Physiology
Fluid Balance
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Get access3 litres of plasma water, 11 litres of interstitial water, 14 litres of extracellular fluid and 28 litres of intracellular fluid
- Total body water:
- 60% intracellular
- 40% extracellular
- 20% intravascular
- 73% interstitial
- 7% transcellular (eg. CSF)
- Plasma osmolality determines plasma osmotic pressure
Sodium accounts for 90% of osmolality but concentration is approximately equal between the intravascular and extravascular compartments
- Sodium: 14 mmol/L intracellularly, 142 mmol/L extracellularly
- Potassium: 140 mmol/L intracellularly; 4 mmol/L extracellularly
- Interstitial fluid production is determined by
- Plasma proteins exerting oncotic pressure
- Albumin contributes 80%
- Globulin contributes 20%
- Fibrinogen contributes a small amount
- Hydrostatic pressure
- Renin-angiotensin aldosterone system
- Angiotensin II is the final product of the pathway and causes
- Vasoconstriction
- Stimulation of aldosterone release
- Thirst
- Increases plasminogen activator
- Promotes microalbuminuria
- Aldosterone (mineralocorticoid hormone)
Increases sodium reabsorption and potassium secretion via mineralocorticoid receptors (which are intracellular, nuclear receptors) in the distal tubule that upregulate expression of the basolateral Na/K pump
Upregulates ENaC receptors in the collecting ducts to further increase sodium reabsorption
- Increases secretion of hydrogen ions via the intercalated cells of the tubules
- Effects take approximately 1 hour and peak at 4 hours
Note also stimulates salt and water reabsorption in the gut, sweat and salivary glands
ADH (peptide hormone): released when hypertonic plasma stimulates hypothalamic osmoreceptors
- Water retention via aquaporin expression in the collecting ducts
- Vasoconstriction