. Nutritional physiology . t is an amount of blood solarge that the single contribution of the ventricle makesa rather small addition to it. The escape of the bloodthrough the terminal twigs cannot cease while there isso much stored under a high pressure in the aorta and itsbranches. The heart may omit or drop a beat withoutnoticeably diminishing the flow through the standstill will be reached only when the arteries haveattained a degree of contraction such that the internalpressure is no higher than that in the veins. The homely THE CIRCDLATION 127 illustration (Fig. 19) which a
. Nutritional physiology . t is an amount of blood solarge that the single contribution of the ventricle makesa rather small addition to it. The escape of the bloodthrough the terminal twigs cannot cease while there isso much stored under a high pressure in the aorta and itsbranches. The heart may omit or drop a beat withoutnoticeably diminishing the flow through the standstill will be reached only when the arteries haveattained a degree of contraction such that the internalpressure is no higher than that in the veins. The homely THE CIRCDLATION 127 illustration (Fig. 19) which accompanies this may be help-ful. The pump delivers water intermittently to the leakytrough, keeping it filled to a level which is nearly constant,though fluctuating a little in the rhythm of the the escape of water through the cracks isall but uniform in its rate. One important difference be-tween the pump and trough, on the one hand, and the cir-culatorv system, on the other, lies in the fact that in the. Fig. 19.—At / the discbarge is in gushes with pauses between—the type of the expulsion of blood from the heart At C the escapethrough the cracks is at a practically constant rate; this is true of theblood flow through the capillaries. first case the driving force is gravity; in the second, it isthe reaction of the enclosing elastic walls. A set of facts which it is well to separate in ones thoughtas completely as possible from considerations of pressureis the body of data respecting the linear velocity of thelilood. By this is meant the rate of advance of the aver-age corpuscle. In any vessel the stream runs more swiftlyin the central axis and lags along the walls. The velocityin the arteries rises and falls as does the pressure, but, on 128 NUTRITIONAL PHYSIOLOGY the whole, is relatively high. The aorta is passed at aspeed of at least a foot in a second. The veins also aretraversed at a high velocity, though the figures are some-what lower than for the
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