L37 – Control of Blood Flow Flashcards Preview

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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
r= radius


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?

Passive influence
Active influence


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
Parasympathetic= vasodilate
Hormonal= vasoconstrict

*Note parasym. innervates very few vessels*


Name some hormones used in extrinsic influence over vessel radius?

Antidiuretic hormone


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?

Without autoregulation:
Flow proportional to pressure, linear

With autoregulation:
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


Passive, active influences

Instrinsic, extrinsic factors

Categorize them.

Passive influence - e.g. transmural pressure + compliance (affected by thickness, composition, pathology and nerves)

Active influence- Intrinsic (myogenic and metabolic) and Extrinsic (neural and hormonal) factors

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