ALVEOLI 



Fig. 19-7. Close relation between the alveolar air 

 sacs and the blood in the lung capillaries. 



walls of the alveolus and capillaries (Fig. 

 19.7), and these membranes are so freely per- 

 meable to the respiratory gases that they 

 cause little delay in the attainment of equi- 

 librium. 



Many early students of respiration believed 

 that the alveolar walls could force oxygen 

 into the blood, especially when the concen- 

 tration of oxygen in the alveolar air is ab- 

 normally low. However, no convincing evi- 

 dence for this view has been obtained. At 

 high altitudes — where the air is rarefied — 

 the oxygen concentration in the alveolar air 

 falls off despite the fact that the breathing 

 becomes deeper and faster. At sea level the 

 total atmospheric pressure approximates 760 

 mm of mercury, of which about 150 mm 

 represents the partial pressure of the oxy- 

 gen. In the alveoli, where the proportion 

 of oxygen is only two thirds that of the out- 

 side atmosphere, the oxygen pressure is 

 therefore only about 100 mm of Hg. Acute 

 respiratory embarrassment begins to appear 

 at about 14,000 feet. Here the outside atmos- 

 pheric pressure falls to about 450 mm of Hg 



Respirafion - 363 



and the oxygen pressure in the alveoli is re- 

 duced to about 55 mm of Hg. Now the blood 

 passing through the lungs fails to absorb 

 enough oxygen to supply the body's needs. 

 The venous blood as it comes to the lungs 

 from the tissues still has an oxygen content 

 equivalent to 40 mm of Hg. Thus the con- 

 centration gradient between the alveolar air 

 and blood has been reduced to 15 mm (55 

 mm in alveolar air at 14,000 feet elevation 

 minus 40 mm in the alveolar blood); and at 

 this gradient the diffusional force that drives 

 O a into the blood begins to be dangerously 

 close to inadequate. 



The symptoms of mountain sickness, in- 

 cluding severe headache, nausea, and emo- 

 tional instability, indicate that the body, and 

 especially the brain cells, are suffering from 

 partial asphyxia, although some of the sym- 

 toms of high altitude distress originate from 

 an excess outpouring of C0 2 from the blood 

 stream. Gradually an individual can become 

 acclimated to high altitude, because altitude 

 stimulates the bone marrow to mobilize a 

 much greater abundance of red cells; and 

 these of course augment the oxygen-carrying 

 capacity of the blood. However, no person 

 can live at altitudes very much higher than 

 14,000 feet, unless equipped with an artificial 

 oxygen supply. When the alveolar oxygen 

 tension drops significantly below 50 mm of 

 Hg, the quantity of oxygen that can reach 

 the blood is insufficient, and the blood is un- 

 able to carry enough oxygen to supply the 

 minimum demands of the body. 



THE OXYGEN-CARRYING CAPACITY 

 OF BLOOD 



Blood is able to absorb and carry large 

 quantities of oxygen— about 50 times more 

 than an equivalent volume of plasma. If 

 blood is exposed to a sea level atmosphere 

 containing 14 percent of oxygen (alveolar 

 air), it continues to absorb oxygen until it 

 contains about 20 cc of pure oxygen per 100 

 cc of blood. This high oxygen-carrying capa- 

 city of blood is mainly due to its content of 



