EFFECT OF RESPIRATION ON THE CIRCULATION 305 



pressure varies in different individuals. At an altitude of about 10,000 feet 

 many persons experience mountain sickness, the symptoms of which are 

 nausea, dizziness, headache, and muscular weakness. At 15,000 feet the 

 pressure approaches the limit which the body can withstand, i.e., an oxygen 

 pressure of about 76 mm. Such pressures produce alternations in the re- 

 lations of the gases of the blood. The tension of the oxygen is no longer 

 sufficient, see figure 238, to allow the hemoglobin in the pulmonary capil- 

 laries to combine with its usual quota of oxygen, hence the tissues do not 

 receive their normal quantity. This condition is called anoxemia. The 

 factor of carbon dioxide elimination is also disturbed. It is observed that 

 the amount of carbon dioxide in the blood is diminished. Henderson has 

 recently given further evidence of the stimulative effects of carbon dioxide 

 in the body on certain functions of nervous and muscular mechanisms. 

 One may infer that the acapnia due to low pressures of the atmosphere may 

 contribute to the symptoms enumerated above, i.e., depression of muscular 

 tone, etc. 



Men are often subjected to higher than normal barometric pressures in 

 caisson work, diving, etc. 



Paul Bert has found in experimenting with animals that the oxygen pres- 

 sures may be gradually increased to a considerable extent without marked 

 effect, even to the extent of 8 or 10 atmospheres, but when the oxygen pres- 

 sure is increased up to 20 atmospheres the oxygen becomes poisonous and 

 the animals experimented upon died with severe tetanic convulsions. How- 

 ever, caisson workers often experience very severe symptoms, such as bleeding 

 from the nose, dyspnea, vascular inco-ordination, etc. These symptoms 

 are due not so much to the great increase in pressure as to the release from 

 the pressure. When the pressure is released too rapidly, the excess of gases 

 in the tissues and in the blood are set free more rapidly than they can be 

 thrown off by excretion processes. Gases, as such, gather in the blood 

 vessels and form embolisms which occlude the finer vessels. This, of course, 

 produces serious disturbances in the nutrition of the parts involved. If 

 these parts happen to be vital, death may result. 



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 during respiration. The disturbance of pres- 

 sure which occurs during inspiration causes, first, a decrease in the intra- 

 thoracic pressure, a decrease 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 



