RESPIRATION 545 



In a diving bell, 200 feet under water, a pressure of seven 

 atmospheres — 5120 mm. Hg — is sustained. As a result of 

 the high pressure of the gases of such an atmosphere, they are 

 dissolved in large quantities in the blood and tissues, and there 

 is great danger in a 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. INTERMEDIATE RESPIRATION. 



1. The Carriage of Gases in the Blood has been already 

 considered (p. 492). 



2. The Passage of Gases between Blood and Tissues- 



1. Oxygen. — In studying the metabolism of muscle (p. 254), 

 which may be taken as a type of all the active tissues, it was 

 seen that oxygen is constantly being used by the muscle. 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, and that, below a pressure of 50 mm., the oxygen is rapidly 

 given off, till at 10 or 20 mm. Hg the oxyhsemoglobin is 

 largely reduced. The influence of the Ch of the blood and of 

 temperature upon the process has already been considered 

 (p. 495). Hence, when the blood is exposed to a low tension 

 of oxygen in the capillaries, the oxygen comes off from 

 the blood and passes into the tissues by the ordinary laws of 

 diffusion. 



The process takes place in three stages. The " head of 

 oxygen," as it may be called in arterial blood, i.e. the difference 

 of tension between that of the HbOg and that of the tissues 

 is, in normal conditions, far in excess of the requirements. 

 The haemoglobin is generally saturated to about 95 per cent., 

 but in various pathological conditions of the lungs, e.g. pneu- 

 monia, it may fall to as low as 50 per cent, saturation and still 

 the same amount of oxygen per unit of volume of blood may be 

 given off to the tissues, the saturation of the venous blood 

 falling proportionately, i.e. the difference between them remain- 

 ing at about 5 per cent. (fig. 222). 

 35 



