Summary
Summary solute and fluid exchange Pt1 – solutes
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- Biomedical Science
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- St Georges
Summary of solute and fluid exchange Pt1 – solutes
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Solute and Fluid Exchange I – SolutesKey Points:
Cell Membrane is a barrier to solute transport
Main solute transport mechanisms
Capillary Structure and types of capillaries
Fick’s Law of diffusion and definition of permeability
Factors affecting endothelial permeability to solutes
Regulation of nutrient transfer across capillaries
[This lecture is about the capillaries and solute movement across them]
Metabolism creates a need to transport solutes and fluids –
The whole point of the CVS is to transport things we need to cells and things we don’t
want away.
O2, glucose, AA, hormones, immune response etc. (contained in H 2O solution) for
energy, growth and repair need to enter the interstitial fluid (so can enter cells) from
plasma
Metabolic end-products e.g. CO2, urea needs to leave cells and enter back into the
plasma through the interstitial fluid.
Cell Membranes are a barrier to solute transport – they have to get from plasma
through blood vessel wall to get into interstitial fluid and then through the plasma
membrane of the cells
Cell membranes provide support and protection, they are also important in cell-to-cell
recognition e.g. immune system. They control what enters or leaves the cell e.g. ion
movement in nerves. They also regulate cell function (be different) e.g. insulin-
mediated glucose uptake.
The cell membrane creates a barrier because it consists of tow layers of amphipathic
phospholipids. With the phosphate heads being polar (hydrophilic) and the fatty acid
tails being non-polar (hydrophobic).
In solution the phospholipids form bilayers.
-Only lipophilic substances can move across and certain small uncharged molecules
The transport of solutes across membranes can be by passive or active mechanisms:
Passive transport
-Is the movement of molecules DOWN a gradient
-Could be concentration, pressure, osmosis etc.
-It does not require energy
-Passive transport can be simple/free (O2,CO2) or facilitated (ions, glucose which use
proteins)
Active transport
- The movement of molecules against a gradient
- Requires energy (uses ATP)
, - E.g. ATP- dependent pumps, endocytosis, exocytosis
The four passive transport processes are:
1) Diffusion: Down a concentration gradient e.g. O2 uptake from lungs into blood
2) Convection: Down pressure gradient e.g. blood flow from heart to vessels
3) Osmosis: Down an osmotic pressure (water) gradient e.g. water uptake by cells
4) Electrochemical flux: Down an electrical AND concentration gradient e.g. ion flow
during an AP in a nerve
The movement of fluid across capillaries is by convection (across a pressure gradient),
this is filtration
This lecture is focusing mainly on diffusion which is responsible for movement of
solutes across capillaries
Solute and Fluid Exchange Occurs at Capillaries
Capillaries are the smallest diameter blood vessels, they are in essence an extension of
the inner lining of arterioles as they are only one cell thick (only endothelium on a
basement membrane), because of this they are semi-permeable.
Capillaries connect the terminal arterioles to the venules and are found near every cell
in the body but are more extensive in highly active tissue (muscles, liver, kidneys and
brain).
They are the site of solute exchange -> O2, glucose, amino acids, hormones, drugs
They are also the site of fluid exchange -> So are responsible for regulation of plasma
and interstitial fluid volumes
Some capillaries are very permeable, some a are moderately while others are not very
permeably. There are three different types of capillaries: continuous capillaries,
fenestrated capillaries and discontinuous capillaries.
Continuous capillaries have a moderate/low permeability, they are made up of a series
of endothelial cells that have tight junctions between the neighbouring cells so there
isn’t much room for molecules to diffuse through. Importantly also the basement
membrane which the endothelial cells are on is continuous producing a firm barrier
against molecules moving out into the interstitial fluid from the plasma. Hence there
will not be much movement of solutes at these capillaries.
These are present in the brain and nervous system (making up the blood brain
barrier).
Fenestrated capillaries are more permeable because the gaps between the endothelial
cells in places involve fenestrations (like cartwheel shaped sieves) which allows
certain molecules of the correct size to move out into the interstitial area i.e. they are
more leaky. Often you will find these capillaries where you want to get a lot of plasma
into the interstitial fluid. So they are found in exocrine glands, salivary/endocrine
glands and other ‘high water turnover tissues’ such as: kidney, synovial joints, anterior
eye, choroid plexus (CSF), gut mucosa.
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