Basic Water and Electrolyte Homeostasis Flashcards
What are the Fluid compartments of the body?
3 Fluid compartments
- Intracellular fluid Compartment
- Extracellular Fluid Compartment and Extravascular Fluid Compartment
What are the Cations within the body?
- Sodium
- Potassium
- Magnesium
- Calcium
- Hydrogen
What are the Anions within the body?
- Chloride
- Bicarbonate
- Phospahte
- Sulphate
- Organic Acids
- Proteins
What are the proportions of different electrolytes within the ECF?
Cations
- Majority is sodium
- Low potassium (most potassium and magnesium is in ICF)
Anions
- HCO3- and Cl- is the majority
- Proteins and other anions are the minority
What is the Anion Gap?
Total positive charge of sodium and potassium minus total negative charge of bicarbonate and chloride. The anion gap approximates the amount of unmeasured anions in the plasma
Anion Gap = ( [Na+]+[K+] ) – ( [Cl-]+[HCO3-] )
- Reference Range = 10-16 mmol/l
The anion gap is important when dealing with acid-base disturbances (covered in other lectures).
What is the importance of keeping electrolytes in the correct compartments?
- Sodium is required in the ECF to maintain blood pressure.
- Other ions e.g. potassium have important roles in intracellular reactions
How does electrolyte movement take place?
Electrolytes will move from an area of higher concentration to an area of lower concentration – sometimes they require assistance to get through the barrier.
Passive transport = Down the concentration gradient
- Diffusion, Facilitated diffusion, Co-Transport
Active transport = Against the concentration gradient
- Requires energy in the form of ATP
- Keeps sodium outside the cell and potassium inside the cell
How does Water Movement Take place?
- Water moves from one area of the body to another to maintain equilibrium.
- It moves from an area of lower concentration to an area of higher concentration until the two areas reach equilibrium
What are the factors affecting water movement?
- Osmotic pressure: Electrolytes, Non-electrolytes e.g. glucose
- Oncotic pressure (AKA colloid osmotic pressure): Proteins
- Hydrostatic pressure: Mechanical pressure generated by the heart
These determine the distribution of fluid and solute molecules between the vasculature and interstitial fluid of the ECF.
What are the main solutes contribtuing to the osmolality in plasma?
- Sodium (Na+)
- Chloride (Cl-)
- Potassium (K+)
- Phosphate (PO43-)
- Urea
- Glucose
What is the difference between Osmolarity and Osmolality?
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
Why Osmolality the preferred measure?
- In plasma, some of the total volume (L) occupied by proteins/lipids.
- Water volume (L) is 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 calculated and measured osmolatity and the equation for Osmolar Gap?
Measured Osmolality: Measure the amount of osmotically active particles present in the plasma.
Calculated Osmolality: Calculate the expected osmolality using other measured analytes
Osmolar Gap: The difference between calculated and measured osmolality
What is the equation for the calculated osmolality?
(2xNa) + K + urea + glucose (if abnormal)
What exerts more pressure, oncotic or osmotic 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 than protein.
Why is it important to keep water in the specific compartments?
- Cells need to keep water in them – otherwise they shrivel up
- Blood needs appropriate pressure so that it can keep circulating and supplying the heart and other organs with essentials
What are causes of Water Depletion?
Inadequate Intake
- Infancy
- Old age
- Dysphagia
- Unconscious
- Obstruction
Excessive Losses
- Renal: tubular disorders, diabetes insipidus, osmotic diuresis
- Gut: diarrhoea (hypotonic fluid loss)
- Skin: sweating (hypotonic fluid loss)
- Lungs: hyperventilation
What are the symptoms of Water Depletion?
- Thirst
- Dry mouth
- Difficulty swallowing
- Weakness
- Confusion
- Weight loss
- Dry mucous membranes
- ↓ skin turgour
- ↓ saliva secretion
- ↓ urine output
- Circulatory failure (severe)
- Cerebral dehydration
How does Cerebral Dehydration occur?
- Water moves to area of higher solute concentration
- Brain cells shrink (cerebral dehydration)
- Blood vessels may tear (haemorrhage)
- Central pontine myelinosis may also occur
How does the brain protect against cerebral dehydration?
- Brain cells synthesise osmolytes
- These raise the osmolality of the brain cell and prevent water leaving the cell
What is the drawback rehydrating too quickly?
- If you rehydrate too quickly, then osmolytes will not clear quickly enough from the brain cells.
- This will cause the ECF to be significantly more dilute than the brain cells.
- Water can rush into brain cells, causing cerebral oedema
What can cause water overload?
Increased Intake
- Overdrinking
- Psychogenic polydipsia
- Brain damage
- Total parental nutrition (TPN)
Decreased Losses
- Severe renal failure
- Increase in the hormone ADH
- Drugs that stimulate/potentiate ADH
- Cortisol deficiency (cortisol is needed for pure water loss)
What are the symptoms of Water Overload?
- Behavioural disturbance
- Headache
- Confusion
- Convulsions
- Coma
How does Cerebral Oedema occur?
- Water moves to area of higher concentration
- Brain cells swell (cerebral oedema)
How does the brain protect against Cerebral Oedema?
- Acheived by reducing the concentration gradient
- Brain cells lose Sodium
- Brain cells synthesise and lose osmolytes
Why must the effect of Water Overload be corrected slowly?
- Brain cell has lost osmolytes and sodium
- Therefore rapid increase of the vasculature osmolality will cause water to rush out of the brain cell
- Which can result in cerebral dehydration
Describe RAAS?
- Angiotensinogen to Angiotensin 1 through Renin
- Angiotensin 1 to Angiotensin 2 throguh ACE
- Angiotensin 2 stimulate the Adrenal Cortex to produce Aldosterone. Ang 2 also triggeres ADH production
- Aldosterone triggers sodium reabsorption in the kidney (CT, DCT) and potassium excretion.
How does sodium regulation take place?
Principal Cells allow for exchange
- Aldosterone stimulates Na+ reabsorption via action on the aldosterone receptor
- This generates an electrochemical gradient which allows K+ and H+ excretion.
- Only ONE K+ OR H+ can move for EACH Na+ reabsorbed
What acts to regulate the potassium?
Aldosterone: Promotes sodium reabsorption and potassium excretion in the distal convoluted tubule
Insulin: Has a direct interaction with the Na+/K+ ATPase, independent of glucose. It drives potassium intracellularly.
Catecholamines: Adrenaline drives potassium intracellularly. Noradrenaline allows potassium to leave cells
How is Water Balance controlled?
- The collecting duct is controlled by the action of ADH to affect water balance.
- ADH is synthesised in the hypothalamus and stored in the posterior pituitary gland.
- When ADH is stimulated and released, it affects the cells lining the collecting ducts. They are usually impermeable to water
- If the body is dehydrated, ADH binds to the V2 receptor on the basolateral surface, which allows insertion of aquaporin 2 channels into the luminal membrane. therefore enabling water reabsorption
- Once in a hydrated state, ADH release stops and further water is not absorbed. Otherwise the body would become too dilute.
What stimulates ADH release? (AVP as well)
Osmotic
- Cells swell/shrink in response to osmolality and this is sensed by Osmoreceptors in the Hypothalamus
- In dehydration, cells shrink and this triggers ADH released
- In hydration, cells swell and this inhibits ADH
Non-Osmotic
- NAUSEA – extremely potent simulator
- ECF HYPOVOLAEMIA
- HYPOTENSION
- Certain drugs
