Summary
Summary solute and fluid exchange Pt2 – fluid
- Module
- Biomedical Science
- Institution
- St Georges
Summary of the solute and fluid exchange Pt2 – fluid
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Solute and Fluid Exchange II – FluidOutline importance of fluid transport
Describe 4 pressures of Starling’s Principle of fluid exchange
Introduce Capillary permeability and reflection coefficients
Filtration along capillaries
Role of the lymphatic system in the control of interstitial fluid
Causes of oedema
This lecture is about fluid movement from plasma in the capillaries into the interstitial
fluid
Importance of Fluid Exchange
Fluid exchange is important for normal physiological function, this is because all our
reactions occur in the presence of water, so we need to control our fluid -> need to keep
bathing our cells, fluid moving from plasma to interstitial space.
Fluid re-absorption is required during haemorrhage and shock, we need to take fluid
from interstitial space back into our plasma in order to maintain our cardiac output.
Abnormalities in fluid exchange lead to oedema and tissue swelling. An example is
having lymphatics removed results in less uptake of fluid from interstitial space so it
accumulates -> lymphoedema
The interstitial space is not just a load of water swirling about, there is a matrix there
and there are pockets of water (i.e. it’s not a swimming pool). There are lots of
connective tissue around etc.
Fluid Movement at the Capillary Wall
, The capillary wall is a semi-permeable membrane and allows water as well as low
molecular weight solutes through. So water (and low molecular weight molecules)
under hydraulic pressure has the ability to move out and into the interstitial fluid.
However large molecules (e.g. plasma proteins) are much harder to get through (you
need special type of capillary), so plasma proteins like albumin stay in the blood plasma.
Because plasma proteins cannot move from blood plasma they create an osmotic
gradient. There will be high plasma proteins in the blood plasma and low plasma
proteins in the interstitial fluid – this means water wants to move from interstitial space
back into plasma, this is the osmotic pressure pulling it in. This type of osmotic pressure
is called oncotic pressure.
So there is a balance, there is hydraulic pressure pushing fluid out of capillaries (due to
action of heart) and you have osmotic pressure pulling fluid back into the capillaries. So
fluid movement across the capillary wall depends on the differences between hydraulic
and oncotic pressures across the capillary wall. A greater hydraulic pressure -> fluid
will move out, greater oncotic pressure -> fluid will be reabsorbed. This is the basis of
Starling’s Law of fluid exchange.
The Four Pressures That Determine Filtration Rate and Starlings Principle of Fluid
Exchange
Across a capillary you have the hydrostatic pressures and you have the colloid osmotic
pressures (oncotic). The equation to determine filtration rate can be explained using the
diagram below ->
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