FOOD DIGESTION AND RESPIRATION 87 



should be kept in mind that haemoglobin is in colloidal solution 

 in water, and about 5 per cent, is the most that can be dissolved. 

 The red corpuscles contain 35 per cent, of haemoglobin, and 63 

 per cent, of water, so that the haemoglobin must be in the form 

 of a moist solid. The following considerations will show that if 

 oxy-haemoglobin (as the oxygenated form is called) is a definite 

 chemical compound, it is unique. There are various compounds 

 which give off oxygen or carbon dioxide at a certain tension of 

 these gases, and combine with it again at a higher tension. But 

 the point is that there is no half-way state. Above a particular 

 "dissociation tension," according to temperature, the whole is 

 in the form of the complete compound. Just below this tension 

 the whole of the gas is given off. It may be stated to be a case 

 of " all or nothing." To get over the difficulty, it has been 

 suggested that there is a series of compounds of haemoglobin with 

 oxygen of the composition HbO. 2 , Hb 2 O 4 , Hb 3 O 6 , etc. These would 

 each obey a different form of the law, deduced from mass action, 

 of the rate of combination in relation to concentration of oxygen, 

 that is, the tension of oxygen. Of course, the difficulty is not 

 present in such a case as that of adsorption by charcoal, where the 

 amount condensed on the surface is in proportion to the tension 

 of the gas, up to the point of saturation. 



Again, it is found that the amount of oxygen taken up by 

 haemoglobin is less at a higher than at a lower temperature, 

 although the rate at which it takes it up or gives it off is greater 

 at the higher temperature. This is one of the peculiarities of 

 adsorption also, owing to the negative temperature coefficient 

 of surface tension. It might be supposed to imply a disadvantage 

 on the part of warm-blooded animals, but it seems to be more 

 important to obtain the oxygen quickly than to have the larger 

 reserve, which is made up for by rapid replacement of the blood 

 by vigorous circulation. 



Haemoglobin, being colloidal, is subject to aggregation by 

 electrolytes, and the fact shows itself in an effect on the form of the 

 dissociation curve such that when acids or salts are present, less 

 oxygen is taken up at a given tension of the gas. The difference 

 is not great at the higher tensions (90-100 mm. of mercury), but 

 marked when it is 15-20 mm. 



The darker colour of blood which has lost oxygen is familiar in 

 the appearance of the veins. Although they look bluish, as seen 

 through the skin by reflected light, the colour of the blood itself 

 may be more correctly described as crimson, when compared with 

 the bright scarlet colour of fully oxygenated arterial blood. The 

 scarlet colour in the arteries is due to the fact that the blood has 

 taken up oxygen in the lungs. As it passes through the tissues, 



