Salt and Water Balance Flashcards
How much water do we require each day?
2-3 L
How much sodium is require each day?
1mmol/kg
How much potassium is required each day?
1mmol/kg
What is the role of sodium in salt and water balance?
essential for regulating body water, therefore its homeostasis us more tightly controlled than any other ion
What is the conc of extracellular sodium?
140mmol
What is equation for plasma osmolality?
2[Na] + [K] + glucose + urea
The normal range for Na+ is
133-146
Which two major systems regulated sodium?
- ADH is releases in response to fluid depletion as this causes an increased plasma osmolality that is sensed by the hypothalamus. ADH release corrects osmolarity by stimulating thirst and increasing renal water absorption
- aldosterone is release when fluid depletion causes a fall in BP. This is sensed in the JGA of the kidney, triggering renin release and downstream production of aldosterone. aldosterone will increase Na respiration in the distal tubule, prompting resorption of water
Hypernatraemia occurs in which 3 situations?
- when sodium is retained in excess of water e.g.low water intake, leading to renal underperfusion and RAAS activation. Can also be due to Conn’s and Cushing’s
- when water is lost in excess of sodium, e.g. diabetes insipidus with indadequate water intake, or it can occur in diarrhoea, vomiting, burn, haemorrhage
- it can be artifactua due to a sudden increase in plasma sodium e..g when a blood sample is taken downstream of an IV saline infusion
What is diabetes insipidus?
How does it present?
- the inability of the pituitary gland to produce ADH or the kidney to respond to ADH
- it presents with polyuria and polydipsia, the increased thirst prevents hypernatreamia and maintains a normal plasma sodium
When does hyponatraemia occur?
- when water is retained in excess of sodium
- the patient may be hypervolaemic, hypovolaemic or euvoleamic
What causes hyponatraemia?
- retain water in oedematous states, SIADH, excessive drinking
- lose Na+ in osmotic diureses, addisons disease, vomiting, burns, fistula
What is SIADH?
- syndromes of inappropriate ADH are caused when here is too much ADH secretion for the osmolarity of the extracellular fluid
- this leads to excessive renal water retention
- causes include: CNS disease, pulmonary diseases porphyria and drugs
If there is hyponatraemia with no oedema, there are several tests that can be performed to distinguish between the differential diagnoses. What is the following caused by: Plasma osmolality = low/normal Urine osmolality - High (>600) Urine sodium >20
SIADH
If there is hyponatraemia with no oedema, there are several tests that can be performed to distinguish between the differential diagnoses. What is the following caused by: Plasma osmolality = low Urine osmolality = low Urine sodium >20
excessive drinking
If there is hyponatraemia with no oedema, there are several tests that can be performed to distinguish between the differential diagnoses.
What is the following caused by:
Plasma osmolality = usually high, may be normal
Urine osmolality - high, may be normal
Urine sodium = high»_space;20
hypovolaemia with renal Na loss
If there is hyponatraemia with no oedema, there are several tests that can be performed to distinguish between the differential diagnoses. What is the following caused by: Plasma osmolality = high Urine osmolality = high Urine sodium = low
hypovolaemia with no renal Na loss
High renal sodium indicates that there is renal Na loss
What can be done in order to distinguish the causes of this?
- short synacthen test (artificial ACTH)
- plasma glucose and cortisol
- full patient history
What are the symptoms of hyponatraemia?
CNS dysfunction as volume of the brain changes
- malaise, confusion, seizures and come
- rapid changes are more likely to be symptomatic
-
When is hyponatraemia treated and how?
When Na is <120mmol/L,
- very slow infusion of 0.9% saline
Why is it imporntat that Na+ levels are restored very gradually?
- to avoid pontine myelinolysis
How is SIADH treated?0
fluid restriction
What is the normal daily IV regimen?
1L of fluid given over 8 hours; this can be normal saline with 20mmol/L of potassium
or 5% dextrose with or without potassium
