S5 Control Of Plasma Osmolarity Flashcards
What changes plasma osmolarity?
ADH and thirst.
ADH decreases renal water excretion. It is the faster, short term unconscious response.
Thirst is caused by plasma osmolarity being too high, so the kidneys are not able to restore it themselves
What are osmoreceptors?
Receptors in the hypothalamus which sense changes in osmolality.
Has a fenestrated leaky endothelium which is exposed to systemic circulation on the plasma side of the BBB. Signals for pathways leading to concentrated urine and thirst.
What is the role of ADH?
Released from the posterior pituitary and is sensitive to osmolality changes.
Acts on the kidney to reduce water excretion by increasing permeability of collecting ducts to water and urea by inserting AQP2 channel.
Describe how osmoreceptors also stimulate thirst
Activated when you’re 10% dehydrated.
A large deficit in water/sodium is only partially compensated for by kidney, so thirst is stimulated. Drinking is induced by increases in plasma osmolarity or by decreases in ECF volume.
There are 3 diseases caused by ADH secretion issues, describe central diabetes insipidus
Central diabetes insipidus: plasma ADH levels are low due to; damaged hypo or pituitary gland, brain injury, tumour, aneurysm
There are 3 diseases caused by ADH secretion issues, describe nephrogenic diabetes insipidus
Nephrogenic diabetes insipidus: acquired insensitivity of the kidney to ADH
There are 3 diseases caused by ADH secretion issues, describe SIADH
SIADH-syndrome of inappropriate ADH secretion from the posterior pituitary, leads to dilutional hyponatraemia where plasma Na+ levels are lowered and the total body fluid is increased
Where are aquaporins 1-7 found?
AQP 1+7: PT and thin descending limb
AQP 2,3+4: CD
What happens when osmolarity changes?
Decreases: limited water reabsorption, leads to diuresis
Increases: kidney must reabsorb water, ADH inserts AQP into apical membrane of CD, less water in urine as it is reabsorbed
What is counter current multiplication?
Generation of a vertical concentration gradient in the kidney
How is the counter-current multiplication set up?
In the descending LoH juxtamedullary nephrons, AQP1 is open and permeable to water to leave, but not to Na so osmolarity in descending limb increases.
Max is 1200 at the tip.
In the thick ascending limb, Na leaves but water stays (hypo-osmotic), so osmolarity in ascending limb decreases (around 100)
What is the function of vasa recta?
Maintains the CC mechanism by acting as a counter current exchanger.
Flow in VR is in the opposite direction to tubule; in the descending vasa recta Na enters, in the ascending limb, water enters.
Describe the vertical osmotic gradient
Large gradient in the interstitial fluid of the medulla, isotonic at the corticomedullary border (300) but hyperosmotic at the papilla (1200).
Why is urea considered an effective osmole?
Urea is hydrophilic so doesn’t freely cross lipid barriers
What can cause hyponatraemia?
Changes in ADH secretion
Heart failure
Kidney or liver disease
Drugs - Thiazide diuretic, PPIs, ACEi
