TRANSPORTATION SYSTEM 



129 



In the veins next to the heart it is on the average less than atmospheric 

 pressure; that is, a "suction" is present there when the auricles relax. 



The high pressure in the arteries is necessary to drive the blood 

 through the capillaries where the resistance is great. It is also needed 

 to send the blood above the pumping organ, as to the head in man. 

 Pressure drops in the capillaries because of the great increase in the 

 aggregate cross section of these numerous vessels, but there must still 

 be a small pressure beyond the capillaries to push the blood (against 

 gravity in much of the system) on to the heart. 



Blood pressure is elevated if heart action is accelerated, also if resist- 

 ance in the vessels is increased. This resistance depends on the diameter 



Course of Circulation 



Fig. 103. — Curve showing decrease of blood pressure in course of circulation in man. 



of the vessels, which is under the control of a nerve center in the medulla. 

 A sUght increase in the carbon dioxide in the blood stimulates this 

 center, the vessel walls contract, and pressure is raised. However, in an 

 active muscle, where the excess carbon dioxide is being produced, there 

 is an opposite effect, a local dilation of the vessels, perhaps a response to 

 higher acidity caused by the extra carbon dioxide or extra lactic acid. 

 The net result is a shunting of the blood to the active organ where it is 

 needed. 



A special situation in the great artery from the left ventricle regulates 

 the heart beat by stimulating it (through a nerve) when the pressure in 

 the artery falls, depressing the heart when this pressure rises. Other 

 stimuli are associated with these, but they all work together to check 

 activity when it becomes too great, stimulate it when it lags. Highly 

 adaptive controls thus depend upon automatic responses of organs to 

 stimuli which the organs themselves Kelp to create. 



