THE CIRCULATION OF THE BLOOD. 365 



through this system hourly and exposed to the respiratory surface is about 

 800 liters. The reason for the existence of the pulmonary circulation is 

 the renewal of the oxygen in the blood and the elimination of the carbon 

 dioxid; for the accomplishment of both objects ample provision is here 

 made. The flow of blood through the cardio-pulmonary vessels is subject 

 to variation during both inspiration and expiration in consequence of their 

 relation to the respiratory apparatus. The mechanism by which these 

 variations are produced will be considered in the chapter devoted to 

 Respiration. 



FORCES CONCERNED IN THE CIRCULATION OF THE BLOOD. 



1. The Contraction of the Heart. The primary forces which keep 



the blood flowing from the beginning of the aorta to the right side of the 

 heart and from the beginning of the pulmonary artery to the left side are 

 the contractions of the left and right ventricles respectively. This 

 is evident from the fact that each ventricle at each contraction not 

 only overcomes the pressure in the aorta and pulmonary artery, the 

 sum of all resistances, but imparts a given velocity to the blood. Since 

 the pressure continuously falls from the beginning to the end of each 

 system, it follows that the blood must flow from the point of high to 

 the point of low pressure. During the interval of the heart's activity, 

 the walls of the arteries, to which the heart's energy was largely trans- 

 ferred, now take up and continue the work of the heart, and by recoil- 

 ing drive the blood forward and into the venous system. Though 

 the heart's energy is probably sufficient to drive the blood into the 

 opposite side of the heart, it is supplemented by -other forces e.g.: 



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 the contrac- 

 tion and relaxation of the muscles there will be exerted an intermittent 

 pressure on the veins. With each contraction, the blood on the prox- 

 imal side will at once be driven forward with increased velocity, while 

 that on the distal side will be retarded, 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 inspira- 

 tion the pressure within the thorax but outside the lungs undergoes a 

 diminution more or less pronounced in accordance with the extent of 

 the movement. As a result, the blood in the large veins outside of the 

 thorax, being subjected to a pressure greater than that in the thorax, 

 flows more rapidly toward the heart. With each expiration the re- 

 verse 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 prevented by 

 the valves, for each retardation is immediately checked by their closure 

 and support given to the column of blood. The influence of gravity 



