234 HUMAN BIOLOGY 



delivery to the active organs. The respiratory adjustments, 

 therefore, maintain in the lungs an adequate supply of 

 oxygen in spite of the extra demand, and they minimize 

 the accumulation of carbon dioxide there in spite of the 

 larger deposit from the blood. 



To understand the circulatory adjustments we must 

 remember that the carriage of oxygen and carbon dioxide 

 is dependent on the red blood corpuscles of the blood and 

 that, although their number can be increased in emergencies, 

 it is nevertheless hmited. In such conditions the only way 

 to increase the carriage of these gases is to increase the use 

 of the carriers; in other words, to multiply the number of 

 trips wh'ch the carriers make between the lungs and the 

 active parts. This in fact takes place, but in addition 

 the processes of loading and unloading are facilitated at the 

 two stations. We shall now consider these adjustments in 

 detail. 



First, in order that there shall be a larger output of blood 

 from the heart there must be a larger return of blood to 

 the heart through the veins. This effect is achieved by a 

 variety of actions when we engage in muscular effort. 

 The nerves governing the size of the blood vessels in the 

 capacious vascular area of the stomach and intestines 

 cause these vessels to contract. In consequence much of 

 the blood is driven out of them and into the vessels of the 

 muscles, which, as we shall see, have a greatly enlarged 

 capacity when the muscles are at work. Now the contracting 

 muscles press more or less rhythmically on the vessels, 

 especially on the small veins, and since there are valves 

 which permit only an onward flow of the blood towards 

 the heart, the rhythmic pressure necessarily promotes 

 that flow. If the left wrist is grasped firmly by the right 

 hand, and the left hand is then rapidly and repeatedly 

 clenched and relaxed, the quick filling of the veins, as seen 

 on the back of the hand, can be readily demonstrated. 

 Another type of pumping action on the veins occurs in the 

 functioning of the great dome-shaped muscle of respiration, 

 the diaphragm, which separates the chest from the abdomen. 

 When it contracts, it flattens, and thereby it somewhat 

 increases the pressure on the great vein which leads the 



