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fracture A break in a bone.

free radicals Also called oxygen free radicals. Molecules produced in the body through normal cell activity or as a result of external factors such as radiation or cigarette smoke that damage or destroy cells. Free radicals are a major cause of disease and aging.

G

gallstone A small, hardened mass composed of cholesterol, calcium salts, and bile pigments. A gallstone can form in the gallbladder or in the bile duct.

gamma globulin The pooled antibodies taken from the blood of healthy donors, used to prevent or treat infections.

genes Segments of DNA that are the basic functional units of heredity. There are about 35,000 genes in each human cell, and each gene carries instructions for making proteins.

genetics The study of heredity.

glaucoma Abnormally high pressure inside the eyeball that damages peripheral (side) vision, causing the visual field to become increasingly narrow, leading to total blindness.

glucose A simple sugar that is the body’s main source of energy.

glucose meter A device used by people with diabetes to measure blood glucose levels.

graft Healthy tissue taken from one part of the body (or from a donor) and surgically implanted in another part of the body to repair or replace damaged tissue.

granulocyte A type of white blood cell that has an important role in the immune system, protecting the body from infections and cancer.

H

hallucination Abnormal sensory perceptions not based on reality that occur without an external stimulus. Hallucinations can involve any of the senses and may be a symptom of some mental disorders.

hay fever See allergic rhinitis.

HDL cholesterol High-density lipoprotein cholesterol . A type of cholesterol made in the liver and transported by the blood. Also called the good cholesterol, HDL cholesterol protects against heart disease by clearing harmful LDL cholesterol from blood vessels.

heart attack Also called myocardial infarction. Sudden death of a section of the heart muscle from lack of blood, usually as a result of a blockage of blood flow in one of the coronary arteries by a thrombus.

heartbeat A contraction of the heart muscle that pumps blood from the heart into the arteries and throughout the body.

heart rate The number of heartbeats per minute. A normal heart rate is between 60 and 100 beats per minute.

hematoma A collection of blood in an organ or tissue caused by bleeding from an injured blood vessel.

hematuria Blood in the urine.

hemoglobin The oxygen-carrying protein in red blood cells.

hemolysis The natural breakdown of red blood cells at the end of their life span. If hemolysis occurs prematurely, it can lead to anemia or jaundice.

hemorrhage Excessive bleeding.

heredity The transmission of traits and disorders from parents to children through genes.

hernia Protrusion of a portion of an organ or tissue through a weakened area in the muscle wall that normally contains it.

high blood pressure Also called hypertension. A condition in which blood pressure is persistently raised.

high-density lipoprotein cholesterol See HDL cholesterol.

hip replacement See arthroplasty.

histamine A chemical released by the body during an allergic reaction that produces signs of inflammation, including redness, swelling, itching, heat, and pain.

HIV Human immunodeficiency virus. A virus that infects specific cells of the immune system, causing AIDS.

Holter monitor A portable device worn around the neck, at the waist, or over the shoulder that records the electrical activity of the heart during a 24-hour period or longer. A Holter monitor is used to detect arrhythmias and other heartbeat abnormalities.

hormone therapy Use of natural or synthetic hormones to treat various diseases and disorders. Treatment of diabetes with insulin is one type of hormone therapy.

hormones Chemical messengers that are produced by a network of glands called the endocrine system and released directly into the bloodstream, which carries them to target organs and