5. Plasma Osmolarity Flashcards
When does plasma osmolarity increase?
When water intake < excretion.
When does plasma osmolarity decrease?
When water intake > excretion.
What is body fluid osmolarity kept at?
275-295mOsm/kg.
How do disorders of water balance manifest?
Changes in body fluid osmolatiry.
How do disorders of Na+ balance manifest?
Changes in volume.
What are changes in plasma osmolarity detected by?
Hypothalamic osmoreceptors.
Where are hypothalamic osmoreceptors located?
Organum vasculoum of the laminae terminalis (OVLT).
Where is the OVLT?
Anterior and ventral to the third ventricle.
How is the OVLT exposed to the systemic circulation?
It has a fenestrated leaky epithelium.
How does the OVLT respond to increase in plasma osmolarity?
Concentrate urine and increase thirst respectively.
When do you begin to feel thirsty?
At 10% dehydration.
How does the posterior pituitary respond to increase in plasma osmolarity?
Releases ADH.
What is the structure of ADH?
It is a small peptide, 99 amino acids long.
What is the role of ADH?
It acts on the kidney to regulate the volume and osmolarity of the urine by increasing permeability of the kidneys to water and urea.
How does ADH affect the apical membrane of the nephron’s collecting duct?
It adds the water channel aquaporin-2 so water can be reabsorbed to decrease plasma osmolarity.
Which aquaporin channels are present on the apical and basolateral membranes?
Apical - aquaporin-2 in presence of ADH.
Basolateral - aquaporin-3, and aquaporin-4 always.
What is the implication of aquaporin-3 and 4 channels always being present on the basolateral membrane?
It is always permeable to water so water that enters across the apical membrane can always pass into the peritubular blood.
How does ADH affect the medullary part of the collecting duct?
It increases permeability to urea so it is reabsorbed and water folllows.
What happens when urea concentration rises?
It passively moves down its concentration gradient into the ascending limb. It then passes back into the collecting duct to be reabsorbed in the medullary portion - i.e. is recycled.
What is SIADH?
Syndrome of inappropriate anti-diuretic hormone secretion. It is secretion of ADH not inhibited by lowering of blood osmolarity.
What is the consequence of SIADH?
An excessive amount of water is retained so blood osmolarity drops and there is hyponatraemia.
What are symptoms of hyponatraemia?
Nausea and vomiting, headache, confusion, lethargy, fatigue, appetite loss, restlessness and irritability, muscle weakness, spasms, cramps, seizures, and decreased consciousness or coma.
How can hyponatraemia caused by SIADH be treated?
ADH receptor antagonists.
How does gradient of osmolarity change over the medullar?
It increases as you descend.
What sets up the osmotic gradient of the medulla?
Active transport of NaCl out of the thick ascending limb and the recycling of urea.
What is the crucial action of the thick ascending limb?
It removes solute without water, diluting the filtrate and increasing intersticium osmolarity.