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Flashcards in Salt and water balance Deck (13)
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1
Q

What is the normal concentration of Na+ and where does it reside?

A

it is normally 140mmol/L extracellularly

It diffuses down its concentration gradient into cells but is pumped out by the ATPase

2
Q

What are the main determinents of plasma osmolality?

A

sodium and potassium

3
Q

What are the two major systems that regulate sodium?

A

The ADH system, this alters the amount of water in the body by increasing thirst drive and increasing water reabsoption in the nephron. This is in response to osmolarity in the hypothalamus.
The RAAS system, aldosterone causes increased reuptake of sodium in the distal tubule and hence water reabsorption.

4
Q

What are the three situations that hypernatraemia occurs in?

A

Hypovolaemia - can be due to diabetes insipidus, dehydration, diarrhoea, vomiting, burns
Sodium retained in excess of water - low water intake common cause, causes renal underperfusion and RAAS activation. Can be caused by Conn’s or Cushing’s
Artefactual - due to sudden Na+ increase e.g. saline infusion

5
Q

What are the two types of diabetes insipidus?

A

Central where there is an inability of the pituitary to produce ADH, Nephrogenic is an inability of the kidney to respond to ADH

6
Q

What are the main causes of hyponatraemia?

A
  • Excess water e.g. oedematous, SIADH, excess drinking

- Increased sodium loss e.g. osmotic diuresis, if hypotonic fluid is given in vomiting etc.

7
Q

What will be the plasma osmolality, urine osmolality and urine sodium if there is hyponatraemia from SIADH?

A

Plasma osmolality - Low/normal
Urine osmolality - High
Urine sodium - >20 - due to sodium remaining in urine being more concentrated due to less water

8
Q

What will be the plasma osmolality, urine osmolality and urine sodium if there is hyponatraemia from excessive drinking?

A

Plasma osmolality - Low
Urine osmolality - Low
Urine sodium - >20 - excreting sodium to try and draw water out of body into urine

9
Q

What will be the plasma osmolality, urine osmolality and urine sodium if there is hyponatraemia from hypovolaemia with no renal Na+ loss?

A

Plasma osmolality - High
Urine osmolality - high
Urine sodium - Low

10
Q

What will be the plasma osmolality, urine osmolality and urine sodium if there is hyponatraemia from Shypovolaemia with renal Na+ loss?

A

Plasma osmolality - high
Urine osmolality - high
Urine sodium -&raquo_space;20

11
Q

If there is high urine sodium in hyponatraemia which tests can be done to distinguish the causes?

A

Short synacthen test (artificial ACTH) to look for pituitary causes, plasma glucose and cortisol

12
Q

What are the clinical findings in hyponatraemia?

A

CNS dysfunction as volume of brain decrease, confusion, malaise, seizures and coma

13
Q

How is hyponatraemia treated?

A

Very slow infusion of 0.9% saline as this is to avoid pontine meylinolysis (damage to myelin sheaths)