6.4.3 Blood Water Potential

• Structure of a Nephron

  A) Cortex

  B) Medulla

  C) Glomerulus

  D) Bowman's Capsule

  E) Proximal Convoluted Tubule

  F) Distal Convoluted Tubule

  G) Loop of Henle

  H) Collecting Duct

  
• Ultrafiltration takes place in the glomerulus and Bowman's capsule
• Ultrafiltration

  1. Blood enters the kidneys via the renal artery at high pressure
  2. Renal artery divides into afferent arteriole then the glomerulus
  3. Water and soluble components are forced out of the glomerulus down a pressure gradient into the Bowman’s capsule
  4. The pressure gradient is aided by the efferent arteriole leaving the glomerulus being narrower than the afferent arterioles
  5. Proteins are left behind in blood (too large)
  6. Glomerular filtrate forms
• The aim of selective reabsorption is to reabsorb glucose from the glomerular filtrate back into the blood
• Selective Reabsorption

  1. Glucose reabsorbed by cotransport from epithelial cells of the proximal convoluted tube to blood capillaries
  2. Carried out by actively transporting Na+ ions creating a low Na+ concentration in the epithelial cells
  3. Na+ moved in from the PCT lumen by facilitated diffusion which also brings in glucose
• Loop of Henle

  1. Na+ actively transported out of the ascending limb creating a low water potential in the interstitial space
  2. The ascending limb is impermeable to water so water can only move out of the descending limb by osmosis
  3. Water then enters blood capillaries by osmosis
  4. At the hairpin of the loop, Na+ ions naturally diffuse out as water potential is at its lowest here
• Distal Convoluted Tubule and Collecting Duct

  1. Water moves out of the DCT and collecting duct by osmosis down water potential gradient
  2. Collecting duct runs parallel to the Loop of Henle so ion concentration increases as you move down into the medulla
• Antidiuretic hormone (ADH) = stimulates the kidneys to conserve water
• Hormones controlling low BWP

  1. Osmoreceptors in the hypothalamus detect blood water potential changes
  2. More ADH released/secretede by posterior pituitary gland into blood
  3. Travels to kidney & binds to receptors on surface of the collecting duct & activates the enzyme phosphorylase
  4. Causes vesicles containing aquaporins to incorporate into the cell surface membrane
  5. This increases water/urea permeability
  6. Urea leaves the collecting duct so water leaves and is reabsorbed into the blood
• When BWP is too low:

  • Detection via osmoreceptors in hypothalamus
  • More ADH released by pituitary gland
  • Walls of collecting duct/DCT become more permeable to water due to aquaporins on CSM
  • Less water leaves body
  • Urine is more concerntrated
• When BWP is too high:

  • Detection via osmoreceptors in hypothalamus
  • Less ADH released by pituitary gland
  • Walls of collecting duct/DCT become less permeable to water
  • More water leaves body
  • Urine is more dilute