Down here is my understanding on the role various organs; especially the role kidney played in evolution and helping us be what we are. So be patient and go through the whole topic and i promise you, you will be having a different perspective on the topic by the end; like seeing from a different view point or like seeing with see through eyes- punn intended!!! OK OK let me start -
Homer. W. Smith (1959), in his article “From fish to philosopher: The story of our internal environment”, suggested that kidneys have played an important role in the evolution of various biological species, especially the land living animals.
He suggested that the earliest single celled organism resided in the salt water environment (the primordial sea) that had a composition similar to our extra cellular fluid. These species could ingest freely from the surrounding sea or excrete waste products into the sea without greatly affecting the composition of the sea, which was very vast to be disturbed.
As evolution progressed from unicellular to multicellular organism; there was development of an open circulatory system, through which the sea water would reach the innermost cells of the organism. Here too, there was no need for the organism to develop extra cellular space as the sea itself, was the extra cellular space for these organisms.
Gradually with the evolution of more complex proto-vertebrates with closed circulatory system, there was development of extra cellular space which was very similar in composition to that of sea water. It was through this extra cellular fluid that the cells of these organisms would take the nutrients and give up their waste products of metabolism. In this way, actually the ECF acted like the sea for the cells of these species.
When these early vertebrates migrated into fresh water streams, the evolution of a relatively water permeable integument was mandatory to avoid fatal dilution from their hypo-osmotic, fresh water environment. Apart from that, a vascular tuft, which we now call the glomerulus, developed enabling the fish to filter the excess fluid from the blood. Since salt preservation was extremely important to maintain the original concentrations of solutes in their ECF, the proximal tubule with its capacity for avid reabsorption of salt evolved. As this proximal tubule was permeable to water, fluid was reabsorbed in an iso-osmotic manner. The proximal tubule thus answered the need for salt conservation but did not allow excretion of hypotonic urine, which is mandatory for organisms ingesting hypotonic fluid from fresh water environment. The need for such excretion was met by the development of the distal tubule, which could dilute urine. In this portion of the nephron, salt was reabsorbed without water, since distal tubular epithelium was relatively impermeable to water. Therefore these fish could excrete the excess salt free water they had obtained from their fresh water environment without concomitantly losing their body salts. The net outcome of this was that, salt free fluid loss to maintain the solute concentration of the ECF became a top priority for the species living in fresh water. In other words these evolving organisms developed new organs or organ systems so that the primordial sea (the ECF) present in them was not altered.
As vertebrates began land inhabitation, the requirement of salt conservation persisted, but the excretion of large amounts of fluids was no longer necessary. In fact paradoxically, conservation and maintenance of ECF volume and solute concentration was of primary importance in this new environment. The kidneys of reptiles, birds, and mammals, however, had glomeruli, which filtered large amounts of fluids and salt, even though excretion of only minute amounts of these substances was needed to maintain daily balance. In reptiles and birds the kidneys responded to this challenge by decreasing the number of capillary loops in their glomerular tufts. In some fish, such as the sea horse and the pipe fish, which may have been the first vertebrates to return to sea, aglomerular kidneys evolved. Tubular secretary systems also evolved in the nephron to allow the elimination of nitrogenous wastes without the need for extremely large filtered volumes of fluid.
In mammals however, the high pressure glomerular filters were maintained, but a system for concentrating urine, namely, the counter current mechanism developed. Mammals along with birds are unique among vertebrates in processing loops of Henle and their ability to compensate for deficits by elaborating urine more concentrated than blood. Thereby, maintaining the volume and solute concentration of their ECF.
Concurrent to the development of renal system was the development of respiratory system. The Respiratory System evolved equally fast, so that the oxygen demand of the organism was maintained along with the excretion of the metabolic waste carbon dioxide. So the concentration of respiratory gases in the ECF was maintained by the respiratory system. (The evolution of respiratory system however relevant is beyond the scope of this book – readers are advised to read relevant books for more understanding)
Hence albeit, we consider ourselves as land living animals, our cells live in the theological remnant of the primordial sea called the extra cellular fluid!!! And these two above mentioned systems, along with some help from other systems of the body are responsible for the maintenance of the volume and solute concentrations of this primordial sea present in each one of us. Understanding the physiology of these systems forms the basis of understanding fluid, electrolyte and acid base balance.
Well i hope that changed your perspective and if you have any questions please feel free to ask - i am online!!!
