, your main artery. If the bulge gets large enough, it can rupture. If it does, your life is in danger.

Aortic aneurysms are usually caused by hardening of the arteries (atherosclerosis). But other causes include genetic conditions, infections, and injury.

Aneurysms need to be repaired if they are large or fast-growing or if they cause symptoms. Smaller ones are usually just checked regularly to see if they are getting bigger.

Aortic dissection is a tear between the inner and outer layers of the aortic wall. The tear can cause the wall to separate and rupture, resulting in life-threatening bleeding and death.

The aorta, like all arteries, is made up of three layers, which are fused together. If the layers begin to separate, it causes bleeding into and around the tear. The bleeding widens the tear and causes the layers to separate. Typically, an aortic dissection occurs in the section of the aorta that leaves the heart and curves down through the chest.

Aortic dissection can be caused by atherosclerosis (hardening of the arteries) and high blood pressure; traumatic injury to the chest, such as hitting the car steering wheel during an accident; and conditions that are present at birth, such as Marfan's syndrome or Ehlers-Danlos syndrome.

Symptoms usually include sudden and severe chest or upper back pain, anxiety, pallor, sweating, and nausea. Aortic dissection usually requires emergency surgery to repair the tear.

The aortic valve separates the lower left chamber (left ventricle) of the heart and the aorta, the large artery that carries blood from the heart to the rest of the body.

The aortic valve works like a one-way gate, opening so that blood from the left ventricle—the heart's main pump—can be pushed into the aorta. When the heart rests between beats, the aortic valve closes to keep blood from flowing backward into the heart.

Aortic valve regurgitation is the backflow of blood from the aorta through the aortic valve into the left ventricle. If enough blood flows back into the heart, it can increase the workload on the left ventricle (lower left chamber), causing damage.

When the heart pumps, the aortic valve opens to let oxygen-rich blood flow from the left ventricle into the aorta. When the heart rests between beats, the aortic valve closes to keep blood from flowing backward into the heart. In aortic valve regurgitation, the aortic valve does not close properly. With each heartbeat, some of the blood pumped into the aorta leaks back (regurgitates) through the faulty valve into the left ventricle.

Medical therapy may delay or minimize the damage caused by aortic valve regurgitation. In some cases, surgery to replace the valve is needed, to avoid damage to the heart chambers and to keep an adequate blood flow to the body.

Aortic valve stenosis is a narrowing of the aortic valve between the lower left chamber of the heart and the aorta, which supplies blood to the body. A narrowed aortic valve forces the lower left chamber of the heart to pump harder to get enough blood through the valve.

Aortic valve stenosis can be caused by a structural problem called bicuspid aortic valve, which develops before a baby is born (congenital heart defect). In these cases, the valve has only two flaps, or leaflets, instead of the normal three.

Aortic valve stenosis also occurs as a person ages and the valve becomes hard and thick from calcium buildup. Most cases of aortic valve stenosis caused by calcium buildup occur in people who are older than 65.

An arterial blood gas (ABG) test measures the levels of oxygen and carbon dioxide in the blood to find out how well the lungs are working. An ABG test checks how well the lungs can move oxygen into the blood and remove carbon dioxide from the blood.

As blood passes through the lungs, oxygen moves into the blood while carbon dioxide moves out of the blood into the airspace of the lungs. An ABG test uses blood drawn from an artery, where the oxygen and carbon dioxide levels can be measured before they enter body tissues and become changed. An ABG test measures pH (acidity or alkalinity) and the levels of oxygen and carbon dioxide in the blood. Abnormal values for pH, oxygen, and carbon dioxide can be caused by changes in:

• Lung function.
• Heart function and blood flow.
• Kidney function.
• How well the body uses food for energy (metabolism).
• The use of some medicines.

An arterial blood gas test is often done for a person who is in the hospital because of severe injury or illness.

An arteriovenous malformation is an abnormal network of blood vessels and veins in the body. It is present at birth (congenital).

The vessel walls of an arteriovenous malformation may become weak and leak or burst, causing bleeding in or around important parts of the body, such as the brain or spinal cord.

Arthritis is inflammation of a joint. Symptoms of arthritis may include pain, swelling, redness, warmth, and limitation of movement.

There are over 100 types of arthritis. Three common types are osteoarthritis, rheumatoid arthritis, and gout.

• Osteoarthritis is a condition in which the cartilage that protects and cushions joints breaks down over time. Eventually, the bones—formerly separated by the cartilage—rub against each other, resulting in damage to the tissue and underlying bone and causing painful joint symptoms.
• Rheumatoid arthritis (RA) is an inflammation of the membranes or tissues lining the joints. Over time, rheumatoid arthritis may destroy the joint tissues, including cartilage, ligaments, tendons and bone, and, in rare but severe cases, may cause organ damage.
• Gouty arthritis (gout) is an inflammatory joint disease that causes acute pain and swelling. It is a form of arthritis that develops when uric acid crystals form in and around the joints, commonly affecting the big toe joint (this symptom is called podagra). People who have gout may have a very painful attack in one or two joints followed by the total disappearance of all symptoms until the next attack.

