Kidney

• Objective


  Description of the proximal convoluted tubule and water reabsorption by the distal convoluted tubule and collecting duct
• One of the main functions of the kidneys is to filter waste products out of the blood and reabsorb useful solutes (e.g. glucose)
• Excretion of waste products

  1. Blood enters the kidney through the renal artery

  2. Passes through capillaries in the cortex (outer layer) of the kidneys

  3. Substances are filtered out of the blood and into long tubules

  4. Useful substances are reabsorbed back into the blood

  5. Remaining unwanted substances pass along to the bladder and are excreted as urine
• Nephrons


  • Long tubules along with the bundles of capillaries where the blood is filtered
  • Around one million nephrons in each kidney
• Ultrafiltration


  1. Blood from the renal artery enters smaller arterioles in the cortex of the kidney

  2. Each arteriole splits into a glomerulus (bundle of capillaries looped inside a Bowman's capsule)

  3. Ultrafiltration takes place in the glomerulus and Bowman's capsule
• Afferent arteriole


  Arteriole that takes blood into each glomerulus
• Efferent arteriole


  Arteriole that takes the filtered blood away from the glomerulus
• The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomeruli is under high pressure
• Filtration through the three layers

  1. Endothelium

  2. Basement membrane

  3. Epithelium of the Bowman's capsule
• Larger molecules like proteins and blood cells can't pass through the filtration layers
• Glomerular filtrate


  The substances that enter the Bowman's capsule
• Selective reabsorption


  1. Glomerular filtrate flows along the proximal convoluted tubule (PCT), through the loop of Henle, and along the distal convoluted tubule (DCT)

  2. Useful substances leave the tubules and enter the capillary network surrounding them
• Proximal convoluted tubule (PCT)


  • Epithelium has microvilli to provide a large surface area for reabsorption
  • Useful solutes like glucose are reabsorbed by active transport and facilitated diffusion
• Water enters the blood by osmosis


  Because the water potential of the blood is lower than that of the filtrate
• Water reabsorption


  1. Water is reabsorbed from the PCT, loop of Henle, DCT and the collecting duct

  2. The filtrate that remains is urine, which passes along the ureter to the bladder
• Urine is usually made up of water and dissolved salts, urea and other substances such as hormones and excess vitamins
• Urine doesn't usually contain proteins or blood cells as they're too big to be filtered out of the blood
• Glucose is actively reabsorbed back into the blood, so it's not usually found in the urine
• Osmoregulation


  The regulation of the water potential of the blood (and urine) to maintain the right amount of water in the body
• If the water potential of the blood is too low


  More water is reabsorbed by osmosis into the blood from the tubules, so the urine is more concentrated and less water is lost
• If the water potential of the blood is too high


  Less water is reabsorbed by osmosis into the blood from the tubules, so the urine is more dilute and more water is lost
• Regulation of water potential


  1. Takes place mainly in the loop of Henle, DCT and collecting duct

  2. The volume of water reabsorbed is controlled by hormones
• Loop of Henle


  • Located in the medulla (inner layer) of the kidneys
  • Has a descending limb and an ascending limb
  • The limbs control the movement of sodium ions so that water can be reabsorbed by the blood
• The movement of water and sodium ions in the loop of Henle, DCT and collecting duct helps to regulate the water potential of the blood
• How the system works

  1. Na" ioms are actively pumped outi the medulla

  2. The ascending limb is impermeable to water, so t says inside the tubule

  3. This creates a low water potential in the ne because there's a high concentration of ions

  4. Water moves out of the descending limb bich is permeable to water) into the medulla by osmosis

  5. This makes the glomerular fibrate more concentrated the ions can't difuse out descending limb isn't permeable to themi

  6. The water in the media's reabsorbed into the blood through the capillary network

  7. Na ions diffuse out in the medulla, further lowering the water potential in the medulla

  8. Water moves out of the distal convoluted tubules (DCT) by osmosis and is reabsorbed into the blood

  9. The fint three stages massively increase the ion concentration in the medulla, which lowers the water potential

  10. This causes water to mo of the collecting duct by osmosis

  11. The water in the medula reabsorbed into the blood through the capillary network
• Volume of water reabsorbed into the capillaries


  Controlled by changing the permeability of the DCT and the collecting duct
• Antidiuretic hormone (ADH)


  Hormone released by the posterior pituitary gland that increases water reabsorption in the DCT and collecting duct
• How ADH works


  1. Water potential of the blood is monitored by osmoreceptor cells in the hypothalamus

  2. When water potential decreases, osmoreceptor cells decrease in volume

  3. This sends a signal to the posterior pituitary to release ADH

  4. ADH binds to receptors on the DCT and collecting duct cells

  5. This inserts aquaporin water channels into the cell membranes

  6. More water is reabsorbed from the DCT and collecting duct into the blood by osmosis

  7. Less water is lost in the urine
• Dehydration - blood water content is too low


  • Water potential of blood drops
  • Osmoreceptors in hypothalamus detect this
  • Posterior pituitary releases more ADH
  • More water reabsorbed from DCT and collecting duct
  • Less water lost in urine
• Hydration - blood water content is too high


  • Water potential of blood rises
  • Osmoreceptors in hypothalamus detect this
  • Posterior pituitary releases less ADH
  • Less water reabsorbed from DCT and collecting duct
  • More water lost in urine