Microcirculation Flashcards
What comprise microcirculatory system?
Arterioles
Capillaries
Venules
What is the overall aim of CVS?
Adequate blood flow through the capillaries
What is the blood flow rate?
Volume of blood passing through a vessel per unit time
What determine blood flow rate?
Flow rate = Pressure gradient/Resistance
Darcy’s law
What is pressure gradient equal to?
Difference in pressure between Pressure A (arteriole) and Pressure B (capillary)
Define resistance
‘Hindrance to blood flow due to friction between moving fluid and stationary vascular walls’
What affects resistance
Vessel length
Vessel radius
Blood viscosity
Which factor affecting resistance changes dynamically?
Vessel radius
The only factor that changes resistance dynamically
What happens to pressure, resistance and flow rate when blood pressure increase?
delta P- increases
R- unchanged
F- increases
What happens to pressure, resistance and flow rate during arteriolar vasoconstriction?
delta P- unchanged
R- increases
F- decreases
What has the biggest influence on resistance?
Arterioles Lots of muscle Lots of capacity to constrict and dilate Biggest change in pressure seen between two ends of an arteriole (50mmHg drop) Greatest resistance in the CVS
What is blood flow to different tissues dependent on?
Resistance
Pressure gradient is the same across all tissues
Resistance determines how much blood flow each tissue gets
How can arterioles control resistance?
Vasoconstriction:
- radius increases
- resistance increases
- flow decreases
Vasodilation:
- relaxation
- radius increases
- resistance decreases
- flow increases
Why are arterioles partially constricted at rest?
Aterirole smooth muscle normally displays a state of partial constriction- this is called vascular tone
At rest arterioles must be partially constricted to allow them to both dilate and further constrict
What are the two functions of adjusting arteriole radii?
Match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)
Help regulate systemic arterial blood pressure
How is matching blood flow regulated?
Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation
How is relating systemic arterial blood pressure regulated?
Regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated
What is matching blood flow chemically driven by?
Chemically driven by:
Increase in metabolites
Increase in O2 usage
Will lead to vasodilation
What is the increase in blood flow in response to increase demand called?
Active hyperaemia
What is matching blood flow physically driven by?
Decrease in blood temperature
Increase in stretch (distension) due to increased blood pressure
Leads to vasoconstriction
What is the decrease in blood flow in response to factors?
Myogenic autoregulation
What would occur skeletal muscle and small intestine arterioles in response to exercise?
SM- active hyperaemia
SI- myogenic vasoconstriction
How do arterioles help regulate arterial blood pressure? (Neural)
Cardiovascular control centre in the medulla
Causes vasoconstriction
Decreases blood flow the specific organs
How do arterioles help regulate arterial blood pressure? (Hormonal)
Arginine vasopressin (ADH)
Angiotensin II
Adrenaline/Noradrenaline
Stimulate vasoconstriction
What is the purpose of capillary exchange?
delivery of metabolic substrates to the cells of the organism [which is the ultimate function of the CVS]
Why is capillary density important?
Depends on how metabolically active the tissue is
Ideally suited to enhance diffusion: Fick’s law
Specially designed to minimise diffusion distance
Maximise the SA and time for diffusion
How does capillary density vary within the body?
Highly metabolically active tissues have denser capillary networks
Skeletal muscle = 100cm2/g
Myocardium/brain = 500cm2/g
Lung = 3500cm2/g
(not related to metabolic activity bur rather gas exchange)
Why is capillary structure important?
For exchange
Thin walls
Small H2O filled gap junctions
Proteins embedded in capillary
What are fenestrations?
Circular holes that allow for slightly larger molecules to pass through the capillary
What is discontinuous capillary?
Need to allow for much larger substances between the blood and tissue
e.g. in bone marrow when white cells need to be transported
and liver
What is bulk flow?
A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
What draws fluid back into he blood?
Oncotic pulling force
protein gradient related
What pushes fluid out into tissues?
Hydrostatic pushing force
What did Starling hypothesise?
“…there must be a balance between the hydrostatic pressure of the blood in the capillaries and the osmotic attraction of the blood for the surrounding fluids. “
” …and whereas capillary pressure determines transudation, the osmotic pressure of the proteins of the serum determines absorption.”
When would ultrafiltration take place?
If pressure inside the capillary > in the IF
When would reabsorption take place?
If inward driving pressures > outward pressures across the capillary
What is the significance of the fact that ultrafiltration is more effective than reabsorption?
Overall net loss
Role of the lymphatic system is to return fluid back to the blood
