Flashcards in L37 – Control of Blood Flow Deck (48):
Blood flow in what pressure gradient?
From high pressure to low
Why does pressure drop in vessels as blood moves through?
Blood flows down pressure gradient from high to low pressure
Energy is used to overcome friction as blood moves through vessels > pressure drops along vessels > generate pressure gradient
Up to which vessels can the effects of heart beat be observed as pulsations in pressure?
Up to arterioles before capillaries
How is blood flow related to resistance and pressure gradient?
Flow = pressure gradient / resistance
Give Poisseuille's equation.
Flow (Q) = [π (P1-P2) r^4] / 8nl
P1-P2 = pressure gradient
n = viscosity
l = length
What is the relationship between flow and radius?
Q ∝ r^4
small change in radius = big change in flow
Given flow = pressure gradient / resistance
Give equation for resistance and state relationship between resistance and radius?
Resistance = pressure gradient/ Flow
resistance = 8nl/πr^4
Resistance ∝ 1/ r^4
Larger diameter = lower resistance
In Poisseuille's equation, which constants are actively controlled to change blood flow?
radius is ACTIVELY changed to alter flow
viscosity and length can change but is Not actively controlled
What smooth muscles contract in vessels for vasoconstriction?
contraction of spirally-arranged smooth muscle in wall
How much of total peripheral resistance is contributed by pre-capillary resistance vessels? (small arteries and arterioles) How?
2/3 of TPR
Pre-capillary resistance vessels have Thick muscular walls
What is the remaining 1/3 of TPR generated by? Why only 1/3? (think SM in walls)
Venules and small veins
Have Longitudinal (not spiral) SM that do not constrict lumen when contracted
What are pre-capillary resistance vessels mainly responsible for?
control of blood flow to organs or tissues
What are the 2 factors that modify blood vessel radius?
What is passive influence in control of blood flow?
Passive stretch of blood vessel wall due to transmural pressure acting on it
What is active influence in control of blood flow?
Factors that modify the EXTENT of Vascular Smooth Muscle Contraction in Arterioles
How is compliance calculated?
Volume change per unit pressure change
Low compliance = hard to stretch
What pressures are exerted in transmural pressure? How is final radius determined?
Internal pressure- blood in vessel push outwards
External pressure - compression of vessel
Final radius determined by Transmural pressure (Internal P - External P) & vessel compliance
What 4 factors affect compliance hence the final radius of a vessel?
1) Wall thickness
2) Wall composition
3) Neurological influence (by SM tone)
4) Pathological conditions
Why do veins retain majority of blood, not arteries? (think compliance)
Thick-walled arteries are less compliant than thin-walled veins
70% of blood volume stays in veins (need to exert the least pressure)
What macromolecules compose vessel walls? Compare their compliance.
Elastin (high amount in arteries) is more compliant than collagen
How does age affect wall compliance?
Ageing > wall composition changes > less elastin more collagen > overall less compliant
How does neurological influence impact vessel final radius?
high sympathetic activity / high adrenaline > increase sympathetic tone > smooth muscle contracts in vessel wall > harder to stretch, less compliant than relaxed smooth muscle
What are the 2 main pathological conditions that can change vessel wall compliance and affect final radius?
How does Hypertension change vessel compliance? (remember NO)
High BP damages endothelium > fails to
secrete factors (e.g. nitric oxide) to relax vascular
smooth muscle > low compliance > hypertension
+ high BP = deposit collagen = less compliant
How does Atherosclerosis change vessel compliance?
1. Obstructing the lumen; and
2. Reducing wall compliance and passive stretch
Atherosclerotic plaque deposit is hard to distend, very low compliance
What are the 2 groups of factors under ACTIVE influences of vessel radius?
Extrinsic factors - from outside tissue
Intrinsic factors - from inside tissue
What are the 4 categories of active influences on vessel radius ?
Which categories of active influences on vessel radius are extrinsic, which are intrinsic?
Extrinsic = Neural, hormonal
Intrinsic = myogenic, metabolic
What is the role of extrinsic factors to modify vessel radius?
Enable ADAPTATION to different situations
Modify blood flow to achieve appropriate distribution of blood flow between different organs and tissues
What is the role of Intrinsic factors to modify vessel radius?
Match supply of blood to demand/ needs of particular organ
Using haemorrhage as an example, how do both extrinsic and intrinsic factors manage the situation?
Extrinsic factors divert blood to adapt to new situation > divert oxygenated blood to vital organs > Intrinsic factors manage the low O2 blood diverted to non-vital organs
What are the individual roles of the extrinsic factors - neural (sympathetic and parasym. ) and hormonal?
Sympathetic = vasoconstrict
*Note parasym. innervates very few vessels*
Name some hormones used in extrinsic influence over vessel radius?
Difference in overall effect between the neural and hormonal extrinsic influences on vessel radius?
Nerves change distribution between different organs
Hormones produce global response with similar effects on most vessels
Why are hormones constrictors and not dilators?
Dangerous to have circulating dilator hormones because low BP may
lead to inadequate blood flow to brain
Explain the action of sympathetic noradrenergic nerves on vessel raidus.
Sympathetic noradrenergic nerves supply ALL vascular SM > Release noradrenaline > act on a- adrenergic receptors in vascular SM > vasoconstriction
How does withdrawal of sympathetic tone lead to vessel dilation?
No vascular SM contraction > Increased wall compliance > Same transmural pressure > vessels dilate passively and radius increases
*few vessels have parasympathetic innervation, rely on transmural pressure for vessel dilation*
How do parasympathetic cholinergic nerves lead to vasodilation?
Parasympathetic cholinergic nerve releases Ach > act on Muscarinic type cholinergic receptors > Increase in cholinergic tone > vasoconstrict
What is the relationship between flow and pressure with and without autoregulation?
Flow proportional to pressure, linear
Flow maintained within a range of different pressures, non-linear
What does autoregulation counter-act in vessels?
Passive response to change in pressure
If pressure drops suddenly in vessel, what is the action of intrinsic influence on vessel radius?
Pressure drop > blood flow decreases > autoregulation induce vasoconstriction to maintain flow
Explain myogenic autoregulation.
Vascular SM respond to stretch by depolarization and contracting back to normal size (negative feedback)
Explain metabolic autoregulation.
Waste chemicals are released from tissue in proportion to rate of metabolic activity, produce vasodilation > keep flow to tissue constant
Give some examples of metabolic autoregulation in intrinsic influence of vessel radius.
Explain the action of metabolic autoregulation if blood flow drops?
low O2 supply to tissue > tissue produces more anaerobic respiration metabolites (e.g. lactate) > accumulation causes vasodilation to receive more blood for O2
Explain the action of metabolic autoregulation during exercise? (Hyperaemia)
Exercise > tissue metabolic activity increase> Require more O2 and produce more metabolites > interstitial conc. of metabolite increases > vasodilation to increase flow
Explain the action of metabolic autoregulation during INTERRUPTED blood flow? (e.g. sitting and compressing on leg vessels)(Reactive hyperaemia)
Interrupted flow to tissue > flow decreased, interstitial metabolite concentration increases, cannot be washed away > stimulation to chemoreceptors (tingling sensation) > vasodilation to restore flow to higher rate until excess metabolites are washed