An artificial heart valve is made of plastic, metal, or pig tissue and is surgically implanted to replace a person's original heart valve. These valves are also called mechanical valves or tissue valves.

A person's heart valve may need to be replaced if it no longer works properly because of a birth defect or because of damage caused by a disease or aging.

When a person has an artificial heart valve, he or she may need to take anticoagulants to prevent blood clots. And he or she may need to take antibiotics before certain procedures to prevent infection.

Asthma is a long-term (chronic) disease of the respiratory system. It causes your airways to tighten and become narrow. This makes it hard to breathe and may cause wheezing and coughing.

Even though asthma is a lifelong disease, most people can lead active lives by following a treatment plan, avoiding things that cause flare-ups, and taking medicine.

The cause of asthma is not clearly known.

Atherosclerosis, sometimes called "hardening of the arteries," occurs when fat (cholesterol) and calcium build up in the inner lining of the arteries, forming a substance called plaque. Over time, the fat and calcium buildup narrows the artery and blocks blood flow through it.

• When atherosclerosis affects the arteries that supply blood to the heart, it can restrict blood flow to the heart muscle, causing heart pain (angina), irregular heartbeats (arrhythmia), and other problems. Plaque may burst open the artery lining, causing blood clots that can block blood flow, which in turn may cause a heart attack and cause damage to the heart muscle. Atherosclerosis in the heart (coronary) arteries is called coronary artery disease.
• When atherosclerosis affects the arteries that supply blood to the brain, it may cause a transient ischemic attack (TIA) or stroke.
• Atherosclerosis can affect arteries in other parts of the body, such as the pelvis and legs, causing poor circulation, slower healing of skin injuries, and erection problems.

A major part of treating atherosclerosis and coronary artery disease involves lifestyle changes (such as quitting smoking) and medicines to help reduce high cholesterol, control high blood pressure, and manage other things that increase a person's risk of heart attack, stroke, and other complications.

Atrial fibrillation (say "AY-tree-uhl fih-bruh-LAY-shun") is a type of irregular heartbeat (arrhythmia) in which the heart's upper chambers quiver, or fibrillate. This increases the risk of blood clots, which can cause a stroke or other problems. The lower chambers beat without a regular rhythm and may beat too fast. This can cause symptoms like lightheadedness or chest pain.

Treatment is done to control your heart rate or heart rhythm and to prevent stroke.

An atrial septal defect is an opening in the wall that separates the upper chambers of the heart. It is one of the most common congenital heart defects, which are structural problems that develop before a baby is born or at birth.

When an atrial septal defect is present, some oxygen-rich blood that should have been pumped to the body flows from one side of the heart to the other. This blood is then pumped to the lungs. This creates extra work for one side of the heart.

If an atrial septal defect is large, heart failure may occur, although this is not common in children. Many children have no symptoms. So this defect may not be found until a child is older or becomes an adult.

A heart catheterization can typically be used to close the opening. The doctor will put a thin tube called a catheter into a blood vessel in your child's groin. The doctor will move the catheter through the blood vessel to the heart. Then the doctor will guide special tools through the catheter to fix the heart problem. The doctor will insert a small closure device into the heart. This prevents blood from flowing between chambers.

Attention deficit hyperactivity disorder (ADHD) is a problem with paying attention and focusing on tasks. People who have ADHD tend to act without thinking and have trouble sitting still.

ADHD may begin in early childhood and can continue into adulthood. Without treatment, it can cause problems at home, at school, at work, and with relationships.

The auditory nerve carries sound impulses from the inner ear to the brain. The brain "unscrambles" the auditory signals to make them recognizable as sounds.

The immune system is the body's defense against foreign substances, such as bacteria or viruses, that may be harmful. An autoimmune disease is an abnormal condition that occurs when a person's immune system attacks its own tissues as though they were foreign substances.

• Normally, when a foreign substance enters the body, the immune system creates special cells to attack and destroy the foreign substance. These cells include antibodies and white blood cells (lymphocytes).
• In a person with an autoimmune disease, the immune system recognizes some of the person's own tissues as foreign substances. The body makes antibodies and other cells that attack and destroy these tissues. This process often leads to inflammation and eventually, if it continues, scarring and destruction of the organs that are made up of those tissues.

Why the body attacks its own cells is not known. Autoimmune diseases include lupus, rheumatoid arthritis, scleroderma, and Sjögren's syndrome. Certain types of diabetes and thyroid disease are related to autoimmune reactions. People who have autoimmune diseases are at an increased risk for infections.

The autonomic nervous system controls all "automatic" body functions, such as breathing, heart rate, blood pressure, sweating, mouth-watering (salivating), and the movement of food through the intestines (peristalsis).

One part of the autonomic nervous system, called the sympathetic nervous system, reacts when a person is facing a dangerous or frightening situation and will automatically increase the heart rate and breathing and move blood to the muscles.

The other part of the autonomic nervous system, the parasympathetic nervous system, helps the body return to normal after the threat is over. It will automatically lower heart rate and breathing and move blood back to the rest of the body (for example, the digestive system or reproductive system).

The autonomic nervous system differs from the voluntary nervous system, which allows a person to control the muscles and body movements.