CHAPTER XIV 

 THE HYGIENE OF THE CIRCULATION 



Vasomotor control of blood distribution. The 

 blood in the human body amounts, as we have seen, to 

 only eight per cent of its weight, and yet it is so accu- 

 rately distributed to all its parts that each of its tissues 

 never fails of getting the right amount at the right time, 

 in spite of the fact that it wants now more, now less, as 

 its activity increases or diminishes. To accomplish this, 

 two methods are employed. The first is the vasomotor 

 adjustment, by which more blood is allowed to escape 

 into the active tissues through the arterioles which are 

 opened more widely for this purpose. As a result, the 

 distention of the arterial walls is decreased and conse- 

 quently their pressure upon the blood which remains 

 within them is lowered. Less blood therefore is forced 

 through them to the inactive tissues of the body. It is 

 conceivable under this arrangement that so many large 

 organs might be actively at work at the same time that 

 a large proportion of the blood in the arteries would 

 escape to them. Such a big reduction in the blood pres- 

 sure might result that the other organs of the body 

 would receive less blood than they needed. This would 

 be especially true of the brain, to which the blood has 

 to be sent uphill. 



Control by heart beat. In order to give complete 

 control of the circulation, the vasomotor control is sup- 

 plemented by variations in the frequency of the heart 



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