Urine concentration and dilution

Question 1

Why does urine osmolality vary?

Answer 1

• Urine is normally concentrated in humans
  
  • Urine is normally hyperosmotic compared to plasma
    ○ When urine more concentrated than the blood plasma
    ○ Blood osmolality=2 mOs/kg but urine=400-1000 mOs/kg
  • Water is reabsorbed from the: PCT, Loh, DCT and CDs
  • When H2O intake is low- lower volume of more concerted urine is produced
  • When H2O intake is high-higher volume of more dilute urine is produced

Question 2

What is the mechanisms by which urine can be concentrated or diluted?

Answer 2

Water reabsorption is regulated by the collecting duct
1. ADH is released by the pituitary gland when there is not enough water
2. ADH is a hormone so travels in the blood
3. ADH diffuses out of the Peritubular capillary and binds to receptors on the collecting duct
4. ADH causes more aquaporins to embed into them membrane
5. More water reabsorbed paracellularly into the blood

Question 3

What is the role of ADH and the collecting ducts?

Answer 3

• If enough water wasn’t drunk or too much fluid was lost the body fluids could become hyperosmotic >285 mOs/kg
• Lower H2O=body fluids are more concentrated so smaller volume of urine is produced
• Collecting ducts are the only place in the nephron which can alter the amount of water re-absorbed – fine tunes the electrolyte concentration
• ADH acts on the collecting duct to increase the impermeable collecting ducts permeability to water
• The corticomedullar hypoosmotic gradient made by the loop of Henle will cause water to move out of the collecting ducts into the medullar/interstitum
• Over-hydration-
  ○ Over hydration is rarely a problem as you can contrail how much urine is produced water is reabsorbed
  ○ Decreased osmolality of extra cellular fluid (les ions in the fluid as there is more water present)
  ○ Hyponatremia is the real problem
  ▪ Hyponatremia is where the Na+ concertation in the blood is lower than normal
  ▪ Over hydration will dilute the Na+ in the blood
  ▪ More Na+ would be lost is so salt in blood is low (water potential is high) water moves into cells. Causes cells to swell and can burst (E.g. swelling of brain can cause death)
  ○ Less of ADH will be released
  ○ Large volume of dilute urine will be produced (can be as low as 100 mOsm/KgH2O
• Dehydration
  ○ Increased osmolality o extracellular fluid (lower volume of H2O will concentrate the ions)
  ○ Lots of ADH will be relaced
  ○ More concentrated and smaller volume of urine will be produced (as high a 1200mOsm/KgH2O)
  ○ ADH causes more aqua porins to be embedded in the collecting duct membranes
  ○ ADH also causes urea to be secreted out of the collecting ducts into the interstium (urea recycling)
  ▪ Urea is a very strong osmolyte (salt) lots of water pull
  ▪ ADH causes urea to join the interstitum to increase the pull of water into the interstitum to reabsorb more water○

Question 4

What happens in the absence of ADH?

Answer 4

• People who can’t produce ADH have central diabetes insipidus
• No ADH mans that you are constantly urinating all the water that you drink/ all the water that enters the collecting ducts is being removed(no water is reabsorbed)
• Diabetes insipidus patients need to keep drinking water and feel very thirsty and are prone to dehydration
• Diabetes = urinate a lot
• Diabetes mellitus people urinate a lot to remove the glucose
• Desmopressin can be given to these patients- Desmopressin is a synthetic ADH is stronger than real ADH
  ○ Can also be given to elderly and children to stop nighttime bed wetting as it stops kidneys producing urine
  ○ Usually given to people will diabetes insipidus
  ○ Need to be careful with how much water is drank as they can dilute the blood
  ○ Too much desmopressin or too little water can cause headaches, dizziness, bloated, hyponatremia and seizures