J de Zoysa: Renal Physiology; salt and water Flashcards
Body composition consisting of water?
Describe the two main components…
60% of our body. (42L)
This is divided in ICF and ECF
ECF:
- many separate compartments
- all external to the cell membrane
- have similar composition
ICF
- many separate compartments
- all within the cell membrane
- all have a similar composition
Normal fluid compartments
What is the interstitial fluid (ECF)
- The fluid which lie in the interstices of all body tissues
- The interstitial fluid bathes all the cells in the body and is the link between the ICF and the intravascular component
- O2 , nutrients, wastes and chemical messengers all pass through the interstitial fluid
What is the transcellular fluid (ECF)?
- small compartment (0.5-1L) that reps all body fluids which are formed from transport activities of cells
- It is contained within epithelial lined spaces
- includes
- CSF, joint fluid, pericardial fluid, aqueous humour etc
Our intravascular component is ~5L, and a couplle of kilograms of this is….
Our RBC’s
Distribution of Na and K in ECF and ICF?
Why is this??
ECF: Na+ high and K+ low
ICF: Na+ low and K+ high
- active pumping forces drive this i order to maintain electrical gradients!
What is Molarity and what’s its constant.
What is Osmolarity?
Molarity:
- Molarity: the number of moles per litre
- 6.022 x 1023 Avogadro’s constant
Osmolarity:
- Measure of solute concentration
- It is the number of Osm of solute per litre of solution
- Osmolarity is tightly regulated
Describe plasma osmolarity..
- Plasma osmolarity is 285-295 mOsm/L
- Regulated by the balance of salt and water
- Hyperosmolarity is defined by too much cation and too little water
- Hypo osmolarity is defined by too little cation and too much water
Describe Tonicity
- TOnicity of a solution refers to what happens to cells in a solution
- If the cells take up water from a solution (ie; swell), then the solution is hypotonic
- If the cells looses water to a solution (ie; shrink) then the solution is hypertonic
- If no change is cell size is observed then the solution is isotonic
What filters water and by how much daily
- In the nephron
- renal artery → arterioles → afferents → glomerulus → efferents
- Regulation of water in prox. tubule, LOH and distal tubule
- Most of the filtration occurs at the glomerulus. free filtration of salt and water occurs here
- GFR: 120ml/min
- 120 x 60 x 24 hours = more than 170L/day
- 200-300mg of protein filtered a day
Where does most of the salt and water reabsorbtion occur?
In the proximal tubules. 65-75% of sodium and water are reabsorbed here
Diuresis drugs are most effective here
Describe the loop of Henle
The descending limb freely filters water
The ascending limb freely filter salt (15-20%)
Diuretics such as frusemide and diurismide act on the LOH luminal surface
The distal convoluted tubule and the collecting tubules
DCT
- Little bit of salt reabsorbed (5%)
- Least effective diuretics act here!
CT
- Salt (5%) and water reabsorbed
- controlled by ADH
- prevents diuresis
Describe ADH and what it does
- Known as vasopressin
- Binds to the vasopressin receptor and affects water in/output
- Made in the hypothalamus, secreted from the pituitary
- increased production in response to BP drop or osmolarity increase
- Increases the reabsorption of water
- Increases BP and reduces osmolarity
Describe aldosterone and what it does
- A mineralocorticoid hormone
- acts on the distal convoluted tubules and the collecting ducts
- Increases sodium reabsorption and potassium excretion and angiotensin II
- Binds to Aldosterone receptor and affects the Na/K ATPase pump
What is the RAAS
When the juxtaglomerular apparatus senses decreased renal perfusion and secretes renin
Renin increases production of angiotensin and angiotensin II
What does Angiotensin II do?
- Potent vasoconstriction
- ADH release stimulated
- Sodium reabsorption in proximal tubule
- Thirst
- Lowers GFR by constriction of mesangial cells, thus reducing the area for glomerular filtration
- Also increases GFR by constriction of efferent arteriole
- Stimulates release of aldosterone
Disorders of sodium are
Abnormality of sodium or and abnormality of water (or both!)
- Hypernatraemia Na > 145mmol/L
- Normal Na 135-145 mmol/L
- Hyponatraemia Na <135mmol/L
Disorders of Sodium: Hypernatraemia
- Impaired thirst/level of consciousness
- no access
- burns/diarrhoea/bloodloss
- Solute diuresis (HONK/DKA)
Can be due occasionally to diabetes insipidus
Disorders of sodium: diabetes insipidus
Huge amount of dilute urine and inc. thirst
- Reduction in amount or efficacy of ADH
- Polyuria and water loss
- Dilute urine (<200mOsm/kg)
- Patient can’t drink enough water to keep up with losses
- Elevated plasma osmolality, hypernatraemia, dehydration
Can be central (50% from traumatic brain injury or pituitary hormone issues) or nephrogenic (problem with aquaporin channels in kidney, partial or complete resistance to ADH)
Disorders of salt: Hyponatraemia
From either
- Excessive sodium loss
- excessive water retention
*pseudohyponatraemia: can be due to lab issues, check the serum osmolarity and only if normal do they have pseudohyponatraemia
What do you do if the patient is hyponatraemic?
- Check urine osmolality
- if low (<100mosm/kg), then they will be passing lots of very dilute urine
- Consistent with polydipsia/water intoxication
- Ddx-psychotropic drugs
- schizophrenia
- beer potomania
It is very important to check the patients fluid levels!!
Describe what you would see with a hypovolaemic patient
- Check pulse,urinary losses BP, tissue turgor, JVP etc
- Dehydrated. urine sodium <20mmol/L
- Sodium loss but relatively less water loss
- diarrhoea, vomiting
- bowel obstruction
- skin losses- burns
- Diuretics
- addisons disease
- ketonuria osotic diuresis, RTA
- Diuretics
Describe Hypervolaemia
- Fluid overloaded
- Sodium retention BUT relatively more water retention
- cirrhosis
- nephrotic syndrome
- HF
- Renal failure
How can you be hyponatraemic and euvolaemic?
- SIADH (syndrome of inappropriate ADH)
- endocrinopathies (hypothyroid/low cortisol)
- check if they’re hyperthyroid or hyperadrenal
- Diuretics
- Fluid replacement
Describe SIADH
- Inappropriate ADH produced in absence of normal stimuli such as low BP
- Body accumulates too much water (stored in cells so patient doesn’t appear to be fluid overloaded)
- Urine osmol; not low- usually greater than 150mosmol/kg
- Urine sodium; not low >20mmol/L
- Plasma osmolality: low
Cause of SIADH
- Trauma (inc surgery)
- Tumours (eg; lung)
- Chronic lung disease
- Head injury
- Medications (eg; SSRIs)
Symptoms of Hyponatraemia are…
Depends on how quickly it’s developed!
Common in the elderly
Slow: brain adaptation; confused, not quite self
Fast: cerebral oedema: confusion, seizures, coma
- Water gain leads to cerebral oedema
- Over time braincells adapt
- Results in correction of cerebral oedema
Treatment of Hyponatraemia
Also depends on speed of onset
THis requires constant check-ups, updates etc
What can happen if you don’t treat hyponatraemia correctly?
- Too rapid → brain doesn’t have time to adapt → brain dehydration = Central Pontine Myelinolysis
- Compression of myelin sheaths; rapid demyelination (mainly in pons)
- quadriparesis
- pseudobulbar palsy
- Locked in syndrome
- Irreversible
