The effects of very gradually increasing or diminishing the amount of circulating hemoglobin have been studied in rabbits. Contrary to expectation it was found that when the pigment was increased by the injection of a small quantity of compatible blood every day during some weeks the erythropoietic tissue did not lessen its activities. The hemoglobin percentage mounted gradually yet considerably when even as little as 1/100 of the amount of blood initially possessed by the animal was injected each day; and the figure it finally attained must in some instances at least have been expressive of a superabundance. To this superabundance the animal itself evidently contributed through its persisting erythropoietic activity.
The results were very different when rabbits were bled daily to the same small amount that was injected into their fellows. The marrow became abnormally active, and this activity continued undiminished throughout the long period of the bleedings. The organism is evidently far more susceptible to blood losses than to blood gains, a fact which is scarcely surprising when one considers that throughout its differentiation as a going concern it has had to cope with exigencies of the first sort only.
Rabbits in which the hemoglobin is very gradually increased by the injection of strange blood become so accustomed to the abundance of pigment that even a slight falling off causes the erythropoietic tissue to become abnormally active to maintain the new status quo. Good reasons exist for referring the habituation thus manifested to readjustments in the functioning of the physiological mechanisms which mediate between oxygen demand and erythropoietic response. Too little recognition has been given to the rôle of these mechanisms in such relation. No evidence was obtained of any effective readjustment to protect the erythropoietic tissue from the stimulus of daily small blood losses.