Page 3, Architecture of the Circulation, Dr. D. Penney

Functional Components (continued..)

The next level of functional components are the precapillary resistance vessels, mainly the arterioles, which control blood flow distribution (Figure 7.01). The cost of such control is a high vascular resistance and a large drop in blood pressure. The final control of flow distribution is exercised by the sphincter section, small smooth muscle bands located immediately ahead of individual capillaries. The major function of capillaries is that of exchange with surrounding cells, 02, CO2, glucose, lactate, etc. Although capillaries are numerous, they are also very small (4-6 um diameter); thus vascular resistance is high.

Blood is gathered from the capillaries into venules, the post-capillary resistance vessels, which also have a limited exchange function. From there, the blood flows into larger veins and venous sinuses, the capacitance vessels, which contain more than half of the body's blood volume at any one time. The blood is then returned to the other pump section for another cycle. The venules and veins have little vascular resistance. All the functional components are in a series arrangement with one another, so that blood must traverse one after the other.