Session 5 - Control Of Plasma Osmolarity Flashcards
What is the normal concentration of plasma osmolarity?
280-310 mmol/L
Where are changes in osmolarity sensed?
Hypothalamic osmoreceptors
What happens when the osmoreceptors in the hypothalamus detect increased osmolarity?
Short term pathway:
- ADH is released from the posterior pituitary
- acts on the kidney to increase the permeability of the collecting duct to water
- more water is reabsorbed and less water is excreted
Long term pathway:
- thirst behaviour is activated
- increased water intake through drinking
What affects does decreased osmolarity have on ADH excretion?
Inhibits ADH secretion from the posterior pituitary
What effect does ADH have on the collecting duct?
Increases the permeability of the collecting duct to:
- water (more aquaporin 2 in the apical membrane)
- urea
Which aquaporin is found in the apical membrane of the collecting duct?
AQP2
Which aquaporins are found in the basolateral membrane of the collecting duct?
AQP3
AQP4
What two factors does the collecting duct use to vary the concentration of urine excreted?
The osmotic gradient of medulla
Antidiuretic hormone
How is the medullary osmotic gradient generated?
Counter Current Multiplication:
Ascending limb of loop of henle is impermeable to water but actively pumps NaCl out of the tubular lumen and into the interstitial fluid.
Concentration of sodium in the interstitial fluid surrounding the loop of Henle increases, giving it a higher osmolarity.
There is now a big difference in osmolarity between the hypo-osmotic fluid in the lumen and the hyperosmotic fluid in the interstitium, creating the osmotic gradient.
Why is urea an effective osmole in the kidney?
Urea is not readily permeable across the lipid bilayer of cells, so can effectively exert an osmotic force as it wont just diffuse down and dissipate its concentration gradient.
What is an ineffective osmole?
A solute that moves freely across a plasma membrane so is therefore ineffective at exerting an osmotic force.
Describe what happens in recycling of urea.
Urea is reabsorbed in the medullary collecting duct.
The cortical collecting duct is impermeable to urea.
Urea diffuses across the interstitium and back into the loop of Henle.
What is the purpose of the vasa recta?
Maintains the concentration gradient of the medulla created by the loop of Henle. Acts as the counter current exchanger.
How does the vasa recta maintain the concentration gradient of the medulla?
Blood flow through the vasa recta is in the opposite direction to fluid flow through the tubule.
In the descending limb of the vasa recta, Na+, Cl- and urea diffuse into the lumen of the vasa recta. Osmolarity of the blood increases as it reaches the tip of the hairpin loop.
In the ascending limb, blood in will now have a higher solute content than the interstitium. Water will therefore move into the ascending limb of the vasa recta from the descending limb of the loop of henle.
Therefore the vasa recta maintains the hyper osmolarity of the renal medulla.
What is central diabetes insipidus?
Insufficient secretion of ADH from the posterior pituitary. This causes inadequate reabsorption of water from the collecting duct creating large volumes of dilute urine.
