fluid exchange Flashcards
why is fluid exchange important? (3)
what do we need for chemical reactions?
what can abnormalities in fluid exchange lead to?
what do we try to control following heamorrhage/sepsis?
We need H2O for chemical reactions
Abnormalities in fluid exchange can lead to oedema/tissue swelling
Controlling blood volume, and interstitial and cell volumes are important goals in medicine following drop in blood pressure/poor end organ perfusion
e.g. haemorrhage, sepsis, during surgery, dehydration
Fluid movement at capillary wall
permeability of membrane? so what can pass?
what moves fluid across membrane? drivers (2)
what can’t pass?
what do they exert? effect of this?
Capillary wall is a semi-permeable membrane – allows H2O to pass through
Fluid moves across membrane into interstitial space due to blood flow driven by heart (Hydraulic pressure) and gravity (Hydrostatic pressure)
Large molecules (e.g. plasma proteins) cannot pass through membrane
So they exert osmotic pressure termed Oncotic pressure
which creates suction force to move
fluid from interstitial space into capillary
what does fluid movement at capillary wall depend upon?
what is this called?
Fluid movement depends on balance between Hydraulic/Hydrostatic and Oncotic pressures
across the capillary wall
- Starling’s principle of fluid exchange -
lumen vs interstitial space
molecules and how do they move?
what dominates moevement?
Hydraulic pressures
Pc = Capillary blood pressure
Pi = Interstitial fluid pressure
Easy movement across membrane
Osmotic pressures
IIp = Plasma proteins
IIi = Interstitial proteins
Movement through intercellular gaps
plasma proteins -> some can get through via intracellular gaps but larg conc still in lumen
Directions of Fluid Movements dominated by Pc and IIp
starling’s principle of fluid exchange
what is the equation?
what are the 3 other factors?
Jv = Lp A { (Pc - Pi) - σ (IIp - IIi) }
Jv (net filtration) is Hydraulic/Hydrostatic pressures difference (Pc - Pi )
- Osmotic pressure difference (IIp - IIi)
What is Lp ? Conductance of the endothelium
How leaky the endothelium is to fluid
What is A? Wall area
What is σ? Reflection coefficient – related to intercellular gaps
reflection coefficient
what is it?
what is it for plasma proteins? meaning of this?
Fraction (σ) of the osmotic pressure is exerted by gaps Effective osmotic pressure = σ x potential osmotic pressure σ for plasma protein is 0.9 i.e. 10% plasma proteins are conducted across capillary wall into interstitial space
σ = 1
small intracellular gaps mean plasma proteins can’t go through at all hence reflection via gaps and more osmotic pressure
what does starlings principle tell us
what is marginal?
what do we look at?
Pi and IIi is marginal so we really look at Pc and IIi
Pc 35 and IIp is 25
Pc but not IIp alters along length of capillary
Pc from 35 to 10 hence filtration from 35 to 25 and reabsoprtion from 25 to 10
What is incorrect with Starling’s principle?
what occurs throughout length of capillaries?
what doesnt occur?
what about interstitial osmotic pressure? how does this relate to plasma osmotic pressure?
what isnt taken into account but is key?
Fluid filtration generally occurs throughout length of capillaries
Reabsorption does not occur - important for fluid replacement
Interstitial osmotic pressure (IIi) is not small
IIp = IIi
Glycocalyx is not taken into account
This structure is now considered central to fluid exchange
clinical importance of incorrect starling principle - colloid fluids
colloid fluids increase osmotic pressure - how does this disprove the principle?
Starling’s principle states that if we increase IIp using colloid fluids (which increase osmotic pressure) we should increase blood volume by increasing reabsorption
But, colloid fluids do not significantly expand plasma volume compared to normal crystaloid fluids (e.g. 0.9% NaCl)
So, we need to revise Starling’s principle
revised starling’s principle of fluid exchange (including glycocalyx)
what does the glycocalyyx mean?
how do plasma proteins move into interstitial place? why?
what happens to plasma proteins in interstitial place? what does this create and how does this change the equatiuon?
glycocalyyx means barrier hence plasma protein can’t move into intercellular space but move via exo/endocytosis
Plasma proteins move from lumen into interstitial space via vesicle system Not via intercellular spaces as glycocalyx acts as a barrier
Stream of fluid filtration into interstitial space carries plasma proteins away from endothelium into IIi
creating low IIg (subglycocalyx region) so IIp = IIi but the IIg has lower osmotic pressure as it is washed away
Osmotic gradient is IIp - IIg (not IIp - IIi)
what does the revised starling’s principle mean?
in relation to filtration?
what happens when Pc decreases at venous end?
how does this affect osmotic pressure? net effect?
explain how this reduce reabsoprtion during haemorrage?
what happens if glycocalyx damaged and colloid fluids given?
