118 NUTRITIONAL PHYSIOLOGY 



at large and back to the right auricle is called the systemic 

 circulation. 



From the right auricle the blood descends to the ven- 

 tricle of the same side, and is sent forth again, this time 

 through the vessel known as the pulmonary artery. 

 This immediately forks into branches which plunge into the 

 two lungs. The smaller pulmonary arteries lead to rich 

 capillary networks which are wrapped around the number- 

 less air-sacs of the lungs. Here the corpuscles are re- 

 charged with oxygen and the carbon dioxid of the blood is 

 reduced to a standard amount. Pulmonary veins return 

 the blood to the left auricle. The relatively short journey 

 of the blood from the right ventricle to the left auricle 

 by way of the lungs is called the pulmonary or lesser cir- 

 culation. 



It is necessary to call attention to the fact that the ad- 

 jectives "arterial" and "venous" are not used in a sense 

 which exactly corresponds with the meanings of the nouns 

 "artery" and "vein." The adjectives have a chemical 

 significance; the nouns, an anatomic one. Arterial blood 

 is blood fully oxygenated; venous blood is blood more or 

 less deficient in oxygen. But an artery, as has been said, 

 is merely a vessel carrying blood away from the heart. 

 The blood within will be arterial if we are observing the 

 systemic circulation, and venous if it is in the pulmonary. 

 So the systemic veins are filled with venous blood, while the 

 pulmonary veins carry that which has just been brought 

 up to the arterial standard through coming into relation 

 with the air in the lungs. 



From what was stated above with respect to the distri- 

 bution of the blood, it is evident that less than one-fourth 

 of the whole volume is in the pulmonary circulation at one 

 time, but the student must be cautioned against the con- 

 clusion that the pulmonary circuit is traversed by less 

 blood than passes through the systemic pathways. The 

 quantities passing along the two routes are inevitably 

 equal, for it is, after all, the same blood which runs through 

 each in turn. If a chain is traveling over two wheels, as 



