PHYSIOLOGICAL 347 



another. This depends largely on the fact that the reactions which 

 yield pyruvic acid are to a large extent reversible, so that the body 

 can form alanine from pyruvic acid and ammonia, as well as carrying 

 out the reverse change ; and fats may also be formed from the same 

 substance. It is easy to see, then, that fats can be formed from either 

 proteins or carbohydrates, by way of pyruvic acid. 



5. The Blood and the Transport of Gases. — As is well known, 

 one function of the blood is to transport food materials from the 

 wall of the intestine to the tissues in which they are used up, or 

 stored, or transformed into other substances. A second function is 

 to transport gases — oxygen from the lungs to the tissues, and carbon 

 dioxide from the tissues to the lungs. But before this can be explained 

 it is necessary to interpolate a description of the blood itself. 



The blood consists of a pale yellow, watery fluid, the plasma, in 

 which are suspended enormous numbers of single cells — so many 

 that they make up nearly half of the whole volume of the blood, 

 and there may be five million in one cubic millimetre, perhaps 

 thirty billion in the whole human body. The great majority of these 

 cells are red blood corpuscles, minute discs without nuclei, which 

 contain the red pigment (haemoglobin) of the blood; there are also 

 white corpuscles (leucocytes) of various kinds, two or three to every 

 thousand of the red cells, and apparently important mainly through 

 the power which most of them possess of engulfing foreign bodies, 

 for example, bacteria. This property (phagocytosis), which is separ- 

 ately discussed, is also shown by certain fixed cells, for example, in 

 the liver; and in many Invertebrates particles of food are taken up 

 by such cells and digested within them. Little, however, is known of 

 the chemistry of the leucocytes, since they are so greatly out- 

 numbered by the red cells of the blood. It is on these red cells that 

 the transport of gases mainly depends. 



Another property of the blood may be noticed here, and that is 

 its power of clotting or coagulating when it escapes through a 

 wound in the blood-vessels. This is by no means merely a question 

 of "drying up", but a very complex affair not yet thoroughly 

 unravelled. By some interaction of unknown substances, a protein 

 in the fluid plasma of the blood becomes transformed into a stringy 

 solid (fibrin), in which the cells of the blood become entangled to 

 form a tlot. A part is also played by certain elements of the blood 

 not yet mentioned, the platelets, extremely small solid bodies with 

 little discernible structure and of very obscure function. 



Oxygen, like other gases, dissolves to some extent in water or any 

 aqueous solution, to an amount depending on various factors, such 

 as temperature, and more particularly on the pressure of the oxygen. 

 It is known to everyone that under standard conditions the pressure 

 of the atmosphere is 15 pounds per square inch, corresponding to 

 a definite height of the barometer (760 millimetres of mercury). 



