hypertrophy (hĪpûrˈtrəfē) [key], enlargement of a tissue or organ of the body resulting from an increase in the size of its cells. Such growth accompanies an increase in the functioning of the tissue. In normal physiology the growth in size of muscles (e.g., in an athlete as a result of increased exercise) and also the enlargement of a uterus in pregnancy are caused by hypertrophy of muscle cells. In pathology the thickening of the heart muscle from overstrain, as in hypertension (high blood pressure), is the result of hypertrophy. An organ subjected to extra work (e.g., the one kidney left to function after surgical removal of the other) usually compensates by enlarging; in such cases hyperplasia, an increase in the number of cells, generally accompanies hypertrophy.