The Kidney and Osmoregulation

• The kidneys regulate blood composition and pH by producing urine with variable water, salt, hydrogen ion, and bicarbonate concentrations.
• It is possible to produce urine that is less concentrated than the blood when water intake is high to prevent blood from becoming too dilute
• Loop of Henle

  1. Sodium and chloride ions are pumped out of the filtrate in the ascending limb into the surrounding medulla region, raising its osmolarity

  2. The ascending limb is impermeable to water, so water is unable to leave the loop here by osmosis

  3. The osmolarity of the ascending limb decreases as it rises back into the cortex due to the removal of solutes and retention of water

  4. The neighbouring descending limb is permeable to water, so water moves out of the descending limb by osmosis due to the high osmolarity of the medulla created by the ascending limb

  5. The osmolarity of the filtrate increases as the descending limb moves down into the medulla due to the loss of water and retention of ions

  6. The water and ions that leave the loop of Henle for the medulla make their way into nearby capillaries

  7. The capillary that flows directly alongside the loop of Henle is known as the vasa recta

  8. The vasa recta also supplies oxygen to and removes carbon dioxide from the respiring cells of the loop of Henle
• Many animals deal with the excretion of the toxic waste product urea by dissolving it in water and excreting it
• Loop of Henle


  Enables the production of urine that is more concentrated than the blood, conserving water
• This method of excretion brings the problem of water loss
• Countercurrent multiplier system


  A system used by the loop of Henle to create a steep concentration gradient across the medulla
• By controlling the volume and composition of the urine, the kidneys help maintain salt and water balance (osmoregulation) in the body.
• Urinary excretion is the primary mechanism used to remove excess substances from the body.
• The kidneys remove excess nitrogenous wastes such as ammonia and urea from the bloodstream.
• Adapted in several ways

  Epithelial cells are adapted to carry out reabsorption
• Proximal Convoluted Tubule


  A part of the nephron where selective reabsorption of substances occurs
• Parts involved in reabsorption

  • Proximal Convoluted Tubule
  • Loop of Henle
  • Collecting Duct
• These substances are therefore reabsorbed into the blood as the filtrate passes along the nephron
• This process is known as selective reabsorption since not all substances are reabsorbed
• Epithelial cells


  Cells that form the lining of the proximal convoluted tubule
• Most of this reabsorption occurs in the proximal convoluted tubule
• Reabsorbed substances

  • Water
  • Salts
  • Glucose
  • Amino acids
• Many of the substances that pass into the glomerular filtrate are useful to the body
• First cell layer in the separation

  • Endothelium of the capillary with fenestrations allowing fluid to pass through but not blood cells
• Second cell layer in the separation

  • Epithelium of the Bowman’s capsule with podocytes having foot-like projections allowing the passage of small molecules
• Substances that pass into the Bowman’s capsule

  Make up the glomerular filtrate
• Red and white blood cells and platelets remain in the blood as they are too large to pass through the fenestrations between the capillary endothelial cells
• Structures within the glomerulus and Bowman's capsule


  • Especially well adapted for ultrafiltration
• Main substances in the glomerular filtrate

  • Amino acids
  • Water
  • Glucose
  • Urea
  • Salts (Na+ and Cl- ions)
• Basement membrane


  Made up of a network of collagen protein and glycoproteins, acts as a sieve allowing small molecules through but preventing passage of large proteins from the blood plasma
• The basement membrane stops large protein molecules from getting through
• Blood in the glomerular capillaries


  Separated from the lumen of the Bowman’s capsule by two cell layers with a basement membrane in between
• Passage of substances into the Bowman’s capsule

  As blood passes through the glomerular capillaries, fenestrations between the capillary endothelial cells, the mesh-like basement membrane, and the gaps between the podocyte projections allow substances dissolved in the blood plasma to pass into the Bowman’s capsule
• Animals that need to conserve water have thicker medulla regions in their kidneys to provide additional space for their long loops of Henle
• Loop of Henle


  A part of the nephron in the kidney that plays a crucial role in the reabsorption of water
• Parts of an animal's body that help conserve water

  • Longer loops of Henle
  • Longer collecting ducts
  • Thicker medulla regions in the kidneys
• Animals that live in dry environments such as deserts depend on their ability to conserve water for survival
• A longer loop of Henle is able to generate steeper concentration gradients and so more water can be reabsorbed by osmosis
• Longer collecting ducts


  Indicates an animal’s ability to conserve water
• Indicators of an animal’s ability to conserve water

  • Loop of Henle length
  • Medulla thickness
• Such animals often have very long loops of Henle
• Longer loop of Henle


  Greater an animal’s ability to conserve water
• Dehydration


  Results if water is lost from the body and not replaced
• Causes of overhydration

  • Overconsumption of water
  • Not replacing sugars and salts after excessive sweating
  • Kidney problems
  • Side effects of certain drugs