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