One of the most important aspects of the relationship of an entity to its environment is given by its intake and output. The concept of hierarchic organization with emphasis on the individuality of the entities, has given a special meaning to the study of these processes. For each entity its proper environment is represented by the secondary part of the entity immediately superior to it. The nuclear sap represents thus the environment from which the chromosomes take the material necessary for their metabolism and where they reject these substances which are no longer needed. Similarly, the cytoplasm represents the environment for the nucleus, the interstitial fluid for cells, the lymph for tissues, the blood for organs and the actual environment for the organism. This systematization, based on the organizational individuality of the entities, has guided the study of the relationship between entities and their environments under normal and abnormal conditions. It is under this aspect that we have investigated the renal excretion, which according to the hierarchic organization, corresponds to the relationship between the organism as an entity and its environment.

Some of the substances excreted come from the metabolism of lower entities. Related to blood, they would represent often noxious undesirable substances, if the higher mechanism of the blood would not intervene. Although that which we see as urine is the result of the relationship between the organism as an entity and its environment, the origin of the different substances forming it, as related to the different other levels, has to be considered. While for certain elements this origin is evident, for many substances only suppositions are available today.

When a systematic analysis of these constituents was attempted, other difficulties arose. The isolated urine samples easily available, have only a very relative value for many of these investigations. The titrimetric data expressed as concentration of various substances are all functions of the amount of water eliminated in the sample. As this often varies widely, the informations obtained are only relative. Balance analyses concerning entire intakes and outputs represent such technical difficulties as to make them unavailable for routine investigation, in which hundreds of subjects are daily studied. We tried to bypass this difficulty by eliminating the factor water excretion, from the considered data. The fact that the concentration of a substance and the specific gravity of the urine, are both direct function of the amount of water present, has permitted to eliminate this factor. The ratio between them appears thus independent of the amount of water present. It relates the amount of a substance to that of the bulk of the substances eliminated through the kidney. An index of excretion was thus obtained by dividing the concentration of the substance by the specific gravity. The opposite ratio would correspond to an index of retention.

From the physiological point of view, these ratios are not affected by the factors which govern the glomerular filtration, which are acting similarly for all the substances present. They are little or not affected also by the back resorption, where the differences between the various substances are reduced. They will show consequently, big variations as resulting from the active reabsorption, which takes place in the distal portions of the convoluted tubes. It is this character which gives the indexes of excretion or retention, as we calculate them, their value. We have utilized for years these indexes for chlorides, sodium, potassium, phosphoric ion, sulfhydryl, calcium, to obtain valuable information which otherwise could not be furnished by the simple analysis of the isolated urine samples. We will come back to these indices during further analyses.