Local regulation of blood flow Flashcards
what are the physical determinants of blood flow within the vascular system?
viscosity of blood
pressure difference
vessel radius
length of vessel
what is the most important factor in local regulation of blood flow?
vessel radius
in the equation radius is raised to the power of 4
what determines vessel radius?
vascular smooth muscle tone
a change in vascular tone of which vessel type is most influential in regulating local blood flow?
arterioles
how is blood flow regulated?
neurologically
humorally
locally
neurological regulation of blood flow
autonomic nervous system
alpha 1 adrenoreceptors - sympathetic NS
humoral regulation of blood flow
angiotensin 2
vasopressin
adrenaline
atrial natriuretic peptide
local regulation of blood floq
autoregulation and active hyperaemia - metabolic hypothesis
what is hyperaemia?
excess of blood in the vessels supplying an organ
flow
Q
how to calculate flow
Q ∝ pressure change x r^4 / n x L
L = vessel length
n = blood viscosity
what is the name of the formula used to calculate flow?
Poiseuille’s formular
what are the main sites of resistance to blood flow?
arterioles, they are the control valves into capillary beds
structure of arterioles
thick layer of smooth muscle in walls - tunica media
normal arteriolar tone
normal blood flow
vasoconstriction of arterioles
increases resistance
decreases blood flow
vasodilation of arterioles
decreases resistance
increases blood flow
what does local regulation of blood flow do?
controls regional/ organ blood flow
humoral factors
vasoactive hormones
neurogenic factors
sympathetic nerves
local factors
active hypereamia
autoregulation
how do humoral factors regulate vascular tone?
adrenaline - beta 2 receptors on vascular smooth muscle and natriuretic peptides - ANP and BNP cause vasodilation
Angiotensin II, vasopressin/ ADH and adrenaline at alpha 1 receptors cause vasoconstriction
how do neurogenic factors regulate vascular tone?
release of noradrenline via sympathetic NS bind to alpha 1 receptors on vascular smooth muscle and cause vasoconstriction
what metabolic changes occur during exercise/ in active tissue?
cell respiration and metabolic demand increase pO2 decreases pCO2 increases lactate increases pH decreases Adenosine/ K+ increases temperature increases there are chemical and physical changes in extracellular fluid
what happens when pCO2 increases in active tissue?
cause release of nitric oxide from vascular endothelium
causes relaxation of underlying vascular smooth muscle
vasodilation and increased blood flow
supply meets demand
what is active hyperaemia?
local/ intrinsic mechanism of increasing blood flow to tissues/ organs to meet metabolic demand
what happens during tissue injury?
histamine, bradykinins, prostaglandins released
cause vasodilation
what do vascular endothelial cells
provide low resistance protective lining to blood vessels
release nitric oxide
release endothelin
release vascular endothelial growth factor
what causes nitric oxide release?
local chemical and physical changes due to increased metabolic demand
increased shear stress on endothelium
release of endothelin
stimulated by angiotensin II and vasopressin
inhibited by nitric oxide
what is endothelin?
potent vasoconstrictor
what does the release of vascular endothelial growth factor do?
promotes angiogenesis
long term adaptation in response to decreased blood flow to a tissue
increased blood flow to skeletal muscle during exercise
blood flow to active skeletal muscle increases by as much as 1000% compared to resting
supplying arterioles vasodilate
increase in blood flow brought on by local/intrinsic factors in response to metabolic changes within tissue extracellular fluid
what else can influence arteriolar vascular tone?
fluctuations in mean arterial pressure and blood supply to a tissue/ organ
what happens when there is an increase in mean arterial pressure or blood supply?
increased stretch of arteriolar smooth muscle
increased Ca2+ influx into smooth muscle
smooth muscle contract
causing increased vascular tone and vasoconstriction
blood flow to tissue/ organ returned to normal
what happens when there is a decrease in mean arterial pressure or blood supply?
decreased stretch of arteriolar smooth muscle
no Ca2+ influx to smooth muscle
smooth muscle relaxes
causing decreased vascular tone and vasodilation
blood flow returned to normal
what is autoregulation?
intrinsic myogenic response to changes in mean arterial pressure or blood supply to a tissue or organ
active hyperaemia
responds to changes in demand
autoregulation
responds to changes in supply
where is local metabolic control of blood flow most important?
cerebral, coronary, renal, pulmonary and exercising skeletal muscle vascular beds
control of blood flow in skeletal muscle
active hyperaemia most influential during exercise due to large increase in metabolic demands - lactate, adenosine and K+
autoregulation contributes in exercise
nerves/ hormones are most influential at rest
sympathetic innervation via alpha 1 adrenoreceptors and circulating adrenaline via beta 2 adrenoreceptors
control of cerebral blood blow
active hyperaemia contributes - sensitive to increases in pCO2 and H+
autoregulation contributes to maintain constant blood flow to brain
nerves and hormones have little influence as cerebral vascular smooth muscle has no alpha 1 receptors
control of coronary blood flow
active hyperaemia is most influential as coronaries are sensitive to hypoxia and adenosine
autoregulation contributes to maintain constant blood flow to heart
nerves and hormones have little influence
control of blood flow to skin
active hyperaemia has little influence except when there is damage and release of histamine
autoregulation has little influence
nerves and hormones had varied sympathetic innervation via alpha 1 adrenoreceptors in response to changes in temperature
control of pulmonary circulation
active hyperaemia but there is vasoconstriction in response to low pO2 - hypoxic vasoconstriction
autoregulation and nerves and hormones have little influence
