MECHANISM AND CHEMISTRY OF RESPIRATION 201 



and moisture is deposited on the glass in the form of small 

 drops of water. This saturation of exhaled air shows that 

 it has extracted water from the body. 



The lining of the lungs is the tissue through which these 

 exchanges are made; it is a membrane, moist because covered 

 with unicellular mucous glands, and thus always flexible and 

 not dried by the ever-changing air. Because of the nature of 

 its structure, gases diffuse through it even more readily than 

 they would go through a pure water film of the same thickness. 

 In the instance of carbon dioxid, diffusion is three times as fast 

 through lung tissue, as through a water membrane of equal 

 thickness. The actual thickness of the wall of a lung alveolus 

 is about 1-6250 of an inch (0.004 mm.) 



Changes in the Blood. The changes in the blood are, of 

 course, just the reverse of those in the air. What the air 

 absorbs, the blood has given up, and what the air has lost, the 

 blood has absorbed. From these simple facts we learn that 

 the blood takes oxygen from the air, but gives up to the air 

 carbon dioxid, heat and moisture. 



How Oxygen Gets into the Blood. When the blood flows 

 through the small capillaries in the walls of the alveoli of the 

 lungs (Fig. 99), it comes very close to the air, so close that 

 gases readily pass from the air to the blood. The air contains 

 oxygen in large amounts and under considerable pressure; 

 as a result some of it is at once absorbed by the liquid plasma 

 of the blood. There is nothing unusual in this fact, for water 

 or any other liquid will absorb gases from the air. Oxygen 

 is forced into water by the pressure of air (15 Ibs. to the 

 sq. in.) on its surface. If water is placed in a closed chamber 

 from which all the air is then removed, this is very evident, 

 for the air dissolved in the liquid will come away from it in 

 bubbles. Contrariwise, if the pressure of air or other gases 

 above the water surface is increased, the gases are absorbed in 

 just the degree that the pressure is increased. So too, carbonic 

 acid gas is forced into water, to form soda or Seltzer water, ancj 



