Osmoregulation

• Changing the external concentration to higher or lower solute concentration causes a nett movement of water either into or out of the cell
• Most cells cannot sustain significant water gain or loss and as a consequence, the solute/water balance in the intracellular environment, blood and tissue fluids needs to be carefully maintained within set tolerance limits
• The homeostatic control of this is called osmoregulation
• Are at least 6 separate hormones that impact osmoregulation
• Antidiuretic hormone (ADH), also known as vasopressin, is a peptide hormone produced in the hypothalamus and stored before release from the posterior pituitary gland 
• Diuretic is an agent that increases the rate of urine formation, therefore an antidiuretic such as ADH decreases urine formation
• The collecting ducts in the nephrons of the kidneys are effectively impermeable to water in the absence of ADH
• Increase in antidiuretic hormone (ADH) secretion

  1. Osmolarity detected by osmoreceptors

  2. Stimulates secretion of ADH from posterior pituitary gland

  3. Stretch receptors detect decreased blood volume

  4. ADH travels to kidneys

  5. Increased ADH results in more aquaporins

  6. Nephrons reabsorb more water

  7. Lower volume of concentrated urine produced
• Osmolarity


  Concentration of solutes in a solution
• Osmoreceptors


  Neurons in the hypothalamus that detect osmolarity
• ADH

  Antidiuretic hormone
• Aquaporins


  Proteins that facilitate water reabsorption in kidneys
• Increased solute concentration detected


  Stimulates secretion of ADH
• Decreased blood volume detected


  Stimulates release of ADH
• ADH travels through the blood to the kidneys
• Increased ADH in the blood results in an increased number of aquaporins inserted into the cell membranes of the collecting ducts in the nephrons in the kidneys
• This allows the nephrons to reabsorb more water back into the capillaries from the filtrate
• As a result, a lower volume of urine is produced that will be more concentrated
• Causes of increased ADH

  • Increased solute concentration in the blood and therefore decreased blood volume and pressure from not drinking enough water
  • Diarrhoea
  • Excessive sweating (intense exercise)
  • Overexposure to heat (dehydration)
  • Increased solute concentration in the blood from consuming products high in salt or sugar
  • Some medications can block receptors involved in sodium and water reabsorption
• Increased solute concentration in the blood

  Decreased blood volume and pressure
• Increased solute concentration in the blood can result from not drinking enough water, diarrhoea, excessive sweating, or overexposure to heat
• Increased solute concentration in the blood can result from consuming products high in salt or sugar
• Decrease in antidiuretic hormone secretion

  • Osmoreceptors in the hypothalamus detect decreased solute concentrations in the blood and stretch receptors detect increases in blood volume and blood pressure
  • Detection of these stimuli inhibits ADH release from the posterior pituitary gland resulting in less ADH in the blood 
  • As a result, the number of aquaporins inserted into the cell membranes of the collecting ducts of the nephrons aldo decreased
  • This results in less reabsorption of water to the capillaries from the filtrate and an increased volume of urine (which is more dilute)
• Salts and sugars are absorbed into the blood from the digestive system, increasing solute concentration
• Some medications can block receptors involved in sodium and water reabsorption, increasing sodium and water loss
• Blocking receptors involved in sodium and water reabsorption decreases blood volume and pressure
• Causes of decreased ASH release are 

  • An increased volume of blood and hence increased blood pressure from factors such as excessive consumption of water 
  • The consumption of alcohol. Alcohol inhibits the release of ADH from the posterior pituitary gland. This results in excessive urination (including the ‘breaking the seal’ effect) and therefore dehydration, resulting in a hangover 
  • Kidney disease can result in receptors on collecting ducts not responding to ADH even though enough is being produced. An example of this is diabetes insipidus (not to be confused with diabetes mellitus)