3.6.4.3 Control of blood water potential

• Blood water potential refers to the balance of water and solutes (e.g., salts, glucose) in the blood.
• Maintaining a stable blood water potential is crucial for maintaining osmotic balance, which ensures that cells neither shrink (lose water) nor burst (gain too much water).
• The kidneys play a key role in maintaining water balance through a process called osmoregulation. This involves:

  1. Ultrafiltration
  2. Selective reabsorption
  3. Loop of Henle
  4. Distal Convoluted Tubule and the Collecting Duct
• Ultrafiltration is the filtration of blood in the glomerulus of the nephron.
• Selective reabsorption is the reabsorption of useful substances like water, glucose, and ions.
• Formation of urine is the removal of excess water and solutes to regulate water potential.
• The hormone antidiuretic hormone (ADH) regulates blood water potential.
• ADH is released by the posterior pituitary gland and affects the permeability of the kidney's collecting ducts to water.
• A change in blood water potential is detected by osmoreceptors in the hypothalamus.
• Response to Low Water Potential (Dehydration):

  • ADH release: The posterior pituitary gland releases more ADH.
  • Effect on Kidneys:
  • ADH makes the walls of the collecting ducts and distal convoluted tubules more permeable to water.
  • Water moves out of the ducts into the blood by osmosis.
  • Outcome: Smaller volume of concentrated urine is produced, conserving water.
• Response to High Water Potential (Overhydration):

  • ADH suppression: The posterior pituitary gland releases less ADH.
  • Effect on Kidneys:
  • Walls of the collecting ducts and distal convoluted tubules become less permeable to water.
  • Less water is reabsorbed into the blood.
  • Outcome: Larger volume of dilute urine is produced, removing excess water.
• The regulation of blood water potential involves a negative feedback mechanism:

  1. Stimulus: Deviation in blood water potential (too high or too low).
  2. Receptor: Osmoreceptors in the hypothalamus detect the change.
  3. Control Center: The hypothalamus signals the posterior pituitary to release or inhibit ADH.
  4. Effector: The kidney alters water reabsorption in the collecting ducts.
  5. Response: Blood water potential returns to normal, and ADH secretion adjusts accordingly.
• Low blood water potential (Dehydration):

  • Symptoms: Dry mouth, dizziness, reduced urine output.
  • Severe cases: Kidney failure or damage.
• High blood water potential (Overhydration):

  • Symptoms: Swelling of cells, nausea, headaches.
  • Severe cases: Brain swelling, seizures.
• The collecting duct is part of the nephron where water reabsorption is regulated.
• Why is maintaining blood water potential important?

  It prevents dehydration, overhydration, and ensures normal cellular function and blood pressure regulation.
• What role does the kidney play in regulating water potential?

  The kidney filters blood, reabsorbs water and solutes, and removes excess water and solutes as urine.
• Describe the role of the hypothalamus in controlling blood water potential. (3 marks)

  • The hypothalamus contains osmoreceptors that detect changes in blood water potential. (1)
  • It sends signals to the posterior pituitary gland to release or inhibit ADH secretion. (1)
  • This adjusts the kidney’s water reabsorption to regulate blood water potential. (1)
• Explain how the body responds to dehydration. (4 marks)

  • Osmoreceptors in the hypothalamus detect low blood water potential. (1)
  • The hypothalamus signals the posterior pituitary gland to release more ADH. (1)
  • ADH increases the permeability of the kidney’s collecting ducts to water. (1)
  • More water is reabsorbed into the blood, producing concentrated urine. (1)
• A person drinks a large volume of water quickly. Explain how their body regulates their blood water potential. (4 marks)

  • Osmoreceptors in the hypothalamus detect high blood water potential. (1)
  • The hypothalamus signals the posterior pituitary to release less ADH. (1)
  • The kidney’s collecting ducts become less permeable to water. (1)
  • Less water is reabsorbed, and dilute urine is produced. (1)
• The human kidney's general structure:

  • An outer fibrous capsule that protects the kidney.
  • A layer called the cortex made up of the Bowman's capsules, convoluted tubules and blood vessels.
  • A layer called the medulla made up of loops of Henle, collecting ducts and blood vessels
  • The renal pelvis, which collects urine into the ureter.
• Process of Ultrafiltration:

  • blood enters the kidney via the renal artery which is under pressure from the heart
  • this divides into the afferent arteriole and then the glomerulus
  • water and soluble substances are forced out of the glomerulus, while large or insoluble proteins remain in blood plasma
• The glomerulus is a complex network of capillaries.
• Process of Selective Reabsorption:

  • glucose is reabsorbed via co-transport from the epithelial cells of the proximal convoluted tubule to blood capillaries
  • it is carried out by actively transporting sodium ions from the epithelial cells to the blood, creating a low concentration of sodium ions in the epithelial cells
  • sodium ions then move in from the lumen of the proximal convoluted tubule by facilitated diffusion, co-transporting glucose with it
  • glucose then diffuses into blood capillaries.
• The Loop of Henle acts as a counter-current multiplier.
• The Loop of Henle:

  • sodium ions are actively transported out of the ascending limb using ATP
  • creates a low water potential between the two limbs
  • the ascending limb is impermeable to water and therefore this means that water only moves out of the descending limb by osmosis into the area of low water potential
  • the water then enters the blood capillaries in this region by osmosis
  • at the hairpin of the loop, the water potential is at its lowest, where sodium ions are naturally diffusing out
• The loop of Henle is made up of the ascending limb and the descending limb.
• The Distal Convoluted Tubule and the Collecting Duct:

  • water naturally moves out of the distal convoluted tubule and the collecting duct by osmosis
  • the collecting duct runs parallel to the loop of Henle and therefore as you move down into the medulla, ion concentration increases
• Water, glucose, amino acids, urea and inorganic ions make up the glomerular filtrate.
• Water, ions (e.g. sodium ions), amino acids and glucose are reabsorbed in the proximal convoluted tubule.