Renal Physiology: salt and water - DeZoysa Flashcards
Approximately how much of a 5L blood volume is plasma?
3L
Sodium and potassium concentrations of intracellular and extracellular fluids
Intracellular: K+ high, salt low
Extracellular: salt high, K+ low
Osmotic concentration
the measure of the solute concentration
Is the number of Osm of solute per litre of solution
Osmolarity is tightly regulated
Plasma osmolarity
Plasma osmolarity is 285-295mOsm/L
Regulated by the balance of salt and water
hyperosmolarity: too much cation and too little water
Hypo: too little cation and too much water
Tonicity
what happens to cells in solution
If the cells take up water from a solution then the solution is hypotonic
If the cells lose water to a solution (shrink) then the solution is hypertonic
If no change in cell size is observed then solution is isotonic.
What is GFR and how much do our kidneys filter per day?
120ml/min
120ml x 60min x 24 hours = > 170L/day
The LoH
Water freely filtered the descending limb and ions filtered out in the ascending limb
Loop diuretics can act in the LoH to induce diuresis
Collecting duct
sodium and water reabsorbed, but this is controlled by antidiuretic hormone
ADH
Vasopressin
Made in the hypothalamus, secreted in the pituitary
Increased production of BP falls or osmolarity increases
Increases the absorption of water
Increases BP and reduces osmolarity
Aldosterone
a mineralocorticoid
acts on the DCT and collecting ducts
Increases sodium reabsorption and K+ excretion.
Stimulated by potassium and angiotensin II
Renin- angiotensin
the junta glomerular apparatus senses decreased renal perfusion and secretes renin
Renin increase production of angiotensin
Angiotensin II - causes
vasoconstriction
ADH release stimulated
Sodium reabsorption in proximal tubule
Thirst
Lowers GFR by contraction of measngial cells thus reducing the area for glomerular filtration
Also increases GFR by contraction of efferent arteriole
Stimulates release of aldosterone
Hypernatremia
impaired thirst / level of consciousness
No access to water
Burns/ diarrhoea / blood loss
Solute diuresis
Hypernatremia due to diabetes insipidus
Reduction in the amount or efficacy of ADH
Polyuria and water loss
Dilute urine (<200mOsm/Kg)
Patient cant drink enough water to keep up with losses
Elevated plasma osmolarity, hypernatremia, dehydration
Types of DI
central - 50% from traumatic brain injury
Nephrogenic
- problem with aquaporion channels in the kidney
- Partial of complete resistance to ADH
Hyponatremia - causes
- what to do?
excessive sodium loss
Excessive water retention
- Check urine osmolarity: if <100mosm/kg = very dilute urine
Consistent with polydipsia/water intoxication
Ddx-Psychotropic drugs
schizophrenia
beer potomania
Pseudohyponatremia
check serum osmolarity
if normal the patient has pseudohyponatremia (may be problems with the way the lab is measuring salt, especially if the glucose is really high)
hypovolaemic
Dehydrated. Urine sodium <20mmol/L
sodium loss but relatively less water loss
- diarrhoea, vomiting
- bowel obstruction
- Skin losses, burns/ sweating
- urinary loses ( diuretics, addisons disease, ketonuria, osmotic diuresis, RTA)
Hypervolaemic
fluid overloaded Sodium retention BUT relatively more water retention - cirrhosis - nephrotic syndrome - Heart failure - Renal failure
Examples of patients with hyponatremia but who are euvolaemic
SIADH (syndrome of innapopriate ADH)
Endocinopathies (Hypothyroid/ low cortisol)
Diuretics
Fluid replacement
SIADH
Inappropriate ADH secreted in absence of normal stimuli such as Low BP
Body accumulates too much water (stored in cells so patient doesn’t appear to be overloaded)
Urine osmol: not low, usually greater than 150mosml/Kg
Urine sodium: not low (>20mmol/L)
Plasma osmolarity: low
Symptoms of hypoonatremia
depends on how quickly it has developed Slow: --> brain adaptation - confused, not quite self Rapid: cerebral oedema - confusion, seizures, coma
Brain adaptation
water causes cerebral oedema (much less of a problem if it develops slowly)
Over time brain cells adapt
resulting in correction of cerebral oedema
Treatment of hyponatremia: rapid versus slow onset
rapid:
- may need more vigorous treatment
- fluid restriction
- normal saline
- 3% (hypertonic) saline in ICU
Slow onset
- Correct gradually
- generally just fluid restriction
- no more than 8mmol/L/day
Complications of treatment of hyponatremia
rapid correction - before brain can adapt
Braindehydraiton = central pontine myelinolysis
Compression of myelin sheaths - rapid demyelination (mainly in pons)
- Quadraparesis
- pseudo bulbar palsy
- Lcoked in syndrome
- Irreversible
