There are four main types of cellular adaptations: hypertrophy, hyperplasia, atrophy, and metaplasia. Hypertrophy refers to an increase in the size of cells resulting in an increased organ size without new cells being produced. Cells that can divide may undergo both hypertrophy and hyperplasia in response to stress. Hypertrophy can be physiological, such as muscle growth from exercise or uterine growth in pregnancy, or pathological like heart muscle growth from hypertension. The mechanisms of hypertrophy involve cells enlarging in response to increased functional demands or stimulation by hormones and growth factors.