water and sodium Flashcards
ECF
• Sodium is the main contributor to ECF osmolality and volume
• Anions chloride and bicarbonate
• Glucose and urea
• Protein = colloid osmotic pressure (oncotic)
20% of body weight
intravascular - around 1/5th of ECF
interstitial - around 4/5ths of ECF
ICF
• Predominant cation is potassium
40% of body weight
interstitial fluid
surrounds the cells, but does not circulate
plasma
circulates as the fluid component of blood
plasma osmolality
• Largely determined by sodium and associated anions
• Estimated plasma osmolality = 2[Na] + 2[K] + urea + glucose mmol/L
• Intra- and extracellular osmolality are equal
• Change in plasma osmolality pulls or pushes water across cell membranes
Under normal circumstances fluid intake = fluid loss
why don’t we give water intravenously?
- It is hypo-osmolar/ hypotonic vs cells
- Water enters blood cells causing them to expand and burst: haemolysis
- However, this only occurs in the vicinity of the intravenous cannula
- If you could achieve instantaneous mixing it wouldn’t occur
water homeostasis
ECF osmolality
• Is very tightly regulated
• Changes in ECF osmolality lead to a rapid response
• Normal plasma osmolality 275-295 mmol/kg
• Water deprivation or loss will lead to a chain of events
ECF volume
• Changes in ECF volume cause a slower response compared to osmolality
causes of water depletion
- Reduced intake
- Sweating
- Vomiting, Diarrhoea, diuresis/diuretics
dehydration signs
Thirst, dry mouth, inelastic skin, sunken eyes, raised haematocrit, weight loss, confusion, hypotension
water excess
Consequences:
• Hyponatraemia
• Cerebral overhydration – headache, confusion, convulsions
volume overload
- ECF volume expansion – heart failure, kidney failure, cirrhosis with ascites
- Loss of intravascular fluid into interstitial space
- Low effective circulating volume stimulates RAAS and ADH
- Renal sodium retention, plus water retention
- Oedema
hydrostatic pressure
pressure difference between plasma and interstitial fluid
oncotic pressure
pressure caused by the difference in protein concentration between the plasma and interstitial fluid
Oedema
- Excess accumulation of fluid in interstitial space
- Disruption of the filtration and osmotic forces of circulating fluid – obstruction of venous blood or lymphatic return, inflammation; ↑ capillary permeability, loss of plasma protein
serous effusion
excess water in a body cavity
pleural effusions
- The normal pleural space contains around 10mL of fluid
- Balance between hydrostatic and oncotic forces in the visceral and parietal vessels, lymphatic drainage
- Pleural effusions result from disruption of this balance
- In pleural effusion, different fluids can enter the pleural cavity
- Transudate is fluid pushed through the capillary due to high pressure within the capillary – low protein content
- Exudate is fluid that leaks around the cells of the capillaries caused by inflammation & ↑ permeability of pleural capillaries to proteins – high protein content
- Pleural fluid protein is measured to differentiate between exudative (eg malignancy, pneumonia) and transudative ( eg LVF, cirrhosis, hypoalbuminaemia, peritoneal dialysis) effusions
- Exudates have a high protein level compared to transudates (and may also contain cells, bacteria, enzymes)
disorders of plasma sodium: general principles
- Normal (reference) range 135-145 mmol/L
- Concentration is a ratio, not a measure of total body content
- High or low [Na] are more often due to gain or loss of water, rather than Na
- Clinical effects are on the brain due to constrained volume (skull)
- Rate of change is more important then absolute levels
Hypernatraemia
high Na
causes: water deficit, sodium excess
effects: cerebral intracellular dehydration (tremors, irratability, confusion)
hyponatraemia
low Na
causes: artefactual, sodium loss, excess water, excess water+ sodium
effects: cerebral intracellular over-hydration (headache. confusion, convulsions)
