BIOLOGY IN HUMAN AFFAIRS 



besides those which are directly involved (muscular and 

 nervous) must operate in a diflferent manner from what 

 they do at rest. Some must carry out their function much 

 more rapidly than before; others must almost cease to 

 function for the time being. The rate and depth of breathing 

 may be increased many fold (in man, from six to one 

 hundred liters of air per minute); this supplies the needed 

 oxygen to the blood. But if the blood moved around 

 with no more rapidity than when the body is at rest, the 

 increased supply of oxygen at the lungs would be useless. 

 Hence, the heart pumps the blood much more rapidly and 

 in greater quantity than before; in man, the amount of 

 blood put out in one minute from one ventricle may increase 

 from about four liters to twenty-five liters or more. This is 

 accomplished by an increase in both the number of strokes 

 per minute and the size of each stroke. But at rest, only 

 about one-third, or less, of the total blood pumped by 

 the heart goes to the muscles, although they comprise 

 about one-half of the total weight or volume of the body; 

 so a further adjustment is necessary. This consists of a 

 widening of the blood vessels in the muscles and, in fact, 

 of an opening up of many stagnant blood capillaries so 

 that the number of capillary vessels through which blood 

 is flowing may be increased forty to one hundred times. 

 But the opening of these vessels alone would not supply 

 enough blood carrying oxygen to the muscle; and, what 

 is more important, it would involve a marked lowering 

 of blood pressure which would secondarily decrease the 

 blood and oxygen supply to the brain — the master tissue 

 of the body— a thing which must not happen if the body 

 is to remain active. So what happens, in order that the 

 muscles and brain may receive an adequate blood supply, 

 is that the supply to all the other organs is greatly depleted 

 by a decrease in the caliber of the blood vessels, and we 

 find, under these conditions, that the intestines get so 



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