Control of Plasma Osmolarity Flashcards
If water intake is less than water excretion, what happens to the plasma osmolarity?
Increases
If water intake is greater than water excretion, what happens to the plasma osmolarity?
Decreases
What is normal plasma osmolarity?
280-310 mOsm/Kg
What senses changes in plasma osmolarity?
Hypothalamic osmoreceptors
What are the two different efferent pathways in response to changes in plasma osmolarity?
ADH -> kidney -> affects renal water excretion
Thirst -> brain -> changes in drinking behaviour
Where are osmoreceptors located?
OVLT of hypothalamus
What causes an increased release of ADH from posterior pituitary?
Conditions of predominant loss of water osmoreceptors in hypothalamus
What does the secretion of ADH do to renal water excretion?
Decreases renal water excretion
Does decreased osmolarity stimulate or inhibit ADH secretion?
Inhibits ADH secretion
If blood volume collapses, is volume or osmlarity more important?
Volume is more important
What induces drinking?
Increases in plasma osmolarity or decreases in ECF volume
What is diuresis?
Increased or excessive production of urine
Does low plasma ADH lead to diuresis or anti-diuresis?
Diuresis
What is central diabetes insipidus caused by?
Plasma ADH levels are too low
Damage down to hypothalamus/pituitary gland
What is nephrogenic diabetes insipidus caused by?
Acquired insensitivity of the kidney to ADH
What happens in diabetes insipidus?
Water is inadequately reabsorbed from the collecting ducts so a large quantity of urine is produced
What characterises SIADH?
Syndrome of inappropriate ADH
Characterised by excessive release of ADH from PP gland o another source
What happens in dilutional hyponatremia?
Plasma Na levels are lowered and total body fluid is increased
Where are AQP1 channels found?
PCT, descending limb
Where are AQP2 channels found?
Apical membrane of distal DCT and apical vessels in collecting duct, principal cells expressed in the presence of ADH
Where are AQP4 channels found?
Basolateral membrane of collecting duct principal cells (potential exit pathways for water entering via AQP2)
Describe how decreases in plasma osmolarity results in diuresis
- No ADH stimulation means no AQP2 in apical membrane, no AQP3/4 on basolateral membrane only of the latter DCT and CD
- Limited water reuptake in latter DCT/CD -excrete lots of urine
- Tubular fluid rich in water passes through the hyperosmotic renal pyramid with no change in water content
- Loss of large amount of hyposmotic (dilute) urine
What happens if plasma osmolarity increases?
Release of ADH causes insertion of AQP2 into apical membrane
Water moves out of CD into hyperosmotic environment if there are AQPs in both the apical and basolateral epithelia
Kidney must absorb as much as water as possible so produces hyperosmotic urine
What nephrons allow us to create the concentration gradient?
Juxtamedullary
Where does ADH act?
DCT and CD
What happens to the permeability of DCT and CD cells to water when there is an increase in plasma osmolarity?
ADH release means that permeability increases - insertion of AQP2 channels
What is diabetes insipidus characterised by?
This condition is characterised by excessive thirst and secretion of copious amounts of dilute urine.
How come the water absorption in the descending limb is passive?
This is because only ions are reabsorbed in the ascending limb so this creates a ‘salty’ medullary insterstitium and forms a concentration gradient
Where is the maximum osmolarity in the loop of henle?
At the tip
Why does osmolarity of the filtrate increase down the descending loop of henle?
This is because the descending limb is permeable to water but impermeable to Na+
Na+ remains in the descending limb and filtrate concentration increases (as water leaves as you go down)
Why does the osmolarity of the filtrate decrease in the ascending limb of the loop of henle?
Ascending limb actively transports NaCl out of lumen into interstitium
IMPERMEABLE to water
NaCl leaves, H2O remains - osmolarity decreases and the fluid leaving the ascending limb has a low osmolality of 100 mOsm/kg
What happens to the concentration of the interstitial fluid surround the loop of henle as Na+ is actively transported out of the ascending limb?
Increases (see counter current multiplication)
What is an example of an effective osmole?
Urea
Why is urea recycling ADH dependent?
It travels through aquaporins
How does vasa recta conserve the concentration gradient?
Flow in vasa recta is in opposite direction to fluid flow in the tubule so the osmotic gradient is maintained
What is the interstitial concentration at the bottom of the medulla?
1200 mOsm
What is the interstitial concentration at the top of the medulla?
300 mOsm
What is the osmolarity at the top of the ascending limb?
100 mOsm
