996 PHYSIOLOGY 



withdrawal of a certain amount of fluid may lower the mean systemic 

 pressure, say from 10 to 5 mm. Hg. It is still possible for the pump 

 to maintain an arterial pressure equal to that produced when the 

 mean systemic pressure was 10 mm. Hg, but to produce this effect 

 the relative distribution of blood must be altered and the veins must 

 be more empty than they were previously. The maintenance of a 

 constant arterial pressure with varying amount of fluid in the 

 system can therefore be accomplished 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 contrac- 

 tion 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 therefore raise the mean systemic pressure. If the heart 

 is contracting the result may be a rise of pressure all round the system, 

 both in arteries and veins, or the rise may be confined to the arteries 

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

 diminished action of the heart and increased 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. 380) 

 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 there- 

 fore to accommodate 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 suc- 

 tion of the blood into the big veins of the thorax. Moreover the vessels 



