Osmoregulation

OSmOREGuLATiOn Stages of Excretion Nephron Nephrons are the functional units of the kidneys • Are situated in the cortex but descend into the medulla Nephrons mediate excretion via three main stages: • Ultrafiltration – filters out all cells and proteins • Selective reabsorption – retains nutrients / solutes • Osmoregulation – controls water retention Ultrafiltration Structure of the Bowman’s Capsule • Glomerular capillaries are fenestrated (have pores), which allows blood to freely exit the glomerulus • The capsule is lined with podocytes that have extensions (called pedicels) that the blood can freely pass between • The only filtration barrier is the basement membrane that lies between the glomerulus and the capsule Podocyte (with pedicels) Basement Membrane Blood Endothelium (fenestrated) Hydrostatic Pressure (ULTRAfiltration) • Blood is forced into a Bowman’s capsule at high pressure • Wide afferent arterioles (entry) lead into narrow efferent arterioles (exit), increasing the pressure in the capsule • Also, the extensive narrow branching of the arterioles increases glomerular surface area available for filtration Selective Reabsorption Selective reabsorption occurs in the convoluted tubules • Involves the reuptake of usable substances from filtrate Materials are actively transported across the tubule’s apical membrane before diffusing across the basolateral membrane • Tubules are lined with microvilli to increase surface area Materials reabsorbed by the convoluted tubules include: • Glucose and amino acids (via symport with sodium ions) • Mineral ions and vitamins (via protein pumps) • Water (follows ions and solutes via osmosis) Distal Convoluted Tubule Bowman’s Capsule Glomerulus Ultrafiltration occurs at the Bowman’s capsule / glomerulus • Separates cells and proteins from blood to form filtrate Filtrate Proximal Convoluted Tubule Loop of Henle Vasa Recta Collecting Duct Osmoregulation Osmoregulation is the control of water balance in the body • Involves the loop of Henle and collecting ducts Establishing a Salt Gradient • The loop of Henle creates a salt gradient in the medulla • The descending limb is permeable to water but not salt • The ascending limb is permeable to salts but not water • This means that as the loop descends into the medulla, the interstitial fluid becomes increasingly hypertonic Antidiuretic Hormone (ADH) • As the collecting duct passes through the medulla, the salt gradient draws water out of the duct (into blood) • The amount of water drawn from the ducts is controlled by ADH (released from the posterior pituitary gland) • ADH produces water channels (aquaporins) to faciliate water reabsorption by the collecting duct • Levels are high when dehydrated and low when hydrated Water Conservation Maintaining water balance is critical to survival (homeostasis) • Dehydration causes blood pressure to drop (⬆ heart rate) • Overhydration causes cells to swell (leads to organ damage) Desert animals will have longer loops of Henle to maximise water conservation (⬆ salt gradient = ⬆ water reabsorption)