This paper is concerned with a variety of questions which bear on the occurrence of hemolysis in vivo, and with the possibility of regarding the contents of the blood stream as a hemolytic system in which a steady state is maintained by the production of new red cells to replace those which are destroyed. The material which is dealt with includes the following.

1. Mixtures of Lysins, Accelerators, and Inhibitors.—The effects of individual accelerators and inhibitors in mixtures, like the effects of individual lysins, are roughly additive in simple systems, the acceleration or inhibition produced by the individual substances being most conveniently measured in terms of R-values.

2. Normal Intravascular Lysins.—These probably play only a small part in red cell destruction unless their concentration rises to unusual levels, or unless their effects are enhanced by accelerators, or by the reduction of the concentration of normal inhibitors. The three normal in vivo hemolytic processes for which there is substantial evidence involve (a) the action of the bile salts and of the soaps derived from chyle, (b) the action of the spleen, and (c) the action of hemolytic substances derived from tissues. The recent observations of Maegraith, Findlay, and Martin on the presence of widely distributed tissue lysins are confirmed except for their conclusion that these lysins are species-specific. Species-specific tissue lysins, if present, are not the only lysins derivable from tissues by simple immersion in saline, for non-species-specific lytic substances can also be obtained, and seem to be similar to the "lysolecithin" which some regard as responsible for the action of the spleen on red cell fragility and shape.

3. Plasma Inhibitors.—About 30 per cent of the total inhibitory effect of plasma for saponin hemolysis is due to the contained cholesterol, while 25 per cent at most is due to the plasma proteins, particularly globulins. The remaining 45 per cent is probably accounted for by enhancing effects among the inhibitors; e.g., the enhancing effect of lecithin on the cholesterol inhibition. The mechanism of the inhibition is still incompletely understood; probably reactions between inhibitor and lysin and reactions between inhibitor and components of the red cell surface are both involved, and it is important to observe that the inhibitory effect of plasma or of a plasma constituent may be greater in systems containing one lysin than in systems containing another. No evidence for diffusible inhibitory substances in plasma has been found, and the variations observed in the inhibitory power of human plasma seem to be related to the combined concentrations of cholesterol, protein, and probably lecithin, rather than to the cholesterol content alone. For this reason the inhibitory power tends to be low under conditions of poor nutrition.

4. The Steady State and the Kinetics of Hemolysis In Vivo.—On the assumption that the steady state is the result of a balance between a process which produces red cells and a process which destroys them, equations have been developed for the way in which cells of different resistances are affected when the rate of destruction changes. A method for analyzing experimental curves is described and illustrated. In general, this part of the paper relates the level of the red cell count in the animal to the intensity of the hemolytic processes taking place in vivo, and does not lend itself to detailed abstraction.

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