CHAP. XII.] RESPIRATION. 



blood of a dark purple colour ; when it enters the left auricle it 

 is arterial blood and of a bright scarlet colour. In passing 

 through the capillaries of the body from the left to the right 

 side of the heart it is again changed from the arterial to the 

 venous condition. The question arises, how is this change of 

 colour effected? 



As we have already seen, the blood in the thin- walled, close- 

 set pulmonary capillaries is separated from the air in the air- 

 sacs by only the' moist delicate membranes which form their 

 respective walls. By diffusion the oxygen in the air passes 

 through these moist membranes into the venous blood in the 

 pulmonary capillaries, combines with the reduced haemoglobin 

 which has lost its oxygen in the tissues, and turns it into oxy- 

 haemoglobin ; the purple colour shifts immediately into scarlet, 

 and the red corpuscles hasten onwards to carry this oxy-haemo- 

 globin to the tissues. Passing from the left ventricle to the 

 capillaries in the tissues the oxy-haemoglobin gives up some of 

 its oxygen, the colour shifts back again to a purple hue, and 

 the red corpuscles return with this reduced haemoglobin to the 

 lungs. 



The oxygen given up by the blood combines with the ele- 

 ments of the tissues and forms with them unstable chemical 

 compounds which are always breaking down to form more sta- 

 ble compounds, called " oxides." In this process, which is called 

 oxidation, both heat and force are produced and such oxidized 

 products as carbonic acid and urea are evolved. 1 The carbonic 

 acid passes by diffusion into the venous blood, and is carried by 

 it to the right side of the heart and thence to the lungs, a 

 certain quantity, however, escaping from the blood through the 

 kidneys and skin. A small and insignificant amount of oxygen 

 is introduced into the blood through the skin, and, with the 

 food, through the alimentary canal ; but, as we have stated in 

 the beginning of this chapter, respiration is the main process by 

 means of which the body is supplied with oxygen and relieved 

 of carbonic acid. 



The respiration and circulation are profoundly and intimately 



1 This process of oxidation may be illustrated by the burning of a fire ; the 

 oxygen which is in the air combines with the carbon of the wood, heat and light 

 are generated, and oxidized products in the form of carbonic acid and ashes pro- 

 duced. 



