When hemoglobin is set free in the circulation the kidney plays an important part in the conservation of iron. When the renal threshold is not exceeded by the hemoglobin in the blood there is little or no excess iron deposited in the kidney but when superthreshold doses of blood hemoglobin are given the epithelium of the convoluted tubules accumulates much iron and the iron analyses may show 5 times normal values.
The normal dog (140 to 150 per cent hemoglobin) has a large reserve store of iron in the liver, spleen and marrow. Diets may modify this storage of iron in these tissues. To bring conclusive proof relating to the individual diet factors, the reserve store of iron should be depleted by an anemia period of 2 to 3 months.
Complete removal of red cells from tissue capillaries is essential for accurate iron assays of fresh tissue. The method described accomplishes this without causing gross tissue edema.
The lowest iron content is observed in the pancreas, stomach, jejunum, colon and urinary bladder. These figures average from 1 to 2 mg. iron per 100 gm. fresh tissue. This shows that smooth muscle and mucous membranes contain little iron.
Striated muscle (heart, psoas) shows a relatively low iron content but uniform values close to 4 mg. per 100 gm. tissue.
Lungs show a considerable fluctuation with low iron values in anemia (3.7 mg.) and higher values in health (6 to 7 mg.).
The spleen shows maximal fluctuations and the highest reserve storage of iron per 100 gm. fresh tissue. The spleen iron analyses show low values in anemia (7 to 15 mg.) and wide differences in controls (25 to 50 mg.). With hemoglobin injections the iron storage is conspicuous and iron analyses may run as high as l50 to 175 mg. iron per 100 gm. fresh tissue.
Bone marrow of the rib runs in parallel with the spleen as regards iron storage following hemoglobin injections and depletion following anemia periods.
The liver because of its weight always contains the main bulk of the iron stored in the blood free tissues of the body. Its store is depleted by anemia even to levels of 4 to 5 mg. iron per 100 gm. fresh tissue. In the normal dog the iron store in the liver averages 25 mg. per 100 gm. tissue. Frequent hemoglobin injections may increase this level to 31 mg. iron per 100 gm. The liver is considered the most active clearing house for iron storage and utilization.