Principles of Evolution of the Excretory Organs and the System of Homeostasis

Abstract

The function of the excretory organs in metazoan invertebrates and vertebrates is aimed at maintaining homeostasis. Two patterns of the morpho-functional organization of these organs can be distinguished: I—a combination of ultrafiltration and subsequent reabsorption with partial secretion of substances, or II—only a secretion of molecules of a certain type. Here we substantiate the principle according to which in the type I organs the key role of the proximal tubule is to reabsorb fluid, which is ideal in terms of its solute composition and concentration, and to secrete a number of organic acids and bases, while the distal tubule is responsible for adjusting the composition of inorganic substances. Everything what does not meet these criteria is to be excreted. The type II organs (salt glands, aglomerular kidneys) are inherent to those animals whose kidneys do not ensure osmoregulation. During evolution, the mammalian kidney developed the mechanism which regulates the redistribution of fluid within the nephron: altered reabsorption of ions and water in the proximal segment can cause an influx of some additional fluid to the distal segment where the regulatory systems adjust the amount of reabsorbed substances. The central tendency in the evolution of the kidney in vertebrates, including humans, is to increase the rates of glomerular filtration and proximal reabsorption. A similarity between molecular mechanisms of transmembrane and transepithelial transport of substances has been revealed in the evolution of excretory organs, with distinctions concerning mainly the structure of the regulatory molecules and the intensity of urine formation.