536 



COLLEGE ZOOLOGY 



high-pitched. In men the cords are usually 

 longer, and the voice is lower. The trachea 

 is a tube about 4^2 inches long with C- 

 shaped rings of cartilage in its walls; it 

 branches into two bronchi, and these branch 

 into many bronchial tubes or bronchioles, 

 each of which terminates in an elongated 

 saccule, the alveolar duct, which bears on its 

 surface air sacs (alveoli). The number of 

 alveoli in the human lungs has been esti- 

 mated to be approximately 750 million. The 

 area of surface exposed to the air within the 

 lungs is over 800 square feet, or more than 

 50 times the skin surface of the body. The 

 lungs (Fig. 389) are membranous sacs, each 

 lying in a cavity lined with a serous mem- 

 brane, the pleura, which lessens friction. 

 When the pleura becomes inflamed, pleurisy 

 results. The quantity of air that can be ex- 

 pelled from the lungs averages from 3500 to 

 4000 cc. 



Breathing and the respiratory 

 function of blood 



Since no circulation of air occurs in the 

 lungs, respiratory movements, which we 

 know as breathing, are necessary. Inspiration 

 is due to the enlargement of the lungs, fol- 

 lowing the enlargement of the chest cavity 

 as a result of the contraction of the muscles 

 of the diaphragm (Fig. 388), and other mus- 

 cles of inspiration which cause the ribs to 

 swing up and out. Air is drawn in through 

 the trachea and bronchi. Expiration is pro- 

 duced by the contraction of the chest cavity 

 due to the relaxation of the inspiratory 

 muscles. The breathing movements are not 

 controlled within the lungs but by a center 

 in the medulla of the brain; if nerves to the 

 lungs are cut, breathing ceases. The changes 

 in the quantity of oxygen and carbon dioxide 

 in the lungs during breathing are not great, 

 as shown by the following figures : 



PER CENT 



Inspired air: 

 Expired air: 



Carbon dioxide, 0.04 

 Carbon dioxide, 4.48 



Inspired air: 

 Expired air: 



PER CENT 



Oxygen, 20.96 

 Oxygen, 16.02 



4.44 gain 



4.94 loss 



Most of the oxygen that diffuses into the 

 blood enters into a loose chemical combina- 

 tion with hemoglobin, producing thereby 

 oxyhemoglobin, which gives the red color to 

 arterial (oxygenated) blood. Oxyhemo- 

 globin at the capillary level gives up its 

 oxygen which diffuses into the cells, leaving 

 the blood dark red or purplish in color 

 (venous blood). The freed oxygen diffuses 

 through the tissue fluid into the cells to take 

 part in cellular or biologic respiration. The 

 carbon dioxide resulting from metabolic 

 processes within the cells diffuses into the 

 blood, where it is less concentrated. Some of 

 it combines with hemoglobin, and the re- 

 mainder is transported in the blood stream 

 to the lungs in the form of bicarbonates. 

 In the lungs, carbon dioxide is freed from 

 these bicarbonates and from the hemoglobin 

 and diffuses through the cells into the air 

 in the lung cavities. From the lung cavities, 

 it enters the sea of air which surrounds man 

 and in which he lives. 



Some problems of space travel 



A great sea of air engulfs the earth, and 

 man lives at the bottom of it. Just as in the 

 ocean, the pressure is greatest at the bottom. 

 When he rises up in air to great heights, dif- 

 ferences between pressures of high altitudes 

 and those on the ground become evident. 

 At sea level, the air man breathes has a 

 pressure of 14.7 pounds per square inch. At 

 high altitudes, the atmospheric pressure de- 

 creases, although the oxygen content varies 

 little. At 18,000 feet the atmospheric pres- 

 sure is only one-half that at sea level, and 

 man must breathe air under pressure in or- 

 der to obtain enough oxygen for the meta- 

 bolic life processes. At altitudes greater than 

 18,000 feet, he suffers seriously from lack 

 of oxygen unless special oxygen equipment 

 is used. At about 43,000 feet, or 8 miles up, 

 a man loses consciousness in 15 seconds and 



