280 KESPIRATION 



Alteration in the Atmospheric Pressure. The normal condition of breath- 

 ing is that the oxygen of the air breathed should be at the pressure of 20.96 

 per cent of the atmosphere, that per cent of 760 mm. of mercury, or 159 mm. 

 But it is found that life may be carried on by gradual diminution of the oxygen 

 pressure to considerably less than one-half of this, to a partial pressure of 

 76 mm. of oxygen, i.e., the oxygen of one-half an atmosphere. This 

 pressure is reached at an altitude above 15,000 feet.* Any pressure 

 less than this may begin to produce alterations in the relations of 

 the gases in the blood, and if an animal is subjected suddenly to a 

 marked decrease of barometric pressure, and so of oxygen pressure 

 (below 7 per cent of oxygen), it is thrown into convulsions. It is found that 

 the gases are set free in the blood-vessels, no doubt carbon dioxide and oxygen 

 as well as nitrogen, although the latter is the only one of the three gases the 

 presence of which has been proven in the vessels in death from this condition 

 of affairs. The other gases are said to be reabsorbed. Other derangements 

 may precede this, bleeding from the nose, dyspnea, and vascular incoordina- 

 tion, etc. On the other hand, the oxygen may be gradually increased to a con- 

 siderable extent without marked effect, even to the extent of 8 or 10 atmos- 

 pheres, but when the oxygen pressure is increased up to 20 atmospheres the 

 animals experimented upon by Paul Bert died with severe tetanic convulsions. 



THE EFFECT OF RESPIRATION ON THE CIRCULATION. 



As the heart, the aorta, and pulmonary vessels are situated in the air- 

 tight thorax, they are exposed to a certain alteration of pressure when the 

 capacity of the latter is varied in respiration. The disturbance of pressure 

 which occurs during inspiration causes, first, a decrease in the intrathoracic 

 cavity, a decrease in pressure which affects all the organs of the thorax the 

 lungs, the great blood-vessels, the heart. The expansion of the elastic lungs 

 counterbalances this change in pressure in part, but it never does so entirely, 

 since part of the pressure within the lungs is expended in overcoming their 

 elasticity. The amount thus used up increases as the lungs become more and 

 more stretched, so that the intrathoracic pressure during inspiration, as far 

 as the heart and great vessels are concerned, never quite equals the intra- 

 pulmonary pressure, and at the conclusion of inspiration is considerably 

 less than the atmospheric pressure. It has been ascertained that the amount 

 of the pressure used up in the way above described varies from 5 to 7 mm. of 

 mercury in ordinary inspiration, to 30 mm. of mercury at the end of a deep 

 inspiration. So it will be understood that the pressure to which the heart 

 and great vessels are subjected diminishes as inspiration progresses, and at 



* For an interesting account of the symptoms produced by diminished atmospheric 

 pressure by very high altitudes, consult Whymper's "Travels amongst the Andes of the 

 Equator." 



