THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 237 



their waste in an emergency. Careful studies have shown 

 that when a muscle is at rest many of its capillaries are not in 

 use or that they have shifts of service, one opening here for a 

 time and then closing down so that no blood runs through 

 it, while another near-by capillary opens and serves its 

 neighborhood. Only the capillaries which contain blood are 

 visible. When an active muscle from one side of the body is 

 compared with the corresponding muscle, inactive, of the 

 other side, the astonishing fact appears that the number of 

 open capillaries in the muscle at work may range from 40 to 

 100 times the number in the muscle at rest. What causes 

 the capillaries to dilate is not yet clear; lack of oxygen, 

 increase of carbon dioxide, or possibly some subtle substance 

 resulting from the wear and tear of the muscle as it pulls, 

 may open the vessels. However they may be opened, the 

 great importance of their being open should not be over- 

 looked. It is in the capillary region of the circulatory system 

 that the exchanges between the blood and the fixed cells 

 occur. Here all the adjustments of that system during 

 physical work that we have been considering have their 

 significance. The blood is bearing sugar and oxygen which 

 the laboring muscles require, it can bear away the carbon 

 dioxide and water which result from the burning that 

 attends contraction. The nearer the flowing blood can be 

 brought to the muscle cells in their need for both these 

 services, the more efficiently will the muscular work be 

 performed. The extraordinary unfolding of the unused 

 capillaries assures intimate relations between the cells 

 and the blood stream. 



We may now complete the circuit of adaptive changes 

 in the circulatory system. It is clear that when the muscles 

 are rhythmically contracting and massaging the vessels 

 within and between them they are pressing on a greater 

 volume of blood than is present when the muscles are at 

 rest. In other words the laboring muscles act as if they 

 were outlying hearts, receiving more blood when they work 

 and pumping that blood back to the central heart and to 

 the lungs for a new service. 



Still another remarkable relation remains to be mentioned : 

 that of the facilitation of the gas exchanges in the capillaries 



