THE CIRCULATORY SYSTEM 133 



Blood pressure is higher in the arteries than the veins. The 

 difference is shown when an artery and a vein are cut; the blood 

 spurts from the artery in a pulsating jet, while the flow from the 

 vein is gentle, more constant, and easily checked. 



Arterial blood pressure has been determined by connecting 

 the femoral artery of a prostrate horse with a vertical glass tube; 

 the blood was found to rise to a height of 10 feet in the tube. 

 Smith states that at each stroke about 2.25 pounds of blood are 

 pumped into the aorta of the horse. From these figures the, work 

 of the left ventricle with each systolic movement is found to be 

 equivalent to raising 22.5 pounds one foot. On account of the 

 greater resistance in the peripheral arteries and capillaries than 

 in their pulmonary counterparts, the work of the left heart has 

 been estimated to be four times that of the right. This should 

 give some idea of the energy expended by the body in simply 

 sustaining the circulation in a state of repose and the importance 

 of keeping it in working order. 



Venous blood pressure may be shown by the same method as 

 that described for measuring the arterial pressure. A vein con- 

 nected up with a vertical tube will force a column of blood but a 

 few inches high into the tube. Venous pressure is largely depend- 

 ent upon external pressure which is exerted in the following ways: 



(1) Respiratory movements, as was explained in the chapter 

 on Respiration, result in expansion and contraction of the thorax. 

 During inspiration there is a negative pressure in the thorax and 

 the blood is sucked into the large intrathoracic veins in much the 

 same way that the air is into the lungs. The greater thickness of 

 the walls of the arteries prevents in large measure this effect 

 on them. During expiration the reverse action would take 

 place, and the blood would be forced back into the tributary 

 veins if they were not provided with valves. As there are no 

 valves in the pulmonary veins and those opening into the right 

 auricle, the blood in them flows into the upper chambers of the 

 heart. 



(2) Muscular contractions result in a shortening and thicken- 

 ing of muscles. This brings about compression of the veins 

 which he in and between the muscles and forces the blood from 

 the smaller into the larger veins, the valves, as already stated, 

 preventing any backward flow. The more rapidly a muscle 

 contracts, the greater is the amount of blood circulating through 



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