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. 
