THE FLUIDS OF THE BODY 61 



were merely dissolved in lymph and plasma, it would be im- 

 possible for the blood to carry it away with sufficient rapidity ; 

 just as it would be impossible for blood to bring sufficient 

 oxygen were it not for the colouring matter (haemoglobin) 

 which forms a temporary, easily divorced union with it. But 

 from a physical point of view it comes to the same thing. As 

 the tension of oxygen in plasma falls, it dissolves more from 

 the haemoglobin. When the tension of oxygen in lymph is 

 less than its tension in plasma, the former borrows from the 

 latter. If the tension of carbonic acid in lymph is higher than 

 in blood, it passes to the blood. The rapidly circulating blood 

 at frequent intervals traverses the lungs. The whole blood of 

 the body is exposed to air in the lungs once every minute. 

 Oxygen tension being higher in pulmonary air than in venous 

 blood, this gas is taken up. Carbonic acid tension being higher 

 in venous blood than in pulmonary air, this gas escapes. 

 The plasma in the capillary vessels which traverse the tissues 

 exchanges gases with the lymph with very great rapidity. 



The specific gravity of blood varies from 1-056 to 1-059. 

 The corpuscles are heavier than the plasma. Its reaction to 

 test-paper is alkaline, owing to the presence of bicarbonate 

 of soda and disodic phosphate. The alkalinity is greatest when 

 the body is at rest ; it is diminished by severe muscular exer- 

 cise. Blood contains about 5,000,000 red corpuscles, and 

 7,000 or 8,000 leucocytes, to a cubic millimetre. Red blood- 

 corpuscles are biconcave discs destitute of nucleus, and, so 

 far as can be seen, devoid of any investing membrane. Seen 

 in profile they appear biscuit-shaped, because the centre is 

 hollowed out. Their largest diameter is 7-5 micromillimetres 

 (Wuir inch) a measurement of great importance to anyone 

 who works with a microscope, because it serves as a standard 

 by which to estimate the size of other objects. They are 

 soft, but fairly tough and highly elastic. In circulating blood 

 a corpuscle may occasionally be seen to catch on the point 

 where two capillary vessels unite. It bends almost double under 

 the pressure of the column of corpuscles behind it, and then 

 springs forward. 



A red corpuscle is a vehicle for haemoglobin. If blood is 

 diluted with water, or if it is alternately frozen and thawed, 

 the haemoglobin separates from the corpuscles, which can then 



