8. Control Of Plasma Osmolarity Flashcards
Where are osmoreceptors located?
In the OVLT of the hypothalamus.
How do osmoreceptors sense changes in plasma osmolarity?
Via fenestrated leaky endothelium exposed directly to systemic circulation.
What two secondary responses are mediated via two pathways triggered by increased plasma osmolarity detection by osmoreceptors?
Concentration of urine (ADH).
Increased water intake (thirst).
What anatomical relationship allows the OVLT to stimulate the production of ADH?
Lies close to the supraoptic nucleus of hypothalamus (where ADH is made), with input from baroreceptors.
When there are changes in blood volume and osmolarity, which is conserved by the body preferentially?
Plasma volume.
What is the analogue of thirst?
Salt ingestion.
What are the two types of salt appetite?
Hedonistic appetite.
Regulatory appetite.
How much of a change in plasma osmolarity is needed to stimulate ADH release?
1% increase.
How much of a change is plasma osmolarity is needed to stimulate the thirst response?
Increase in osmolarity of 10% (or decrease in volume).
Where is ADH released from?
Posterior pituitary.
What does ADH increase the permeability of the collecting duct to help control osmolarity?
Water and urea.
What condition results from plasma ADH levels being too low?
Central (ADH release affected) Diabetes Insipidus.
What conditions results from acquired kidney insensitivity to ADH?
Nephrogenic diabetes insipidus.
How would you manage diabetes insipidus clinically?
ADH injections or ADH nasal spray treatments.
Give 2 pathologies that can result in plasma ADH levels being too low.
Damage done to hypothalamus or pituitary
gland.
A brain injury, particularly a fracture of the base of the skull.
A tumour.
Sarcoidosis or tuberculosis.
An aneurysm.
Some forms of encephalitis or meningitis.
The rare disease Langerhans cell histiocytosis
What conditions results form excessive release of ADH from the posterior pituitary gland or another source?
Syndrome of inappropriate antidiuretic hormone secretion (SIADH).
What happens to plasma sodium levels and total body fluid in increased ADH release?
Dilutional hyponatraemia in which the plasma Na+ levels are lowered and total body fluid is increased.
What happens to aquaporin expression in the kidneys if plasma osmolarity decreases?
No ADH stimulation, no AQP2 in apical membrane, AQP3 and AQP4 on basolateral membrane only in the latter distal convoluted tubule and collecting ducts. Diuresis.
What happens to aquaporin expression in the kidneys if plasma osmolarity increases?
ADH release, insertion of AQP2 channels into the apical membrane of the collecting duct, to increased water reabsorption (needs a hypertonic interstitium).
Hyperosmotic urine produced.
What is the medullary counter current mechanism?
The entire functional organisation in the medulla to result in the concentration of urine.
What 4 mechanisms make up the medullary counter current mechanism?
Juxtamedullary nephron – long loop of Henle establish (create) the vertical osmotic gradient.
Vasa recta help to maintain (preserve) this osmotic gradient.
Collecting duct of all nephrons use the gradient along with hormone ADH (vasopressin) to produce urine of varying concentration.
Urea also helps in urine concentration mechanism.
What features of the loop of Henle lead to counter current multiplication?
Descending limb is highly permeable to water due to AQP1, but not permeable to Na+.
Ascending limb actively transports NaCl out of the tubular lumen into the interstitial fluid, but is impermeable to water.
So the interstitial fluid is isotonic at the corticomedullary border, and the medullary interstitium is hyperosmotic at the papilla.
What is an effective osmole?
Solutes which are not able to freely cross the nephron membrane, making them able to exert an osmotic force across the membrane. Eg urea.
What happens in the recycling of urea?
Cortical collecting duct cells are impermeable to urea. ADH causes the insertion of urea channels into the collecting duct, so that urea is reabsorbed (helps water to also be reabsorbed). The urea then moves into the interstitium and diffuses back into the loop of Henle.
What is the maximum osmolarity at the tip of the loop of Henle?
1200 mOsm/Kg.
The concentration gradient in the kidney is produced by the loop of Henle acting as a counter current multiplier, but what maintains it?
The vasa recta acting as a counter-current exchanger.
What feature of vasa recta blood flow allows the osmotic gradient to be maintained?
Blood flow in the vasa recta is in the opposite direction to fluid flow in the tubule.
Describe what happens in the descending and then ascending limb of the vase recta?
Isosmotic blood in vasa recta enters hyperosmotic milieu of the medulla ( high conc. Na+ ions, Cl- ions and urea). Na+, Cl- and urea diffuse into the lumen of vasa recta. The osmolarity of the blood in vasa recta increases as it reaches tip of hairpin loop.
Blood ascending towards cortex will have higher solute content than surrounding interstitium and so water moves in from the descending limb of the loop of Henle.
Why does blood flow slowly through the vasa recta?
Lower total plasma blood flow (5-10% of RPF), but still needs to deliver nutrients and maintain medullary hyper-tonicity.
