William D. Zeothout, Ph.D. W.W. Tuttle, Ph.D., Textbook of Physiology, 11th ed., C.V. Mosby, 1952


According to the law of volume flow, the amount of blood flowing through the combined capillaries in a unit of time is the same as that passing through the aorta (if measured over a considerable length of time). Although the diameter of an individual capillary is microscopic, 0.01 mm., because of the countless number of capillaries in the body the total capillary bed is estimated to be from four to eight hundred times that of the aorta.* From the law of velocity we can deduce that the flow must be correspondingly slower in the capillaries than in the larger vessels. In the aorta this has been found to be about 320 mm. per second. As the blood passes into the branches of the aorta, the bed becomes progressively larger and the velocity decreases correspondingly. When the blood reaches the capillaries with their tremendously large bed, microscopic observation reveals a velocity of from 0.5 to 1.0 mm. per second. Since the exchange of materials between the blood and the tissues takes place in the capillaries, the value of this greatly reduced velocity can be readily appreciated. As the blood leaves the capillaries and courses through the veins, the bed becomes gradually smaller and the velocity increases correspondingly. But as a vein is generally larger than the corresponding artery,** the blood in a vein never attains the velocity in the artery.

* In the average-sized adult of 30 years, the aorta has a diameter of about 1.8 cm.

** An artery supplying a definite area of the body and a vein draining the area are said to be corresponding vessels; for example, the renal artery and the renal vein.