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B cell: White blood cells that come from bone marrow, and produce antibodies to fight off disease.

bacteria: Tiny single-celled organisms. Some bacteria cause disease, although most are harmless.

balance: Ability to maintain equilibrium while stationary or moving.

balloon angioplasty: A procedure to open clogged heart arteries. A surgeon inserts and inflates a tiny balloon. It widens the blocked artery then expands a small wire mesh tube to keep the artery open.

balloon dilation: A surgical procedure to open a narrowed vessel or tube, such as the urethra, esophagus, or artery. A small, deflated balloon is inserted into the area and inflated to widen it.

bariatric surgery: One of several types of weight loss surgery performed on people who are dangerously overweight, to restrict or reduce food intake and/or absorption.

barium study: An imaging test that allows doctors to see the inside of the esophagus and upper stomach. It involves swallowing a barium solution, which coats the esophagus and makes it possible for x-rays to see the inside of the intestine.

Barrett's esophagus: The abnormal growth of stomach or small intestine cells in the esophagus, resulting from damage caused by the reflux of stomach acid; occasionally may transform into cancer.

basal cell carcinoma: The most common skin cancer. Basal cell carcinoma doesn't spread to internal organs.

basal ganglia: Clusters of nerve cells deep in the brain that play an important role in movement.

baseline EKG: An electrocardiogram (EKG) tracing taken in a healthy individual for later comparison to subsequent EKGs.

basilar artery: The artery that supplies blood to the cerebellum, the brainstem, and the back of the brain.

benign: Harmless; often used to refer to a tumor that is not cancerous and does not usually spread.

benign orgasmic headache: A severe headache that occurs when orgasm is reached.

benign prostatic hyperplasia: A noncancerous enlargement of the prostate gland that obstructs the flow of urine. Often called BPH.

benzodiazepines: Anti-anxiety medications that work by helping to maintain levels of the neurotransmitter gamma-aminobutyric acid (GABA) in the brain.

bereavement: The period of grief and mourning after a death.

beriberi: A nervous system or heart disorder caused by lack of the vitamin thiamine (B1).

Bernstein test: A test to try to reproduce heartburn symptoms; used by doctors to diagnose GERD (gastroesophageal reflux disease).

beta blockers: Medications that block epinephrine (adrenaline) and norepinephrine from attaching to certain parts of nerve cells known as beta receptors. Used to treat high blood pressure, heart rhythm problems, migraines, panic attacks, and other conditions.

beta carotene: A richly colored compound (red, yellow, or orange) found in many plants, fruits, and vegetables that the body can convert into vitamin A.

beta cells: Cells that make and secrete insulin; located in the islets of Langerhans of the pancreas.

beta agonists: A medication that opens airways by relaxing the muscles around the airway; used to treat asthma or COPD (chronic obstructive pulmonary disease).

beta amyloid: An abnormal protein deposited in the brain in Alzheimer's disease.

beta blockers: A class of drugs that slow the heartbeat, lessen the force of each contraction, and reduce the contraction of blood vessels in the heart, brain, and throughout the body by blocking the action of beta-adrenergic substances such as adrenaline (epinephrine) at the beta receptor. Beta blockers, also known as beta adrenergic blocking agents, are used to treat many cardiovascular conditions, including abnormal heart rhythms, angina, and high blood pressure. They also improve survival after a heart attack.

biguanides: Medications that stop the liver from making excess glucose (sugar) and improve sensitivity to insulin.

bile: A thick, yellow-green fluid produced by the liver that aids in digestion.

bile acids: Fatty substances made by the gallbladder that aid in digestion.

bilevel positive airway pressure: A machine that helps people get more air into their lungs when sleeping by increasing the pressure or force of air when breathing in; often used to treat sleep apnea.

binge drinking: Heavy bouts of drinking interspersed with periods of abstinence; often refers to the consumption of five or more alcoholic beverages within one day.

binocular vision: The ability of both eyes to focus on an object and form a single visual image.

Binswanger's dementia: A type of dementia caused when blood flow is interrupted to the white matter of the brain, which lies beneath the cerebral cortex.

bioavailability: How quickly and completely the body can absorb and use a nutrient.

biochanin A: A natural compound found in soybeans that may help prevent cancer from spreading. It is a type of flavonoid.

biochemical recurrence: Usually used regarding prostate cancer. It refers to a post-treatment increase in the level of prostate-specific antigen in the bloodstream, indicating that prostate cancer has recurred or spread following the original treatment. Also called biochemical failure.

biofeedback: An treatment that helps people learn to gain control over normally unconscious body functions, such as breathing and heart rates.

biological variability: Normal fluctuations over time in the levels of a substance being measured (such as cholesterol).

