68 THE MECHANISM OF LIFE 



represents. The pulmonary artery (carrying venous blood) enters 

 the lung, and also divides and subdivides again and again until, 

 in the end, each ultimate twig of the artery breaks up into a 

 network of capillaries on the outer wall of an air sac. 



Air is drawn up into the lungs and expelled out from them by 

 the upward and downward movements of the diaphragm (the 

 muscular partition between the chest and abdominal cavities) 

 in men, or by upward and downward movements of the ribs and 

 chest wall in women. Thus the sacs are full of air which is 

 continually being renewed (we do not really draw air right down 

 to the " bottom " of the lungs, but into the upper parts only). 

 The air of the smallest sacs renews itself, however, by diffusion 

 into the fresh air that is continually entering the larger passages. 

 Now the venous blood that enters the air-sac capillaries contains 

 (in solution) less oxygen and more carbonic acid than that which 



i P^ 5 5 ^--^6 



Fig. 17. — Blood-Corpuscles. 



1, White; 2, human; 3, human in " rouleaux "; 4, human, seen from 

 the side; 5, from a frog; 6, from a fish. 



is contained in the air of the sacs, and therefore an interchange 

 occurs such that oxygen goes through the very delicate membrane 

 of the sac and the equally thin wall of the capillaries from the air 

 into the blood, while carbonic acid gas passes from the blood into 

 the air. 



In respiration, then, we take oxygen from the outer air and 

 transfer it to the blood-stream, and we take carbonic acid from 

 the blood-stream and transfer it to the air. And so the venous 

 blood, which goes to the lungs and comes from the body, contains 

 more carbonic acid and less oxygen than the arterial blood, 

 which comes from the lungs and goes to the body. 



Just a word as to how the oxygen and carbonic acid are carried 

 by the blood. The latter, when seen under the microscope, 

 becomes a clear, colourless liquid in which there are enormous 

 numbers of minute, biscuit-shaped bodies called the red blood- 

 corpuscles. 



