lecture 15 - microcirculation Flashcards
What are the 2 functions of the microcirculation?
Transport of nutrients/o2 to tissues, transport of cellular waste products away from tissues
What does blood enter and then leave capillaries through?
Enters via an arteriole, leaves through a venule
Why do capillaries have a very low velocity of blood flow
To allow sufficient capillary residency time for exchange to occur
What are the layers of a capillary?
Single layer of endothelial cells surrounded by a basement membrane
What is the name for the spaces between adjoining endothelial cells in a capillary?
Intercellular clefts
What is the function of the intercellular clefts of capillaries?
They are water filled spaces that allow water and lipid insoluble solutes (glucose, amino acids, drugs, etc.) to pass through
How are capillaries able to undergo constriction?
Although the have no smooth muscle, pericytes (vascular mural cells) on the outside of the capillary can generate constriction
What are the 3 classes of capillary?
Continuous, fenestrated, sinusoidal/discontinuous
Why does the brain have continuous capillaries?
To maintain the blood-brain barrier, there must be less permeability
Why do the kidneys have fenestrated capillaries?
Because they have high water turnover, but still need to make sure large molecules and proteins are not filtered out.
Why does the liver have discontinuous capillaries?
Because it produces lots of proteins, which are large in size and must squeeze into the circulation via large gaps in the capillaries
What maintains and alters the gradient which allows diffusion across capillaries?
Cellular utilisation or production of metabolites determines the diffusion gradient, and therefore how much of the solute will be able to diffuse based on how much is needed.
What factors determine the rate of diffusion of gases across a membrane (Fick’s Law)?
Surface area of membranes, membrane thickness, pressure difference across membrane
What are the 3 key factors that affect diffusion across capillaries?
1.) Molecule type (solubility, polarity, etc.)
2.) Molecule size (molecular weight)
3.) Metabolic demand (O2 demand, capillary recruitment)
What is the relatinship between molecular weight and diffusion across a capillary?
Molecules with low molecular weight have higher permeability
What are the 2 pressures that capillary exchange depends on?
Hydrostatic pressure, colloid osmotic pressure
What is hydrostatic pressure exerted by?
Fluid in the blood or interstitial fluid
What is colloid osmotic pressure exerted by?
Proteins in the plasma or interstitium
If capillary pressure os greater than oncotic pressure, what will be the net change to plasma volume?
Net loss from plasma (more filtration than absorption)
If capillary pressure is less than oncotic pressure, what will be the net change to plasma volume?
Net gain to plasma (more absorption than filtration)
What types of clinical conditions would lead to capillary pressure exceeding oncotic pressure, and therefore a net loss from the plasma?
Inflammatory response, heart failure, hepatic failure
What types of clinical conditions would lead to oncotic pressure exceeding capillary pressure, and therefore a net gain to the plasma?
Haemorrhage/shock
Why does the inflammatory response lead to net loss from plasma?
Vessels become leaky, increasing capillary pressure and driving filtration
Why does heart failure cause a net loss from plasma?
Poor right heart function causes blood to back up in the veins, which increases capillary pressure resulting in peripheral oedema.
Why does haemorrhage/shock cause processes that cause a net gain in plasma?
Decreased blood volume decreases capillary pressure, driving reuptake/absorption from the interstitium
Why does hepatic failure cause a net loss from plasma?
Decreased production of proteins such as albumin reduces the colloid osmotic pressure of the blood, reducing capillary pressure and driving loss from the plasma and into the interstitium.
