topic 11.3: Kidney and Osmoregulation

• Osmoregulators


  Animals that tightly regulate their body osmolarity, which always stays constant, irrespective of their environment
• Osmoconformers


  Animals that maintain an internal condition that is equal to the osmolarity of their environment
• Malpighian tubule system


  • In insects, carries out osmoregulation and removal of nitrogenous wastes

  
• Kidney


  • Carries out osmoregulation and removal of nitrogenous wastes

  
• Composition of blood in the renal artery


  Different from that in the renal vein
• Glomerulus and Bowman's capsule


  • Their ultrastructure facilitates ultrafiltration
• Ultrafiltration

  1. Occurs in the Bowman's/renal capsule

  2. In the kidney cortex

  3. Afferent arteriole has larger diameter than efferent arteriole, creating high pressure in the capsule

  4. Smaller molecules are forced out of the glomerulus/blood into the capsule

  5. Through basement membrane and fenestrations/pores in the capillary wall

  6. Filtration slits between foot projections of podocyte cells also act as a filter

  7. Molecules in the filtrate include water, urea, glucose, amino acids, salts

  8. Plasma proteins, platelets, and cells are too large so remain in the blood

  9. Glomerular filtrate is then transported through the nephron

  
• Proximal convoluted tubule (PCT)

  • Selectively reabsorbs useful substances by active transport
  • All glucose, amino acids, vitamins and hormones are reabsorbed here, along with approx 80% of the mineral ions and water
  • Microvilli cell lining to increase the surface area for absorption
  • Carrier proteins and pumps are present to actively transport substances
  • A large number of mitochondria provide ATP for active transport
• Selective reabsorption in the PCT

  1. Amino acids, hormones, mineral ions & vitamins are actively transported into the PCT cells

  2. Glucose is actively transported across the membrane in symport with sodium

  3. Water follows the movement of the ions passively (by osmosis)

  4. Due to high concentrations of recovered substances in PCT cells, the substances can passively diffuse into the bloodstream (along the concentration gradient)

  
• Loop of Henle


  • Maintains hypertonic conditions in the medulla
• ADH


  • Controls reabsorption of water in the collecting duct

  
• The type of nitrogenous waste in animals is correlated with evolutionary history and habitat
• Nitrogenous waste in different animals

  • Mammals metabolize ammonia into urea
  • Reptiles and birds package their nitrogenous waste as uric acid
  • Fish and amphibians flush their nitrogenous waste primarily as ammonia
• Terrestrial animals, because they have less access to water, have been under selective pressure to "repackage" their toxic ammonia as less toxic molecules which can be flushed with less water
• Proximal convoluted tubule (PCT) cell

  • Reabsorbs amino acids, hormones, mineral ions & vitamins
  • Glucose is actively transported across the membrane in symport with sodium
  • Water follows the movement of the ions passively (by osmosis)
  • High concentrations of recovered substances allow passive diffusion into the bloodstream

  
• Selective reabsorption of useful substances from the proximal convoluted tubule (PCT)

  1. Amino acids, hormones, mineral ions & vitamins are actively transported into the PCT cells

  2. Glucose is actively transported across the membrane in symport with sodium

  3. Water follows the movement of the ions passively (by osmosis)
• Symport


  A type of cotransport where a molecule of glucose and sodium are moved together in the same direction
• Loop of Henle


  • Generates a high concentration of solutes (low concentration of water) in the tissue fluid of the medulla compared to the filtrate in the nephron
  • Aids the reabsorption of water in the collecting duct
• Osmoregulation


  The control of water and solute concentrations in the body fluids (e.g. the blood plasma)

  
• ADH


  Balances the water concentration of the blood by changing the permeability of the collecting duct

  
• The loop of Henle maintains hypertonic conditions in the medulla
• ADH controls reabsorption of water in the collecting duct

  
• Kangaroo rat


  • Has a much longer loop of Henle than other rodents
  • This allows the kangaroo rat's kidneys to be especially efficient and produce only small quantities of highly concentrated urine
  • Can concentrate urea to 3,500 mmol/l, whereas humans can only concentrate urea to 400 mmol/l
• Length of the loop of Henle

  Positively correlated with the need for water conservation in animals
• The higher the ion concentration in the medulla, the more water can be reabsorbed in the collecting duct making the urine more concentrated

  
• Dehydration


  • Loss of water from the body so body fluids become hypertonic
  • Causes thirst, small quantities of dark coloured urine, lethargy, low blood pressure, raised heart rate, inability to lower body temperature, and in severe cases seizures, brain damage and death
• Overhydration


  • Over-consumption of water
  • Causes clear urine, swelling of cells due to osmosis, headache, disruption of nerve function, and in more serious cases delirium, blurred vision, seizures, coma and death
• Kidney failure

  A condition in which the kidneys fail to adequately filter waste products from the blood, leading to death if not treated

  
• Treatment of kidney failure

  1. Hemodialysis

  2. Kidney transplants
• Hemodialysis


  • A process of purifying the blood of a person whose kidneys are not working normally
  • Lasts about four hours and is done three times per week
  • The dialyser contains a semi-permeable membrane that allows small particles (e.g. urea) to diffuse through, but larger molecules and cells remain in the blood
  • The fresh dialysate contains no urea to encourage diffusion from the blood, and has optimal concentrations of glucose and other useful molecules to minimize loss from the blood, and a high solute concentration to remove excess water

  
• Kidney transplant


  • The best long-term treatment for kidney failure
  • Donors and the recipient have to be a close match in both blood and tissues to minimize the chance of rejection
  • The transplanted kidney is grafted in to the lower abdomen with the renal artery, renal vein and ureter connected to the recipient's blood vessels and bladder

  
• Urinalysis


  Diagnostic tests of urine samples that can detect the presence of blood cells, glucose, proteins, and drugs (or their breakdown products)

  
• Processes that affect the metabolism affect the metabolic waste that is produced and then excreted in urine