COMPOSITION OF THE BLOOD 337 



In addition to the red and white blood corpuscles certain other 

 minute formed elements are also found in shed blood, namely the Blood- 

 platelets. They are only from one-fifth to one-third of the diameter 

 of the red corpuscles and they do not contain nuclei. There has been 

 very much discussion as to whether they exist in the circulating blood 

 as such, or are not artefacts arising out of the shedding of the blood. 

 They have been regarded by various observers as preformed constit- 

 uents of the circulating blood, as detritus from the destruction of 

 leukocytes and as protein coagula or sphere crystals, which appear in 

 the blood whenever the endothelium of the bloodvessels is injured. 

 They have, however, been observed by Osier in the blood contained 

 in the freshly excised capillaries of the mesentery, so that injury to the 

 bloodvessels, or shedding of the blood from the vessels is not an 

 essential prerequisite to their formation. On standing in shed blood 

 the platelets swell and finally break up and disappear and there is some 

 .indication that those agencies which prevent the disintegration of the 

 platelets also hinder the Coagulation of the blood. They appear to 

 consist of protein with a very high admixture of a phospholipin which 

 resembles Lecithin. 



The Specific Gravity of the blood necessarily varies with its total 

 dilution, that is, with the amount of fluid which has recently been 

 absorbed from the intestine. As a rule it remains between the upper 

 and lower limits of 1.060 and 1.054, averaging 1.058 in males and a 

 little less in females. In newborn infants the blood has a higher 

 specific gravity, about 1.066. 



The Chemical Composition of the Blood is very constant in certain 

 respects and highly variable in others. Thus we have seen that the 

 reaction, osmotic pressure and relative proportions of the various 

 inorganic constituents are exceedingly invariable. The concentrations 

 of proteins, glucose, cholesterol and so forth are, on the contrary, very 

 variable. The following analytical data, cited after Abderhalden, 

 are therefore not to be regarded as affording fixed criteria of the com- 

 position of the blood in the different species enumerated, but simply 

 as indications of an approximate average composition. Furthermore 

 the estimations of the inorganic constituents are, as Abderhalden 

 points out, merely of comparative value, since the analytical errors 

 involved in the estimations were high, although presumably of similar 

 magnitude in each of the types of blood investigated. 



At the time that the above analyses were made the whole of the 

 Glucose in the blood was supposed to be confined to the plasma (or 

 serum). It has since been ascertained by Rona and Masing, however, 

 that the glucose in the blood is contained partly in the erythrocytes 

 and partly in the plasma. It is not, however, distributed between these 

 two elements in proportion to their relative volumes. In addition to 

 the various substances enumerated in these analyses, it must also be 

 remembered that blood contains small amounts of Amino-acids, derived 

 by absorption from the intestine, and of waste products such as Ammo- 

 nia and Urea derived from the metabolic activities of the tissues. 

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