BW&EH Flashcards
What are the main fluid compartments?
Intracellular (interstitial fluid, lymph), extracellular fluid compartment (Pleural fluid, pericardial fluid, CSF) and vascular (plasma)
Which fluid compartment is used in routine measurements?
A. Intracellular fluid compartment
B. Extracellularfluid compartment
C. Vascular
C
What percentage of the water is plasma?
Firstly 60% of body weight is water. Calculate water first 0.6xbody weight= water
But
- 6xBWx0.083=plasma (L)
- 3%
Match the where the majority of specific electrolyes are most common in compartments.
A. ICF
B. ECF
- Potassium
- Sodium
- Magnesium
A1,3
B2
Why is there an increase in potassium during haemolysis?
Because the K can shoot up coming from ICF into the plasma, which is being measured
Causes for raised anion gaps?
Increase in the unmeasured anions
- Lactate, Ketones and alcohols (ethanol)
Rarely decrease in umeasured cations
- Calcium, Magnesium
Describe with example the importance of electrolytes in the correct compartment?
Sodium is required to be in the ECF to maintain blood pressure
Potassium have important roles in intracellular reactions
Thus, the two electrolytes relationship with compartment is maintained by the Na-K-ATPase
Factors affecting water movement
Osmotic pressure
- Created by electrolytes and non-electrolytes
Oncotic pressure
- Created by proteins
Hydrostatic pressure
- Mechanical pressure generated by the heart
What occurs in oncotic pressure?
Using vascular and extra vascular space.
Water is remained at the vascular space because the proteins are too big to cross the capillary walls, therefore controlling the amount of water in vascular and extravascular space
Describe osmotic pressure.
High osmotic pressure refers to the amount of osmotically active particles in the space.
- Movement of water occurs low to high osmotic pressure in order to dilute the high osmotic pressure
Difference between osmotic and oncotic pressure
Osmotic pressure is determined by the number for particles.
- E.g. more sodium particles in the ECF than there are protein particels. Therefore sodium exerts a greater effect than protein
Define osmolality
The number of solute particles /kg of solvent
Define osmolarity
Number of particles /L of solution
- Affected by temperature
- Includes the solute space
Define omsolarity/osmolality
Conc of solutes in a solution that contribute to the osmotic pressure
Why is osmolality more frequently used as a measurement rather than osmolarity?
It is not affected by solute space or temp changes
In water: osmolarity=osmolality
In plasma some of the volume is occupied by proteins/lipids making water volume less than 6% total volume
What is measured osmolality?
Measures the amount of osmotically active particles present in the plasma
What is calculated osmolality?
Calculate the expected osmolality using other measured analytes
=(2xNa) + K + urea + glucose (if abnormal)
How is the osmolar gap calculated?
Difference between measured and calculated osmolality
Importance of water retention in cells?
- No water= shrivel up
- Blood needs appropriate pressure to ensure the blood can keep circulating and supply the heart and organs with essentials
- To do this, water moves when required. Controlled by different forces
Describe cerebral dehydration.
Describes the process where water moves from brain cell (low solute) to dehydrated vasculature (higher solute)
- This causes the brain cell to shrink - cerebral dehydration
- Blood vessels may tear (haemorrhage)
- Can cause central pontine myelinosis
Mechanism to limit cerebral dehydration
The brain cell is able to synthesis osmolytes to raise its osmolality and prevent water loss to vasculature.
Describe the rehydration process of cerebral dehydration
Rehydration must occur slowly. If it occurs quickly then the osmolytes will not clear quickly enoguh from the brain cells
- Causes the ECF to be significantly more dilute than the brain
- Water rush into the brain cells causing cerebral oedema
What is water overload?
When the kidneys excretion rate of up to 20mL/min is exceeded
What is cerebral oedema?
- Describes when water moves to area of higher conc. Overloaded vasculature -> brain cell
- Brain cells swells, cerebral oedema
Protective mechanims against cerebral oedema?
Achieved by reducing concentration gradient.
Step 1 - Brain cells lose sodium
Step 2 - Brain cells synthesis and lose osmolytes
How would correction of cerebral oedema occur?
Correction must be slowly administered.
- Brain cell has lost osmolytes and sodium
- Therefore rapid increase in vasculature osmolality will cause water to rush out the brain cell resulting in cerebral dehydration
Describe the sodium and water relationship.
Sodium is the major ECF electrolye and exerts majority of the osmotic effect. Sodium balance is therefore inextricably linked to water balance - water follows sodium
Describe the axis that regulate sodium regulation. (Draw for extra points)
Sodium levels are regulated by the renin-angiotensin-aldosterone axis. The axis is stimulated by low Na, low aterial pressure, low ECF volume, sympathetic NS stimulation.
- Liver is stimulated to release angiotensinogen which is cleaved by renin (from the kidney) to cleave angiotensin I.
- Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE)
- Angiotensin II stimulate ADH and the adrenal cortex to release aldosterone.
- Aldosterone act on kidney to reabsorb sodium and excrete potassium
Describe sodium regulation in the distal tubule.
Principal cells - aldosterone
- Aldosterone binds to its receptor to stimulate reabsorption
- Generates an electrochemical gradient to allow K+ and H+ excretion
- One H+ or K+ for each Na+ reabsorbed
By what hormones are potassium regulated by?
Aldosterone, insulin, catecholamines
Describe effect of aldosterone on potassium regulation.
Aldosterone promotes sodium reabsorption and potassium excretion in the distal convoluted tubule
Describe effect of insulin on potassium regulation.
Has a direct interaction with the Na/K ATPase, independent of glucose
- Drives potassium intracellularly
- Hypercalcaemia in plasma
Describe effect of catecholamines on potassium regulation.
Adrenaline drives potassium intracellularly
Noradrenaline allows potassium to leave cells
Match the following hormone with effect to potassium regulation
A. Aldosterone
B. Noradrenaline
C. Insulin
D. Adrenaline
- Allows potassium to leave cells
- Drives potassium intracellularly
- Excrete potassium in the distal convoluted tubule
- Drives potassium intracellularly by Na/K-ATPase interaction
A3
B1
C4
D2
Describe the control of water balance
System between the collecting duct and ADH.
- Draw the axis
- Stored in pPituitary
- Release of ADH
What are some osmotic factors stimulating ADH?
Osmoreceptors in the hypothalamus
- Swell/shrink in response to omsolality
- Dehydration - contract - ADH release
- Hydration - swell -ADH inhibited
What are some non-osmotic factors stimulating ADH?
Nausea, ECF hypovolaemia, hypotension, certain drugs
Describe the specific actions of ADH
The luminal cells lining the collecting ducts are impermeable to water. During rehydration, channels (aquaporin II) are inserted into the membrane to allow water reabsorption
- ADH binds V2 on basolateral surface, which allows insertion of aquaporin 2 channels into the luminal membrane - to enable water reabsorption
What is the effects of aldosterone on electrolyte composition in DCT?
Stimulates Na+ absorption by actions of the aldosterone receptor
- Generates a gradient to allow H+ and K+ to leave
