142 THE ENGINES OF THE HUMAN BODY 



bustion chambers throughout the body. Also we have 

 to see how carbon dioxide, a product of oxidation or 

 combustion, is brought back from the tissues of the body 

 and is discharged into the minute respiratory chambers of 

 the lungs. 



The mechanism is very simple. In fig. 35 the walls of 

 a cluster of air sacs are shown, covered by a close-meshed 

 network of capillary vessels. Into the meshwork opens 

 a terminal artery ; issuing from it is a commencing vein 

 or venule. The artery is a twig from the great vessel 

 which springs from the right ventricle of the heart — the 

 pulmonary artery ; the venule ultimately joins one of the 

 veins which discharges its blood into the left auricle of the 

 heart. Every one of the air sacs, which we have roughly 

 estimated as numbering six millions, is surrounded by a 

 capillary field ; the fields are fed by arterioles and drained 

 by venules. The right ventricle is the pump which keeps 

 this extensive capillary field constantly flooded with 

 venous blood. Every pumpful of arterial blood which is 

 thrown into the aorta by the left ventricle is accompanied 

 by an equally great discharge of venous blood, thrown 

 into the pulmonary artery by the right ventricle. Files 

 of red blood-discs are constantly being hurried through the 

 capillary fields in the walls of the air sacs. They have been 

 robbed of their oxygen while passing through the tissues 

 of the body. The red substance of the discs has a remark- 

 able affinity for oxygen, and when the capillary fields of the 

 lung are reached the discs quickly renew their loads and 

 are then sped onwards in the veins leading to the left 

 chambers of the heart. The blood also brings back from 

 the tissues charges of carbon dioxide carried in its plasma 

 or liquid part. The carbon dioxide diffuses into the air 

 chambers as the blood circulates through the capillary 

 fields. Between the blood circulating in the capillaries 

 and the air in the respiratory chamber there is but a thin 

 delicate partition which is always moist and through 

 which gases in solution can diffuse very readily. 



We now ask the question — Why should not the re- 

 spiratory chambers be completely emptied of air ? The 



