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Glossaries - H

We've defined thousands of terms related to health care. This page discusses glossary terms with the letter H.

H2-Blockers (aytch-too BLAH-kurz)

Medicines that reduce the amount of acid the stomach produces. They block histamine2 (HIH-stuh-min-too). Histamine signals the stomach to make acid. Prescription H2-blockers are cimetidine (suh-MEH-tuh-deen) (Tagamet), famotidine (fuh-MAH-tuh-deen) (Pepcid), nizatidine (nih-ZAH-tuh-deen) (Axid), and ranitidine (ruh-NIH-tuh-deen) (Zantac). They are used to treat ulcer symptoms. Nonprescription H2-blockers are Zantac 75, Axid AR, Pepcid-AC, and Tagamet-HB. They are for GERD, heartburn, and acid indigestion.


the time required for the decay of half a sample of a radioactive substance; may also apply to pharmacologic agents (see elimination half- life - the time required for half the amount of a substance to be eliminated from the body or to be converted to another substance(s).

Heartburn (HART-burn)

A painful, burning feeling in the chest. Heartburn is caused by stomach acid flowing back into the esophagus. Changing the diet and other habits can help to prevent heartburn. Heartburn may be a symptom of GERD. See also Gastroesophageal Reflux Disease (GERD).

Tips to control heartburn:

  • Avoid foods and beverages that affect lower esophageal sphincter pressure or irritate the esophagus lining.

  • Lose weight if overweight.

  • Stop smoking.

  • Elevate the head of the bed 6 inches.

  • Avoid lying down 2 to 3 hours after eating.

  • Take digestive enzymes.

Helicobacter pylori (H. pylori) (HELL-uh-koh-BAK-tur py-LOH-ree)

A spiral-shaped bacterium found in the stomach. H. pylori damages stomach and duodenal tissue, causing ulcers. Previously called Campylobacter pylori.

Helper T cell

lymphocyte bearing the CD4 marker. Helper T cells are the chief regulatory cells of the immune response. They are responsible for many immune system functions, including turning antibody production on and off, and are the main target of HIV infection. (See also CD4+ T lymphocyte).

Hemoglobin A1C (HbA1C)

The substance of red blood cells that carries oxygen to the cells and sometimes joins with glucose (sugar). Because the glucose stays attached for the life of the cell (about 4 months), a test to measure hemoglobin A1C shows what the person's average blood glucose level was for that period of time.

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Hemochromatosis (HEE-moh-kroh-muh-toh-sis)

A disease that occurs when the body absorbs too much iron. The body stores the excess iron in the liver, pancreas, and other organs. May cause cirrhosis of the liver. Also called iron overload disease.

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Hemorrhoidectomy (HEM-roy-DEK-tuh-mee)

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Hemorrhoids (HEM-roydz)

Swollen blood vessels in and around the anus and lower rectum. Continual straining to have a bowel movement causes them to stretch and swell. They cause itching, pain, and sometimes bleeding.


relating to bleeding.

Hemostasis (he-moh-STAY-sis)

A halt to bleeding, either by the physiological processes of vasoconstriction and coagulation or by surgical means.

Heparin (HEP-uh-rin)

A glycosaminoglycan composed of mixed polysaccharides of various molecular weights; present in many tissues. Heparin has potent anticoagulant activities that result from binding to and enhancing the activity of antithrombin III and from inhibiting a number of coagulation factors, especially factor Xa.

Heparin Cofactor II

A plasma thrombin inhibitor that, similar to antithrombin, undergoes activation in the presence of heparin. To achieve the same level of thrombin inhibition with this inhibitor, heparin cofactor II requires a heparin concentration approximately ten times higher than antithrombin.

Heparin-Associated Thrombocytopenia (HAT)

Thrombocytopenia associated with administration of heparin. Two forms are recognized: a mild, self-limiting type called HAT type I, and a delayed and more severe type called HAT type II that is associated with thromboembolic complications. Also known as Heparin-induced Thrombocytopenia (HIT).

Heparinoid (HEP-uh-rin-oid)

A heparin-like compound.

Hepatic (heh-PAT-ik)

Related to the liver.

Hepatic Encephalopathy (heh-PAT-ik en-SEF-uh-LAWP-uh-thee)

A condition that may cause loss of consciousness and coma. It is usually the result of advanced liver disease. Also called hepatic coma.

Hepatitis (heh-puh-TY-tis)

Irritation of the liver that sometimes causes permanent damage. Hepatitis may be caused by viruses or by medicines or alcohol. Hepatitis has the following forms:

Hepatitis A

A virus most often spread by unclean food and water.

Hepatitis B

A virus commonly spread by sexual intercourse or blood transfusion, or from mother to newborn at birth. Another way it spreads is by using a needle that was used by an infected person. Hepatitis B is more common and much more easily spread than the AIDS virus and may lead to cirrhosis and liver cancer.

Hepatitis C

A virus spread by blood transfusion and possibly by sexual intercourse or sharing needles with infected people. Hepatitis C may lead to cirrhosis and liver cancer. Hepatitis C used to be called non-A, non-B hepatitis.

Hepatitis D

A virus that occurs mostly in people who take illegal drugs by using needles. Only people who have hepatitis B can get hepatitis D.

Hepatitis E

A virus spread mostly through unclean water. This type of hepatitis is common in developing countries. It has not occurred in the United States.

Hepatologist (HEH-puh-TAW-luh-jist)

A doctor who specializes in liver diseases.

