VELOCITY OF BLOOD FLOW
The velocity, or speed, with which blood flows differs in the various parts of the vascular system. Velocity is inversely related (meaning as one value goes up, the other goes down) to the cross-sectional area of the particular segment of the vascular system. Refer to Fig. 13–9 as you read the following. The aorta receives all the blood from the left ventricle, its cross-sectional area is small, about 3 cm2 (1 sq. inch), and the blood moves very rapidly, at least 30 cm per second (about 12 inches). Each time the aorta or any artery branches, the total cross-sectional area becomes larger, and the speed of blood flow decreases. Think of a river that begins in a narrow bed and is flowing rapidly. If the river bed widens, the water spreads out to fill it and flows more slowly. If the river were to narrow again, the water would flow faster. This is just what happens in the vascular system.
The capillaries in total have the greatest cross-sectional area, and blood velocity there is slowest, less than 0.1 cm per second. When capillaries unite to form venules, and then veins, the cross-sectional area decreases and blood flow speeds up.
Recall that it is in capillary networks that exchanges of nutrients, wastes, and gases take place between the blood and tissue fluid. The slow rate of blood flow in capillaries permits sufficient time for these essential exchanges. Think of a train slowing down (not actually stopping) at stations to allow people to jump on and off, then speeding up again to get to the next sta-tion. The capillaries are the “stations” of the vascular system.
The more rapid blood velocity in other vessels makes circulation time quite short. This is the time it takes for blood to go from the right ventricle to the lungs, back to the heart to be pumped by the left ven-tricle to the body, and return to the heart again. Circulation time is about 1 minute or less, and ensures an adequate exchange of gases.