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Abdominal cavity- the part of the body between the bottom of the ribs and the top of the thighs, containing most of the digestive and urinary systems along with some reproductive organs

ABO blood groups- The system by which human blood is classified, based on proteins occurring on red blood cells; the four classification groups are A, AB, B, and O

Abortion- termination of a pregnancy; can occur because of natural causes (called a miscarriage) or be a medical intervention

Abscess- an accumulation of pus in a body tissue, usually caused by a bacterial infection

ACE inhibitor- a drug typically used to treat high blood pressure (Angiotensin-Converting Enzyme inhibitor)

Achilles tendon- the tendon at the back of the lower leg that connects the calf muscle to the heel bone

Acid-base balance- the mechanisms that the body uses to keep its fluids close to neutral (neither basic nor acidic) so that the body can function properly

Acidosis- a condition marked by abnormally high acid levels in the blood, associated with some forms of diabetes, lung disease, and severe kidney disease

Acid reflux- a disorder in which acid in the stomach comes up into the esophagus, because the valve separating the stomach and esophagus does not function properly