Blood is pumped by the heart into the aorta, the largest artery in the body, and then into increasingly smaller arteries, arterioles, and capillaries. The velocity of blood flow decreases with increased cross-sectional blood vessel area. As blood returns to the heart through venules and veins, its velocity increases. The movement of blood is encouraged by smooth muscle in the vessel walls, the movement of skeletal muscle surrounding the vessels, and one-way valves that prevent backflow.
Somewhat counterintuitively, the velocity of blood flow decreases as it enters blood vessels with smaller diameters. If a hose is squeezed, decreasing its diameter, water will squirt out faster and harder, but this does not occur when blood moves into blood vessels with smaller diameters. This is because blood does not simply move from one blood vessel into a smaller one, but travels from a blood vessel into multiple smaller blood vessels. The total cross-sectional area of these smaller blood vessels is greater than that of the original blood vessel. Additionally, the decreased diameter of individual vessels creates increased resistance. Therefore, as blood enters smaller blood vessels, it slows down, providing time for gas exchange to occur through the walls of small capillaries.
Blood flow is directed by vasodilation and vasoconstriction. Chemical signals can cause blood vessels to dilate, increasing blood flow, or constrict, decreasing blood flow. In this way, the body can selectively provide more oxygen and nutrients to muscles than the gastrointestinal tract during a flight-or-fight response, and similarly provide the gastrointestinal tract with more oxygen and nutrients during food consumption.
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