. Principles of modern biology. Biology. 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. ), 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 alti
. Principles of modern biology. Biology. 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. ), 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 Oa 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, altho
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