Salt and water balance

Question 1

How is plasma (ECF) osmolality determined?

Answer 1

2[Na+] + [K+] + glucose + urea

Question 2

What are the causes of hypernatraemia?

Answer 2

Sodium retention in excess of water…

1) Low water intake (Can have significantly elevated sodium)
2) Primary hyperaldosteronism (Conn’s)
3) Cushing’s disease (hypercortisolism)

Water loss in excess of sodium…

1) Diabetes insipidus
2) Diarrhoea, vomiting, burns, haemorrhage

Question 3

What are the two types of diabetes insipidus?

Answer 3

CENTRAL –> Posterior pituitary gland does not produce ADH/vasopresin so body cannot reabsorb water at the kidneys

NEPHROGENIC –> Kidneys do not respond to ADH/vaspopressin

Question 4

What can cause hyponatreamia?

Answer 4

Total body sodium normal or high (LOTS OF WATER)

1) Oedematous states - nephrotic syndrome, cardiac / renal /liver failure
2) Syndrome of inappropriate ADH (SIADH)
3) Excessive drinking (psychogenic polydipsia)

Total body sodium low

1) Renal – osmotic diuresis (DKA; HONK), diuretic use, hypocortisolism (Addison’s)
2) Non-renal – diarrhoea, vomit, burns,

Question 5

What is SIADH?

Answer 5

Syndrome of inappropriate ADH production.

The body makes too much antidiuretic hormone (ADH) –> Leads to the retention of too much water

Question 6

What plasma osmolality, urine osmolality, and urine sodium results would you expect in addison’s disease?

Answer 6

Plasma osmolality –> LOW (due to insufficient mineralcorticoid

Urine osmolality –> HIGH (In proportion to plasma osmolality)

Urine sodium –> HIGH

Question 7

What is the Synacthen test?

Answer 7

A test of adrenal insufficiency.

Based on the measurement of serum cortisol before and after an injection of synthetic ACTH.