Fluid pathophysiology Flashcards
what % of total body water is in the intracellular and extracellular fluid compartments
- 66% intracellular
- 33% extracellular
How is the extracellular fluid further subdivided
- Interstitial is 75% of ECF (25% total water)
- intracellular is 25% of ECF (8% tota water)
how are the intracellular and extracellular fluid compartments separated
Semi-permeable membrane.
- freely permeable to water but solutes (electrolytes) only by pumps.
- allows generation of electrochemical gradient across the membrane.
How does the semi permeable membrane differ from the vascular endothelium?
- Vascular endothelium permeable to most ions and solutes as well as water.
- not permeable to larger colloid molecules (albumin. globulin and fibrinogen).
What is the difference in osmolality and osmotic pressure (tonicity)?
osmolality is a measure of ALL solutes and particules dissolved in a compartment, wether or not they can cross the membrane.
What is the equation for a rough calculation of plasma osmolaity
2(Na + K) + glucose + BUN
normal arund 300mosm/ug dogs (310 cats)
What is Colloid Osmotic pressure (oncotic pressure)?
Contribution of colloid molecules and their associated electrolytes to plasma osmotic pressure.
- Albumin most important, highly negatively charged, so carriers multiple cations which also contribute.
- only about 0.5% of overall osmotic pressure
What is the glycocalyx
a gel like matrix of glycoproteins and proteoglycans on the luminal surface of endothelial cells.
what are the proposed functions of the glycocalyx
- Control vascular permeability
- mechanotransduction of shear stress to vessel walls.
- blood cell/vessel wall interactions
- binding of mediators of inflammation and coagulation.
What is the theory of the revised starling principle
- beneath glycocalyx = subglycocalyx space, small, low protein.
- oncotic pressure gradient between plasma and the subglycocalyx space is key to fluid flux (not interstitium)
What is the significance of glycocalyx shedding?
- associated with poor outcomes in people.
- numerous conditions eg trauma and sepsis can cause.
- exacerbated by hypervolaemia, be cautious.
What are the key difference in solutes between intravascular, interstitial and intercellular fluid?
- Intravascular and interstitial are the same except proteins (both extracellular).
- intracellular fluid has high K+ and low Na+, extracellular the opposite.
- intracellular also has lower chloride and higher protein.
Define hypovolaemia
A deficit of fluid from the intravascular space = decreased effective circulating volume.
What is relative hypovolaemia?
Abnormal vasodilation causes blood pooling in kidneys, depleting the central circulation giving shock signs even when total vascular volume remains unchanged.
What is dehydration
Fluid deficit from the interstitial and intracellular spaces. (intravascular not effected until dehydration is severe).
What are the immediate homeostatic responses to fluid loss
- Baroreceptors detect decreased stretch - simulate SNS. (increase HR, contractility, vasconstrict and stim RAAS)
- transcapillary refill, reduced capillary hydrostatic pressure causes fluid flux into the vessel (starling).
Transcapillary refill a reason for TS drop in HAem
How does the RAAS compenstae for hypovolaemia?
in summary
It causes sodium and water retention and stimulates thirst
What are the steps in the RAAS
- Sympathetic stimularion from baroreceptors = renin release.
- Renin converts angiotensinogen to angiotensin 1.
- ACE produced in lung converts angiotensin 1 to angiotensin 2.
- angiotensin 2 = vasoconstriction, sodium and water absorption at proximal tubule and release of ADH and aldosterone.
What is the role of aldosterone
- release from adrenals stimulated by angiotensin 2
- causes sodium resorption at the distal tubule.
What is the role od ADH (vasopressin)
- increased osmolality and angiotensin 2 sitimulate ADH release from the pituitarty.
- increases renal resorption of water at the collecting duct
