210 



THE CIRCULATION OF THE BLOOD 



lose heat to the environment until the outflowing or venous blood is at 

 exactly the same temperature as the environment; for example, if the 

 hand is placed in water that is a little cooler than that of the blood, 

 and the temperature of the blood in one of the large veins of the hand 

 is measured, it will be found to be the same as that of the water in the 

 water-bath. 



To measure the rate of flow, therefore, we must ascertain: (1) how 

 much heat has been given out by the part to the water surrounding it 

 in a given time, and (2) the difference in temperature of the inflowing 

 (arterial) and outflowing (venous) blood. We measure the amount of 



Fig. 63. Plethysmograph for recording volume changes in the hand and forearm. By observ- 

 ing the rate with which the volume increases when the arm is compressed, the mass movement of 

 the blood can be determined. (From Jackson.) 



heat given out to the water in calories, a calorie being the amount of 

 heat required to raise the temperature of 1 c.c. of water from C. 

 .to 1 C. Suppose, for example, a hand were placed in 3,000 c.c. of 

 water at 33 C., and that after ten minutes the temperature had risen 

 to 33.5 C., then the amount of calories given out would be 3,000 x 0.5= 

 1500. Since calories equal cubic centimeters multiplied by change in 

 temperature, it follows that if we divide the figure representing them by 

 the actually observed difference in temperature between inflowing and 

 outflowing blood, the result must equal the number of cubic centimeters 

 of blood that has flowed through the part. The temperature of the in- 

 flowing blood has been found to be practically identical with that of the 



