Capillaries Flashcards
How are capillaries involved in exchange?
Diffusion e.g. gases
Filtration - water
What are the components of capillaries?
Mostly all endothelium.
Small amount of collagen in the basement membrane.
No smooth muscle so cannot constrict themselves.
What is the size of capillaries?
Very small.
Some cells passing through are bigger than the capillary.
Red blood cells take up lots of the capillary.
Larger cells are deformable, so can get through the small capillary without clogging the vessel up.
What is the structure of continuous capillaries?
Continuous, no significant gaps.
Surrounded by pericytes - wrap around epithelial cell to regulate blood flow and for barrier function.
Continuous basement membrane.
Most common type.
What is solute transfer in continuous capillaries?
Paracellular
Transcellular - active, passive, transcytosis.
Transcytosis is where things are carried by vesicles.
What is the location of continuous capillaries?
Lungs, muscle, adipose tissue.
Central nervous system - including the blood brain barrier (neurovascular unit).
What is the neurovascular unit?
Maintains integrity of the blood brain barrier.
Endothelial cell with a very tight tight junction.
Pericytes also prevent movement out the vessel.
Astrocytes have foot processes and sit around the capillary.
Intercellular clefts are also very small.
What are the pores of capillaries?
Capillaries are permeable to small molecules at the points where the endothelial cells meet - intercellular clefts.
What is the structure of fenestrated capillaries?
Openings in their endothelium - fenestrations.
60-100nm diameter, any items smaller than this can pass through.
Although the basement membrane still acts as a barrier.
What is the solute transfer in fenestrated capillaries?
Not selective in what passes through, as long as it is small.
Water, ions, small molecules.
What is the location of fenestrated capillaries?
Organs where exchange is critical - kidneys, intestines, endocrine glands.
What is the structure of discontinuous capillaries?
Large gaps in the endothelial layer and the basement membrane.
Provide little resistance to movement.
What is the solute transfer in discontinuous capillaries?
Water, ions, small molecules and large molecules.
Cells can get through.
What is the location of discontinuous capillaries?
Liver, spleen, bone marrow, adrenal glands, endocrine glands.
In the bone marrow, RBCs are produced, so need to get into the blood through sinusoids.
Why does the liver have discontinuous capillaries?
The liver is important for detoxification (of bacteria and toxins) and metabolism of various compounds, it has direct access to plasma.
Hepatocytes produce albumin, so direct contact with blood allows for efficient secretion.
It also provides additional compartment for cell-cell interactions, protecting from shear stress.
What is the space of Disse?
In between the sinusoid vessel and the hepatocyte is the space of Disse.
This allows direct access to the plasma without significant filtration before it.
How are the capillaries the bulk of the circulatory system?
Total surface area is 6000m^2.
So provides a very large surface area for exchange.
What is the relationship of cross sectional area in capillaries?
Flow = velocity x cross sectional area
Q = u x A
Velocity decreases in capillaries, and cross sectional area increases, so flow decreases.
Why are the capillaries the bulk of the circulatory system?
Slowing down the blood in the capillaries is very good for filtration and diffusion.
It increases transit time, so there is more time for exchange.
How is blood flow regulated in capillaries?
Arterioles are the main regulators of capillary blood flow.
Capillaries can also autoregulate blood flow using pre-capillary sphincters at their proximal end.
What are meta-arterioles?
Meta-arterioles come off arterioles, then capillaries branch off meta-arterioles.
Meta-arterioles allow shunting of blood from one side to the other id the capillaries are closed.
What nutrients can be exchanged in the capillaries?
Plasma proteins cannot leave unless they have a specific transporter, or it is a discontinuous capillary, or the capillary is damaged.
Proteins with specific receptors on the vessel wall side can go into the tissues.
Small water soluble substances can pass through intercellular clefts.
Lipid soluble molecules can pass straight through the endothelium.
What are the mechanisms of capillary exchange?
Diffusion
Bulk flow
Vesicular transport
Active transport
What is diffusion?
Movement of a molecule from an area of high concentration to an area of low concentration.
What is Fick’s law of diffusion?
See picture.
The rate at which stuff diffuses = surface area x size of concentration difference / distance to travel.
What are the types of diffusion?
Simple diffusion, through endothelial wall - oxygen, carbon dioxide, lipid soluble, water.
Facilitated diffusion, carrier proteins and channel proteins.
What is bulk flow?
Fluid and electrolytes move together through pores and intracellular clefts.
This is driven primarily by pressure differences between the capillary and tissue.
What is vesicular transport?
Big molecules are transported across the endothelium in vesicles.
Bind to proteins on the luminal side of the capillary, form vesicles, which are then translocated to the apical (tissue) side.
What are caveolae?
Invaginations of cell membrane, have receptors for big plasma and other proteins.
e.g. receptor for albumin, insulin, so big proteins can cross endothelial cell.
Proteins bind on luminal side, form vesicles, transported to apical side and are released.
What is active transport?
Molecules are actively transported against their concentration gradients.
Requires energy (primary) or uses another molecule’s concentration gradient (secondary).
What are the forces favouring filtration?
Capillary hydrostatic pressure.
Interstitial oncotic pressure
What are the forces favouring reabsorption?
Interstitial hydrostatic pressure
Capillary oncotic pressure
What is the equation for net filtration?
See picture
Net filtration = the filtration coefficient x the hydrostatic pressure promoting fluid movement out of the capillary
Take away the reflection coefficient x the oncotic pressure promoting fluid movement out of the capillary.
