The cardiovascular system depends on the energy provided by hemodynamic pressure gradients to move materials over long distances (bulk flow) and the energy provided by concentration gradients to move material over short distances (diffusion). Both types of movement are the result of differences in potential energy. As we have seen, bulk flow occurs because of differences in pressure. Diffusion occurs because of differences in chemical concentration.
Hemodynamic Pressure Gradients Drive Bulk Flow; Concentration Gradients Drive Diffusion
Blood circulation is an example of transport by bulk flow. This is an efficient means of transport over long distances, such as those between the legs and the lungs. Diffusion is accomplished by the random movement of individual molecules and is an effective transport mechanism over short distances. Diffusion occurs at the level of the capillaries, where the distances between blood and the surrounding tissue are short. The net transport of molecules by diffusion can occur within hundredths of a second or less when the distances involved are no more than a few microns. In contrast, minutes or hours would be needed for diffusion to occur over millimeters or centimeters.
Bulk Flow and Diffusion Are Influenced by Blood Vessel Size and Number
The aorta has the largest diameter of any artery, and the subsequent branches become progressively smaller down to the capillaries. Although the capillaries are the smallest blood vessels, there are several billion of them. For this reason, the total cross-sectional area of the lumens of all systemic capillaries (approximately 2,000 cm2) greatly exceeds that of the lumen of the aorta (7 cm2). In a steady state, the blood flow is equal at any two cross sections in series along the circulation. For example, the flow through the aorta is the same as the total flow through all of the systemic capillaries. Because the combined cross-sectional area of the capillaries is much greater and the total flow is the same, the velocity of flow in the capillaries is much lower. The slower movement of blood through the capillaries provides maximum opportunity for diffusional exchanges of substances between the blood and the tissue cells. In contrast, blood moves quickly in the aorta, where bulk flow, not diffusion, is important.
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