CHAP. XIIL] RESPIRATION. 161 



cubic feet (20,642 cubic decimetres), and this space should 

 be accessible by direct or indirect channels to the outside air. 



Effects of respiration upon the blood. While the air in passing 

 into and out of the lungs is robbed of a portion of its oxygen 

 and loaded with a certain quantity of carbon dioxide, the blood 

 as it streams along the pulmonary capillaries is also undergoing 

 important changes. As it leaves the right ventricle it is venous 

 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 hsemoglobin 

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

 hsemoglobin ; the purple colour shifts immediately into scarlet, 

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

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

 capillaries in the tissues the oxy-hsemoglobin 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 readily combines with the 

 unstable chemical compounds of which the tissues are composed. 

 In this process, called oxidation, 1 complex bodies are broken up 

 into simpler ones, such as carbon dioxide and water, and there 

 is thus liberated a great deal of energy which is manifested in 

 the increasing of muscular activity, and in the production of 

 heat. The carbon dioxide 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 insig- 



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 carbon dioxide and ashes 

 produced. 



