Kidney - osmoregulation

• Kidneys

  Responsible for filtering blood and producing urine for osmoregulation
• Kidneys

  • Composed of millions of nephrons
• Blood supply to kidneys

  1. Renal artery branches into arterioles

  2. Afferent arteriole brings blood towards the glomerulus where ultrafiltration occurs between it and the bowman's capsule

  3. Efferent arteriole carries blood away and branches into capillaries towards the rest of the nephron to ensure for short diffusion pathway for faster diffusion for absorption
• Ultrafiltration

  1. Occurs between the glomerulus and bowman's capsule due to the high hydrostatic pressure creating a pressure gradient

  2. Caused by the smaller diameter in the efferent arteriole compared to the afferent arteriole

  3. Only allows the passage of glucose, water, ions, urea

  4. Prevents blood and proteins

  5. Substance needs to pass through specific gaps in the endothelium of the arteriole, basement membrane, and podocytes on the lining of the bowman's capsule

  6. The solution formed after filtrate is called glomerular filtrate
• Selective reabsorption

  1. Occurs in the proximal convoluted tubule (PCT) to reabsorb useful substances such as water, glucose and ions

  2. PCT contains sodium ions in both the tubule lumen and epithelial cell

  3. Na+ ions are actively transported out of the epithelial cell into the blood which creates a concentration gradient for more Na+ ions and water to enter the epithelial cell

  4. Na+ ions travel through the tubule lumen into the epithelial cell via facilitated diffusion co-transport proteins which also transport glucose

  5. Water leaves the tubule lumen via osmosis into the epithelial cell and then into the blood

  6. Adaptations of the PCT: lots of mitochondria, large number and variety of co-transport proteins, microvilli to increase surface area
• Additional absorption - Loop of Henle

  1. Descending limb is permeable to water

  2. Ascending limb has Na+ ions exit

  3. Na+ ions leaving the ascending limb increases the water potential of the medulla causing water to leave the permeable descending limb into the medulla

  4. Counter current flow speeds up the process

  5. Water and ions in the medulla are reabsorbed back into the blood
• Osmoregulation - Distal convoluted tubule, Collecting duct

  1. ADH increases permeability of the DCT and collecting duct by binding onto its cell walls

  2. Hypothalamus contains osmoregulatory centres

  3. If water potential in blood too high, ADH not produced

  4. If water potential in blood too low, ADH produced and stored in posterior pituitary gland to be released when stimulated by hypothalamus
• Diuresis: increased production of urine.
• Antidiuretic hormone (ADH): hormone that regulates the amount of water excreted from the body.