122 NUTRITIONAL PHYSIOLOGY 



to do with the behavior of the organs concerned when they 

 are viewed as living structures. Under the first class are 

 included the facts of blood-pressure, velocity of flow, the 

 resistance overcome, etc. The interpretation of such data 

 is almost a science in itself and is known as hemodynamics. 

 Among the questions of the second sort are the mysterious 

 automaticity of the heart and the nervous government of 

 the quantity and the distribution of blood flow. These 

 difficult matters may well be left to be briefly dealt with 

 toward the end of the book, when the general work of the 

 central nervous system will be presented. We must, 

 however, devote some space at this time to the fundamen- 

 tals of hemodynamics. 



The Character of the Blood Flow in Vessels of Different 

 Classes. When an artery is cut the blood escapes in a 

 forcible jet which may spring to a distance of several feet. 

 The stream is not steady, but mounts and declines in a 

 rhythm corresponding with the heart-beat. A good deal 

 of pressure must be applied to restrain the bleeding. If a 

 vein is severed the flow of blood is rapid and copious, but 

 easily repressed. It is uniform so far as can be judged. 

 These observations lead to the conclusion that the blood 

 in the arteries is under a high average pressure, with large 

 fluctuations from moment to moment. The pressure in 

 the veins is evidently very low. 



The diminution of pressure between the arteries and the 

 veins can be simply explained. When the blood is started 

 on its course through the systemic circulation the high 

 pressure which it exerts against the elastic wall may be 

 regarded as a measure of the energy which the ventricle 

 has impressed upon it. When it draws near the right side 

 of the heart, the goal of its journey, its abated pressure is 

 the sign that the initial energy has been spent. How has 

 it been consumed? There is but one possible answer: It 

 has been transformed into heat in overcoming the friction 

 encountered in the vessels. Analogous conditions can be 

 demonstrated for any tubular system through which liquid 

 is driven. In the mains which carry a city water-supply 



