EESPIKATION 319 



leads to shortness of breath, palpitation, and even to sickness 

 (mountain sickness). These symptoms soon pass off, increased 

 pulmonary ventilation and increased heart action increasing the 

 intake of oxygen. Hence residence in high altitudes tends to 

 increase the power of the respiratory muscles and the strength 

 of the heart. It also increases the richness of the blood in 

 erythrocytes and in hsemoglobin. 



On the other hand, the atmospheric pressure may be 

 enormously increased without any change in the metabolism. 

 In a diving bell 200 feet under water a pressure of 5120 mm. 

 Hg seven atmospheres is sustained. As a result of the high 

 pressure of the gases of the atmosphere they are dissolved in 

 large quantities in the blood and tissues, and there is great 

 danger in too sudden relief of pressure, since this may cause 

 bubbles of gas to be given off in the vessels, and these may 

 lead to air embolism and a plugging of the smaller vessels 

 (Caisson disease). 



B. INTERNAL RESPIRATION 



1st. The Passage of Oxygen from Blood to Tissues. 



In studying the physiology of muscle, which may be taken 

 as a type of all the active tissues, it was seen that oxygen 

 is constantly being built up into the muscle molecule, and 

 that the living tissues have such an affinity for oxygen that 

 they can split it off from such pigments as alizarin blue. 

 The tension of oxygen in muscle is therefore always very 

 low. We have seen that the tension of oxygen in arterial 

 blood is nearly 100 mm. Hg; therefore, when the blood is 

 exposed to a low tension of oxygen in the tissues, the oxygen 

 comes off from the blood and passes into the tissues by the 

 ordinary laws of diffusion. 



But it must be remembered that this takes place in three 

 stages. 



(1) The tissue elements are always taking up oxygen from 

 the lymph, because of the very low pressure of oxygen in 

 the protoplasm and because the protoplasm has an affinity 

 for oxygen as for other nourishing substances. 



(2) As a result of this the oxygen pressure in the lymph 

 falls and becomes lower than the oxygen pressure of the blood 

 plasma, and thus the gas passes from the blood through the 



