L9 Salt and Water balance

Question 1

What are the compositions of the 2 divisions of total body water in ions - na+, k+, cl and specific compartments

Regulated by Na/K/atpase

Answer 1

1. ICF: High K+, low Na+, low Cl-
2. ECF: low K+, High Na+. High Cl-
   - interstitial (links ICF and Intravascular fluid),
   - transcellular: body fluids formed from transport activities of cells - eg. CSF, joint fluid..
   - intravascular: plasma

Question 2

What is plasma osmolarity : define hypo and hyperosmolarity

Answer 2

The number of osmols of solute (cation) per litre of solution. Normal = 285-295 mOsm/L.

Hypo: too little cation: too much water

Hyper: Too much cation: too little water

This is regulated by the kidney

Question 3

Describe the route of filtration of blood by the kidneys

Answer 3

1. Afferent arteriole–> Efferent arteriole through Bowmans capsule : Free filtration of salt and water - 170L/day
2. Proximal tubule: 65-75% Na+ an Water reabsorbed via active processes

LoH
3. Descending limb; H2O is reabsorbed

4. Ascending limb: 13-20% more sodium resorption
5. Distal tubule: 5% more sodium resorbed
6. Collecting duct: 5% more sodium resorbed but also potential water reabsorption if ADH is here

Question 4

Compare the production, stimulation of release and actions of ADH (vasopressin) and Aldosterone (mineralocorticoid)

Answer 4

ADH
-made in hypothalamus and secreted by pituitary in response to

• decrease in BP or increase of osmolarity
• Acts on collecting duct to increase the absorption of water to increase BP and reduce osm

Aldosterone
- Produced by adrenal gland

• Stimulated by K+ and angiotenin 2 to
• Act on distal convoluted tubule + collecting ducts NaKatpase to

Increase Na+ reabsorption in blood and K+ excretion

Question 5

What are the values and what are causes of Hypernatraemia

Answer 5

Na+ >145 mmol/L - high ECF (plasma) osmolarity

Either caused by volume depletion:

• No access to fluids,
• Impaired thirst/level of consciousness
• Burns/Diarrhoea/blood loss

or Increased solute diuresis :
eg. due to increase blood glucose

Question 6

What is Diabetes Insipidus and how does it work

Answer 6

Reduction in amount or efficacy of ADH.

1. Central (traumatic brain injury)
2. Nephrogenic (aquaporin problems/partial or complete resistance to ADH)

• Leads to polyuria and water loss + dilute urine which doesn’t match up to high plasma osm
• Patient is not able to drink enough to keep up with losses, so there is hypernatraemia, dehydration and high plasma osm

Question 7

What are the values and what are causes of Hyponatraemia

- Attached to different findings (5)

Answer 7

Na+ <135 mmol/L

• Excessive Na+ or Water retention.
  1. Check serum osm for pseudohyponatremia

2. Check Urine osm.
   a) If very dilute (<100mOsm/kg)
   = polydipsia/water intoxication - patients on psychotropic drugs for schizophrenia, beer potomania
3. If Hypovolaemic: more sodium than water loss.
   = diarh, vom, bowel obstruction, skin loss, urinary losses (diuretics/addisons/ketonuria etc.
4. If Hypervolaemic: more water retention > sodium
   = cirrhosis, nephrotic syndrome, HF, Renal F.
5. Euvolaemic:
   = SIADH, endocrinopathies (hypothyroid/low cortisol), diuretics, fluid replacement

Question 8

What is a false cause of hyponatraemia and how do find this out. -what causes this false result

Answer 8

1. After hyponatraemia result, firstly recheck serum osmolarity.

If normal: pseudohyponatremia

• > there is another cation to balance out
  eg. In hyperglycaemia
• Osmotic shift of water from cells to the blood stream
  eg. infusions of mannitol, hypertriglyceridaemia, IVIV, paraproteinaemia or pancreatitis

Question 9

What is SIADH, why does it happen and what is the pathology

Answer 9

Syndrome of Inappropriate ADH

1. ADH produced in absence of normal low BP stimuli
2. Leads to too much water being stored in cells so appears to be euvolaemic
3. Urine Osm is not low, urine Na+ is not low but plasma Osm is low

Caused by
- Trauma, tumours, chronic lung disease, head injury and medications (SSRIs)

Question 10

How does hyponatraemia present

Answer 10

2 ways
- Slow: brain cells can adapt–> confused, not quite self

• Rapid: Cerebral oedema–>
• Confusion, seizures, coma

Question 11

How is hyponatraemia treated

Answer 11

1. If rapid onset (<48hr) - more vigorous treatment
   - Need fluid restriction, normal saline or 3% saline in ICU
2. Slow onset:
   Correct gradually with just fluid restriction

• Don’t want the change more than 8 mmol/L per day because rapid correction before the brain can adapt leads to brain dehydration=

Central pontine myelinolysis

Question 12

What is Central pontine myelinolysis

Answer 12

Compression of myelin sheaths : causing rapid demyelination mainly in pons: IRREVERSIBLE

• Quadraparesis
• Pseudobulbar palsy
• Locked in Syndrome