12 2 Kidneys The vertebrate kidney is responsible for several crucial osmoregulatory functions filtration of the blood active secretion of solutes from the blood reabsorption of solutes into the blood and excretion By regulating solute and water balance the kidney also regulates blood pressure and pH balance in the body There is a lot of variation in kidney structure which has been shaped by the selective pressure of osmoregulating in different environments such as aquatic habitats terrestrial habitats food availability and diet In general the vertebrate kidney has become more extensive over evolutionary time with the transition from seawater to freshwater and with the transition from water to land Around 400 mya the first amphibians transitioned to land In larval forms amphibians have pronephros from Greek before kidneys for formation and excretion of urine These are fairly simple structures with tubules nephric ducts that open into their coelom body cavity Capillary aggregates called glomi singular glomus are closely associated with the openings of the ducts to facilitate filtration of the blood Like in aquatic molluscs and annelids blood is filtered between leaky podocytes in the capillaries due to blood pressure in the glomi Water and small solutes are able to squeeze between the podocytes the blood retains its cells and macromolecules The filtrate is collected by the ducts and conducted to the outside of the body for excretion Recall that in their larval stages amphibians breathe water using gills and also exchange a lot of gas through their skin Like freshwater fish there is little need for water retention and they excrete large amounts of dilute urine As the amphibian metamorphoses into an adult its gills are resorbed and its lungs mature Although it is still dependent on water to maintain the moisture of its skin to facilitate CO2 release and O2 absorption to a lesser degree it now breathes air and lives on land where it has adaptations to retain water Its bladder restructures to facilitate water reabsorption and its kidneys restructure into more organized efficient units called nephrons The glomi become more organized forming blood filtration units called glomeruli singular glomerulus The figure below illustrates the larval amphibian kidney pronephros and adult amphibian kidney mesonephros or metanephros in some More recently evolved vertebrates also go through these stages during development but only the adult stage metanephros is functional This is an illustration of recapitulation theory that as animals develop they progress through embryonic stages that resemble their ancestors adult forms in order of evolution Although there are many examples of this also phrased as ontogeny recapitulates phylogeny there are as many counterexamples For example a chick does not resemble an adult reptile during any point of its embryonic development However this theory posited by many but most often attributed to Ernst Haeckel 1834 1919 is an interesting observation on embryonic development and possibly one that reflects how much animal genomes have in common across taxa If animals develop using many of the same genes it is not surprising that they would go through similar stages during ontogeny Nephrons from Greek kidneys are the functional units of the vertebrate kidney Nephron diversity is illustrated in the figure above with several divisions indicated In this lesson you will learn about the structure and function of each section It is important to keep in mind the principles of electrochemical gradients passive vs active transport and the selective permeability of the cell membrane which lets small uncharged particles through but charged and larger particles need transport proteins Each kidney contains many nephrons on average each human kidney contains 1 million nephrons A single nephron comprises a tubule the renal tubule which is divided into segments based on cellular composition and function and vasculature As you saw in the amphibian nephron the vasculature in the kidney transports the blood to the nephron and intimately associates with it The capillaries of the glomerulus facilitate filtration of the blood while the capillaries of the vasa recta facilitate active secretion of some solutes from the blood to the filtrate and reabsorption of other solutes and water from the filtrate back to the blood Like other osmoregulatory organs we have learned about the renal tubule is lined with epithelial cells that express various profiles of transport proteins allowing the animal to osmoregulate and ionoregulate its ECF Many of these cells respond to hormones which allows the animal to alter its osmotic and ionic balances in response to changes in environment diet metabolic activity etc Each segment of the tubule has epithelial cells that differ in structure and express a different profile of transport proteins endowing each segment with the following functions Bowman s capsule collects initial filtrate from glomerular capillaries Proximal tubule majority of solute and H2O reabsorption to the peritubular capillaries some secretion Loop of Henle additional solute and H2O reabsorption to the vasa recta in mammals Distal tubule secretion from the peritubular capillaries additional reabsorption to the peritubular capillaries hormonal regulation Collecting duct secretion and final reabsorption hormonal regulation gathers urine from multiple nephrons for excretion Filtration In the vertebrate kidney the initial filtrate is collected by a cup of epithelial cells called Bowman s capsule Arterioles transport blood into the capsule which contains a ball of capillaries the glomerulus As you saw in the adult amphibian nephron the capillary walls comprise leaky podocytes that allow water and small solutes to be squeezed out of the blood due to there being lower pressure in the capsule than in the capillary These solutes include ions glucose amino acids uric acid or urea depending on the species and some vitamins toxins and drugs There are no active transport mechanisms so the initial filtrate will be isoosmotic to the blood in the glomerular capillaries The capillaries are surrounded by mesangial cells which control blood pressure and thus filtration or glomerular filtration rate Between all of the nephrons in both kidneys the filtration power of the glomeruli is astonishing all of the blood in the human body 3 L gets filtered approximately every 30 minutes Reabsorption Once the initial
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