How the Cells Are Provided with Oxygen 231 



PROBLEM 4, 



dinary breathing. A particularly forci- 

 ble expansion of the chest brings some 

 of them into play, but even this will not 

 send air into the most distant air sacs. In 

 the same way, exhaling never leaves the 

 lungs empty of air. Yet the air in these 

 remote parts mixes slowly to some ex- 

 tent with the air you inhale because the 

 CTases within the lungs move about by 

 diffusion. 



Changes in the air within the air sacs. 

 You can determine in a crude way how 

 exhaled air differs from inhaled air. Can 

 you devise a simple experiment? If not, 

 you will find suggestions in Exercise 6. 

 Scientists have analyzed the air breathed 

 in and the air breathed out. Thus they 

 know what goes on inside. Ordinary air 

 contains about 78 per cent of nitrogen, 

 less than 2 1 per cent of oxygen, a fraction 

 of I per cent of carbon dioxide, and 

 varying amounts of water vapor. Water 

 vapor is not counted in the percentages. 

 The air that comes out of the lungs con- 

 tains the same per cent of nitrogen but 

 only about 16 per cent of oxygen, 4 per 

 cent of carbon dioxide, and much more 

 water vapor than it had when it went in. 



While the heart is beating normally, 

 three and one half to four or more quarts 

 of blood are brought to the lungs each 

 minute. Here the blood flows through 

 capillaries so narrow that the red cor- 

 puscles move practically in single file. 

 Each corpuscle is separated from the sup- 

 ply of oxygen by the thin capillary mem- 

 brane and the thin membrane of the air 

 sac. Each membrane consists of cells only 

 one layer thick. These are excellent con- 

 ditions for diffusion. Since the blood 

 arriving in the lungs has come from all 

 parts of the body it has recently lost a 



part of its oxygen supply to the cells and 

 it has acquired an extra amount of car- 

 bon dioxide from the cells. Therefore 

 oxygen will diffuse from the air sacs into 

 the capillary; and carbon dioxide will 

 diffuse from the capillary into the air 

 sac. The oxygen that enters combines 

 directly with the hemoglobin. However, 

 there is much more oxygen in the air 

 sacs than the red cells can pick up. For 

 this reason there is much oxygen left in 

 the exhaled air. 



Bieathing and respiration. As applied to 

 animals, the term respiration means the 

 exchange of gases between an organism 

 and its surroundings. In an organism like 

 man this is a complex process which is 

 most easily described in two parts. You 

 have just read of the entrance of oxygen 

 into the lung capillaries and the passage 

 of carbon dioxide and water vapor out 

 of the lungs. This was called breathing. 

 The oxygen taken in goes to all the cells. 

 Now the movement of oxygen into cells, 

 the oxidation with the release of energy 

 which follows, and the movement of car- 

 bon dioxide out of the cell is the sec- 

 ond part of respiration, called cellular 

 respiration. Respiration in man, therefore, 

 includes breathing and cellular respira- 

 tion. Botanists use the term respiration 

 to mean only the oxidation of food in 

 the cells. 



What makes the chest muscles work? 

 Regular messages travel along the nerves 

 to the muscles of the ribs and to the dia- 

 phragm so that regular contractions take 

 place. Normal breathing is thus an auto- 

 matic process that goes on during sleep 

 as well as during waking hours. These 

 messages start from a small region at the 

 base of the brain called the breathing 



