GENERAL ZOOLOGY 



that the blood will actually seem to boil. The reduced atmospheric pressure 

 at high altitudes makes it difficult or even impossible for men to breathe. 

 Consequently, adequate air pressure must be artificially maintained in aircraft 

 operating at high altitudes, and above 48,000 feet pilots are obliged to wear 

 pressurized suits. With advances in physiology, as well as in aviation and 

 space technology, new altitude records are constantly being set. At 126,000 

 feet, an altitude reached some time ago, 99.6 per cent of the atmospheric mass 

 lies below. Living creatures sent into space in rockets have returned alive; 

 mice, dogs, monkeys, and even men have now traveled successfully in space, 

 beyond the limits of our atmosphere, in properly equipped, pressurized cap- 

 sules. A thorough understanding of the physiology of respiration and asso- 

 ciated circulatory functions in the normal individual, together with experi- 

 mentation to achieve safeguards for these bodily requirements, is a necessary 

 prerequisite to man's conquest of outer space. 



Ventilation of the lungs requires muscular work. As the muscles of the 

 chest, or thoracic cage, contract, air moves from the external environment 

 into the air passages and lungs. In a 24-hour period of normal muscular 

 activity, something like 10,000 liters of air are moved into the lungs of man, 

 about 500 ml. at each inhalation. The same volume is, of course, exhaled 

 when the respiratory muscles relax and the inherent elasticity of the thoracic 

 structures snaps the lungs back to their collapsed capacity. When it is 

 necessary, or if an individual desires, a much greater volume of air can be 

 moved as a result of more vigorous muscular contractions which not only pull 

 air into but, also, actively force it out of the lungs. Under these conditions, 

 the volume of air inhaled may amount to 4500 ml. for a single maximal 

 muscular effort, depending on the age and size of the individual. In an adult 

 man, about 1 liter of air always remains in the lungs in spite of maximal 

 effort to blow out all the air. This is important because it means that any 

 fresh air pulled into the lungs is mixed with and diluted by the partly used 

 air that cannot be expelled. Therefore, air in the alveoli of the lungs where 

 gas exchange occurs contains only about 15 per cent of oxygen instead of the 

 20 per cent present in the outside air, and 5.6 per cent of carbon dioxide 

 instead of 0.04 per cent. 



A survey of the multicellular animals reveals that many different structures 

 have been evolved to capture oxygen from the atmosphere for eventual use 

 in cellular oxidation. All have a common feature — the medium containing 

 oxygen, be it water or air, comes in close approximation, through a very thin 

 membrane, with blood or other body fluid. In man, this membrane is the 

 epithelium lining the alveoli of the lungs. It has a surface area of nearly 

 1000 square feet, about 50 times that of the skin of the body. The respiratory 

 membrane of man is composed of the alveolar membrane and the endothelium 

 of the blood capillary; it is about 4 microns thick (Fig. 3.10). 



Free gases in a mixture exert a tension or pressure in proportion to their 

 concentration. If a membrane separates two mixtures of oxygen which exert 

 different partial pressures because the concentration is different, oxygen will 



80 



