292 ANIMAL BIOCHEMISTRY 



(leucocytes) and the blood platelets, constitute only about 0.2 per cent 

 of the volume of the corpuscles. 



The average specific gravity of normal human blood varies from 

 1.055 to 1.060, although in abnormal situations specific gravities from 

 1.035 to 1.075 have been observed. There is a tendency for the specific 

 gravity of blood to be lower during sleep than during waking hours, 

 and it is increased by exercise and decreased slightly after meals. There 

 is so much variation between individuals that a value normal for one 

 person may be an index of a pathological condition in another. The 

 corpuscles have a higher specific gravity than the plasma, and erythro- 

 cytes are heavier than leucocytes. 



The pH of blood is approximately 7.4 with an extreme normal 

 range of 7.3 to 7.5. This remarkably uniform pH is the result of the 

 buffering capacity contributed by hemoglobins, plasma proteins, phos- 

 phate ions, and bicarbonate ions. Bicarbonate ion concentration is 

 controlled through carbon dioxide excretion from the lungs at a rate 

 proportional to its concentration in the blood. 



The osmotic pressure of blood is also kept very nearly constant in 

 spite of continual exchange of nutrients, waste products, and water 

 on the circuitous passage through the tissues. The rate of water 

 absorption from the intestine and the rate of loss through the skin 

 and the lungs are dependent on external conditions. The kidneys 

 serve an important function in controlling the water and salt content 

 and, therefore, the osmotic pressure of the blood. The osmotic pres- 

 sure of the human blood is equivalent to that of a 0.9 per cent solution 

 of NaCl. The latter is known as physiological saline solution and is 

 sometimes used for intravenous injections. Ringer's solution is pre- 

 ferred to physiological saline because it contains a better mixture of 

 cations. Ringer's solution for mammals consists of NaCl (0.90 per 

 cent), CaClo (0.26 per cent), and KCl (0.03 per cent). 



On the basis of body weight, the amount of blood in man varies 

 from 6 to 8 per cent for men and 5 to 7 per cent for women. Blood 

 accounts for 2.5 to 5.0 per cent of the body weight of swine, 7.7 to 10.0 

 per cent for chickens, and 10 per cent for horses. The blood volume 

 increases in the summer time and decreases in winter because of 

 surface capillary constriction in colder weather. 



Erythrocytes. The number of erythrocytes per unit volume varies 

 in different animals. In man the average number is about 5,000,000/ 

 cu. mm., whereas the blood of women averages about 4,500,000. Indi- 

 viduals fully acclimated to higher altitudes show a higher red cell 

 count and an increased hemoglobin content. The erythrocytes con- 

 tain about 30 per cent hemoglobin, 3 per cent stroma protein, lipides, 



