282 ESSENTIALS OF PHYSIOLOGY. 



lactic acid, which stimulates the respiratory centre ; and the breathing 

 becomes deeper, with the result that the partial pressure of carbonic 

 acid in the alveoli decreases and that of oxygen rises. At a high 

 altitude, when the alveolar oxygen pressure is low, even a small rise in 

 the partial pressure of oxygen appreciably increases the extent to which 

 haemoglobin can be saturated with oxygen as the blood passes through 

 the lungs. For example, if the alveolar pressure of the oxygen rises 

 from 40 mm. Hg to 45 mm. Hg, the saturation of haemoglobin may 

 be increased from 70 per cent, to 75 per cent. At the same time, 

 acceleration of the heart takes place, and consequently the blood is 

 carried more rapidly through the lungs and round the body. 



Owing to these changes, the blood not only takes up more oxygen 

 in the lungs, but transfers it more rapidly from the lungs to the 

 tissues. The presence of lactic acid also assists the dissociation of 

 oxyhsemoglobin in the capillaries, so that more oxygen is available for 

 the tissues. 



Secondly, a more gradual compensatory process takes place in 

 persons who remain at a high altitude for a long period. The number 

 of red corpuscles and the percentage of haemoglobin in the blood are 

 increased, and the oxygen-carrying capacity becomes much greater. In 

 one series of observations, made at a level of 14,000 feet, the percentage 

 of haemoglobin in the blood rose in the course of some weeks from 115 

 to 154. When the subjects returned to a low level, the percentage of 

 haemoglobin rapidly fell to normal. 



The extent to which adaptation takes place varies in different 

 individuals ; in many cases the supply of oxygen to the tissues is 

 barely sufficient during rest, and exercise frequently leads to an in- 

 adequate supply of oxygen to the tissues and brings on mountain 

 sickness. This occurs less readily, however, in the trained than in the 

 untrained person, since the former uses his muscles more economically, 

 and therefore his dSmands for oxygen are not so great. 



(2) The Effect of Raised Pressure. Men engaged in the building 

 of bridges and tunnels are often compelled to work in caissons, which 

 are filled with compressed air to prevent the inrush of water. They 

 suffer no inconvenience, and the respiratory movements are not affected 

 while they are in the caisson, even though the pressure may be three 

 or four atmospheres. Under this pressure, however, the blood dissolves 

 an increased amount of oxygen and nitrogen, and when a man leaves 

 the caisson, and the pressure to which the blood and tissue fluids are 

 exposed is reduced to one atmosphere, most of the nitrogen, previously 

 in solution, is evolved as bubbles, which may obstruct the flow of blood 

 along the blood-vessels or through the heart. 



