i8o 



ESSENTIALS OF PHYSIOLOGY. 



The complete circulation consists therefore of two parts, the one 

 from the right side of the heart through the lungs and back to the left 

 side of the heart, known as the pulmonary or lesser circulation, the 

 other from the left side of the heart throughout the rest of the body 



and back to the right auricle, forming the 

 systemic or greater circulation. As the 

 blood traverses the lungs it takes up 

 oxygen, becoming scarlet in colour (arterial 

 blood). Arterial blood is found in the 

 pulmonary veins, in the left side of the 

 heart, and in the systemic arteries. Dur- 

 ing its passage through the capillaries in 

 the various tissues the blood loses much 

 of its oxygen, receives carbonic acid, and 

 becomes darker in colour (venous blood). 

 The venous blood is carried along the 

 systemic veins to the right side of the 

 heart and into the pulmonary artery to 

 take up a further supply of oxygen from 

 the lungs. 



THE BLOOD PRESSURE. 



FIG. 54. Diagram showing the 

 course of the circulation. 



When an artery is cut across, the 

 blood spurts out from its central end (the 

 end nearest the heart) with considerable 

 force for some distance ; arid, evidently, 

 the blood contained in the arteries is exert- 

 ing a high pressure upon the vessel walls. When a vein is divided, the 

 blood escapes from its peripheral end in a slow steady stream. 



The arterial blood pressure can be measured by allowing the blood 

 to flow into a vertical glass tube tied into the central end of an artery. 

 The blood will be seen to attain a height of three or four feet or more, 

 and to show oscillations corresponding with each heart beat. The 

 method is unsatisfactory, partly because the clotting of the blood in 

 the tube soon brings the experiment to an end, partly because, in a 

 small animal, the loss of blood from the body may interfere with the 

 circulatory mechanism. It is customary, therefore, to place in the 

 artery a small cannula, filled with half-saturated sodium sulphate solu- 

 tion to delay clotting, and to connect the cannula with one limb of a 

 U-shaped tube ("manometer") containing mercury, which is so heavy 

 that a column of mercury 100-150 mm. high suffices to counter- 



