26 A MANUAL OF PHYSIOLOGY 



The Relative Volume of Corpuscles and Plasma in Unclotted 

 Blood, or, what can be converted into this by a small correction, 

 the relative volume of corpuscles and serum in defibrinated 

 blood, can be easily determined, with approximate accuracy, 

 by comparing the electrical conductivity of entire blood with 

 that of its serum.* Another method, more suitable for clinical 

 work, though not so accurate, is the so-called haematocrite 

 method. A small quantity of blood is centrifugalized in a 

 graduated glass tube of narrow bore until the corpuscles have 

 been collected into a solid ' thread ' at the outer extremity of 

 the tube. Their volume and that of the clear plasma which has 

 been separated from them are then read off on the scale. The 

 haematocrite must rotate at such a high speed (10,000 turns a 

 minute) that separation of the corpuscles from the plasma is 

 accomplished before clotting has occurred. Dilution of the blood 

 with liquids which prevent clotting is not permissible for exact 

 work (Practical Exercises, p. 59). By these and other methods 

 too elaborate for description here, it has been shown that the 

 plasma or serum usually makes up rather less than two-thirds, 

 and the corpuscles rather more than one-third, of the blood. 

 But this proportion is, of course, liable to the same variations 

 as the number of corpuscles in a cubic millimetre of blood. It 

 depends, further, the number of corpuscles being given, on the 

 average volume of each corpuscle. For instance, when the mole- 

 cular concentration, and therefore the osmotic pressure (p. 398), 

 of the plasma is reduced, as by the addition of water or the 

 abstraction of salts, water passes into the corpuscles and they 

 swell ; when the molecular concentration of the plasma is in- 

 creased, by the abstraction of water or the addition of salts, 

 water passes out of the corpuscles, and they shrink. In human 

 serum the average depression of the freezing-point below that 

 of distilled water, which is a measure of the molecular concentra- 

 tion and of the osmotic pressure, is about 0-56 C. (Practical 

 Exercises, p. 64). For clinical purposes, the determination of the 

 relative volume of corpuscles and plasma is most useful in cases 

 where the average size of the erythrocytes departs from the 

 normal, and where, accordingly, the enumeration of the corpuscles 

 would give an erroneous idea of their total mass. 



Laking of Blood, or Haemolysis. Even in thin layers blood 

 is opaque, owing to reflection of the light by the red corpuscles. 



* The formula #>=,,-(- ( T 74-M&)), where/? is the number of c.c. of 



A(S) 



serum in 100 c.c. of blood ; \(b), \(s), the conductivity respectively of 

 the blood and serum (both measured at or reduced to 5 C., and expressed 

 in reciprocal ohms multiplied by io 8 ), may be used in the calculation. A 

 reciprocal ohm is the conductivity of a mercury column 1*06^ metres long 

 and i square millimetre in section. 



