236 HUMAN BIOLOGY 



blood, causes an increase of pressure in the veins (note the 

 prominence of the veins beneath the skin during exertion). 

 This increased pressure continues and accentuates the 

 nervous effects just mentioned, for when it is appHed to the 

 right side of the heart, it starts a reflex which suppresses 

 still more the vagus check on the heart rate and thereby 

 the beat becomes still faster. The sympathetic nerves, 

 also, which are known to be excited when muscular exertion 

 is very strenuous and especially when emotional excitement 

 accompanies the eff"ort, as in competitive games, may play 

 an important part in making the heart contract more 

 rapidly. All these influences working in harmony provide 

 for adequate reception of the greater volume of blood 

 flowing back through the veins, for adequate delivery of 

 the blood to the lungs where the deeper ventilation cares 

 for the larger exchange of the respiratory gases (oxygen 

 and carbon dioxide), and for adequate driving of the oxygen- 

 laden blood into the great "arterial tree." 



The more ample discharge from the heart into the arteries 

 is attended by a rise of pressure in the arteries. In tests 

 made on a man riding a stationary bicycle the arterial blood 

 pressure rose at the start from 130 millimeters of mercury 

 to 180, and during the continuation of the exercise it 

 remained high, between 165 and 170 millimeters (i.e. at 

 about 7 inches instead of the resting level, about 5 inches). 

 The value of the increased pressure we can best appreciate 

 when we consider that there is a dilation of the arterioles 

 and capillaries in the active muscles. If the arterial pressure 

 were barely sufficient to keep the blood in circulation, a 

 widening of the vessels in one region would provide such a 

 way of escape for the blood from the arteries into the veins 

 that it would run through them and thus would leave other 

 regions without an adequate supply. The increased arterial 

 pressure not only prevents any such failure of the delivery 

 of blood to quiet regions, but it also assures rapid flow 

 through the dilated vessels of active regions, i.e. where 

 the need for the materials which the blood carries is greatest. 



The dilation of the blood vessels, arterioles and capillaries 

 in active muscles is one of the most remarkable adjustments 

 for bringing supplies to the cells and for carrying away 



