The Kidney- The structure and function of the nephron

The Kidney The structure and function of the nephron Structure of the nephron  The nephron is the functional unit of the kidney  There are about a million in each kidney.  They consist of: a cup-shaped Bowman’s capsule immediately below the capsule a twisted region called the proximal convoluted tubule. Structure of the nephron followed by the long hair-pin like loop of Henle, which runs deep into the medulla and then back into the cortex this is followed by another twisted region called the distil convoluted tubule This is joined to the collecting duct which carries urine through the medulla to the pelvis of the kidney Glomerulus, a knot of capillaries afferent arteriole Glomerulus, a knot of capillaries afferent arteriole efferent arteriole Glomerulus, a knot of capillaries afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole Bowman’s capsule afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole Bowman’s capsule afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole proximal tubule distil tubule Bowman’s capsule afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole proximal tubule distil tubule loop of Henle Bowman’s capsule afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole proximal tubule distil tubule ascending loop descending loop loop of Henle Bowman’s capsule afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole proximal tubule distil tubule ascending loop descending loop Collecting duct loop of Henle Bowman’s capsule afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole proximal tubule distil tubule ascending loop descending loop Collecting duct loop of Henle to renal pelvis Bowman’s capsule afferent arteriole efferent arteriole Glomerulus, a knot of capillaries venuole proximal tubule distil tubule ascending loop descending loop capillary net Collecting duct loop of Henle to renal pelvis Structure of the nephron  Each nephron has a rich blood supply  Each Bowman’s capsule is supplied with blood by an afferent arteriole  This branches inside the Bowman’s capsule to form the glomerulus  Taking blood away from the capsule is the efferent arteriole.  The afferent arteriole is much wider than the efferent arteriole….. Structure of the nephron ….. So there is more blood carried to the glomerulus than is carried away, and pressure in the glomerulus is high. Ultrafiltration  ..the filtering of small molecules (under pressure) out of the blood and into the Bowman’s capsule. Bowman’s Capsule Bowman’s Capsule glomerulus Bowman’s Capsule glomerulus afferent arteriole Bowman’s Capsule glomerulus efferent arteriole afferent arteriole Bowman’s Capsule glomerulus efferent arteriole distil convoluted tubule afferent arteriole Bowman’s Capsule glomerulus efferent arteriole proximal convoluted tubule distil convoluted tubule afferent arteriole Bowman’s Capsule glomerulus efferent arteriole proximal convoluted tubule distil convoluted tubule capsular space afferent arteriole Ultrafiltration  The blood entering the glomerulus is separated from the space inside the Bowman’s capsule, by two cell layers and a basement membrane. podocyte cell podocyte cell nucleus podocyte cell nucleus Basement membrane podocyte cell nucleus Basement membrane capillary endomethium podocyte cell nucleus Filtrate in lumen of capsule Basement membrane capillary endomethium Ultra filtration  The first layer is the wall or endothelium of the capillary. In the glomerulus this single layer of cells has thousands of gaps.  The basement membrane between the two cell layers is composed of glycoprotein and collagen fibres. Its mesh-like structure acts as a filter during ultrafiltration. Ultra filtration  The second cell layer is the wall of the Bowman’s capsule  The epithelial cells in this wall are called podocytes  They have foot-like processes and do not fit tightly together so have gaps between them. Ultra filtration  Most molecules in the blood can pass through the gaps in he capillary and the podocyte.  BUT the basement membrane acts as a filter preventing large molecules such as proteins and blood cells from passing through.  Only the small soluble molecules can pass through due to the high pressure in the kidney Reabsorbtion Molecule or ion water protein glucose amino acids urea inorganic ions Approx. concentrations / g dm-3 Plasma Filtrate 900.0 900.0 80.0 0.0 1.0 1.0 0.5 0.5 0.3 0.3 7.2 7.2 Reabsorbtion  Up to 70% of water and solutes are removed from the plasma as it passes through the glomerulus  These include some useful substances such as aa and glucose, these must be reabsorbed back into the body.  This process is called selective reabsorbtion. Reabsorbtion  All the glucose, aa, vitamins and many Na and Cl ions are actively transported out of the proximal tubule and back into the blood. Adaptations of the proximal tubule  The cells possess microvilli to provide a large surface area for absorption  Numerous mitochondria are present to provide ATP for active transport. Reabsorbtion  The uptake of these substances means that the blood in the capillaries surrounding the nephron now has a a relatively high solute concentration.  So large amounts of water passes from the filtrate back to the blood in the proximal tubule by osmosis The Loop of Henle  Its function is to create an area of high solute concentration deep in the medulla.  The collecting ducts of each nephron pass through this area and so a lot of water can be reabsorbed from the ducts by osmosis  A concentrated urine can be produced as a result Descending limb Descending limb Ascending limb, impermeable to water but permeable to NaCl Descending limb Descending very permeable to water Ascending limb, impermeable to water but permeable to NaCl Ascending limb – very thick Descending limb Descending very permeable to water Ascending limb, impermeable to water but permeable to NaCl Ascending limb – very thick Descending limb Water moves out tubule into back into capillary Descending very permeable to water Ascending limb, impermeable to water but permeable to NaCl Ascending limb – very thick Descending limb Water moves out tubule into back into capillary Descending very permeable to water Na and Cl ions move out of tubule Ascending limb, impermeable to water but permeable to NaCl Ascending limb – very thick Descending limb Water moves out tubule into back into capillary Descending very permeable to water Na and Cl ions move out of tubule Ascending limb, impermeable to water but permeable to NaCl Increasing salt conc. in interstitial fluid Ascending limb – very thick Descending limb Decreasing salt conc. in interstitial fluid Water moves out tubule into back into capillary Descending very permeable to water Na and Cl ions move out of tubule Ascending limb, impermeable to water but permeable to NaCl Increasing salt conc. in interstitial fluid Ascending limb – very thick Descending limb isotonic filtrate Decreasing salt conc. in interstitial fluid Water moves out tubule into back into capillary Descending very permeable to water Na and Cl ions move out of tubule Ascending limb, impermeable to water but permeable to NaCl Increasing salt conc. in interstitial fluid Ascending limb – very thick Descending limb isotonic filtrate Decreasing salt conc. in interstitial fluid Water moves out tubule into back into capillary Descending very permeable to water Na and Cl ions move out of tubule Ascending limb, impermeable to water but permeable to NaCl hypertonic filtrate Increasing salt conc. in interstitial fluid Ascending limb – very thick Descending limb isotonic filtrate hypotonic filtrate Decreasing salt conc. in interstitial fluid Water moves out tubule into back into capillary Descending very permeable to water Na and Cl ions move out of tubule Ascending limb, impermeable to water but permeable to NaCl hypertonic filtrate Increasing salt conc. in interstitial fluid The loop of Henle  The ascending limb is more permeable to salts and less permeable to water  As filtrate moves up Na and Cl ions move out passively and then actively  This cause water to move out of the descending limb by osmosis.  Thus filtrate become more concentrated as it passes down the descending limb The loop of Henle  The solute conc. in any part of the loop is always lower in the ascending limb than the descending limb  This mechanism is called the countercurrent mulitplier mechanism. The loop of Henle  As the collecting ducts pass through the medula to the pelvis they pass through this region of high solute concentration  So water is also drawn out the collecting ducts due to osmosis resulting in a far more concentrated urine.