Oedema Flashcards
Another name for capillary beds
microcirculation
What is the input into capillaries?
arteriole blood
What kind of blood flow is travelling through the arterioles?
smooth flow
How is the blood flow controlled to go to specific capillaries from the arterioles?
arterioles have pre-capillary sphincter
smooth muscle which can be contracted and relaxed, which opens or closes the capillary beds
How do venules control where blood goes from the capillaries?
post-capillary sphincter
smooth muscle which can be contracted and relaxed, makes it harder or easier for blood to travel through that venule
Function of capillaries
sites of exchange
Can capillaries control where the blood flows to?
no - have no smooth muscle in their walls
What is the blood flow velocity in capillaries like?
And what does it depend on?
not constant - can vary from 0 to 8mm/s
depends on contractile state of the arterioles/pre-capillary vessels
What is the solute/solvent movement across capillaries like?
And what does it depend on?
not constant
depends on the permeability which can vary between tissues, at different times and along the capillary bed
What is solute/solvent movement across capillaries dricen by?
diffusion - largely
filtration - fluid, find space
pinocytosis - larger molecules, lipid insoluble packed into vesicles
What are the prime determinants for diffusion?
Capillary permeability of substance and surface area
What is the law associated with the rate of diffusion?
Fick’s Law
What type of substances readily pass through endothelial cells?
lipid soluble substances
eg. O2 and CO2
How does fluid move?
filtration and reabsorption
What is filtration and reabsorption?
bulk flow
movement of a volume of protein-free fluid out of the capillary (filtration) and back (reabsorption)
Why is bulk flow important?
helps to determine distribution of ECF
Driving forces for filtration
capillary hydrostatic pressure (Pc)
interstitial fluid colloid osmotic pressure (πi)
Driving forces for reabsorption
capillary (plasma) colloid osmotic pressure (πc)
interstitial fluid hydrostatic pressure (Pi)
What is capillary hydrostatic pressure (Pc) a major determinant of?
fluid movement
What does capillary hydrostatic pressure depend on?
and give an example
pre/post capillary resistances due to:
venous pressure
(arterial pressure)
If arteriole constricts, how does Pc change?
increased pressure upstream
decreased pressure downstream
therefore increased resistance and precapillary constriction reduces Pc
If venule constricts, how does Pc change?
increased pressure upstream
decreased pressure downstream , less flow
postcapillary constriction increases Pc
How important is interstitial fluid colloid osmotic pressure on fluid movement?
minor determinant
What does interstitial fluid colloid osmotic pressure depend on ?
presence of protein in interstitium, therefore the capillary premeability to protein which is normally very low
How important is capillary colloid osmotic pressure on fluid movement?
and why?
major determinant
lots of protein in the blood
What does capillary colloid osmotic pressure depend on ?
synthesis/breakdown of protein - liver
capillary permeability to protein
abnormal protein loss - kidney damage
Symptoms of end-stage liver failure
lose fluid to tissues and abdomen
hear water sloshing in abdominal cavity
lost protein in blood, decreased colloid osmotic pressure
How important is interstitial fluid hydrostatic pressure on fluid movement?
minor determinant
What does interstitial fluid hydrostatic pressure depend on?
interstitial fluid volume
compliance of organ
effective drainage
What provides drainage for the cappilaries?
lymphatic system
Characteristics of lymphatic system which allows for drainage
valved and highly permeable to protein
What is the purpose of the lymphatic system?
returns excess filtered fluid and 95% of protein lost from vascular system back into circulation close to jugular vein
What occurs if the lymphatic system does not function?
build up of fluid in tissues
swelling
Equation for fluid movement
fluid movement ∝ filtration forces - reabsorption forces
Q ∝ (Pc + πi) - (πc + Pi) Q = Kf [(Pc + πi) - (πc + Pi)] where Kf (filtration coefficient) is a constant that depends on permeability and surface area available
What does capillary fluid transfer depend on and why?
capillary hydrostatic pressure
plasma colloid osmotic pressure
πi and Pi are normally negligble
If capillary hydrostatic pressure is increased, how does this affect fluid movement?
increased filtration
If plasma colloid osmotic pressure is increased, how does this affect fluid movement?
increased reabsorption
If Pc > πc, what occurs?
net filtration
If Pc < πc, what occurs?
net reabsorption
How does Pc and πc present in a typical capillary?
higher Pc pressure on arteriolar side
same pressures mid-capillary
higher πc pressure on venular side
How does Pc and πc change moving from arteriole to venules? and what does this mean for the net movement of fluid?
Pc decreases
πc remains constant
net filtration until Pc = πc, then no movement
net reabsorption once Pc < πc
How would Pc and πc be affected by pre-capillary vasoconstriction?
and therefore movement of fluid?
give an example of where this occurs
reduced Pc, no change in πc
therefore less time for filtration and therefore increased reabsorption
eg. lungs - not driving fluid into alveoli and do not interfere with gas exchange
How would Pc and πc be affected by pre-capillary vasoconstriction?
and therefore movement of fluid?
why might this occur?
increase Pc due to increased venous pressure
no change in πc
increased filtration
RHS heart failure - swollen ankles
LHS - breathlessness due to increased hydrostatic pressure in pulmonary veins
How would Pc and πc be affected by hypoproteinaemia?
Examples of when this would occur
πc reduced, no change in Pc
reduction in net reabsorption, increased filtration,
liver failure - comprimised capacity for protein synthesis
renal failure - lose protein in urine
Causes of oedema
lymphatic obstruction - reduced drainage
increased venous pressure - congestion (HF)
hypoproteinaemia - renal damage
hypervolaemia inflammation - vasodilation and increased permeability
