When the minimal renal threshold for blood hemoglobin is exceeded there is observed a deposit of iron staining pigment in the epithelium of the renal convoluted tubules. At a certain point this epithelium cannot take up more hemoglobin and this coincides with the minimal renal threshold level.

When the injections of blood hemoglobin are kept below the minimal renal threshold level we note a complete absence of iron staining pigment in the renal tubular epithelium.

Given a deposit of iron staining pigment in the tubular epithelium, it will slowly disappear during rest periods with no hemoglobin injections. Anemia due to bleeding will accelerate this removal of pigment from the renal epithelium and this indicates a conservation of material by the kidney for use in construction of new hemoglobin.

Pigment giving a positive stain for iron will be found in the liver and spleen when hemoglobin injections are given, regardless of the renal threshold. Removal of this pigment is accelerated by anemia due to bleeding and as a rule an anemia period of 2 months at a level of 1/3 normal (40 to 50 per cent hemoglobin) will render the spleen, liver and kidney free from iron staining pigment.

Pigment giving a positive iron stain is frequently observed in the mesenteric and lower retroperitoneal lymph glands. This is merely a drainage of pigment and phagocytes including pigment from some organ in which the pigment deposit was primary.

In stock dogs in this laboratory the hemoglobin level is quite high when the animals are in a perfectly normal state. The blood hemoglobin averages 120 to 150 per cent hemoglobin. In such dogs iron staining pigment in the spleen is a common finding and on occasion is observed in the liver.

To establish an accurate base line for the study of iron and iron staining pigment storage due to diet intake one must submit these dogs to a preliminary anemia period of at least 2 months.

Muscle hemoglobin has a very low renal threshold and escapes freely into the urine when given intravenously. Contrasting with blood hemoglobin this musde hemoglobin under identical conditions does not cause the deposit of iron staining pigment within the epithelium of the renal tubules.

A pigment giving no iron staining reaction may be found in the epithelium of the convoluted tubules of the kidney. Whether this is due to dietary or other factors is uncertain but this indicates pigment conservation by the kidney.

Finally we would emphasize again the fact that the kidney is of considerable importance in the conservation of hemoglobin and hemoglobin split products which presumably are utilized to build up new hemoglobin.

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