Blood flow is slowest in the capillaries, which allows time for exchange of gases and nutrients. As the total cross-sectional area of the vessels increases, the velocity of flow decreases. The rate, or velocity, of blood flow varies inversely with the total cross-sectional area of the blood vessels.
Blood flows in the same direction as the decreasing pressure gradient: arteries to capillaries to veins. Like all fluids, blood flows from a high pressure area to a region with lower pressure. Pressure is a measure of the force that the blood exerts against the vessel walls as it moves the blood through the vessels. Blood Flowīlood flow refers to the movement of blood through the vessels from arteries to the capillaries and then into the veins. The net result of the capillary microcirculation created by hydrostatic and osmotic pressure is that substances leave the blood at one end of the capillary and return at the other end. Fluid movement across a capillary wall is determined by a combination of hydrostatic and osmotic pressure. Oxygen and carbon dioxide move across the capillary wall by diffusion. Substances pass through the capillary wall by diffusion, filtration, and osmosis. In addition to forming the connection between the arteries and veins, capillaries have a vital role in the exchange of gases, nutrients, and metabolic waste products between the blood and the tissue cells. Acknowledgements Physiology of Circulation Roles of Capillaries
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