. Elements of human physiology. Physiology. 200 PHYSIOLOGY filled the system to a slight extent. Under these circumstances, so long as the heart is not beating, the pressure in all parts of the system will be the same, and this pressure, which may be called the mean general blood-pressure, amounts in a large dog to about 10 mm. Hg. It will be seen at once that the pump or heart cannot alter this mean general pressure, but can only give rise to an unequal distribution of the pressure. Thus it may diminish the pressure in the veins and increase the pressure in the arteries by pumping the fluid f
. Elements of human physiology. Physiology. 200 PHYSIOLOGY filled the system to a slight extent. Under these circumstances, so long as the heart is not beating, the pressure in all parts of the system will be the same, and this pressure, which may be called the mean general blood-pressure, amounts in a large dog to about 10 mm. Hg. It will be seen at once that the pump or heart cannot alter this mean general pressure, but can only give rise to an unequal distribution of the pressure. Thus it may diminish the pressure in the veins and increase the pressure in the arteries by pumping the fluid from the veins into the arteries. We may take Fig. 103 to repre- sent the vascular system, which has a definite capacity and contains a definite quantity of blood. If the heart (ii) is not acting, and the fluid is motionless, the pressures at all parts of the system will be the same (10 mm. Hg.). If now the heart begins to act, it pumps blood from the veins into the arteries, so that the latter become distended at the expense of the former, and. the arterial pressure rises above, and the venous pressure sinks below, the mean pressure of the system. It must be remembered that in the body the tubes forming the veins are much more distensible at low pressures than are the tubes on the arterial side. Hence, when the heart begins to beat, we get a large rise of pressure on the arterial side (from 10 mm. to 120 mm.) and only a small fall of pressure on the venous side (from 10 mm. to 5 mm., 0 mm., or near the heart about —5 mm.). It is evident that in such a system, while the resistance remains constant, the venous pressure will vary inversely with the arterial pressure, and not directly with the latter, as is the case with an open circuit. There is one other important fact arising from the closed condition of the circulatory system. In the body, changes in the peripheral resistance are effected by contractions of the smaller arteries and of some of the veins. Now the contractions o
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