The basic mechanisms that underlie alterations in the physiology of pregnancy are virtually unknown. Basal oxygen consumption increases by some 50 mL/min in pregnant women at term. Blood volume increases gradually over gestation as does red cell mass. Cardiac output increases by some 50% by mid-third trimester. Stroke volume and heart rate increase over the course of pregnancy with heart rate increasing gradually until term. The heart of the pregnant woman remodels dramatically in the first few weeks of pregnancy; end diastolic volume increases. Stroke volume is augmented by the increase in end diastolic volume and maintenance of ejection fraction through a possible increase in contractile force. Systolic and diastolic blood pressures drop during normal pregnancy. There is evidence of blood vessel remodeling in all vessels. Venous compliance and venous blood volume are increased. Renal plasma flow increases by some 70% in pregnancy with glomerular filtration rate increasing by 50% by unknown mechanisms. The complex hormonal environment is changing throughout pregnancy. In summary, under the influence of circulating chemical mediators blood flow is redistributed to the uterus, breast, and kidney.