920. How much blood does the human body contain?
From twenty-five to thirty-five pounds. (See [623].)
921. How does the blood ascend in the veins, in opposition to gravitation?
In addition to the muscular coats of the veins, and the influence of muscular action upon them, there are in the veins numerous semi-circular valves, which are not found in the arteries. These valves extend from the sides of the veins in such a manner that they allow the free passage of the blood upwards, but a backward motion of the blood would expand the cup-like valves and stop the passage; so that the blood can only move in one direction, and that towards the heart.
922. How frequently does the total amount of blood circulate through the system?
The blood circulates once through the body in about two minutes. If, therefore, we estimate the amount of blood at twenty-four pounds, it follows that no less than twelve pounds of blood pass through the heart every minute; and it is estimated that if the blood moved with equal force in a straight line it would pass through one hundred and fifty feet in a minute.