64 AX AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cent, of the body-weight is not usually followed by a fatal result. Just what 

 percentage of loss can be borne by the human being has not been deter- 

 mined, but it is probable that a healthy individual may recover without 

 serious difficulty from the lo.-s of a quantity of blood amounting to as much 

 as 3 per cent, of the body-weight. It is known that if liquids that are iso- 

 tonic to the blood, such as a 0.9 per cent, solution of NaCl, are injected into 

 the veins immediately after a severe hemorrhage, recovery will be more certain ; 

 in fact, it is possible by this means to restore persons after a hemorrhage that 

 would otherwise have been fatal. In addition to the mechanical effects on 

 blood pressure such an infusion tends to put into circulation all the red cor- 

 puscles. Ordinarily the number of red corpuscles is greater than that neces- 

 sary for a barely sufficient supply of oxygen, and increasing the bulk of liquid 

 in the vessels after a severe hemorrhage makes more effective as oxygen-carriers 

 the remaining red corpuscles, inasmuch as it insures a more rapid circulation. 

 If a hemorrhage has not been fatal, experiments on lower animals show that 

 the plasma of the blood is regenerated with great rapidity, the blood 

 regaining its normal volume within a few hours in slight hemorrhages, and 

 in from twenty-four to forty-eight hours if the loss of blood has been 

 severe ; but the number of red corpuscles and the hemoglobin are regenerated 

 more slowly, getting back to normal only after a number of days or after 

 several weeks. 



Blood-transfusion. — Shortly after the discovery of the circulation of the 

 blood (Harvey, 1628), the operation was introduced of transfusing blood from 

 one individual to another or from some of the lower animals to man. Ex- 

 travagant hopes were held as to the value of such transfusion not only as a 

 means of replacing the blood lost by hemorrhage, but also as a cure for various 

 infirmities and diseases. Then and subsequently, fatal as well as successful 

 results followed the operation. It is now known to be a dangerous under- 

 taking, mainly for two reasons: first, the strange blood, whether transfused 

 directly or after defibrination, is liable to contain a quantity of fibrin ferment 

 sufficient to cause intravascular clotting; secondly, the serum of one animal 

 may be toxic to another or cause a destruction of its blood-corpuscles. Owing 

 to this globulicidal and toxic action, which has previously been referred to 

 (p. 36), the injection of foreign blood is likely to be directly injurious instead 

 of beneficial. In cases of loss of blood from severe hemorrhage, therefore, it 

 is far safer to inject a neutral liquid, such as the so-called " physiological salt- 

 solution " — a solution of NaCl of such a strength (0.9 per cent.) as to be iso- 

 tonic with the blood-serum. The volume of the circulating liquid is thereby 

 augmented, and all the red corpuscles are made more efficient as oxygen- 

 carriers, partly owing to the fact that the bulk and velocity of the circulation 

 are increased, and partly because the corpuscles are kept from stagnation in the 

 capillary areas. 



