Microcirculation Flashcards
What are first order arterioles?
elements of circulation specific to each organ branch off 1st order capillaries
1st order –> terminal arterioles –> capillaries –> post capillary venules –> venules –> veins
What determines blood flow through capillaries?
Q = P/R
flow can be regulated depending on need
P = P (1st order) - P (capillaries)
R affected by viscosity, vessel radius, vessel length
What is a consequence of increased BP?
increase flow due to increase P difference
but BP does not cause increased flow to all tissues
How is flow affected in arteriolar vasoconstriction?
increased resistance –> decreased flow
How does the flow equation change for tissues?
usually Pa = MAP (93mmHg), Pb = 37mmHg (leaving arterioles)
in tissues Pb (venous pressure) = approx 0 therefore Pa = MAP so that Q is almost entirely dependent on resistance of the tissue/organ
Forgan = MAP/Rorgan
What is vascular tone?
state of partial constriction displayed by arteriolar smooth muscle to allow subsequent vasoconstriction/dilation
What happens in arterioles and why?
radii are independently adjusted to accomplish 2 functions
- match blood flow to metabolic needs of tissues
- regulate arterial BP
How is blood flow matched to metabolic needs of specific tissues?
- regulated intrinsically (local tissue) without endocrine/nervous action
- either a chemical or physical environment response
CHEMICAL - muscle more metabolically active - O2 usage increases - vasodilation - increase metabolites = active hyperaemia
PHYSICAL ENVIRONMENT
- tissue responds to local temperature changes
e. g. decrease blood temperature - vasoconstriction to reduce heat lost in blood flow
e. g. increase BP –> increase stretch - myogenic autoregulation = increase BP increases stretch on cells to stimulate vasoconstriction
What is active hyperaemia?
increase organ blood flow with increase metabolic activity of organ/tissue
How is blood flow adjusted to regulate arterial BP?
via extrinsic controls –> nerves/blood
centrally coordinated
NEURAL
regulated by CVS control centre in medulla
increased BP causes vasoconstriction to decrease blood flow to organs
adrenoreceptors facilitate vasoconstriction (a) and dilation (b)
HORMONAL
brain mimics SNS to control heart beat by stimulating production of adrenaline and norepinephrine
vasopressin/angiotensin II/A and NA regulate arterial BP
How does myogenic autoregulation impact autoregulatory map?
increase flow with increase resistance and perfusion pressure means not enough blood to perfuse all tissue in body maximally
increased flow is detected by stretch receptors that cause vasoconstriction and decrease blood flow to normal levels
What is the function of capillaries?
- deliver metabolic substrates to cells of organism
- 7micrometres wide, 1 thick (enhance diffusion via Ficks Law)
- highly branched
- increase SA
- increase diffusion time
What happens during exercise?
myogenic autoregulation in the gut
What is the capillary density in the... skeletal muscle myocardium/brain lung adipose tissue?
100cm2/g
500
3500
very low in adipose tissue
High metabolically active tissues have denser capillary networks as they require lots of O2
Skeletal muscle has high capacity but a limited flow at rest and shuts most capillaries via pre-capillary sphincters (10% flow rate) without exercise
What are 3 types of capillary?
continuous
- single cell thick
- most substances move through cells
- H20 filled gap for water soluble/lipid soluble substances
fenestrated
- 80nm gaps
- allow passage of larger molecules
discontinuous
- larger holes for passage of large molecules
- leukocytes move from BM to blood
- metabolites entering liver tissues
