928 PHYSIOLOGY 



empty than they were previously. The maintenance of a constant arterial 

 pressure with varying amount of fluid in the system can therefore be accom- 

 plished either by alterations in the work of the heart or by alterations in 

 the peripheral resistance, and therefore in the ease with which the blood is 

 allowed to escape from the arterial to the venous side. 



Alterations of the capacity of the system will have the inverse effect 

 to alterations of its contents. Thus diminution in the volume of veins, 

 such as might be caused in the living body by the contraction of their 

 walls and which may be imitated in our model by pressure on the veins 

 from without, will drive the fluid into other parts of the system and there- 

 fore raise the mean systemic pressure. This rise of pressure may be con- 

 fined to the arteries by increased action of the heart, or it may be confined 

 to the veins by diminished action of the heart or decreased constriction of 

 the arterioles forming the peripheral resistance. 



Similar change in capacity may be brought about if we bring in the 

 effects of hydrostatic pressure. If in the model illustrated (Fig. 395) we 

 allow the thin-walled vein to hang over the edge of the table, the pressure 

 of the column of fluid within it causes it to dilate and therefore to accom- 

 modate more fluid, and this increased capacity might be so great that the 

 pressure in the section of the vein near the heart might sink to nothing 

 and the heart receive no blood when it started to contract. The whole 

 arterial system might in this way be allowed to drain under the influence 

 of gravity into the distensible dependent segment of the venous tube. 



All the conditions in our artificial schema have their exact analogue in 

 the living body. The determination of the mean systemic pressure in 

 the living body is difficult to carry out with accuracy. If, for instance, 

 we stop the heart, which we can do by stimulation of the vagus nerve, the 

 arteries will gradually empty themselves through the peripheral resistance 

 into the veins, and this process will tend to go on until the pressures are 

 identical throughout the system. Before this equilibrium is arrived at 

 however, reaction takes place on the part of the animal, tending to restore 

 the failing circulation. Thus the vessels contract strongly, so diminishing 

 the capacity. Movements take place, causing pressure on the veins of the 

 abdomen and the suction of the blood into the big veins of the thorax. 

 Moreover the vessels in an animal are not all on one plane and, if the animal 

 is in a vertical position, the hydrostatic pressure of the column of blood 

 between the heart and the dependent parts of the body may distend the 

 veins to such an extent that the whole of the blood is taken up in these 

 veins and none returned to the heart. The fact that, after stoppage of the 

 heart, the pressure is positive at all parts of the vascular system in the 

 animal with open thorax shows that there is actually a mean systemic 

 pressure, i. e. under normal circumstances, when the animal is in a hori- 

 zontal position, .all parts of the system are slightly distended. Direct - 

 measurement shows that this mean systemic pressure is about 10 mm. Hg. 

 The smallness of this figure shows moreover that, under the influence of 

 gravity alone, the pressure will be easily reduced to nothing at all in the 



