THE BREATHING SYSTEM 



487 



bon dioxide and has taken on a load of 

 oxygen. Just how is this done? 



The oxygen molecules in the air dissolve 

 in the watery mucus covering the lining 

 of the alveoli and pass directly through by 

 diffusion to the blood which is flowing very 

 close to the walls of the alveoli, in fact, only 

 two cells away (one cell layer in the capil- 

 lary wall plus one cell layer in the alveolar 

 wall) (Fig. 18-9). Oxygen passes from the 

 alveolar air to the blood by simple diffu- 

 sion, since the concentration of the oxygen 

 molecules is much greater in the alveoli 

 than in the blood. Although oxygen mole- 

 cules are moving freely across the mem- 

 branes in both directions, there are more 

 going into the blood than are going into the 

 alveoli, hence the net movement is toward 

 the blood stream. Conversely, the carbon 

 dioxide moves in the opposite direction for 

 exactly the same reason. The inert nitrogen 

 gas remains approximately the same during 

 breathing. The over-all result, then, is that 

 the blood leaving the lungs is rich in oxy- 

 gen and low in carbon dioxide, whereas 

 expired air is rich in carbon dioxide and 

 low in oxygen. With the continued breath- 

 ing movements there is a continual ex- 

 change of these gases, and interruptions 

 cannot be tolerated, at least for any pro- 

 longed periods of time. 



The amount of oxygen in the alveoli de- 

 pends on the amount in the air, which 

 changes with atmospheric pressure. At sea 

 level it is 760 mm. of mercury, of which 

 about 150 mm. represents the partial pres- 

 sure of oxygen. There is a gradual drop in 

 pressure as one rises above the earth's sur- 

 face and with this comes a drop in the 

 pressure of oxygen in the alveoli, even 

 though the breathing rate and depth are 

 increased. At about 14,000 feet, no matter 

 how fast or deep the breathing, symptoms 

 called mountain sickness appear. These are 

 due directly to a shortage of oxygen in the 

 tissues, resulting from too little oxygen in 

 the alveoli and hence too little in the blood 

 (Fig. 18-10). Airplanes at or above 14,000 



feet must, therefore, supplement the oxygen 

 supply. This is done in passenger liners by 

 pressurizing the entire cabin to that of 

 about 4000 feet, no matter how high the 

 plane goes. People who live at high alti- 

 tudes ( 10,000 feet ) soon become accli- 

 matized to the rarefied air by producing 

 more red corpuscles, thus increasing the 

 oxygen-carrying capacity of the blood. 



In the tissues, where internal respiration 

 occurs, exactly the opposite condition pre- 

 vails with respect to the movement of the 

 two gases (Fig. 18-9). The oxygen diffuses 

 from the blood into the tissue cells because 

 the amount of oxygen is higher in the blood 

 than in the tissues, and the carbon dioxide 

 diffuses out of the tissues into the blood for 

 a similar reason. Not all of the oxygen 

 leaves the blood at the tissues, however. 

 Only about 7 cc. out of a possible 19 cc. 

 ( per 100 cc. of blood ) actually diffuses into 

 the tissue cells. 



GASEOUS TRANSPORTATION 

 BY THE BLOOD 



The value of the blood as a transporting 

 agent for oxygen and carbon dioxide can be 

 determined easily by measuring the amount 

 of these gases that will be dissolved in 

 equal quantities of blood and water. It will 

 be found tliat for both Oo and COo many 

 times more will be picked up by blood 

 than by water. This is owing primarily to 

 that amazingly complex organic compound 

 called hemoglobin. Plasma is little better 

 than water in its capacity to carry these two 

 gases. For example, only 0.2 cc. of oxygen 

 and 0.3 cc. of carbon dioxide will dissolve 

 in 100 cc. of plasma, whereas the same 

 amount of whole blood will absorb 20 cc. 

 of oxygen and 50 or more cc. of carbon 

 dioxide. If it were not for hemoglobin, the 

 blood would need to circulate 35 times 

 faster than it does to accomplish the same 

 job. 



About 98 per cent of the oxygen is car- 

 ried in combination with hemoglobin and 



