34« LIFE : OUTLINES OF GENERAL BIOLOGY 



Since oxygen makes up 21 per cent, of the air, the partial pressure or 

 tension of oxygen under these conditions is 21 per cent, of 760, 

 i.e. 160 millimetres of mercury. If the total atmospheric pressure is 

 increased, or the amount of oxygen in the atmosphere is increased, 

 then in either case the tension of oxygen is increased and a larger 

 amount will dissolve in water. The amount dissolved is always in 

 equilibrium with the atmosphere above it, so that it is f>ossible also 

 to speak of the tension of the dissolved oxygen. There are, of course, 

 certain substances which combine with oxygen (such as pyrogallol) 

 to form compounds from which the gaseous oxygen cannot be 

 obtained again readily; but there are also compounds which combine 

 loosely with oxygen to an extent depending on the oxygen tension; 

 and one of these is haemoglobin, the red pigment of the blood cor- 

 puscles. The haemoglobin combines with oxygen when the blood is 

 passing through the lungs, where the oxygen tension is relatively 

 high, and gives up the oxygen again as the blood passes through the 

 tissues, where the oxygen tension is low. The compound which 

 the pigment forms with oxygen is called oxyha^moglobin ; and it is 

 found that it contains two added-on atoms of oxygen for every 

 atom of iron (there are probably four atoms of iron in every 

 molecule of haemoglobin, but this is uncertain), so that the contrac- 

 tion HbOj is often used for oxyhaemoglobin. 



Carbon dioxide is carried by the blood in a somewhat similar way, 

 that is to say, it can all be removed from blood by sufficiently 

 lowering the carbon dioxide tension of the atmosphere in contact 

 with it — in a vacuum, for example; but it can also be driven out of 

 blood by adding strong acids. The latter property is shown also by 

 solutions of sodium bicarbonate (NaHC03), and indeed the plasma 

 does contain inorganic bicarbonates which aid in the transport of 

 CO,. The carbon dioxide thus carried is called the "alkali reserve" 

 of the blood. This, however, is greater than might at first appear, 

 because of a curious series of reactions which go on. Of these the 

 first merely expresses the passing of the gas into solution : 



CO, + H,0 = H.CO^ 



Then follows the reaction of the carbonic acid with the sodium 

 chloride of the plasma: 



H,C03 + NaCl = NaHC03 + HCl 



but as hydrochloric is a strong and carbonic a very weak acid, this 

 reaction would take place only to a very slight degree, if it were not 

 that the hydrochloric acid, as soon as it is formed, tends to pass 

 into the corpuscles (where it is neutralised). This mechanism — 

 called the "chloride shift" by some and the "ionic interchange" by 

 others — allows the plasma to carry more carbon dioxide than it 

 could if the corpuscles were not there to take up the hydrochloric 



