This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Pressure Or Force With Which The Blood Is Urged on its course by the heart must not be confused with its velocity. The velocity is at its minimum in the capillaries; the pressure diminishes from quitting the heart till the return to it, being dissipated by the friction of the tissues which resist it. From experiments on the lower animals, it is calculated to be equal in large arteries, such as the carotid, to the support of a column of mercury more than six inches high, and in small arteries, like those of the foot, to be about a fourth less; while in the veins, after having experienced the resistance of the tissues in the capillaries, it is only about a twelfth of what it is in the arteries. These observations, together with the fact that defibrinated blood has been injected through the body of a dog with less pressure than that exerted by the heart (Sharpey), point out that the heart is the motive power which causes the blood to flow through the whole system. It must not, however, on that account be supposed that the tissues have no influence whatever on the circulation, for we have proof to the contrary in the fact that in interference with respiration, the unaerated blood fails to pass the capillaries, and that in inflammations, examined microscopically in the web of the frog's foot, blood corpuscles are seen arrested in their course without any obstruction existing in the channel beyond.