Blood Volume and Blood Vessels

The volume of blood that is pumped by the heart each minute is called the minute-volume, whereby the heart beats (contractions) per minute (cardiac stroke rate) eject a typical quantity of blood per beat. This rate is altered by the body's activity and by the volume of blood returning to the heart from the veins each minute. If the venous blood volume is adequate, then increase in stroke rate can increase minute-volume. The increased stroke rate, however, involves a decrease in the ventricular filling time, and as a result, the ventricles do not fill completely. Thus, the stroke volume is decreased; at rapid heart rates, even the minutevolume may be decreased, so that it offsets the stroke rate. During systole, the ventricles do not empty completely. A small residual volume of blood remains in them. An increased venous return may cause more complete filling and emptying of the ventricles, thus increasing the cardiac output without changing the stroke rate.

The vessels at various points in the circulatory path differ anatomically and functionally. The great arteries have thick walls heavily lined with smooth muscle and contractile tissue to enable them to transport blood under pressure from the heart to peripheral tissues. The arteries become smaller and thinner-walled as they branch out toward the periphery. The systemic arteries deliver blood to the microcirculatory beds of the tissues and organs. These "capillary beds" consist of microscopic arterioles, capillaries, and venules. The contraction (vasoconstriction) and relaxation (vasodilation) of the smooth muscles in the terminal branches of the arteries play an important role in regulating blood flow in the capillary bed. Control of the arteriole muscles is mediated by sympathetic neurotransmitters, hormones, and local effects. From the arterioles, the blood enters the capillaries, minute vessels whose walls consist of a single layer of cells, facilitating transfer of oxygen and nutrients to the tissues and the loading of metabolic waste and carbon dioxide, all via the extracellular fluid. Their density depends on the need of the particular tissue for nutrients and oxygen. The capillaries drain into small, thin-walled but muscular vessels called venules, whence the blood begins its return to the heart through the veins. The veins have elastic walls but are without muscles. The venous vasculature serves as a reservoir, storing about 60 percent of the blood.

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