Basic Water and Electrolyte Homeostasis Flashcards
(31 cards)
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)
