Control of blood water potential

• Roles of the kidney:

  • Forms filtrate through ultrafiltration
  • Reabsorption of glucose and other substances through the proximal convoluted tubule
  • Osmoregulation of Na+ gradient at the loop of Henle and reabsorption of water at the distal convoluted tubule and collecting ducts
• Kidney structure:

  • Fibrous capsule
  • Cortex
  • Medulla
  • Renal pelvis
  • Ureter
  • Renal artery
  • Renal vein
• Osmoregulation:

  • Osmoreceptor cells in the hypothalamus detect fall in water potential
  • ADH secreted into the capillaries by the posterior pituitary gland
  • ADH travels to kidneys in blood and increases permeability of cells in the distal convoluted tubule (DCT) and collecting duct
  • ADH causes vesicles bound with aquaporin proteins to fuse with the cell membrane of cells of the DCT and collecting duct
• Formation of glomerular filtrate:

  • Afferent arteriole is wider and efferent arteriole is narrower
  • Ultrafiltration of small molecules like urea, water and mineral ions
  • Pass through the basement membrane
  • Blood cells and proteins remain
  • Fluid travels through fenestrations in capillary walls and then through podocytes into the Bowman's capsule
• Reabsorption of glucose and water by the proximal convoluted tubule:

  • Na+ actively transported out of the cell into the blood
  • Creates concentration gradient of Na+
  • Na+ diffuses into the cell through channel protein
  • Glucose moves into the cell through protein
  • Glucose moves into the cell against its concentration gradient
  • Glucose diffuses into the blood and is removed from filtrate
• Maintaining a gradient of Na+ in the medulla by the loop of Henle:

  • Na+ actively transported out of the ascending arm using ATP
  • Into intestinal fluid so lowers water potential
  • Water moves out of the descending limb by osmosis
  • Water moves into blood capillaries by osmosis
  • Water potential decreases further down the medulla
  • Urea is concentrated in the filtrate
• Reabsorption of water by the distal convoluted tubule and collecting duct:

  • DCT reabsorbs water and salts
  • Changes permeability of wall
  • Lower water potential further down the medulla
  • Water moves out of the collecting duct by osmosis
  • Water then reabsorbed into the blood
  • Substances left in the collecting duct becomes urine
  • Permeability changes
• Cells of the PCT:

  • Lots of microvilli so larger surface area for absorption
  • Lots of mitochondria to provide ATP for active transport
• Describe how urea is removed from the blood:

  • High hydrostatic pressure produced
  • Ultrafiltration at the glomerulus
• Explain how urea is concentrated in the filtrate:

  • Reabsorption of water
  • At the proximal convoluted tubule
  • Active transport of Na+ ions out of the ascending limb in the loop of Henle
• A thicker medulla means the loop of Henle is longer
• Describe how ultrafiltration occurs:

  • High hydrostatic pressure due to wider afferent and narrow efferent arterioles
  • Forces urea, mineral ions and water out of the blood in capillaries and out of the fenestrations in the basement membrane
  • Past the podocytes
• The effect of a thicker medulla:

  • Thicker medulla leads to a longer loop of Henle
  • More Na+ and Cl- ions are actively transported out of the ascending limb of the loop of Henle into the medulla
  • More water reabsorbed from the loop of Henle by osmosis into the blood
• Describe and explain how the secretion of ADH affects urine produced by the kidneys:

  • ADH makes cells more permeable to water
  • More water absorbed from the collecting duct into capillaries by osmosis
  • Smaller volume of urine produced
  • Urine becomes more concentrated
• The concentration of the fluid in the proximal convoluted tubule remains constant because water is also reabsorbed
• Explain how lack of insulin affects reabsorption of glucose in the kidneys:

  • Higher blood glucose concentration in the proximal convoluted tubule
  • Reabsorption of glucose by facilitated diffusion
  • Using carrier proteins
  • These carrier proteins become saturated so not all glucose can be reabsorbed
• Desert mammals have longer loops of Henle and secrete large amounts of ADH. Explain these adaptations:

  • More water reabsorbed from filtrate by osmosis
  • From collecting duct to longer loop of Henle
  • As more ADH acts an collecting duct
  • Making cells more permeable to water
  • Na+ and Cl- ion absorbed from filtrate into the ascending limb