232 THE BLOOD 



3. Composition. 



Since the function of the blood is to act as common nutritive 

 medium to all the parts of the body, it has to convey food material 

 from the digestive organs and oxygen from the lungs to the tissues. 

 From these, it receives in exchange their waste products, viz., 

 results of nitrogenous metabolism, CO 2 and H 2 O, and carries them 

 away to the excretory organs, kidneys, lungs, skin, etc., by which 

 they are eliminated. It is therefore evident that the composition 

 of the blood must vary from time to time and from place to place, 

 according to the activity and the function of the organ which it is 

 traversing. The cells of the body are adjusted to respond to 

 very minute changes in the composition of the blood and, 

 therefore, changes are kept within infinitesimal limits. 



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

 content of the vascular system, i.e. to the fluid plus formed 

 elements. 



I. Fluid or Plasma. 

 (a) Physical Characters. 

 (i) Colour, light straw. 



(ii) Opacity, practically transparent. 



(iii) Specific Gravity, about 1030. 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 (see specific gravity of blood, p. 277). 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 ~ 7 . It is acid to phenolphthalein 

 and therefore has an H-ion concentration greater than 10 ~ 8 . Exact 



