Salt and Water Balance

Question 1

How much water do we require each day?

Answer 1

2-3 L

Question 2

How much sodium is require each day?

Answer 2

1mmol/kg

Question 3

How much potassium is required each day?

Answer 3

1mmol/kg

Question 4

What is the role of sodium in salt and water balance?

Answer 4

essential for regulating body water, therefore its homeostasis us more tightly controlled than any other ion

Question 5

What is the conc of extracellular sodium?

Answer 5

140mmol

Question 6

What is equation for plasma osmolality?

Answer 6

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

Question 7

The normal range for Na+ is

Answer 7

133-146

Question 8

Which two major systems regulated sodium?

Answer 8

• 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

Question 9

Hypernatraemia occurs in which 3 situations?

Answer 9

• 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

Question 10

What is diabetes insipidus?

How does it present?

Answer 10

• 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

Question 11

When does hyponatraemia occur?

Answer 11

• when water is retained in excess of sodium

- the patient may be hypervolaemic, hypovolaemic or euvoleamic

Question 12

What causes hyponatraemia?

Answer 12

• retain water in oedematous states, SIADH, excessive drinking
• lose Na+ in osmotic diureses, addisons disease, vomiting, burns, fistula

Question 13

What is SIADH?

Answer 13

• 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

Question 14

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

Answer 14

SIADH

Question 15

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

Answer 15

excessive drinking

Question 16

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&raquo_space;20

Answer 16

hypovolaemia with renal Na loss

Question 17

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

Answer 17

hypovolaemia with no renal Na loss

Question 18

High renal sodium indicates that there is renal Na loss

What can be done in order to distinguish the causes of this?

Answer 18

• short synacthen test (artificial ACTH)
• plasma glucose and cortisol
• full patient history

Question 19

What are the symptoms of hyponatraemia?

Answer 19

CNS dysfunction as volume of the brain changes
- malaise, confusion, seizures and come
- rapid changes are more likely to be symptomatic
-

Question 20

When is hyponatraemia treated and how?

Answer 20

When Na is <120mmol/L,

- very slow infusion of 0.9% saline

Question 21

Why is it imporntat that Na+ levels are restored very gradually?

Answer 21

• to avoid pontine myelinolysis

Question 22

How is SIADH treated?0

Answer 22

fluid restriction

Question 23

What is the normal daily IV regimen?

Answer 23

1L of fluid given over 8 hours; this can be normal saline with 20mmol/L of potassium
or 5% dextrose with or without potassium