28 ORGANISM AND ENVIRONMENT 



pressure of the gas leaving the liquid is thus equal 

 to the gas pressure outside. If, as in the lungs, a 

 mixture of gases is in contact with the liquid, the 

 pressure of each of the gases in the liquid becomes, 

 if no interference to their passage inwards or out- 

 wards occurs, equal to the pressure of the correspond- 

 ing gas in the gas-mixture. This holds good even 

 if the liquid contains substances which form well- 

 defined compounds with the gas; but in the latter 

 case the amount of gas which the liquid has to take 

 up before equilibrium occurs may be very large. ' If no 

 such chemical combinations occur the volume of gas 

 taken up by the liquid is in ordinary cases directly 

 proportional to the pressure of the gas. 



As we have already seen, the red corpuscles of the 

 blood contain a coloured albuminous substance, 

 haemoglobin, which enters into chemical combination 

 with oxygen. The compound, oxyhaemoglobin, has 

 the remarkable property of dissociating freely as the 

 pressure of oxygen in the surrounding liquid falls, 

 and re-forming as it rises. The oxyhaemoglobin thus 

 acts as a reservoir of oxygen, enabling the blood to 

 take up or give off far more oxygen with varying pres- 

 sures of oxygen than water would take up or give off, 

 and thus to act as a very efficient carrier of oxygen 

 from the lungs, where the oxygen pressure is high, to 

 the capillary vessels of the body tissues, where it is 

 low in consequence of the constant consumption of 

 oxygen. Human blood saturated in the lungs is capa- 

 ble of giving off about 18 cc. of oxygen per 100 cc. of 

 blood, whereas water would only give off about 0.3 cc. 



