Regulation of body fluid osmolality

Question 1

How is ADH secreted and what is its function?

Answer 1

• All body fluids are isomolar at 285mOsm/KgH2O and a variation of +/-3mOsm/KgH2O will cause the body to change something to bring osmolality back
  
  • The only organ that has a varying osmolality is the kidneys where the renal medulla surrounding the Loh and CD osmolality varies/ isn’t at 285 and has a gradient of ions
  • Osmolarity increases if you have lots of salt in the diet will cause cells to shrivel as water will move out of cells by osmosis but Osmolarity decreased you have less of salt in the diet will cause cells to burst as water will move in to cells by osmosis
    1. Stimulus-change in osmolality
    ○ If too much water is drank or not enough salts eaten the osmolality of the blood will become lower than normal (less concentrated)
    ○ If too little water is drank or too much salts the osmolality of the blood will increase (more concentrated)
    ○ ADH will usually respond to osmolality rather than volume
    2. Receptors-detection of osmolality
    ○ Hypothalamus detects a change in osmolality using specialist receptors called osmoreceptors
    § Osmoreceptors are very sensitive to even small changes
    § small change in osmolality makes a bigger difference than a big change in blood loss
    § Hypothalamus regulates the relace of ADH and modulates the H2O retention or loss
    § If there is too much salt in blood you will produce less urine to stop to you getting to salty body (stop concentrating of the blood with salt)
    - More salt=less urination
    - Concentrated blood=more ADH releases
    ○ Osmoreceptors sends impulse to the posterior pituitary gland where the ADH is stored
    3. Control centre- ADH is released
    ○ Hypothalamus makes the ADH but is stored in the posterior pituitary gland as it is unstable when circulation and has a half-life of 10 minuets
    ○ Posterior pituitary gland secrets more ADH is blood osmolality is high and less is blood osmolality is low
    ○ Thirst receptors are in the hypothalamus so when you are slaty you will be thirsty
    4. Effector- change to DCT and CD
    ○ More ADH released If osmolality is high
    ○ Less ADH released if osmolality is low
    ○ If you get and ADH question think osmolality (and vice versa)
    ○ Causes insertion of aquaporins in the collecting duct and DCT which increases the permeability of the impermeable collecting duct (and DCT) to water
    Increases urine concertation

Question 2

How does ADH modify plasma osmolality?

Answer 2

• More ADH released If osmolality is high
  
  • Less ADH released if osmolality is low
  • If you get and ADH question think osmolality (and vice versa)
  • Causes insertion of aquaporins in the collecting duct and DCT which increases the permeability of the impermeable collecting duct (and DCT) to water
  • Increases urine concertation
    *

Question 3

How does ADH lead to aquaporins insertion into collecting ducts?

Answer 3

• Fluid goes down the lumen
  
  1. ADH is released from the posterior pituitary gland and travels to the kidneys in the blood
  2. ADH binds to the V2 receptors outside the cell (vasopressin/ADH receptors) and causes cAMP levels to raise
  3. Causes vesicles with aquaporins to merge with the plasma membrane to release aquaporins (AQP2) onto apical side
  4. More water will be absorbed by osmosis down a concertation gradient

Question 4

How does ADH dilute or concentrate urine produced?

Answer 4

please see notes for the table
* Diabetes insipidus
○ Diabetes insipidus is where the patient can produce ADH
○ 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
○ Picture bellow if from rats with Diabetes insipidus
○ From rats with Diabetes insipidus
○ Central Diabetes insipidus is 1st type
○ Nephro Diabetes insipidus is 2nd type
○ Pictures are from the second type
○ No ADH produced so rats urinate a lot
○

• Drinking Sea water
  ○ Sea water has an osmolality of 2000mOsm/KgH2O urine can reach 1400mOsm/KgH2O
  ○ Shows that if you drink 1 litter of sea water you will have to urinate 1.4 litters to maintain blood osmolality
  ○ Dehydration, muscle cramps, deliria, dry mouth and thirst will be symptoms of sea water drinking