Filtration occurs across length of capillaries
Less Pc at venous end means plasma proteins diffuse into subglycocalyx region IIi = IIg
So filtration occurs at venous end, even with low Pc
venous side has less capillary pressure so acts less like a river and less plasma protein swept away so the osmotic pressure is the same and lower gradient hence there is no drive to get fluid to lumen but continous filtration happening
Explains why a significant decrease in Pc, e.g. haemorrhage, can only produce a relatively small reabsorption
In sick patient, shedding of glycocalyx explains why colloid fluids do not expand blood volume
Low capillary pressure (Pc) - increased IIp
how is there low Pc?
how much is internally transfused at this point? why?
how? when does thiss top?
Drop in blood volume leads to drop in CO and drop in BP (BP=CO x TPR)
so Pc is reduced
500 ml interstitial fluid ‘internally transfused’ into the blood over 0.5 hr after a haemorrhage
Life-preserving: Supports CVP, Increases CO, Rises BP
Greater end organ perfusion
Only about 500 ml as this stops when IIp = IIg
(see previous slide)So even the low Pc causes filtration
If we have constant filtration, where does this fluid go?
where does that go?
what do these vessels have to help with flow? (2)
what else help flow?
Lymphatic circulation
Lymphatic circulation returns excess tissue fluid/solutes back to the cardio-vascular system
Lymph vessels have valves and smooth muscle
Spontaneous contractions of the smooth muscle contributes to lymph flow
Surrounding skeletal muscle contractions / relaxation also contributes to lymph flow
organisation of lymphatic system
descrive the route and organisation
initial lymphatic plexus collecting lymphatic afferent lymphatic high endothelial venule lymphocyte lymph node efferent lymphatic cysterna chyli lacteal thoracic duct into left subclavian vein
what does overall control of extracellular fluid balance depend on?
3 things
and chnages in what?
Capillary filtration
Capillary reabsorption
Lymphatic system
Changes in:
Starling’s factors
Efficiency of lymphatic system
Influence fluid balance between the intravascular and interstitial spaces
what if overall control of extracellular fluid balance is not maintained? what can it lead to?
Imbalance between filtration, reabsorption, lymphatic function, glycocalyx function leads to
Excess interstitial fluid – Oedema
factors promoting filtration
increased Pc, IIg and Lp and decreased IIp
factors promoting absorption
decreased Pc and increased IIp
Increased capillary pressure (Pc)
3 issues
Gravitational oedema – ‘standing up for long periods’
Deep venous thrombosis
Cardiac failure - eject less blood hence build up of pressure in heart e.g LV + LA hence more pressure in pulmonary veins
DVT
why? where does it occur? what causes filtration?
Prevention of venous return
(below clot) Increases venous pressure causes ‘back-up’ of pressure leading to:
Increased PC across capillaries therefore increased arterial pressure and more filtration
Increased filtration
decreased plasma osmotic pressure (IIp) examples
3 examples
Low protein oedema
Malnutrition/malabsorption, hepatic failure, nephrotic syndrome
Nephrotic syndrome: Urinary protein loss»_space;> Replaced by liver production
Liver disease (not enough endogenous albumin produced)
decreased plasma osmotic pressure (IIp) process
what is the net result?
Reduced plasma protein concentration Reduced plasma oncotic pressure (IIp) Greater influence of Pc Fluid efflux from capillaries into the interstitial fluid Leads to Oedma
Inflammatory-mediated oedema
what causes inflammation? examples
what causes swelling?
what makes it more leaky?
what drives filtration?
Inflammation
e.g. Insect bites/stings, infection, trauma, autoimmune disease
Swelling is triggered by local chemical mediators of inflammation e.g increased histamine
Increase capillary permeability so its more leaky
Inflammation = increasedLp, (also decreased o, increased protein permeability (now IIp = IIg = IIi)
Enhanced filtration
as intracellular junctions are opened up so it’s easier for fluids to be filtered out therefore large proteins can move out and no oncotic gradient across + all the same therfore drives filtration
Lymphatuc obstruction
condition? what happens?
Filariasis/elephantitis – nematode infestation, larvae migrate to lymphatic system grow/mate/form nests –> block lympathetic drainage
Lymphatic removal
caused by? why?
Lymphoedema - caused by surgery to treat testicular cancer –> removal of lymphatics
when glycocalyx breaksdown
what happens when you increase fluids? effect of this?
what does this drive? what does this lead to?
what is the caution?
It is increasingly apparent that a sick patient has a dysfunctional glycocalyx
e.g. inflammation, sepsis, following surgery
In these conditions increasing fluids (either crystalloids or colloids) also causes movement of plasma proteins through intercellular gaps as well as vesicles
So: IIp = IIg = IIi - nullifies oncotic pressures
This drives filtration as no force to drive fluid back into lumen
Pc dominants – filtration – potential oedema if the lymphatic system doesn’t reabsorb
Considerable caution must be given when giving fluid replacement therapy in an attempt to expand blood volume
