THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 235 



blood upward through the abdomen from the legs. Since 

 valves prevent the backward flow of the blood into the 

 legs the pressure favors the onward flow. At the same time 

 that the pressure in the abdomen is increased by contraction 

 of the diaphragm, the pressure on the veins in the chest is 

 decreased. The result is that with each inspiratory act 

 conditions are estabhshed which promote the flow of a 

 larger volume of blood into the heart. During expiration 

 the returning venous blood accumulates in the veins outside 

 the chest, in the arms, neck and abdomen. At the next 

 inspiration, however, the conditions just described recur 

 and the accumulated blood is driven to the heart. Thus to 

 the pumping action of the limb muscles is added the pumping 

 action of the diaphragm as a factor favoring the greater 

 utilization of the blood. Note that the bodily organization 

 is such that the contracting muscles, which need extra 

 oxygen because of their contractions, automatically favor 

 the securing of the needed oxygen by returning the blood 

 which carries it; and that the diaphragm, which is made to 

 pump more vigorously during exercise, not only maintains 

 the oxygen supply for loading the oxygen carriers, but also 

 aids to speed up the circulation of the carriers. 



Although the capacity of the heart chambers can be 

 enlarged, that adaptation is limited. The greater return 

 of blood to the heart in a given time resulting from the 

 pump-like actions just described must be received and 

 sent forth, therefore, by a heart that beats faster. As with 

 the red corpuscles, limitation is compensated for by more 

 rapid service. The faster heart beat is brought about and 

 maintained by a variety of agencies. We have noted that 

 the very act of making a motion is accompanied by increase 

 of respiration, because nerve impulses, attending the act, 

 excite the respiratory center in the brain. Similarly when 

 we start to move, the heart beats faster because vagus 

 nervous influences, which are continuously holding the 

 heart in check, are more or less suppressed. These are devices 

 for prompt adjustment to need, that appear in two diff"erent 

 systems which are, however, clearly related in their coopera- 

 tive functions. The pump-like action of the limb muscles 

 and of the diaphragm, that drives onward the venous 



