A "HEAD OF PRESSURE" 113 



This is the head of pressure maintained in the aorta or 

 compression chamber by the human ventricular pump. 

 If the head of pressure falls below 24 inches, then the 

 force in the aorta is not sufficient to keep up a flow 

 through the capillary fields of the body, and death soon 

 follows. Our life and health depend on the aortic head 

 of pressure. 



We must now examine for a moment the mechanism 

 by which a head of pressure is maintained in our arteries, 

 for there is no need to emphasise how important this 

 matter is for the welfare of the entire machinery of the 

 human body. We have only to look at the compression 

 chamber attached to the simple pump in fig. 29 (p. no) 

 to see that the head of pressure may be altered in two 

 ways : (1) increasing the inflow by speeding up the pump, 

 or by giving the piston a greater length of stroke, or by 

 both means ; (2) diminishing the outflow by turning the 

 stopcock still further ofF. Pressure will be diminished 

 by turning on the stopcock more fully or by slowing 

 the action of the pump. It will also be noticed that the 

 head of pressure is no indication of the amount of fluid 

 passing through the compression chamber ; if the stop- 

 cock of the outflow pipe be nearly closed, then a very 

 slight inflow from the pump will maintain a high head 

 of pressure. Now Nature uses both of these means for 

 regulating the head of pressure in the arterial compression 

 chamber of the human body ; the strokes of the heart 

 may be increased or diminished in number or in volume 

 or in both. She has established the most elaborate 

 system of stopcocks, which can be turned on or off 

 to any degree. By these means the head of pressure 

 in arteries can be regulated to the exact needs of the 

 machine. 



The heart is thus a muscular engine-pump. We have 

 seen that when all the muscular engines of the body are 

 in action they must be supplied with seven or eight times 

 the amount of fuel and oxygen needed while in a state of 

 rest. When the body is in action, then, the heart has to 

 pump at a much greater rate. Physiologists estimate that 



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