Water And Electrolyte Distribution Flashcards
How are electrolytes distributed within the ECF and the ICF?
The proportion of electrolytes in each compartment varies depending on the electrolyte.
Potassium and chloride are required for intracellular reactions so they are stored inside cells.
In the extracellular fluid, most of the positive ions are sodium.
This is balanced so that total positive and negative ions are balanced.
What is the anion gap?
Sodium and potassium are positively charged.
The total positive charge in the plasma is made up of sodium, potassium and a small amount of ‘other’ analytes – including H+, Ca and Mg ions.
Bicarbonate and chloride are negatively charged.
They do not account for as much of the overall negative charge (compared to the contribution of sodium and potassium).
But we routinely measure these four, therefore don’t account for the small amount of proteins and anions contributing to the negative charge - this is the anion gap.
The anion gap approximates the amount of unmeasured anions in the plasma - important for working out the cause of metabolic acidosis.
Anion Gap = ([Na+]+[K+]) – ([Cl-]+[HCO3-])
Reference range = 10-16
The anion gap is important when dealing with acid-base disturbances
Why is it important to keep electrolytes in the correct compartments?
Sodium is required in the ECF to maintain blood pressure, particularly the vascular compartment. Because water follows sodium!
Whereas other ions e.g. potassium have important roles in intracellular reactions.
Equal amount of intracellular sodium as extracellular potassium. Gradient is maintained by Na/K ATPase pumps.
How do electrolyte move between compartments?
Electrolytes will move from an area of higher concentration to an area of lower concentration – sometimes they require assistance to get through the barrier.
All methods occur at different rates, all more slowly than water
Active transport keeps sodium outside the cell and potassium inside the cell. If you run out of ATP the potassium will leak out of the cells to remove the gradient. This s what occurs when a sample is left on the bench after collection. The ATP stores run out and there is no way to generate anymore so all the potassium is in the ECF and results appear very high.
What factors affect water movement?
Water movement is affected by:
- Osmotic pressure
- Electrolytes
- Non-electrolytes e.g. glucose - Oncotic pressure (AKA colloid osmotic pressure)
- Proteins - Hydrostatic pressure
- Mechanical pressure generated by the heart
The oncotic pressure and the hydrostatic pressure are primarily important in the transfer of fluid between the vasculature and the ECF, thus not really talking about cells, just proteins and pressures generated by the heart.
What is osmotic pressure?
Osmotically active particles (Na, K, urea and glucose (also chloride and phosphate) moving from an area of lower osmotic pressure (fewer osmotically active particles), through a semi permeable membrane to an area of higher osmotic pressure (more osmotically active particles).
What is the difference between osmolality and osmolarity?
Osmolarity/osmolality: The concentration of solutes in a solution that contribute to the osmotic pressure.
Osmolality: The number of solute particles /kg of solvent
Osmolarity: The number of solute particles /L of solution
- Affected by temperature
- Includes the solute space
In water osmolarity = osmolality
In plasma, some of the total volume (L) occupied by proteins/lipids. Water volume (L) 6% less than total volume. Therefore osmolality is the more frequently used measurement as it is not affected by solute space or temperature changes.
What is the osmolar gap?
Measure the amount of osmotically active particles present in the plasma.
Calculate the expected osmolality using other measured analytes:
Osmolality = (2xNa) + K + urea + glucose (if abnormal)
(multiply Na by 2 to account for negative anions (also osmotically
active, but less frequently measured) that are present in similar
amounts to sodium, to maintain charge balance)
The osmolar gap is the difference between the calculated (expected) and the measured osmolality (observed) - if large it indicates some other osmotically active substance is present, eg alcohol.
Osmolar gap = measured osmolality – calculated osmolality
Which exerts a greater effect, osmotic pressure or oncotic pressure?
The osmotic effect is determined by the NUMBER of particles.
There are more sodium particles in the ECF than there are protein particles.
Therefore sodium exerts a greater effect on water than proteins do.
What happens in water depletion?