biomarker: A distinctive biological indicator of an event, process, or condition.

biopsy: The removal of a small piece of tissue from the body for examination under a microscope.

bisphosphonate: Medications, including alendronate and etidronate, used to prevent and treat osteoporosis by slowing the breakdown of bone.

blackout: An episode of temporary memory loss resulting from the ingestion of alcohol or other drugs.

bladder neck: Where the bladder and urethra meet.

blepharoplasty: Cosmetic surgery to improve the appearance of droopy eyelids by removing excess skin and fat.

blister: A small pocket of fluid that develops between the upper layers of skin; often caused by friction or burns.

blocking agent: Substance that prevents a biological activity or process.

blood alcohol concentration: A measure of the amount of alcohol in the blood.

blood clot: A coagulated mass that occurs when blood cells stick together and form a solid.

blood pressure: The force blood exerts against the walls of the arteries. Optimal blood pressure is less than 120/80 mm Hg.

blood urea nitrogen test: A test that measures levels of urea in the blood to assess how well the kidneys are functioning.

blood vessels: Hollow tubes that transport blood throughout the body; includes arteries, veins, and capillaries.

BMD: Abbreviation for bone mineral density, the amount of mineralized bone tissue in a given area.

BMI: Abbreviation for body-mass index, a measure of body fat estimated from a person's height and weight. A healthy BMI is defined as 18.5 to 24.9. BMI = weight (in kilograms) divided by height (in meters) squared. Using English units, multiply weight in pounds by 703, then divide the result by height in inches, and divide that result by height in inches.

body mass index: A measure of body fat estimated from a person's height and weight. A healthy BMI is defined as 18.5 to 24.9. BMI = weight (in kilograms) divided by height (in meters) squared. Using English units, multiply weight in pounds by 703, then divide the result by height in inches, and divide that result by height in inches.

bolus: A soft mass of chewed food. Alternatively, a single large dose of a medication given intravenously.

bone mass: The total amount of bone tissue in the body.

bone mineral density: The amount of mineralized bone tissue in a given area.

bone scan: A test in which radioactive material is injected into a person's bloodstream to help produce images of bones; often used to detect cancer or bone diseases.

borborygmi: Stomach growling; the rumbling noises caused by gas moving through the intestine.

Botox: Brand name for a drug made of botulinum toxin type A that is injected into muscles and weakens them to ease the appearance of wrinkles.

Bouchard's nodes: Hard, bony growths that form on the middle joints of fingers in people with osteoarthritis.

bowel: The small or large intestine.

BPH: Abbreviation for benign prostatic hyperplasia, a noncancerous enlargement of the prostate gland that obstructs the flow of urine.

brachial plexus: A network of nerves that are rooted at the cervical spine and provide sensation and movement to the shoulder and arm.

brachytherapy: Treatment in which a surgeon implants seeds or pellets of radioactive material in the body to destroy cancer cells.

bradycardia: A slow heart rate, usually below 60 beats per minute.

brain imaging: Technologies that allow doctors to view the structure of the brain or see how different parts of the brain function; examples include computed tomography (CT), magnetic resonance imaging (MRI), functional MRI (fMRI), and positron emission tomography (PET).

brain stem: The part of the brain that connects the brain with the spinal cord and controls movement, sensation, and reflexes.

brain waves: Electrical impulses generated by the firing of nerve cells in the brain (neurons).

breast augmentation: Cosmetic surgery to increase the size of the breasts.

breath focus: A form of meditation aimed at bringing on a state of relaxation.

Broca's area: The part of the brain (in the frontal lobe of the left hemisphere) responsible for language comprehension and speech.

bronchial tubes: The airways that connect the lungs to the trachea (windpipe) and allow air to pass into and out of the lungs.

bronchiole: A small airway in the respiratory system that connects to the alveoli (air sacs); a branch of the bronchial tubes.

bronchodilator: Medication that eases breathing by relaxing the muscles surrounding the bronchial tubes.

bruit: Unusual sound, heard through a stethoscope, that blood makes when it rushes past an obstruction, like a blockage in an artery.

bunion: A bump of bone or tissue that forms at the big toe joint, causing inflammation and considerable pain.

bunionette: A small, painful bony bump that forms on the outside of the foot, at the base of the small toe.

bursa: A protective, fluid-filled sac located in or near the joints that cushions the movement of bone against tendons, skin, and muscle.

bursitis: Pain and swelling of the bursa, the small fluid filled pads that act as cushions in or near the joints.

bypass: A procedure used to divert the flow of blood or other fluids. When referring to the heart, shorthand for coronary artery bypass surgery, used to divert blood flow around a blocked coronary artery.