Hepatology (HEH-puh-TAW-luh-jee)

The field of medicine concerned with the functions and disorders of the liver.

Hepatotoxicity (heh-PAT-oh-tawk-SIS-uh-tee)

How much damage a medicine or other substance does to the liver.

Hernia (HUR-nee-uh)

The part of an internal organ that pushes through an opening in the organ's wall. Most hernias occur in the abdominal area.

Herniorrhaphy (hur-nee-AWR-uh-fee)

An operation to repair a hernia.

Heterotrophic (het-er-o-tro-phic)

Requiring a supply of organic compounds (food) from the environment.

Hiatal hernia

Hiatal Hernia (Hiatus Hernia) (hy-AY-tul HUR-nee-uh)

A small opening in the diaphragm that allows the upper part of the stomach to move up into the chest. Causes heartburn from stomach acid flowing back up through the opening. See also Diaphragm.

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High Blood Pressure

When the blood flows through the vessels at a greater than normal force. High blood pressure strains the heart; harms the arteries; and increases the risk of heart attack, stroke, and kidney problems. Also called hypertension.

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Hirschsprung's Disease (HURSH-sprungz duh-zeez)

A birth defect in which some nerve cells are lacking in the large intestine. The intestine cannot move stool through, so the intestine gets blocked. Causes the abdomen to swell. See also Megacolon.

Hirudin (hih-ROO-din)

The active principle secreted by the salivary glands of leeches; it acts as an anticoagulant by blocking the activity of thrombin.


A chemical in the body tissues, produced by the breakdown of histidine. It is released in allergic reactions and causes widening of capillaries, decreased blood pressure, increased release of gastric juice, fluid leakage forming itchy skin and hives, and tightening of smooth muscles of the bronchial tube and uterus.


the correspondence between the microscopic structure of cells and tissues and their function.

HLA Antigens

Proteins on the outer part of the cell that help the body fight illness. These proteins vary from person to person. Scientists think that people with certain types of HLA antigens are more likely to develop insulin-dependent diabetes.

HLA class I

molecules that exist on all nucleated cells and identify the cell as "self." In addition, if the cell is infected by a virus or other microbe, the cell displays the invaders antigens in combination with the cells HLA class I molecules. The presence of the foreign peptide antigen with the HLA class I molecule activates CD8+ CTLs specific for that antigen.

HLA class II

molecules that are found on antigen-presenting cells such as macrophages. These cells process soluble antigens such as toxins or other proteins made by microbes and then display them on their surface as peptide antigens in combination with HLA Class II molecules. Helper T cells specific for these antigens are then able to be activated and respond to the presence of the invading microbe.


When the body is working as it should because all of its systems are in balance.


similar in appearance, structure and usually function.


Chemical messengers released inside the body that circulate through the bloodstream to produce physiological responses. Hormones are important for regulating a diverse range of body functions relating to sleep, energy production, sexual function, emotions, stress response, and many others.


a plant or animal harboring another organism.

Humoral immunity

Hydrochloric Acid (hy-droh-KLOR-ik ASS-id)

An acid made in the stomach. Hydrochloric acid works with pepsin and other enzymes to break down proteins.

Hydrogen Breath Test (HY-droh-jen breth test)

A test for lactose intolerance. It measures breath samples for too much hydrogen. The body makes too much hydrogen when lactose is not broken down properly in the small intestine.


Therapy that takes place in water.

Hyperalimentation (HY-pur-al-uh-men-TAY-shun)

Hyperbilirubinemia (HY-pur-bil-ee-roo-buh-NEE-mee-uh)

Too much bilirubin in the blood. Symptoms include jaundice. This condition occurs when the liver does not work normally. See also Jaundice.


Too high a level of glucose (sugar) in the blood; a sign that diabetes is out of control. Many things can cause hyperglycemia. It occurs when the body does not have enough insulin or cannot use the insulin it does have to turn glucose into energy. Signs of hyperglycemia are a great thirst, a dry mouth, and a need to urinate often. For people with insulin-dependent diabetes, hyperglycemia may lead to diabetic ketoacidosis.


increased production of gonad-stimulating hormone from the anterior pituitary gland.

Hyperkeratosis (hi-per- ker-ah-TOE-sis)

a disease resulting in the abnormal growth of the corneous (hornlike) layer of the ski.


Too high a level of fats (lipids) in the blood.


abnormally increased pigmentation.

Hyperplasia (hi-per- PLAY-zee-ah)

an abnormal increase in the number of normal cells in a tissue.


Blood pressure that is above the normal range. See also: High blood pressure.


Too low a level of glucose (sugar) in the blood. This occurs when a person with diabetes has injected too much insulin, eaten too little food, or has exercised without extra food. A person with hypoglycemia may feel nervous, shaky, weak, or sweaty, and have a headache, blurred vision, and hunger. Taking small amounts of sugar, sweet juice, or food with sugar will usually help the person feel better within 10-15 minutes. See also: Insulin shock.

Hypogonadism (high-po-GO- nad-izm)

below normal gonad (sex gland) function.


Low blood pressure or a sudden drop in blood pressure. A person rising quickly from a sitting or reclining position may have a sudden fall in blood pressure, causing dizziness or fainting.


an area of the forebrain which regulates pituitary gland secretion among many other functions.


a tentative statement or supposition, which may then be tested through research.

Hypoxemia (high-POCK-see-me--ah)

below normal oxygen content in arterial blood.

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