212 



THE CIRCULATORY SYSTEM. 



[CH. XVIII. 



f// 1 



-" 



continuous with each other at the far end by means of the 

 capillaries. 



From the right auricle the blood passes to the right ventricle, 

 then by the pulmonary artery, which divides into two, one for 

 each lung, then through the pulmonary 

 capillaries, and through the pulmonary veins 

 (two from each lung) to the left auricle. 

 From here it passes into the left ventricle, 

 which brings us back to where we started 

 from. 



The complete circulation is thus made up 

 of two circuits, the one, a shorter circuit from 

 the right side of the heart to the lungs and 

 back again to the left side of the heart ; the 

 other and larger circuit, from the left side 

 of the heart to all parts of the body and 

 back again to the right side. The circula- 

 tions through the lungs and through the 

 system generally are respectively named the 

 Pulmonary and Systemic or lesser and 

 greater circulations. It will be noticed also 

 in the same figure that a portion of the 

 stream of blood having been diverted once 

 into the capillaries of the intestinal canal, 

 and some other organs, and gathered up 

 again into a single stream, is a second time 

 divided in its passage through the liver, 

 before it finally reaches the heart and completes a revolution. 

 This subordinate stream through the liver is called the Portal 

 circulation. A somewhat similar accessory circulation is that 

 through the kidneys, called the Renal circulation. The difference 

 of colours in fig. 213 indicates roughly the difference between 

 arterial and venous blood. The blood is oxygenated in the lungs, 

 and the formation of oxyhaemoglobin gives to the blood a bright 

 red colour. This oxygenated or arterial blood (contained in the 

 pulmonary veins, the left side of the heart, and systemic arteries) 

 is in part reduced in the tissues, and the deoxygenated haemoglobin 

 is darker in tint than the oxyhaemoglobin ; this venous blood passes 

 by the systemic veins to the right side of the heart and pulmonary 

 artery to the hmgs, where it once more receives a fresh supply of 

 oxygen. 



N.B. It should, however, be noted that the lungs, like the rest of the 

 body, are also supplied with arterial blood, which reaches it by the 

 bronchial arteries. 



Fig. 214. Minute artery 

 viewed in longitudinal 

 section, e. Nucleated 

 endothelial membrane, 

 with faint nuclei in 

 lumen, looked at from 

 above, i. Elastic mem- 

 brane, m. Muscular 

 coat or tunica media. 

 a. Tunica adventitia. 

 (Klein and Noble 

 Smith.), x 250. 