Causes are either excessive loss (uncontrollable loss of water without adequate replenishment) of water or inadequate intake of water:
Inadequate intake:
- Infancy
- Old age
- Dysphagia (can’t swallow)
- Unconscious
- Obstruction
Excessive loss:
- Renal: tubular disorders, diabetes insipidus, osmotic diuresis
- Gut: diarrhoea (hypotonic fluid loss)
- Skin: sweating (hypotonic fluid loss)
- Lungs: hyperventilation
Symptoms:
- Thirst - Dry mouth
- Difficulty swallowing - Weakness
- Confusion - Weight loss
- Dry mucous membranes -↓ skin turgour
- ↓ saliva secretion -↓ urine output
- Circulatory failure (severe) - Cerebral dehydration
What is cerebral dehydration?
Normal brain cell and severely dehydrated vasculature:
Water moves out of the brain cell to the area of higher concentration, i.e. the vasculature.
This causes the brain cells shrink, causing cerebral dehydration.
Cerebral dehydration can result in:
- Blood vessels can tear (haemorrhage)
- Central pontine myelinosis
In order to protect themselves the brain cells synthesise osmolytes (synthesised in response to stress - increase the osmolality in the brain cell by increasing the concentration of solutes, reducing the concentration gradient and lowering the volume of water that leaves the cell).
Treatment:
REHYDRATION MUST BE SLOW
- If rehydrate quickly – osmolytes will not clear quickly enough from the
brain cells
- It would cause the ECF to be significantly more dilute than the brain
cells
- Water would rush into brain cells, causing cerebral oedema.
What happens in water overload?
A healthy kidney can excrete up to 20mL/min. If exceed this, water overload can occur:
- due to increased intake
- or due to reduced losses
Increased intake:
- Overdrinking
- Psychogenic polydipsisa
- Brain damage
- TPN - total parental nutrition
Decreased losses:
- Severe renal failure
- Increased ADH
- Drugs that stimulate/potentiate ADH
- Cortisol deficiency (cortisol is needed for pure water loss)
Symptoms: Depend on the severity and the rate of onset - Behavioural disturbance - Headache - Confusion - Convulsions - Coma
What is cerebral oedema?
Normal brain cell and overloaded, dilute vasculature:
Water into the brain cell to the area of higher concentration(more solutes), i.e. the brain cells.
This causes the brain cells to swell, causing cerebral oedema.
As a result sodium moves out of the brain cells (quickly), as do newly synthesised osmolytes (slowly). The aim is to increase the concentration of the vasculature.
In order to protect themselves the brain cells synthesise osmolytes (synthesised in response to stress - increase the osmolality in the brain cell by increasing the concentration of solutes, reducing the concentration gradient and lowering the volume of water that leaves the cell).
Treatment:
TREATMENT (fluid restriction) MUST BE SLOW
- brain cells have lost solutes
- Therefore rapid increase of the vasculature osmolality will cause water
to rush out of the brain cell
- Water would rush into brain cells, causing cerebral oedema.
- Which can result in cerebral dehydration
How is sodium regulated?
The renin – angiotensin – aldosterone axis. Stimulated by: - Low sodium - Low arterial pressure - Low ECF volume - Sympathetic NS stimulation
Aldosterone stimulates Na+ reabsorption via action on the aldosterone receptor, allows sodium in through ENac channel (where sodium goes water follows)
This generates an electrochemical gradient which allows K+ and H+ excretion - Only ONE K+ OR H+ can move for EACH Na+ reabsorbed
Cortisol is prevented from acting on the aldosterone receptor by 11β-HSD. Which converts cortisol to cortisone, to prevent cortisol doing the same job inappropriately. This is becacau aldosterone and cortisol have such similar structure, they can inappropriately act on each other’s receptors)
How is potassium regulated?
Aldosterone promotes sodium reabsorption and potassium excretion in the distal convoluted tubule
Insulin has a direct interaction with the Na+/K+ ATPase, to force potassium into of cells, independent of glucose. Drives potassium intracellularly
Adrenaline drives potassium intracellularly, whereas Noradrenaline allows potassium to leave cells
The acid-base status
