Fluid and electrolyte balance Flashcards
What are the major divisions of fluid compartments?
Intracellular
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Extracellular:
Subdivided into:
- Plasma
- Interstitial
- Synovial
(- Intra-ocular)
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Describe the major compartments and volumes for a 70kg man.
Total body fluid : 42L
28L intracellular fluid (including ~2L blood cells)
11L interstitial fluid: between membrane and capillary wall
3L plasma: bound by capillary wall
Describe the barrier between the plasma and interstitial fluid.
Capillary wall:
- Leaky
- Protein stays inside vessels (in health) to produce oncotic pressure
- When not healthy… oedema occurs
Describe the barrier between extracellular fluid and intracellular fluid.
Plasma membrane:
- channels/ membranes
- water follows freely as ions take channels into/out of cells
What are the intracellular, interstitial and plasma levels of K+?
Intracellular: 150 mM
Interstitial: 4.5 mM
Plasma: 4.5 mM
What are the intracellular, interstitial and plasma levels of Na+?
Intracellular: 10 mM
Interstitial: 130mM
Plasma: 130mM
What are the intracellular, interstitial and plasma levels of Mg2+?
Intracellular: 2.5 mM
Interstitial: 0.85 mM
Plasma: 0.85 mM
What are the intracellular, interstitial and plasma levels of Cl-?
Intracellular: 10 mM
Interstitial: 110mM
Plasma: 110 mM
What are the intracellular, interstitial and plasma pHs?
Intracellular: ~7.0
Interstitial: 7.4
Plasma: 7.4
What are the gains of body fluid?
Food and water intake;
Oxidation of food
What are the losses of body fluid?
Urine (average 1500 ml)
Faeces (average 100 ml)
Swear (average 50 ml)
Insensible losses (average 900 mL)
TOTAL LOSSES: (average): 2550 mL
What is insensible water loss?
- Insensible losses are solute free
- Trans-epidermal diffusion: water that passes through the skin and is lost by evaporation
- Evaporative loss from respiratory tract
Describe the source, solute loss and role of sweat.
Source: from specialized skin appendages called sweat gland
Solute loss: yes, variable
Role: body temperature regulation
Describe the source, solute loss and role of insensible fluid.
Source: from skin (trans-epithelial)
Solute loss: none
Role: cannot be prevented; evaporation of insensible fluid is a major source of heat loss from the body each day but is not under regulatory control
Where are the sensors for body fluid found?
- Osmoreceptors: in hypothalamus
- Low pressure baroreceptors: in right atria and great veins
- High pressure sensory: in carotid sinus and aorta
What is the key driver if total volume?
Total sodium
What occurs in total sodium drops and osmolality stays the same?
The total volume falls (including plasma volume)
What occurs in total sodium rises and osmolality stays the same?
The total volume will rise
What occurs if osmolality rises?
- Increase in thirst
- Increase in release of ADH
- Increase in water intake/retention
= increase in volume
What occurs if osmolality falls?
- Decrease in thirst
- Decrease in release of ADH
- Decrease in water intake/retention
= Decrease in volume
What occurs in an increase in volume?
Increase in stretch of vascular system:
- Baroreceptors (high pressure areas; low pressure areas)
- Decrease in renin release
- Decrease in aldosterone release
- Increased release of ANP (cardiac myocytes)
= Decreased sodium and water retention
What occurs in an decrease in volume?
Decrease in stretch of vascular system: - Baroreceptors (high pressure areas; low pressure areas) - If pressure (from decreased volume) falls, also influences ADH release and thirst centres - Increase in renin release - Increased levels of AII - Increase in aldosterone release - Decreased release of ANP = Increased sodium and water retention
Describe the control of plasma Na+.
- Hormones controlling sodium balance must act on the kidney
- DCT is the area of control in the nephron
- Noreceptors detecting Na+
- Controlled indirectly via volume sensors
- Changes in Na+ lead to changes in blood volume
- Net sodium excretion = Na+ filtered – Na+ reabsorbed
Describe the control ok K+.
- K+ is freely filtered
- Predominantly reabsorbed again in the PCT with controlled secretion at the DCT
- Secretion is linked to Na+ reabsorption (sodium pump)
- K+ is tightly regulated
- 98% of K+ is inside cells
- Significant & variable intake of potassium from diet - Intracellular potassium acts as a reservoir (attenuates change)
What occurs with increased K+ plasma?
- Increases activity of basolateral sodium pump
- More K+ enters the cell
- Increased secretion across simple diffusion channels on apical membrane
- Increased secretion of aldosterone
- NOT driven by AII, but by direct detection of raised K+ levels by the aldosterone-secreting cells of the adrenal cortex
What is the effect of aldosterone on the DCT?
- Increases activity of sodium pump (basolateral)
- Increases the number of sodium pumps (basolateral)
- Increases the number of sodium and potassium channels in apical membrane
Result: increased reabsorption of sodium and increased secretion of potassium
What is Conn’s syndrome?
Hyperaldosteronism leading to:
- hypertension from increased fluid volume
- hypokalaemia
Describe IV fluid - crystalloids and where they distribute.
- 5% dextrose (glucose)
Initially distributes through ISF and plasma; glucose
metabolised so effectively adding just water. Further distributes into cells as well as ISF and plasma. - 0.18% NaCl 4% dextrose
- 0.9% NaCl (isotonic saline)
- Plasmalyte
Distributes through ISF and plasma; does not enter cells
Describe IV fluid - colloids and where they distribute.
- 4.5% albumin
Supplied in 0.9% NaCl
Tends to stay in plasma; does not enter cells
Blood product - Hydrolysedgelatin
Supplied in 0.9% NaCl
Initially tends to stay in plasma; does not enter cells
Protein metabolised over time so then equivalent to 0.9% NaCl - Blood
Stays in the vasculature and increases blood volume
What are the clinical features of diabetic keto-acidosis?
Hyperglycaemia
- Dehydration
- Tachycardia
- Hypotension
- Clouding of consciousness
Acidosis
- Air hunger (Kussmaul’s respiration)
- Acetone on breath
- Abdominal pain
- Vomiting
What can cause dehydration in DKA?
Hyperglycaemia
Vomiting
Kaussmaul respiration
Altered conscious level - reduced intake
