22.8: Blood Flow

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.

Slow Blood

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.

Regulation of Blood Flow

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.

Blood flow is driven by the heart which must be strong enough to push blood throughout the body into all of the major arteries, arterials and smallest capillary beds.

When blood leaves through the aorta and enters smaller arterials and capillaries, its high velocity and high pressure decrease, as a result of the increased combined diameters of the many blood vessels in comparison to the diameter of the aorta alone.

Such slow rate of movement provides adequate time for gas and nutrient exchange through the walls of the small blood vessels.

In contrast, blood travels faster through the venules, veins and back to the vena cava with the aid of smooth muscle in the vessel walls and compression from neighboring skeletal muscles preventing blood from pooling. Plus one way valves in veins prevent backflow despite the pull of gravity.

Ultimately, the process of blood flow is controlled by the body's needs and can be regulated by neurological signaling and hormones. For example, while exercising, blood is targeted to muscles due to vasodilation and away from the digestive system via vasoconstriction, directing blood where it is needed most.

Thus not all of the capillary beds have blood flowing through at all times.