VERTEBRATE KIDNEYS
Excretory organs in protochordates are very different from the higher vertebrates. Balanoglossus (Hemichordata) has a glomerulus in the proboscis region to excrete nitrogenous wastes from the blood. Herdmania (Urochordata) has a neural organ near the solid nerve ganglion located in between the two siphons. Amphioxus (Cephalochordata) possesses protonephridia that carry hundreds of flame cell-like solenocytes that excrete wastes in the atrial cavity and to the outside.
Kidneys evolved in primitive fresh water vertebrates to excrete excess water that was continuously entering the body by osmosis. Later, kidneys also acquired the function of removing the waste materials from body. In invertebrates and also in hag fishes, body fluids are isotonic to sea water and hence they do not have problem of osmoregulation. Cartilaginous fishes and also coelacanths retain considerable amount of urea in the blood so that blood is isosmolal to sea water and osmotic problems are avoided.
It is believed that kidneys in all modern vertebrates evolved from a hypothetical kidney known as Archinephros or Holonephros, which extended from anterior to the posterior end of the body, with segmentally arranged glomeruli and nephrostomes. Glomeruli received blood from the dorsal aorta and filtered nitrogenous wastes into the coelom, from where they were collected by the nephrostomes and transported to outside via collecting ducts and archinephric ducts. Such kidneys resemble segmentally arranged nephridia of annelids and can still be observed in the larval hagfishes and apoda.
The English embryologist Balfour suggested that the three types of kidneys found in modern vertebrates, namely, Pronephros, Mesonephros and Metanephros evolved from the anterior, milddle and posterior regions of this archinephros.
PRONEPHROS
Also called the head kidney because of its location in the anterior region of body is still a functional kidney in Myxine and some primitive teleosts. It has very few (3-15) collecting tubules, each with a nephrostome that collect waste materials from a single glomus. Pronephros becomes lymphoid in function in other vertebrates.
MESONEPHROS
Also called the Wolffian body, mesonephros is the functional kidney of the larvae as well as the adults of fish and amphibian and functional kidney in the embryonic stage of amniotes, i.e. reptiles, birds and mammals. The number of uriniferous tubules increases to thousands and glomeruli are enclosed in a cup-like Bowman’s capsule; the latter two together are called Renalcorpuscle or malpighianbody. In addition, there is also a nephrostome attached to the collecting tubule, which all meet the mesonephric duct that carries the wastes to the outside. Nephrostomes disappear in most of the higher elasmobranchs, teleosts and amphibians. In sharks and urodeles, mesonephros is elongated kidney that extends up to the posterior end of body and hence is called Opisthonephros.
METANEPHROS
Metanephros occurs in the adult reptiles, birds and mammals. Reptilian kidney is elongated, more so in snakes where the body lumen available is narrow and long. Bird kidney is trilobed and mammalian kidney is bean-shaped, with a depressed area at the junction of ureter. Kidney is enclosed in a protective tunic that is made of fibrous and adipose layers. Kidney tissue is divided into an outer cortex that carries renal corpuscles and convoluted tubules and inner medulla which houses collecting ducts and loop of Henle. The number of nephrons (uriniferous tubules plus renal corpuscles) runs into millions, thereby increasing the efficiency of kidneys to extraordinary levels. In mammals renal arteries bring blood into the kidneys and renal veins take it away, but reptiles and birds possess a renal portal system.
Uriniferous tubule of mammals carries a loop of Henle that is designed to reabsorb all water from the filtrate by counter current mechanism. Reptiles do not have loop of Henle and birds have a reduced one, both groups being uricotelic do not excrete much water any way.
OSMOREGULATION IN FISHES
Elasmobranchs maintain high level of urea in blood to make the blood isotonic to sea water and therefore they can have large glomeruli. Sharks also have low ion permeability in gills so that not much salt enters through the gills. Also they excrete excess salt from the body through rectal gland that is attached to the large intestine.
Freshwater bony fishes gain a lot of water through food, gills and body surface and hence they possess large glomeruli in kidneys to excrete excess water and reabsorb salts. Also they have salt absorbing cells in the gills. Freshwater fishes do not drink water at all and their DCT is impermeable to water.
Marine teleosts on the other hand have opposite osmotic problem that they lose water through osmosis and drink plenty of sea water along with food. Therefore, their kidneys have reduced glomeruli or they are altogether absent. In addition, they have chloride secreting cells in gill to actively excrete sodium chloride. Kidneys excrete magnesium sulfate, calcium and urea but very little water. Intestine also secretes magnesium sulfate and calcium through faeces.