Salt and water balance

Question 1

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

Answer 1

it is normally 140mmol/L extracellularly

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

Question 2

What are the main determinents of plasma osmolality?

Answer 2

sodium and potassium

Question 3

What are the two major systems that regulate sodium?

Answer 3

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.

Question 4

What are the three situations that hypernatraemia occurs in?

Answer 4

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

Question 5

What are the two types of diabetes insipidus?

Answer 5

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

Question 6

What are the main causes of hyponatraemia?

Answer 6

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

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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

What are the clinical findings in hyponatraemia?

Answer 12

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

Question 13

How is hyponatraemia treated?

Answer 13

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