326 TEXT-BOOK OF PHYSIOLOGY. 



2. Muscle Contraction. As a result of the relation which the veins 



bear to the muscles in all parts of the body it is clear that with 

 each contraction and relaxation of the muscles there will be exerted 

 an intermittent pressure on the veins. With each contraction 

 the blood on the proximal side will at once be driven forward 

 with increased velocity, while that on the distal side will be re- 

 tarded, will accumulate and distend the veins, owing to the 

 closure of the valves; with the relaxation of the muscle the elastic 

 and contractile tissues in the walls of the veins will come into 

 play and force the blood forward. 



3. Thoracic Aspiration. The inspiratory movement aids the flow 



of blood through the venae cavae and their tributaries. With 

 each inspiration the pressure within the thorax but outside the 

 lungs undergoes a diminution more or less pronounced in ac- 

 cordance with the extent of the movement. As a result, the 

 blood in the large veins, now subjected to a pressure greater than 

 that in the thorax, flows more rapidly toward the heart. With 

 each expiration the reverse obtains. 



4. Action of the Valves. It is quite probable that gravity opposes 



to some extent the flow of blood through the veins below the level 

 of the heart. This opposition to the upward flow is largely pre- 

 vented by the valves, for each retardation is immediately checked 

 by their closure and support given to the column of blood. 

 The influence of gravity is shown when the relation of the arm to 

 the heart is changed. Thus, if the arm be allowed to hang pas- 

 sively by the side of the body, the veins, especially on the back of 

 the hand, will become distended with blood. If now the arm be 

 raised, the blood will flow rapidly toward the heart, as shown by 

 the rapid emptying of the veins. 

 Work Done by the Heart. The work which the left ventricle 



performs^at each contraction when it discharges its contained volume 



of blood into the aorta is : 



1. To overcome the total resistance of the systemic vascular appa- 



ratus expressed in terms of aortic pressure; and 



2. To impart velocity to the blood. 



The pressure in the aorta is not absolutely determined, though 

 f or many reasons it may be assumed to be about 250 mm. Hg, 

 or its equivalent, a column of blood 3.21 meters in height As the 

 heart discharges 188 grams, the work done may be calculated by 

 multiplying the weight by the height: viz.,o.i88 X 3.2 = 0.6016 kilo- 

 grammeter. 



^The velocity of the blood in the aorta has been approximately 

 estimated at 0.5 meter per second. The work done in imparting this 

 velocity to 188 grams is estimated by squaring the velocity and dividing 

 by the accelerating force of gravity (^ ) and multiplying the 



