. The animans and man; an elementary textbook of zoology and human physiology. closed. (After Dalton.) the tissues of the body. Its branches go to the tissuescarrying blood laden with food and oxygen. The pulmonary artery (8) arises in the right ventricle andcarries the blood that is to be distributed to the air chambersof the lungs. Its branches go to the walls of the lungs filledwith blood laden with carbon dioxide which is there ex-changed for oxygen. The openings from the ventriclesinto the arteries are guarded by semilunar valves shownin fig. 162. Their position in action is shown in figs
. The animans and man; an elementary textbook of zoology and human physiology. closed. (After Dalton.) the tissues of the body. Its branches go to the tissuescarrying blood laden with food and oxygen. The pulmonary artery (8) arises in the right ventricle andcarries the blood that is to be distributed to the air chambersof the lungs. Its branches go to the walls of the lungs filledwith blood laden with carbon dioxide which is there ex-changed for oxygen. The openings from the ventriclesinto the arteries are guarded by semilunar valves shownin fig. 162. Their position in action is shown in figs. 163 THE BLOOD AND CIRCULATION 321 and 164. Thus we see that the flow of blood from auricleto ventricle, and from ventricle to artery, can take placein one direction only. In case of diseased valves there maybe a backward flow or regurgitation. How the heart auricles are constantly fill-ing with blood from the great veins. Both auricles contractat the same time, the auriculo-ventricular valves open andthe blood is driven into the ventricles. The ventricles con-. FIG. 164. Right cavities of the heart; auriculo-ventricular valves closed,arterial valves open. (After Dalton.) tract at once and send the blood out into the great arteries. The heart beat is the alternate contraction (systole) andrelaxation or expansion (diastole) of the walls of the man this occurs about seventy-five times per minuteunder normal conditions. A childs pulse is more rapid. Systemic circulation.—We have seen that the aortacarries food and oxygen to the tissues. It passes anteriorlyfrom the left ventricle, then arches and passes posteriorly 322 THE ANIMALS AND MAN and dorsally, forming thus the arch of the aorta (fig. 162,7).The dorsal aorta passes downward through the diaphragmnear the spinal cord (fig. 167) to the lower part of the abdom-inal cavity. Here it divides into two great arteries, one toeach of the lower limbs, the common iliac arteries. Branches of the small
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