PROPERTIES OF PLASMA 301 



composition of the blood must vanj from time to time ami from place 

 to place, act'orciing to the activity and the function of the organ 

 which it is traA ersing. The cells of the body are adjusted to respond 

 to very minute changes in the composition of the blood and, there- 

 fore, changes are kept within infinitesimal limits. 



The term blood or whole blood is usually applied to the fluid 

 content of the vascular system, plus the formed elements suspended 

 in it. 



I. Fluid or Plasma 

 (a) Physical Characters. 



(i.) Colour, light straw. 



(ii.) Opacity, practically transparent. 



(iii.) Specific Gravity, about 1,030. The specific gravity is 

 lowered after a meal because of the dilution of the plasma by 

 ingested water. Conversely, exercise and profuse perspiration 

 cause a slight increase in the specific gravity on account of the loss 

 of water. Variation in activity will therefore produce a diurnal 

 variation — a decrease during the day and an increase during rest 

 at night. The night worker, of course, has this reversed. It 

 varies greatly in individuals, so that a figure which is normal for one 

 person may be pathological for another. 



(iv.) Viscosity. At body temperature (37° C.) plasma has a 

 viscosity about twice that of distilled water, i.e. 1-7-2-09. Salt 

 solutions have almost the same viscosity as water. This factor 

 is due to the emulsoid colloids present {q.v.), one of which by 

 forming a gel under certain conditions may produce so great an 

 increase in viscosity that the flow of plasma may be entirely 

 stopped. The plasma is then said to clot (see fibrinogen and also 

 viscosity of blood). 



(v.) Reaction. Plasma turns red litmus blue and therefore has 

 an H-ion concentration under 10~^. It is acid to phenolphthalein 

 and therefore has an H-ion concentration greater than 10"^. Exact 

 determinations have shown that the jjH. of plasma is 7-4, i.e. just 

 on the alkaline side of neutrality. This alkalinity is due to the 

 presence of sodium bicarbonate (see below). 



(vi.) Colligative Properties. It is of academic interest to ascer- 

 tain the values of the vapour pressure, osmotic pressure, and 

 depression of the freezing-point of plasma, and many attempts 

 have been made to correlate changes in these values with the 

 symptoms of disease. As we have seen in studying the colligative 

 properties of dilute solutions (Chaps. V. and VI.), the temperature 

 at which the determinations are made is of great importance. 

 Grollman has determined the values for " separated " plasma 



