The circulation is a continuously moving stream of blood that supplies the tissues with nutrients and oxygen and removes the by-products of metabolism from the tissues. Movement of blood depends upon pressure; that is, a force is applied to one portion of the system that drives the fluid contained within into another portion. The pressure involves four components of the circulatory system.
The heart is a compact muscular pump, which is about the size of a fist in the human adult. On contraction, it ejects about 2 ounces of blood (a tea cupful). The force that is generated on blood flow is known as hemodynamic pressure.
Hemodynamic pressure generated by the heart is transferred into the arteries. A momentary expansion of the elastic arteries with each heartbeat reflects the increased pressure and is responsible for the peripheral pulse. The pressure at its peak defines systole. Between contractions when the heart relaxes and the blood moves outward into smaller branches of the arteries, hydrostatic pressure falls sharply. The lowest pressure denotes diastole; it occurs during the elastic recoil of the arteries.
The smallest arteries (or arterioles) lead into the capillaries, extremely fine tubes of a single layer of cells. These vessels thread intimately among cells of all the body tissues. Here, the vital exchange of nutrients and gases occurs between the blood and the cells. This exchange occurs in an aqueous medium, the interstitial fluid, in which every cell in the body bathes. Hydrostatic pressure is very low in the capillaries; here, blood flow slows markedly. This delay allows several seconds with each passage for the exchanges between the blood and the interstitial fluid. The diffusion of fluids between serum and cells across the semi-permeable membrane is determined principally by the concentration of non-permeable proteins; the difference in density determines osmotic pressure.
The capillaries merge into tiny veins (or venules) as blood is collected from the tissues. Blood moves into the highly distensible veins that merge into larger and larger veins much as twigs on a tree merge into branches and branches converge into the trunk. Finally, the largest vein empties into the heart, thus completing the circuit (and hence the term circulation, a word coined by William Harvey in 1628).
The hemodynamic pressure from the heart that forces blood through the arteries and then the capillaries is virtually dissipated before the blood moves into the venous circulation. Consequently, other mechanisms must come into play to complete the flow of blood from the tissues back to the heart. The challenge is all the more demanding when the individual is upright. The actions recruited for this task make up the venous pump.