THE CIRCULATION OF THE BLOOD. 325 



This view is supported by the difference in the thickness of the walls of 

 the right and left sides of the heart. The pressure in the pulmonary 

 artery of the dog was shown by Bentner to be about one-third that 

 in the aorta; by Bradford and Dean to be one-fifth. The velocity 

 of the blood-stream in each of the three divisions of the system can ^ J. 

 not well be determined. The time occupied by a particle of blood 

 in passing from the right to the left ventricle has been estimated at 

 one-fourth the time required to pass from the left to the right ven- 

 tricle. \Assuming the latter to be thirty seconds, the former would be 

 seven and one-half seconds.^ 



The capillary vessels are spread out in a very elaborate manner 

 just beneath the inner surface of the pulmonary air-cells, and form 

 by their close relation to it, a mechanism for the excretion of carbon 

 dioxid and the absorption of oxygen. The extent of the capillary 

 surface is very great. It has been estimated at 200 square meters. 

 The amount of blood flowing 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 

 volume 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. 



The Contraction of the Heart. The primary jorces which keep 

 the blood flowing from the beginning ofThe 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 transferred, 

 now take up and continue the work of the heart, and by recoiling 

 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. : 



