Monday, January 30, 2012

Velocity of Capillary Blood Flow

When a continuous stream moves through consecutive sets of tubes, the velocity of flow decreases as the sum of the cross-sectional areas of the tubes increases. This is precisely the case in the cardiovascular system. The blood velocity is very great in the aorta, slows progressively in the arteries and arterioles, and then slows markedly as the blood passes through the huge cross-sectional area of the capillaries. Slow forward flow through the capillaries maximizes the time available for substances to exchange between the blood and interstitial fluid. The velocity of flow then progressively increases in the venules and veins because the cross-sectional area decreases. To reemphasize, flow velocity is not dependent on proximity to the heart but rather on total cross-sectional area of the vessel type. The cross-sectional area of the capillaries accounts for another important feature of capillaries: Because each capillary is very narrow, it offers considerable resistance to flow, but the huge total number of capillaries provides such a large cross-sectional area that the total resistance of all the capillaries is much less than that of the arterioles.