water and sodium

Question 1

ECF

Answer 1

• 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

Question 2

ICF

Answer 2

• Predominant cation is potassium

40% of body weight

Question 3

interstitial fluid

Answer 3

surrounds the cells, but does not circulate

Question 4

plasma

Answer 4

circulates as the fluid component of blood

Question 5

plasma osmolality

Answer 5

• 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

Question 6

why don’t we give water intravenously?

Answer 6

• 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

Question 7

water homeostasis

Answer 7

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

Question 8

causes of water depletion

Answer 8

• Reduced intake
• Sweating
• Vomiting, Diarrhoea, diuresis/diuretics

Question 9

dehydration signs

Answer 9

Thirst, dry mouth, inelastic skin, sunken eyes, raised haematocrit, weight loss, confusion, hypotension

Question 10

water excess

Answer 10

Consequences:
• Hyponatraemia
• Cerebral overhydration – headache, confusion, convulsions

Question 11

volume overload

Answer 11

• 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

Question 12

hydrostatic pressure

Answer 12

pressure difference between plasma and interstitial fluid

Question 13

oncotic pressure

Answer 13

pressure caused by the difference in protein concentration between the plasma and interstitial fluid

Question 14

Oedema

Answer 14

• 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

Question 15

serous effusion

Answer 15

excess water in a body cavity

Question 16

pleural effusions

Answer 16

• 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)

Question 17

disorders of plasma sodium: general principles

Answer 17

• 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

Question 18

Hypernatraemia

Answer 18

high Na

causes: water deficit, sodium excess
effects: cerebral intracellular dehydration (tremors, irratability, confusion)

Question 19

hyponatraemia

Answer 19

low Na

causes: artefactual, sodium loss, excess water, excess water+ sodium
effects: cerebral intracellular over-hydration (headache. confusion, convulsions)