Osmoregulation

• Ultrafiltration in Bowman's Capsule

  Function of the renal capsule is to filter the blood arriving from the afferent arteriole from the renal artery. This process is called ultrafiltration. High hydrostatic pressure in the glomerulus pushes water and small, soluble molecules such as glucose, ions, urea from the blood in the glomerulus into Bowman's capsule. The fluid formed by ultrafiltration is known as glomerular filtrate and flows from Bowman's capsule into the proximal convolute tubule
• Three "layers" of cells that the water and small solutes must pass through

  • Endothelial cells lining the glomerular capillaries-contain tiny pores
  • Basement membrane-a fine "mesh" between glomerulus and Bowman's capsule which acts as a filter-doesn't let proteins through
  • Epithelial cells which line the inside of the Bowman's capsule called podocytes with finger-like processes and gaps in between them
• Loop of Henle

  Creates a high concentration of salt (Na) in the tissue fluid in the medulla to help more water to be reabsorbed back into the blood from the distal convoluted tubule (DCT) and collecting duct. Ascending limb is impermeable to water, but the descending limb is permeable to water. Na+ is actively transported out of the ascending limb into the surrounding tissue fluid in the medulla. Some Na+ diffuse into the descending limb. This lowers the water potential
• Components of the filtrate in Bowman's Capsule

  • Water
  • Small solutes such as glucose, urea, and sodium ions
• Hydrostatic pressure in the glomerulus

  Is high because the afferent arteriole is wider than the efferent arteriole
• Selective Reabsorption in the Proximal Convoluted Tubule

  Most of the volume of the filtrate must be reabsorbed into the blood, including glucose, mineral ions, amino acids, and water. Around 85-99% of the filtrate is reabsorbed in the proximal convoluted tubule. Useful substances are reabsorbed by facilitated diffusion and active transport. Reabsorption of glucose decreases the water potential, leading to water leaving the filtrate by osmosis. Filtrate not reabsorbed continues through the rest of the nephron
• Filtrate in Bowman's Capsule does not contain cells or proteins as they are too large to fit through
• Role of ADH in Osmoregulation

  1. Negative feedback homeostatic mechanism to maintain steady blood water potential

  2. Blood water potential is monitored by osmoreceptors in the hypothalamus in the brain

  3. If the water potential of the blood is too low, water leaves osmoreceptor cells by osmosis, triggering them to send impulses to the hypothalamus

  4. The posterior pituitary responds by releasing more ADH

  5. ADH is a peptide hormone synthesised in the hypothalamus and stored in the posterior pituitary gland to be released into the bloodstream

  6. ADH uses the second messenger model of hormone action when reaching its target cells in the collecting duct and DCT

  7. Cells lining the DCT and collecting duct contain aquaporins, which are special membrane channel proteins for water transport

  8. ADH causes vesicles containing aquaporins to fuse with the membrane, insert the aquaporin channels, and increase permeability to water
• When water potential of blood is too high

  • Less ADH is released into the blood
  • Less water is reabsorbed from the collecting duct by osmosis, decreasing the water potential of the blood
  • The kidney makes a large volume of dilute urine
  • Less aquaporins are inserted into the membranes of cells lining the collecting duct and DCT, reducing permeability to water
• Reabsorption of water from the distal convoluted tubule (DCT) and collecting duct

  1. The ascending limb is impermeable to water, but the descending limb is permeable to water

  2. Na+ is actively transported out of the ascending limb into the surrounding tissue fluid in the medulla

  3. Some Na+ diffuse into the descending limb

  4. This lowers the water potential of the tissue fluid

  5. This causes water to move out of the collecting duct and DCT into the tissue fluid by osmosis, down a water potential gradient

  6. This water then moves back into the capillaries by osmosis
• When water potential of blood is too low

  • Water leaves osmoreceptor cells by osmosis, triggering them to send impulses to the hypothalamus
  • The posterior pituitary responds by releasing more ADH
  • More water is reabsorbed from the collecting duct by osmosis, increasing the water potential of the blood
  • The kidney makes a small volume of concentrated urine
  • More aquaporins are inserted into the membranes of cells lining the collecting duct and DCT, increasing permeability to water