CHAPTER X 

 THE PLASMA 



THE plasma in which the blood-corpuscles float is like the middle- 

 man, and barters between the tissues. Its composition summarizes 

 the building-up and breaking-down processes of the body. It carries 

 the necessary nutriment from the alimentary tract to the tissues, 

 and foodstuffs to and from the food depots of the body. It carries 

 also the necessary water and salts to the tissues, thereby surrounding 

 them with a medium in which they can carry out their activities. 

 Protoplasm yields 70 to 90 per cent, of water, and this proportion 

 has to be maintained, or damage, and eventually death, ensue. 



The electrolytes in solution are of the greatest importance, as de- 

 monstrated by the fact that a frog's heart beats for days in a solution 

 containing potassium, sodium, and calcium salts in the proportion 

 contained in blood. If, however, there be too much soluble calcium 

 salts present, the heart ceases to beat, stopping tightly contracted in 

 the state known as systole. If too much soluble potassium salt be 

 present, the heart stops beating in a flabby, relaxed condition known 

 as diastole. The mammalian heart may beat for days in the same 

 solution if it is warmed to body temperature and oxygenated by 

 bubbling oxygen gas through it. 



The unfertilized eggs of sea-urchins and other animals can be 

 made to develop into larvae by altering the concentration of certain 

 electrolytes in the sea-water. It is possible that some of the conditions 

 of ill-health and disease may be due to an altered relationship of the 

 salts of the plasma, disturbing the vital processes. At present it is not 

 possible for us to investigate this proportion of salts; it is apt to be 

 little considered or even overlooked altogether. Salts are constantly 

 leaving the body in the urine, and unless they are replaced in 

 the proper amount salt starvation occurs. An animal deprived 

 of salts dies just as soon as an animal cut off from food. Besides 

 the waste products formed in the different organs as a result of 

 their activity, the plasma may receive from certain tissues special 

 substances which have been elaborated by them. Such bodies are 

 the so-called internal secretions. They play a great part in 

 regulating the various functions of the body. Sometimes, too, a 

 special messenger, or " hormone," is added to the plasma. For 

 example, blood leaving the upper part of the small intestine during 

 digestion takes away in the plasma a messenger, or hormone, named 

 secretiii. The secretin on reaching the pancreas in the course of the 



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