374 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 appara- 

 tus consists of a closed system of vessels extending from the right ventricle 

 to the left auricle, and includes the pulmonic artery, capillaries, and pul- 

 monic veins. In its anatomic structure and physiologic properties it closely 

 resembles, with, the systemic apparatus. 



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

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

 terminations of the pulmonic 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- 

 monie 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 ran- 

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

 pulmonic 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 pulmonic capillaries and the imperfect development 

 of an arteriole muscle at the periphery of the pulmonic 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 pulmonic 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 cannot 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. Assuming the latter to be thirty seconds, the former would be 

 seven and one-half seconds. 



The presence of vaso-motor nerves in the walls of the arterioles of the 

 pulmonic artery has not been definitely determined. Adrenalin which con- 

 stricts blood vessels supplied with vaso-motor nerves is stated by some 

 investigators to have this effect, but by others to be without it. If vaso-motor 

 nerves are present their action is relatively slight. 



The capillary vessels are spread out in a very elaborate manner just 

 beneath the inner surface of the pulmonic 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 



