FOOD DIGESTION AND RESPIRATION 85 



Thus it is not, strictly speaking, correct to refer to the energy 

 value of fat, for example. The system possessing the potential 

 energy is fat plus oxygen. 



Animals of small size or of flat form obtain their oxygen by 

 free diffusion to the tissue cells. But as soon as larger dimensions 

 and more complex forms appear, the necessity of special arrange- 

 ments for conveying oxygen to the tissues becomes evident. 



In insects, \ve find a peculiar branching system of fine tubes, 

 called " tracheae," which contain air and are distributed to all 

 organs (E., p. 200). The air is changed by squeezing movements 

 which press it out through certain openings on the side of the 

 body, while fresh air enters when the pressure is relaxed. 



In Crustacea, molluscs, and vertebrates, a liquid, the blood, is 

 carried to all parts of the body by a tubular system. The 

 arrangements of this system will be described in a later chapter. 

 For the present, it will suffice to remember that a supply of 

 oxygen is conveyed to all tissues in this way. But oxygen is only 

 slightly soluble in water, and a very copious current of such a 

 solution would be necessary to provide enough oxygen for the 

 vigorous movements of the vertebrates ; in fact, so great a 

 current would be required as to be mechanically impossible. 

 Accordingly, we find in the blood certain very small red bodies, 

 the red corpuscles, which contain that remarkable compound, 

 haemoglobin, about whose chemical nature we have already 

 learned some facts. The most important one is that it takes up 

 oxygen when the pressure of this gas is about that which it has in 

 the atmosphere, and gives it up again when the pressure is lower, 

 as in the tissues, where the gas is being continually used up 

 (E., p. 200). 



We must at this point understand what is meant by the tension 

 of a gas, since the expression is often to be used. Suppose that 

 we have a mixture of air with carbon dioxide, such that in one 

 hundred volumes of the mixture there are 95 of air and 5 of carbon 

 dioxide. And further, that the mixture of gases is at atmospheric 

 pressure. The five volumes of carbon dioxide are diffused through- 

 out the space, and if we imagine the air removed, it is clear that the 

 pressure would be only five-hundredths of an atmosphere, since 

 the five parts fill the space of one hundred. The pressure of the 

 carbon dioxide in the mixture is therefore five-hundredths of 760 

 mm. of mercury ; that is, 38 mm. This is known as the partial 

 pressure or tension of the carbon dioxide in the mixture. Similarly, 

 the tension of the oxygen, which forms 21 per cent, of air, will be, 

 in our mixture, 21/100x95/100x760=151.6 mm. A liquid in 

 contact with such a mixture will dissolve carbon dioxide until the 

 tension of the gas in the liquid is the same as that in the gas phase, 



