Urinary: Control of Plasma Osmolarity Flashcards
How is a change in plasma osmolality detected?
Detected by osmoreceptors in the organum vasculosum of the lamina terminalis (hypothalamus)
Act via 2 efferent pathways:
Secretion of ADH or thirst (there has to be significant decrease in volume for us to feel thirsty)
When is ADH released?
ADH is released from the posterior pituitary when there is predominant loss of water
ADH concentration in the plasma is never 0 - there is always a small maintenance amount present
When does the body not act to maintain osmolality?
When there is a big decrease in effective circulating volume eg massive haemorrhage. The kidneys continue to conserve water even though this will reduce osmolality because volume is more important.
What is the ultimate compensation for a decrease in osmolality?
Large deficits in water are only partially compensated for by the kidney - the ultimate compensation is ingestion (thirst).
We stop feeling thirsty when sufficient fluid has been consumed - even though the water has not yet been absorbed by the GI tract. Mechanism unknown
What is the effect of thirst on the collecting duct?
Increases permeability of the CD to water and urea
What are some medical conditions that affect ADH secretion?
Central diabetes insipidus: ADH levels too low
It is due to damage to the hypothalamus or pituitary gland
Nephrogenic diabetes insipidus: an acquired insensitivity of the kidney to ADH. Can be managed by ADH injections or ADH nasal spray
SIADH (syndrome of inappropriate ADH secretion): excessive ADH release from posterior pituitary or other source. Dilutes plasma excessively
When is there diuresis and anti-diuresis?
Diuresis occurs when plasma osmolality decreases
Anti-diuresis occurs when plasma osmolality increases
What is the role of the juxtamedullary nephron and vasa recta in the osmotic gradient of the parenchyma?
The juxtamedullary nephron’s long loop of henle creates the vertical osmotic gadient via a counter current mechanism.
The vasa recta maintain the gradient
Why is urea an ‘effective osmole’ in the kidney?
Urea is not freely permeable therefore is effective at exerting an osmotic force across the membrane.
How is urea recycled?
Urea is reabsorbed from the medullary collecting duct in the presence of ADH.
Urea moves out of the CD with water through AP channels and then diffuses back into the filtrate in the thin ascending limb down its conc gradient.
Acts to increase osmolarity of the interstitium.
Describe the mechanism of the counter current multiplication eg how it creates a vertical conc gradient
The thick asc limb has very effective NaK2Cl transporters which pump out NaCl into the interstitium - increasing the osmolarity.
The desc limb are very permeable to water so water moves out into the interstitium until the osmolarity of the filtrate matches that of the interstitium.
(now imagine the fluid moves round)
The NaK2Cl transporters now pump out more NaCl which increases the osmolarity of the interstitium even more, and then water leaves the desc limb to match the osmolarity.
(repeating the process again and again creates a vertical gradient - max 1200mOsm at the tip)
How do the vasa recta maintain the vertical conc gradient?
The vasa recta blood flow is in the opposite direction to the flow of the filtrate in the loop of henle.
The osmolarity of the blood matches that of the interstitium because there is no active transport in the vasa recta (so NaCl enters blood and water leaves).
Therefore the asc portion of the vasa recta takes in the water leaving the desc limb of the LoH so the water doesnt ruin the vertical conc.
