Fluid homeostasis

Question 1

In a healthy human, what percentage of body weight is made up of water?

Answer 1

60%

Question 1

How can fluid compartments be divided?

Answer 1

• Intracellular
• Extracellular:
• Intravascular
• Extravascular (interstitial)

Question 2

For a 70kg man, what volume of fluid would be found in each of the following compartments:

• Intracellular
• Extracellular
• Intravascular
• Interstitial
• Transcellular

Answer 2

• 25 L of intracellular water
• 19 L of extracellular water.
• Of the extracellular water:
  ■ 3 L is in blood plasma
  ■ 15 L is interstitial fluid
  ■ 1 L is transcellular fluid, e.g. CSF, peritoneal fluid, intraocular fluid.

Question 3

Define diuresis

Answer 3

Increase in the amount of urine produced by the kidneys

Question 4

What are the two types of diuresis?

Answer 4

Water
Osmotic

Question 5

What is water diuresis?

Answer 5

• Occurs when water is ingested or administered in excess of body’s requirements.
• Antidiuretic hormone (ADH) secretion is suppressed.
• Collecting ducts become relatively impermeable to water and excess water is lost without solute.
• The kidney can therefore adjust to excretion of water without markedly affecting its handling of solutes.

Question 6

What is osmotic diuresis?

Answer 6

Occurs when more solute is presented to the kidney’s tubules than they can reabsorb

Examples include:
* Diabetes, where the concentration of glucose in the
plasma rises so that the filtered load exceeds the
* The administration of mannitol, which is filtered but is a non-reabsorbable solute.
* Inhibition of tubular function, e.g. by drugs that block reabsorption of sodium chloride in one or more parts of the tubule.

Question 7

How does water enter the body?

Answer 7

Consumption
In solid food
Oxidation of metabolites

Question 8

How does water leave the body?

Answer 8

• evaporation via the respiratory system: 500 mL
• skin (insensible): 400 mL
• faeces: 100 mL
• urine (obligatory): 500 mL

Question 9

What volume of solute must be excreted each day?

Answer 9

600 mosmol

Question 10

What is the maximum osmolality of urine?

Answer 10

1200mosmol/L

Question 11

What is the obligatory urine output per day?

Answer 11

500mL

Question 12

Discuss fluid and solute mobilisation between compartments - what does this mean?

Answer 12

Movement of certain ions and proteins between compartments is restricted, although water can move freely. As such, the osmolality of all compartments is identical (maintained at 285-295 mosmol/L)

Question 13

What is the mechanism through which osmolality is regulated?

Answer 13

Water loss > water gain –> osmolality increases

This leads to
1. thirst increasing leading to increased water consumption
2. ADH secretion leading to decreased water loss

Question 14

How are thirst and ADH secretion directly regulated?

Answer 14

determined by the osmolality of plasma-perfusing nuclei in the hypothalamus

Question 15

How do the receptors regulating ADH secretion and thirst differ? Why is this important?

Answer 15

Thirst receptors have osmotic threshold about 10 mosmol higher meaning that thirst is not experienced until ADH release has ensured that ingested water is retained by the kidneys

Question 16

Name four stimulants of increased ADH release

Answer 16

• Increased osmolality of plasma perfusing nuclei in hypothalamus
• reduced arterial blood pressure (signals via carotid and aortic baroceptors)
• reduced central venous pressure (signals via atrial low pressure receptors)
• increased angiotensin II in the brain.

Question 17

What is the most common cause of water depletion?

Answer 17

Sodium depletion

Question 18

What are some causes of pure water depletion?

Answer 18

• Diminished oral intake:
  
  • Exhaustion
  • Inability to swallow, e.g. comatose
  • Restricted intake after gastrointestinal surgery.
• Loss of fluid from the lungs.
• Hyperventilation with unhumidified air.
• Diabetes insipidus.
• Diuretic phase of acute renal failure.

Question 19

How is pure water depletion reflected biochemically? What is this associated with?

Answer 19

Hypernatraemia

• increase in plasma osmolality
• concentrated urine
• a low urine sodium concentration despite the hypernatraemia.

Question 20

What are the clinical manifestations of pure water depletion? What is the mechanism of these?

Answer 20

Driven by hypernatraemia –> CNS depression (lethargy/coma)

Question 21

What is the treatment of pure water depletion?

Answer 21

5% dextrose in water

Question 22

Name some causes of water intoxication. What is the most common cause in surgical patients?

Answer 22

• Impaired renal excretion of water, e.g.
  ■ renal failure with excessive intake
  ■ excessive administration of 5% dextrose in the postoperative period when ADH secretion is high
  ■ ADH-secreting tumours.
• Cardiac failure.
• Liver disease.
• Hypoalbuminaemia.

Question 23

What is the clinical presentation of water intoxication?

Answer 23

• peripheral oedema
• raised JVP
• pulmonary oedema.