Salt + Water Balance

Question 1

What are the average daily requirements of a person?

Answer 1

2-3 litres of water
100-200 mmol NA
60-80mmol K

Question 2

How does the composition of Intracellular fluid differ from extracellular fluid?

Answer 2

ICF:
Na+ = 12mmol/L
K+= 150 mmol/L

ECF:
Na+= 140mmol/L
K+= 4 mmol/L 
Glucose= 5mmol/L
Urea= 5mmol/L

Question 3

How can you work out the osmolality of ECF (plasma) ?

Answer 3

2x [Na+] + [K+] + glucose + urea == 300 mmol/L

Question 4

How is the ECF Na+ concentration determined?
What are the normal reference values?
What are the consequences of increased water volume?

Answer 4

ECF sodium concentration/ ECF water volume

133-146 mmol/L

Decreased sodium concentration

Question 5

How is sodium concentration regulated?

Answer 5

Indirectly by regulating plasma water volume via antidiuretic hormones and aldosterone

Question 6

How does ADH regulated plasma water volume?

Answer 6

Increased plasma osmolality due to fluid depletion detected by osmoreceptors in hypothalamus

ADH stimulated to be released from posterior pituitary

Acts on collecting duct to increase permeability via aquaporins to increase water re-absorption

Fluid excess causes low plasma osmolality which inhibits ADH release and promotes increased excretion of fluid

Question 7

Why is aldosterone released and how is it involved in regulation of plasma water volume?

Answer 7

Fluid depletion causes decreased ECF vol and decreases BP
Arterial baroreceptors stimulate symp NS to activate to cause vasoconstriction
Leads to decreased renal artery perfusion which is detected by juxtaglomerular apparatus
JGA stimulates increased renin release= increased angiotensin II == increased aldosterone
Aldosterone acts to increase Na+ reabsorption in the distal CT

Question 8

Would the urine sodium of a fluid depleted patient be high or low? Describe the reasons behind this.

Answer 8

Low
Fluid depletion is associated with RAAS activation and increased aldosterone secretion which acts to increased sodium reabsoprtion to promote fluid retention

Therefore: decreased conc of Na+ excreted in urine

Question 9

What is hypernatraemia and what are the causes?

Answer 9

Serum sodium > 145mmol/L

1. Sodium retention > water retention
   - low water intake (MOST COMMON) activates RAAS due to renal underperfusion
   - Primary hyperaldosteronism (Conn’s)
   - Cushings (hypercortisolism)
2. Water loss > sodium loss
   - diarrhoea
   - vomiting
   - burns
   - haemorrhage
   - insufficient water intake with diabetes insipidus (RARE)

Question 10

What is diabetes insipidus and when can it lead to hypernatraemia?

Answer 10

Inability of pituitary to produce ADH or kidney insensitive to ADH leading to polyuria and thirst

Fluid restriction leads to hypernatraemia due to excess loss of water compared with sodium

Question 11

What is hyponatraemia and what are the causes?

Answer 11

<135 mmol/L

ECF water retention > sodium retention i.e. BODY SODIUM NORMAL OR HIGH

• oedematous state
• syndrome of inappropriate ADH (SIADH)
• excessive drinking

Sodium loss > water loss i.e. BODY SODIUM LOW

Osmotic diuresis (DKA or diuretic stages of CKD or diuretics or Addison’s)
Diarrhoea/vomit/burns/fistula

Question 12

What is the most common cause of hyponatraemia in the community?

Answer 12

Thiazides diuretics

Question 13

What condition should always be considered when previously healthy person presents with hyponatraemia in combination with postural hypotension and no polyuria?

What results would these people present with and why? What is a differential diagnosis for these results?

Answer 13

Addison’s disease i.e. ADRENAL FAILURE

Low plasma osmolality
High urine osmolality
High urine sodium

Adrenal failure leads to decreased aldosterone production
I.e. hypoadolsteronism= decreased sodium and water retention

SIADH

Question 14

What is SIADH and why does it causes hyponatraemia? How can SIADH be diagnosed?

Answer 14

ADH secretion occurs inappropriately for the ECF osmolality or volume

I.e. increased volume retention

Results:
High urine osmolality
Low plasma osmolality
High urine sodium

Diagnosis of exclusion i.e. other causes of hyponatraemia excluded

Question 15

What would you expect the biochemical tests to be for the following conditions?

SIADH
Excessive drinking
Hypovolaemia (renal Na loss)
Hypovolaemia (non-renal Na loss)

Answer 15

SIADH
Low plasma osmolality
High urine osmolality
High urine sodium (>=20)

Excessive drinking
Low “”
Low “”
High urine sodium (>=20)

Hypovolaemia w/ renal loss
High plasma osmolality
High urine osmolality
V high urine sodium (»20)

Hypovolaemia w/o renal loss
High plasma osmolality
High urine osmolality
Low urine sodium

Question 16

What are the consequences of sodium depletion?

Answer 16

CNS dysfunction when Na+ <120mmol/L due to determining brain volume:

• malaise
• confusion
• seizures
• coma

Question 17

How do you treat hyponatraemia? (Think about the cause)

When should symptomatic total body loss hyponatraemia be treated? What is the risk of treatment outside this criteria?

Answer 17

SIADH= fluid restriction due to being dilutional hyponatraemia

Symptomatic total body losses
Slow IV saline (no more than 12mmol/L/24h
Loop diuretic
Vasopressin anatagonists

When Na+ <120mmol/L
Brain adapts to hypoosmolar environment meaning rapid correct can lead to central pontine myelinolysis

Question 18

What are examples of different IV fluids? What are their compositions and are they isotonic or hypotonic?

Answer 18

0. 9% Saline= 154 mmol/L Na+ isotonic
1. 45% Saline= 77 mmol/L Na+ hypotonic

Dextrose saline= 30 mmol/L Na+ and 40g/L glucose Hypotonic

5% dextrose= 50g/L glucose Hypotonic

Hartmann’s= 131 mmol/L Na+ w/ 5mmol/L K+ and 29 mmol/L HCO3-

Question 19

What are the normal daily IV requirements?

Which fluids are considered maintenance fluids?

Answer 19

25-30ml/kg water
1 mmol/kg Na+/Cl-/K+
50-100g glucose

0.45% NaCl
0.18% NaCl + 4% glucose
5% glucose

Question 20

What IV fluids are required when there is excess loss of volume and sodium?

Answer 20

Hartmann’s or 0.9% saline