THE CIRCULATION OF THE BLOOD 



339 



capillaries, venules, and veins may therefore be schematically arranged 

 (Fig. 150) in a manner identical with the schematic arrangement of tubes 

 represented on page 336. The heart, with which they are in connection, 

 when filled with blood may be compared with the reservoir filled with water, 

 and the intra-ventricular pressure developed during the contraction, to 

 the downward pressure of the water when the stopcock at A is opened. 



The Stream-bed. The stream-bed, the path along which the blood 

 flows, varies widely in its total sectional area in different parts of its course, 

 being least in the aorta and venae cavae, and greatest in the capillaries. In 

 passing from the base of the aorta toward the capillaries the sectional area of 

 individual arteries, in consequence of repeated branching, diminishes, 

 though their total sectional area increases and In direct proportion to their 



Veins 



FIG. 150. SCHEMATIC ARRANGEMENT OF THE VASCULAR APPARATUS. 

 a, x, the arteriole muscle by which the peripheral resistance can be increased or decreased. 



distance from the heart. In the capillary system the sectional area of an 

 individual capillary attains its minimal value, though the total sectional 

 area attains its maximal value. Comparing one with the other, it has been 

 estimated that the total sectional area of the aortic bed is to the total sectional 

 area of the capillary bed as i is to 600 or 800. This statement is based on 

 the estimated values of the sectional area of the aorta, 615 mm., the velocity 

 of the blood in the aorta 300 mm. per second, the average velocity in the 

 capillaries 0.5 mm. per second. If these estimates are accepted as approxi- 

 mately correct, then the sectional area of the capillary system or bed is 

 obtained by the following formula: 



x = - or 369,000 mm., a ratio of the aortic sectional area to the 



