Kidneys + Osmoregulation

• Define osmoregulation?

  Control of blood water potential
• Describe the structure of a nephron?

  • Bowman's capsule - surrounds glomerulus, inner layer of podocytes
  • Proximal convoluted tube (PCT) - loops surrounded by capillaries, walls made of epithelial cells with microvilli
  • Loop of Henle - hairpin loop
  • Distal convoluted tube (DCT) - fewer capillaries than PCT
  • Collecting duct - DCT from several nephrons empty into collecting duct
• Describe the blood vessels associated with a nephron?

  • Afferent arteriole is wide in diameter
  • Efferent arteriole is narrow diameter
  • Glomerulus capillaries
• Explain how glomerular filtrate is formed?

  Ultrafiltration in Bowman's capsule

  • High hydrostatic pressure in glomerulus forces small molecules (urea, water, glucose, mineral ions) out of capillary pores/fenestrations against osmotic gradient
  • Basement membrane acts as filter, plasma proteins remain is capillary as they are too large to leave
• State what happens during selective reabsorption and where it occurs?

  Useful molecules (e.g., glucose) are reabsorbed into the blood.

  Occurs in PCT
• Outline the transport processes involved in selective reabsorption?

  1. Glucose from glomerular filtrate moves to cells lining PCT by co-transport with Na+ ions
  2. From PCT to intercellular spaces by active transport
  3. From intercellular spaces to capillaries by diffusion
• How are cells in the PCT adapted for selective reabsorption?

  • Microvilli - large surface area
  • Many mitochondria - produce ATP for active transport of glucose
  • Many carrier proteins - for active transport of glucose
• What happens in the loop of Henle?

  1. Active transport of Na+ & Cl- out of ascending limb
  2. Water potential of tissue fluid decreases
  3. Osmosis of water out of descending (ascending limb is impermeable to water)
  4. Water potential of filtrate decreases going down descending limb
• Explain the role of the DCT?

  Reabsorption:

  1. of water via osmosis
  2. of ions via active transport
• Explain the role of the collecting duct?

  Reabsorption of water from filtrate into tissue fluid via osmosis through aquaporins
• Explain why it is important to maintain an Na+ gradient?

  Countercurrent multiplier - filtrate in collecting ducts is always beside an area of tissue fluid that has a lower water potential.

  Maintains water potential for maximum reabsorption of water
• Explain the role of the hypothalamus in osmoregulation?

  1. Osmosis of water out of osmoreceptors in hypothalamus causes them to shrink
  2. This triggers hypothalamus to produce more ADH
• Explain the role of the posterior pituitary gland?

  Stores and secretes the ADH produces by the hypothalamus
• Explain the role of ADH in osmoregulation?

  1. Stimulates addition of channel proteins (aquaporins) into membrane
  2. Make cells/membranes more permeable to water
  3. More water reabsorbed by osmosis
• Name the two parts of a nephron where osmoreceptors are found?

  1. Collecting duct
  2. Distal convoluted tube