9 Control of Plasma Osmolarity Flashcards
Between what range does urine osmolarity vary?
50-1200 mOsm/Kg
1200 (maximum possible)
What is the normal osmolarity of cells?
280-310 mOsm/Kg
Juxtamedullary nephrons make up 10-15% of the total nephrons in the kidneys. What is there function and how does their structure differ from the cortical nephron?
Responsible for making concentrated urine
How is the vertical concentration gradient in the kidney maintained?
Vasa recta and long loope of henle
Why are there no aquaporin channels in the ascending loop of henle?
Diluting action of filtrate
Removes solute without water
Increase osmolarity in interstitium
In which patients might there be a lack of vertical concentration gradient within the kidney?
Patients on prolonged loop diuretics
Transplant patients
At what point is the osmotic gradient within the loop of henle at its highest?
At deepest point of loop of henle
How is urea reabsorbed from the lumen of the tubule in the proximal convulted tubule?
Small changes in plasma concentration cause small hormonal changes. What happens as a result of these hormonal changes? Where does this hormonal regulation take place?
- Alter water uptake in kidney
- Stimulate thirst
Where?:
Late distal tubule
Collecting duct in nephron
Hormone= aldosterone
Outline the thirst response. (how is it sensed, where and how is it perceived?)
- Perceived:
- Peripherally eg drying of oral mucosa
- Sensed:
- Lateral pre-optic area of hypothalamus- responsive to raised plasma osmotic pressure/reduced ECF volume
Outline the thirst mechanism. (when is it active)
Thrist mechanism active when level of hyperosmotic dehydration surpasses protective capacity of kidneys
What is normal osmolarity in the body?
- 280-310 mOsm/kg
- 290 in interstitial fluid
- 291 in blood plasma due to plasma proteins ==> oncotic pressure
How do you alter plasma osmolarity?
What nephrons are responsible for concentrating urine and how do they do this?
- Juxta medullary long LOH generates gradient
- Vasa recta from efferent arteriole running paralell with Loop of Henle and blood running countercurrent maintains gradient
- Counter current multiplier system
What is the difference in transport between the ascending and descending limb?
When may you see this concentration gradient between the loop of henle and the medullary interstitium and what are the consequences of this?
- Newly transplanted kidney or long term loop diuretics as they block NKCC2 so no gradient can be established
- Lots of dilute urine made
How can urea be used to help reabsorb more water?
- Uptake in the PCT, 50% filtered is taken back
- Under ADH influence, urea reabsorbed from medullary CD
- Urea increases osmotic gradient in the interstitium so more water reabsorbed
- Urea then just taken back up into loop and cycles round
How do we supply blood to the medulla of the kidney without washing away the osmotic gradient needed to reabsorb water?
- Vasa recta with very slow flow
- No active transport just passive absorption through endothelial cells
- Flow opposite direction to tubular fluid flow
- Equilibriates at each level
How is a change in plasma osmolality detected in the body and what are the 2 pathways used to restore osmolarity?
- Osmoreceptors in hypothalamus in OVLT
- Cells in supraoptic nucleus containing baroreceptors sit close to OVLT so if low pressure means low volume and means high osmolarity so ADH secreted
- (organum vasculosum of the lamina terminalis)*
What is the plasma osmolarity feedback loop?
Always a little bit of ADH, never 0
How does the effectiveness of ADH change with changes in plasma volume?
Changes in B.P (plasma volume) have an effect on response to changes in osmolarity
What happens once you have corrected a high plasma osmolality by taking a large drink?
What is diabetes insipidus? What are the 2 types of diabetes insipidus?
What is SIADH and how can it cause issues with plasma osmolarity?
What will happen to the osmolality of urine when plasma osmolality increases/decreases?
What effect does ADH have on the receptors in the collecting duct?
- Always AQP on basolateral membrane but not apical
- When ADH present, AQP from vesicles are added to apical membrane
How may someone with hyponatraemia present?
- Can be confused, lethargic, muscle paralysis, blurred vision, muscle cramps
- Serum conc lower than 135mmol
How may someone with hypernatraemia present?
What are the three processes involved in the counter-current multiplier system?
- Active secretion of NaCl
- Urea recycling
- Vasa recta maintaining gradient
What are some causes of hyponatraemia?
- Severe diarrhoea and vomiting (losing water too so no osmolality change)
- Diuretics/Renal failure
- Peritonitis
- Burns
- Na/Water imbalance
- Anything that changes ADH secretion
What are some causes of hypernatraemia?
Why do we need to be careful when treating someone with hyponatraemia?
If we rapidly correct Na levels then rapid rise of Na pulls water from neurones, especially in brainstem, so neurones shrink, leading to
CENTRAL PONTINE MYELINOLYSIS
How does heart failure lead to hyponatremaia?
Too much water in ECF
What are some causes of hypovolemic hyponatremia?
When would nephrogenic diabetes insipidus present?
- As baby
- Salt restrict, give thiazide diuretics and monitor fluid balance
- DONT GIVE NORMAL SALINE
Fill in the following table with true or false.
If you had a low plasma osmolarity what might you crave?
SALT
If someone presented with the following blood parameters what may you think the diagnosis is and how may you treat it?
- Serum osmolality 259 (decreased)
- Urine osmolality 522 (decreased)
- Urine Na 81 (increased)
- SIADH, kidneys are uneccessarily diluting urine
- Fluid restrict
- Wean off any meds like valproate that cause inappropriate ADH release
What are some conditions that can change ADH secretion from the hypothalamus?
- SIADH
- Heart failure
- Liver/Kidney disease
- Tumours e.g small cell lung
- Meds e.g diuretics, PPIs, ACE inhibitors
Apart from fluid restriction, how can you treat hyponatremia?
Infusion of hypertonic saline and furosemide
If a patient has abnormal serum sodium what are three things you need to establish?
- Patient’s volume status?
- How much sodium being lost in urine?
- Is patient symptomatic?
A 30 year old woman has been feeling light headed and nauseated for the past two days, she has not eaten or drunk much due to this and her B.P is 90/50, how do the macula densa cells respond?
Stimulate JGA to release renin
How does the ENaC drive further reabsorption in the DCT?
Not electroneutral so drives paracellular transport of Cl-
A 36 year old man is suspected to have primary hypersaldosteronism, what would you expect his U and E’s to be?
- Increased Na, decrease K+
- Normal urea
- Decreased renin due to hypertension
Conn’s syndrome