What are the coefficients in net filtration?
Kf is filtration coefficient - leakiness to water.
σ is reflection coefficient - leakiness to protein, mostly to albumin.
How can the net filtration be explained?
Difference in hydrostatic pressure inside vs outside
And the difference in oncotic pressure inside vs outside
Comparing the driving force to push fluid out against the driving force to keep fluid in.
What is capillary hydrostatic pressure?
The pressure exerted on the capillary walls by the blood.
It is affected by arterial and venous pressure.
It is always positive so that fluid moves out of blood vessel.
What is interstitial oncotic pressure?
The force to move fluid to where the proteins are.
Oncotic pressure is the osmotic pressure due to proteins.
What is interstitial hydrostatic pressure?
Pressure of fluid in the tissue, exerted on the vessel wall.
In normal health, should be low, as little fluid in tissue compared to blood vessels.
What is capillary oncotic pressure?
It is high because there are lots of plasma proteins in the blood - albumin, so draws water in to the capillaries.
What are Starling’s forces at the arteriole end of the capillaries?
The capillary hydrostatic pressure is high because it is driven by the arteriole pressure and blood volume.
The capillary oncotic pressure is also high because of albumin.
Normally, interstitial hydrostatic pressure is low as fluid doesn’t collect in tissues.
Interstitial oncotic pressure is close to 0.
So there is a positive value of net filtration about 10mmHg, which pushes fluid out of blood vessel.
What are Starling’s forces at the venule end of the capillaries?
As fluid is taken out of the blood, it reduces hydrostatic pressure, and the capillary resists flow, so reduces pressure.
There is no change in capillary oncotic pressure, as proteins cannot leave the vessel.
Net filtration of -10mmHg, fluid goes the other way.
So there is a net absorption pressure of 10mmHg.
What are the lymphatics?
Lymphatics are one way vessels, that exist to return fluid from tissues to the blood.
They are present everywhere, then form larger lymphatic vessels which drain out of ducts and back into the back into the blood.
What is the classical Starling hypothesis?
As the blood moves through the capillaries, it flips from filtration to absorption.
90% of water is reabsorbed at the end, 10% is lost to the lymphatics.
This has been replaced by another Starling hypothesis.
What is the problem with the classical hypothesis (Starling’s)?
The amount of fluid reabsorbed in the venous end of the capillaries is less than expected.
Protein permeability studies showed that the capillaries are not completely impermeable to proteins.
The lymphatic flow is much higher than the classical hypothesis would suggest.
And the glycocalyx was discovered.
What is the glycocalyx?
A gel-like structure, that sits around the capillary wall, on the surface of the endothelial cell.
Acts as an additional barrier to fluid movement.
What is the revised Starling’s forces model?
The glycocalyx is a barrier on the endothelial surface that complicates fluid exchange.
Capillaries are slightly permeable to proteins, but is highly regulated by the glycocalyx.
The sub-glycocalyx space is important in managing oncotic pressure across the vessel wall.
The lymphatics remove all superfluous tissue fluid, with no net reabsorption across the capillary.
What is Starling’s revised law?
Although hydrostatic gradient decreases, so does osmotic pressure gradient.
There is never a point where hydrostatic gradient (pushes fluid out) goes below the osmotic pressure gradient (keeps fluid in).
Net filtration = the differences in hydrostatic pressure take away the difference of the capillary oncotic pressure and the sub-glycocalyx space.
What is the effect of high capillary hydrostatic pressure on the capillaries?
There is a very high hydrostatic pressure gradient.
There is a high oncotic pressure gradient.
This favours filtration out through intercellular clefts, due to there being a big distance between capillary oncotic pressure and sub-glycocalyx oncotic pressure.
What is the effect of reducing capillary hydrostatic pressure on the capillaries?
The fluid flows slower.
There is still a high hydrostatic pressure gradient.
And a high oncotic pressure gradient.
So there would be no net movement, however this is affected by proteins.
How does protein diffusion affect the capillaries?
As the flow slows through the cleft, proteins can diffuse into it.
This increases the oncotic pressure of the sub glycocalyx, which reduces the oncotic pressure gradient.
So it restores filtration.
What is an oedema?
The build-up of fluid - cellular and tissue (interstitial).
Occurs due to excess fluid filtration.
Compromises tissue function.
How does oedema compromise tissue function?
Extramural compression of vessels.
Increased diffusion distance - moves tissue away from the vessel wall.
Impaired cell trafficking, propensity for infections.
Compression of other external structures.
What are the types of oedema?
Hydrostatic
Oncotic
Permeability
Lymphoedema
What is the cause of hydrostatic oedema?
Caused by increased venous pressure and blood volume, due to renal failure and sodium retention. Decreased arteriolar resistance, increased pressure in capillaries. Increase hydrostatic pressure gradient, fluid moves out of vessel easier.
What is the cause of oncotic oedema?
Oncotic oedema is caused by changes in oncotic pressure gradient.
Decreased albumin production, by liver disease or malnutrition.
Causes oedema as less protein in blood vessel keeping water in.
What is the cause of permeability oedema?
Permeability oedema is caused by increased permeability in blood vessel, due to inflammation, sepsis, more fluid moves out.
What is the cause of lymphoedema?
If there is any problem with lymph drainage, fluid collects in the tissue, and interstitial space increases.
