STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 303 



into the blood from the tissues, and which are to be gotten rid of 

 in the excretory organs. The most important of these are urea, 

 creatinine, uric acid, and similar bodies. The plasma contains 

 also the various hormones, mentioned in a previous paragraph, 

 and a group of substances, known as biological reagents, which are 

 part of the disease-resisting mechanism. These will be considered 

 in detail in later paragraphs. 



The plasma carries in solution a certain amount of oxygen, 

 nitrogen, and carbon dioxid, but no more than a similar amount 

 of pure water would dissolve at the same temperature. Most of 

 the oxygen carried by the blood is in combination with the hemo- 

 globin of the red corpuscles; most of the carbon dioxid is in com- 

 bination with sodium, forming sodium carbonate or bicarbonate. 



Summary. Practically the composition of the blood may be 

 thus stated: It consists of (1) plasma, consisting of watery solu- 

 tions of serum albumin, paraglobulin, fibrinogen, grape-sugar, 

 sodium and other salts, and extractives of which the most constant 

 are urea, creatinine, and uric acid; (2) red corpuscles, contain- 

 ing rather more than half their weight of water, the remainder 

 being mainly hemoglobin, other proteins, and potash salts; (3) 

 white corpuscles, consisting of water, various proteins, glycogen, 

 and potash salts; (4) the platelets; (5) gases, partly dissolved in 

 the plasma or combined with its sodium salts, and partly com- 

 bined (oxygen) with the hemoglobin of the red corpuscles. 



Quantity of Blood. The total amount of blood in the Body is 

 difficult of accurate determination. It is about ^V of the whole 

 weight of the Body, so the quantity in a man weighing 75 kilos 

 (165 Ibs.) is about 3.8 kilos (8.5 Ibs.). Of this at any given moment 

 about one-fourth would be found in the heart, lungs, and larger 

 blood-vessels; and equal quantities in the vessels of the liver, and 

 in those of the muscles which move the skeleton; while the remain- 

 ing fourth is distributed among the remaining parts of the Body. 



Blood of Other Animals. In all animals with blood the white 

 corpuscles are pretty much alike, but the red corpuscles, which 

 with rare exceptions are found only in Vertebrates, vary con- 

 siderably. In all the classes of the mammalia they are circular 

 biconcave disks, with the exception of the camel tribe, in which 

 they are oval. They vary in diameter from 0.02 mm. dinnr fach) 

 (musk deer) to 0,011 mm. GrzW inch) (elephant). In the dog they 



