circulation Flashcards
flow: explain the physical principles, recall Poiseuille's equation, explain the effect of gravity, explain the control of capillary blood flow, and explain the concept of vascular capacitance and compliance
2 types of blood flow
laminar and turbulent
define laminar flow
velocity of fluid is constant at any one point and flows in layers; flows fastest closest to lumen centre due to friction with endothelial lining
define turbulent flow
blood flows erratically, forming eddys at branching, and is prone to pooling; associated with pathophysiological changes to endothelial lining
what is parabolic velocity profile
the further from the wall, the faster the velocity
what is the tangent at any point on the parabolic velocity profile
shear rate (change in velocity/change in radius)
how to calculate shear stress
shear rate x viscosity
diagram of parabolic velocity profile
benjis
what does shear stress govern
how well endothelial cells work
when does laminar shear stress occur
at a high level of shear stress
what does laminar shear stress promote
endothelial cell survival, allowing secretions to promote vasodilation and anticoagulation
when does turbulent shear stress occur
at a low level of shear stress
what does turbulent shear stress promote
endothelial proliferation, apoptosis and shape change, allowing secretions to promote vasoconstriction, coagulation and platelet aggregation
what can turbulent shear stress lead to
occlusion
turbulent shear stress and age
worsens with age
how do you use turbulent flow to measure blood pressure
release of cuff leads to turbulent flow that can be heard with a stethoscope
where is blood pressure ususally measured and why
upper arm as easily accessed and at heart-level
Poiseuille’s equation
resistance = (8 x length x viscosity)/(pi x radius^4)
how is regulation of flow achieved
variation in resistance in vessels while pressure remains relatively constant
explanation of Poiseuille’s equation
changing radius changes amount of flow, so relatively small changes in vascular tone can produce large changes in flow
in Pouiseuille’s equation, what can be taken as effectively constant
length and viscosity of blood vessels, so only radius changes rapidly
Poiseuille’s equation for blood flow
R = 1/r^4 (so halving radius decreases flow 16 times)
define vascular capacitance/compliance
ability of a vessel to distend and increase its volume with increasing transmural pressure
at a given pressure, what happens to volume at high and low compliance
at high compliance, volume increases by large degree; at low compliance, volume doesn’t increase much
compliance equation
change in volume/change in pressure
define elastance
ability of a vessel to return to original shape
elastance equation
change in pressure/change in volume
what produces elastance in vessels
elastin fibres in vascular wall
compliance properties of veins
very compliant so can dilate substantially and store a large volume of blood inside
compliance properties of arteries
low compliance so do not distendas they have a larger elastic layer so have high elastance, recoiling to maintain pressure
what is the Windkessel effect
recoil of arteries ensures continual flow despite pulastile effect of heart (e.g. during diastole); gets less significant with age
clinical external modulation of compliance
stockings apply external pressure to veins, preventing large increase in volume and reducing risk of pooling
internal modulation of compliance
renin-angiotensin aldosterone system, endogenous vasodilators/constrictors, vasoactive drugs
what does gravity do to blood
pulls it towards ground
effect on hydrostatic pressure by gravity when standing and outcome
standing causes increased hydrostatic pressure, so blood transiently pools in veins due to high compliance
what happens to venous return in response to pooling and further outcomes
decreases, so without compensation will reduce cardiac output and blood pressure, causing fainting
why is pressure gradient from left to right heart maintained
ensure unidirectional flow
effect of small leg movement on venous return
activates skeletal muscle pump to increase venous return
what ensures unidirectional blood flow in veins
valves
effect of respiratory pump on venous return
negative intrathoracic pressure increases venous return
define varicose veins
incompetent valves causing dilated superficial veins in leg
cause of oedema in feet due to gravity
prolonged elevation of venous pressure even with intact compensatory mechanisms
what are the local mechanisms of flow intrinsic to
smooth muscle (important for reflexive control)
wht are the systemic mechanisms extrinsic to
smooth muscle (hormones and autonomic nervous system to control radius)
define flow autoregulation
intrinsic capacity to compensate for changes in perfusion pressure by changing vascular resistance
myogenic theory of autoregulation to decrease resistance to increase flow
smooth muscle fibres respond to tension in vessel wall, so increased pressure caues contraction, and reduced perfusion causes relaxation
metabolic theory of autoregulation to decrease resistance to increase flow (active hyperaemia)
as blood flow decreases, metabolites accumulate, causing dilation to increase flow and wash metabolites away
how does injury cause constriction
serotonin release from platelets causes constriction
endoelium derived vasodilators
nitric oxide, prostacyclin
non-endothelium derived vasodilators
kinins, ANP
endothelium-derived vasoconstrictors
thromboxane A2, endothelins
non-endothelium derived vasoconstrictors
ADH, (nor)adrenaline, angiotensin II
calculate flow rate
pressure gradient/resistance
what is the pressure gradient in capillary blood flow
difference in pressure between arteriole and capillary
what is the normal venous pressure
37 mmHg
capillary beds in different tissues
highly metabolically active tissues have denser capilarry networks; skeletal muscle have large capacity limited flow at rest; myocardium and brain are most heavily perfused except lung
function of pre-capillary sphincters and relevance to skeletal muscle beds
allow some capillaries to be completely closed off, allowing skeletal muscle bed flow to be reduced at rest and larger during exercise
features of continuous capillaries
junctions between endothelial cells are filled with water; water soluble and small molecules diffuse over gap junctions; large and water soluble molecules require transport proteins; small and lipid soluble molecules can diffuse straight across
features of fenestrated capillaries
many small gaps (fenestrations) make walls leaky e.g. glomerulus
features of discontinuous capillaries
large gaps between cells e.g. bone marrow/liver
what type of capillaries are involved in the blood-brain barrier
continuous capillaries with tight rather than gap junctions, and many transporter proteins
