The end-diastolic volume is called the preload. At a given end-diastolic volume, the amount of blood ejected depends on contractility. Strength of contraction is increased by sympathoadrenal stimulation. The venous flow of blood to the heart is aided by the action of skeletal muscle pumps and the effects of breathing. Edema occurs when there is an accumulation of tissue fluid.
Aldosterone acts on the kidneys to promote the retention of salt and water.
According to Poiseuille's law, blood flow is directly related to the pressure difference between the two ends of a vessel and is inversely related to the resistance to blood flow through the vessel. Extrinsic regulation of vascular resistance is provided mainly by the sympathetic nervous system, which stimulates vasoconstriction of arterioles in the viscera and skin. The heart normally respires aerobically because of its high capillary supply, myoglobin content, and enzyme content. During exercise, when the heart's metabolism increases, intrinsic metabolic mechanisms stimulate vasodilation of the coronary vessels and thus increase coronary blood flow. Just prior to exercise and at the start of exercise, blood flow through skeletal muscles increases due to vasodilation caused by cholinergic sympathetic nerve fibers.