Fluid balance

Question 1

Which organ regulates fluid?

Answer 1

The kidneys are essential for regulating the volume and composition of bodily fluids.

Question 2

Which hormone directly control water excretion in the kidneys?

Answer 2

Vasopressin/ADH

Question 3

Where is ADH made?

Answer 3

ADH is a peptide hormone secreted mostly by the hypothalamus.
ADH is also produced by the posterior pituitary gland.

Question 4

What does ADH do?

Answer 4

ADH causes the insertion of water channels into cell membranes of the collecting ducts.
This allows water reabsorption to occur.
Without ADH, little water is reabsorbed in the collecting ducts and dilute urine is excreted.

Question 5

What influences the secretion of ADH?

Answer 5

• Special receptors in the hypothalamus that are sensitive to increasing plasma osmolarity (when the plasma gets too concentrated).
• Stretch receptors in the atria inhibit ADH secretion if the blood volume is too great.
• Stretch receptors in the aorta and carotid bodies which stimulate ADH secretion when BP falls

Question 6

Why is osmolarity control important?

Answer 6

The osmolarity of bodily fluids is tightly regulated.
Cells may shrink, swell or get damaged if osmolarity is uncontrolled.
Regulation of osmolarity must be integrated with regulation of volume.
Plasma osmolarity is controlled by aldosterone.

Question 7

What is aldosterone?

Answer 7

This is a steroid hormone produced by the adrenal cortex.
The secretion is controlled 2 ways:
-Adrenal cortex directly senses plasma osmolarity, aldosterone secretion is inhibited if the osmolarity increases above normal.
-Kidneys sense low BP, RAAS system leads to the secretion of aldosterone.

Question 8

What are the disorders of ADH secretion?

Answer 8

• Diabetes Insipidus

- Syndrome of inappropriate ADH secretion (SIADH)

Question 9

What is diabetes insipidus?

Answer 9

This is a rare condition.
Either too little ADH is produced by the pituitary gland (cranial diabetes insipidus)
The kidney becomes insensitive to ADH (nephrogenic diabetes insipidus)
Large quantities of dilute urine produced -polyuria
To compensate people drink lots of water-polydipsia

Question 10

Which electrolyte imbalance can occur in people that don’t drink enough water?

Answer 10

Hypernatraemia

Question 11

What is SIADH?

Answer 11

This is much more common than diabetes insipidus.
Many stimuli may override osmolality control and cause the release of high amounts of ADH.
Urine has high sodium content while hyponatraemia develops as water is retained.
Diagnosis made by checking paired serum osmolality and urine osmolality.

Question 12

Causes of SIADH?

Answer 12

Lung diseases- cancer, pneumonia
Brain lesions- tumour, head injury and bleed, stroke
Drugs- carbamazepine, SSRIs
Miscellaneous causes

Question 13

Assessment of fluid balance

Answer 13

Pulse 
BP 
Skin turgor 
Mucous membranes
JVP
Pitting oedema 
Urine volume

Question 14

Diagnosis of SIADH

Answer 14

Serum sodium <133 mmol/L
Low serum osmolality 
Urine sodium >20 mmol/L 
Normal renal, adrenal, thyroid and pituitary function. 
Exclusion of relevant drug therapy 
Response to fluid restriction

Question 15

What is hyponatraemia?

Answer 15

Serum Na+ below the reference range i.e. <133 mmol/L

This is one of the commonest electrolyte abnormalities found in hospitalised patients.

Question 16

Signs and symptoms of hyponatraemia

Answer 16

Headache
Dizziness
Nausea
Coma (if severe)

Question 17

Classification of hyponatraemia

Answer 17

In a patient with low serum sodium, the result should be interpreted in relation to the patient’s hydration status:

• Hypervolaemia
• Hypovolaemia
• Euvolaemia

Question 18

Why does hypervolaemia cause hyponatraemia?

Answer 18

• Patients with increased extracellular fluid and oedema have an excess of both sodium and water.
• The primary problem is a reduced circulating blood volume due to heart failure or low serum albumin (due to cirrhosis or nephrotic syndrome)
• ADH secretion is stimulated as well as aldosterone
• Net water gain relative to sodium
• Hypervolaemia is clinically obvious, further investigation is not required.
• Treatment often involves the use of diuretics and fluid restriction.

Question 19

Clinical features of hypervolaemia

Answer 19

Tachycardia/ bounding pulse
Raised JVP 
Pulmonary oedema or pleural effusion
Ascites 
Peripheral oedema

Question 20

Why does hypovolaemia cause hyponatraemia?

Answer 20

Sodium loss may be renal or extrarenal.
If the cause is not obvious, random urine sodium can be helpful.
Patients require sodium and water replacement ideally orally but sometimes IV.
Isotonic saline is usually given but hypertonic saline is required in severe, acute secretions.

Question 21

What causes extrarenal loss of sodium?

Answer 21

Urine sodium <20 mmol/L
Causes include: 
Vomiting 
Diarrhoea 
Excess sweating 
Fistulae 
Small bowel obstruction
Trauma 
Heat exposure

Question 22

What causes renal loss of sodium?

Answer 22

Urine sodium >20 mmol/L 
Caused by: 
Diuretics 
Self-losing nephritis 
Adrenal insufficiency (mineralocorticoid deficiency)

Question 23

Clinical features of hypovolaemia

Answer 23

Cool peripheries/prolonged capillary refill 
Tachycardia/ can be weak, thread pulse 
Postural hypotension
Confusion 
Dry mucous membranes 
Reduced skin turgor

Question 24

Why does euvolaemia cause hyponatraemia?

Answer 24

The primary problem is with water excretion. 
The main causes are: 
Excess hypotonic fluid replacement 
Psychogenic polydipsia 
Hypothyroidism (rare) 
Glucocorticoid deficiency 
SIADH 
Drugs including carbamazepine, chlorpropamide.

Question 25

What is hypernatraemia?

Answer 25

This is defined as serum sodium greater than 146 mmol/L

Question 26

What causes hypernatraemia?

Answer 26

• This occurs as a result of either sodium gain or much more commonly water loss.
• It is much less common than hyponatraemia.
• This usually occurs in elderly patients who have dementia which has impaired their sense of thirst or patients in medically induced comas in ITU.

Question 27

Classification of hypernatraemia

Answer 27

In a patient with an elevated serum sodium, the result should always be interpreted in relation to the patient’s hydration status:
Hypervolaemia
Hypovolaemia
Euvolaemia

Question 28

Why does euvolaemia cause hypernatraemia?

Answer 28

As a result of pure water loss and the clinical cause is usually obvious.
However, if there is clinical doubt, it can be helpful to measure the plasma and urine osmolality.
Extrarenal or renal cause

Question 29

Extrarenal loss of water in hypernatraemia?

Answer 29

Inadequate water intake in CVA, confusion, skin loss & low intake- fever, burns

Question 30

Renal loss of water in hypernatraemia?

Answer 30

Diabetes insipidus

Question 31

Why does hypervolaemia cause hypernatraemia?

Answer 31

This form is relatively rare and biochemistry investigation doesn’t have a major role in most cases.
Causes include:
Iatrogenic- IV sodium bicarbonate infusion
Salt ingestion- Accidental, seawater drowning
Mineralocorticoid excess

Question 32

Why does hypovolaemia cause hypernatraemia?

Answer 32

Loss of both sodium and water but loss of water is greater. 
-Measurement of urine and plasma osmolality can be helpful. 
-Extrarenal loss causes include: 
GI- vomiting, diarrhoea, fistula 
Excessive sweating 
Renal loss: 
Hyperosmolar diabetic coma 
Renal failure