238 HUMAN BIOLOGY 



of the lungs and of the muscles. We have seen that the 

 adjustments in the circulation when work is being done are 

 all directed towards increasing the number of trips of the 

 red blood corpuscles from lungs to muscles and from muscles 

 to lungs again in a given time. Although the blood flow 

 in the capillaries is slower than anywhere else in the circuit, 

 when the rate is increased it is increased in the capillaries 

 as well as elsewhere. That means, of course, that less time 

 is allowed for the carriers to unload carbon dioxide in the 

 lungs and take on oxygen and to perform the reverse proc- 

 esses in the muscles. The beautiful fact has been discovered 

 that excess of carbon dioxide hastens the unloading of 

 oxygen from the corpuscles and that excess of oxygen hastens 

 the unloading of carbon dioxide. When the muscles work, 

 therefore, and produce extra carbon dioxide and need more 

 oxygen, the extra carbon dioxide forces the unloading of 

 oxygen from the corpuscles more rapidly at a time when 

 the faster flow through the muscle capillaries requires a more 

 rapid unloading. And when the corpuscles, laden with carbon 

 dioxide, reach the lungs, the higher concentration of oxygen 

 there drives out the carbon dioxide more rapidly when the 

 faster flow through the pulmonary capillaries requires a 

 more rapid unloading. In each place the gas which drives 

 the other out seizes the vacated place in the carrier for itself 

 and holds it until it in turn is driven out. There is no more 

 fascinating interplay of processes than this in any part of 

 the organism. 



One more striking provision for assuring an adequate 

 delivery of oxygen in case of need is seen in the sudden 

 rise in the number of red blood corpuscles when muscular 

 exertion is vigorous. This is the only aspect of the adaptation 

 of the organism to oxygen want that resembles a resort to 

 the supply depots. As we have seen, when muscular exertion 

 is severe and prolonged, glucose is mobilized from the liver 

 stores and distributed by the blood for use wherever required. 

 There is a store of red blood corpuscles in the spleen; the 

 concentration of the corpuscles there may be as much as 

 twice that in the general circulation. In strenuous exercise 

 the spleen is made to contract by sympathetic nerve impulses 

 and squeeze out its contents. The addition of the con- 



