364 TEXT-BOOK OF PHYSIOLOGY. 



The capacity of the venous system is considerably greater than that of 

 the arterial system, as there are usually two and even three veins accom- 

 panying each artery. This, taken in connection with its greater disten- 

 sibility, makes of the venous system a reservoir in which blood can be stored. 

 On this reservoir the arterial system can call for that amount of blood 

 necessary for the maintenance of its normal volume and pressure, and into 

 it any excess can be discharged. The relative amounts of blood contained 

 in the two systems are regulated by the degree of contraction of the arteriole 

 muscles and this in turn by the vaso-motor nerves. The movement of the 

 blood through the veins is accomplished by the cooperation of several 

 forces, reference to which will be made in a following paragraph. 



THE PULMONIC VASCULAR APPARATUS. 



The pulmonic vascular apparatus consists of a closed system of 

 vessels extending from the right ventricle to the left auricle, and includes 

 the pulmonary artery, capillaries, and pulmonary veins. In its anatomic 

 structure and physiologic properties it closely resembles, with, the systemic 

 apparatus. 



The stream-bed widens from the beginning of the pulmonary artery to 

 the middle of the capillary system; it again narrows from this point to the 

 terminations of the pulmonary veins. 



The movement of the blood from the beginning to the end of the system 

 is due to a difference of pressure between these two points, the result of the 

 friction between the blood and the vascular walls. The pressure in the pul- 

 monary artery of the dog has been shown by Beutner to be about one-third 

 that in the aorta; by Bradford and Dean to be one -fifth. Wiggers has 

 recently shown that in normally breathing dogs with arterial pressures rang- 

 ing from no to 112 mm. of mercury, the maximal or systolic pressure in 

 the pulmonary artery averaged 36 mm., and the minimal or diastolic 

 averaged 5 mm. The reason for the low pressure may be found in the 

 large size and rich development of the pulmonary capillaries and the 

 imperfect development of an arteriole muscle at the periphery of the- 

 pulmonary artery, the result of which is a diminution in the friction. 

 Inasmuch as the friction is relatively low, the work of the right heart is 

 less than that of the left heart and hence its walls are not so well developed. 

 The pulmonary pressure being low the intraventricular pressure of the 

 right heart is relatively low as compared with that of the left heart. The 

 velocity of the blood-stream in each of the three divisions of the system 

 can 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 ventricle. As- 

 suming 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 



