L37 – Control of Blood Flow

Question 1

Blood flow in what pressure gradient?

Answer 1

From high pressure to low

Question 2

Why does pressure drop in vessels as blood moves through?

Answer 2

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

Question 3

Up to which vessels can the effects of heart beat be observed as pulsations in pressure?

Answer 3

Up to arterioles before capillaries

Question 4

How is blood flow related to resistance and pressure gradient?

Answer 4

Flow = pressure gradient / resistance

Question 5

Give Poisseuille’s equation.

Answer 5

Flow (Q) = [π (P1-P2) r^4] / 8nl

P1-P2 = pressure gradient
n = viscosity 
l = length 
r= radius

Question 6

What is the relationship between flow and radius?

Answer 6

Q ∝ r^4

small change in radius = big change in flow

Question 7

Given flow = pressure gradient / resistance

Give equation for resistance and state relationship between resistance and radius?

Answer 7

Resistance = pressure gradient/ Flow

resistance = 8nl/πr^4

Resistance ∝ 1/ r^4

Larger diameter = lower resistance

Question 8

In Poisseuille’s equation, which constants are actively controlled to change blood flow?

Answer 8

radius is ACTIVELY changed to alter flow

viscosity and length can change but is Not actively controlled

Question 9

What smooth muscles contract in vessels for vasoconstriction?

Answer 9

contraction of spirally-arranged smooth muscle in wall

Question 10

How much of total peripheral resistance is contributed by pre-capillary resistance vessels? (small arteries and arterioles) How?

Answer 10

2/3 of TPR

Pre-capillary resistance vessels have Thick muscular walls

Question 11

What is the remaining 1/3 of TPR generated by? Why only 1/3? (think SM in walls)

Answer 11

Venules and small veins

Have Longitudinal (not spiral) SM that do not constrict lumen when contracted

Question 12

What are pre-capillary resistance vessels mainly responsible for?

Answer 12

control of blood flow to organs or tissues

Question 13

What are the 2 factors that modify blood vessel radius?

Answer 13

Passive influence

Active influence

Question 14

What is passive influence in control of blood flow?

Answer 14

Passive stretch of blood vessel wall due to transmural pressure acting on it

Question 15

What is active influence in control of blood flow?

Answer 15

Factors that modify the EXTENT of Vascular Smooth Muscle Contraction in Arterioles

Question 16

How is compliance calculated?

Answer 16

Volume change per unit pressure change

Low compliance = hard to stretch

Question 17

What pressures are exerted in transmural pressure? How is final radius determined?

Answer 17

Internal pressure- blood in vessel push outwards

External pressure - compression of vessel

Final radius determined by Transmural pressure (Internal P - External P) & vessel compliance

Question 18

What 4 factors affect compliance hence the final radius of a vessel?

Answer 18

1) Wall thickness
2) Wall composition
3) Neurological influence (by SM tone)
4) Pathological conditions

Question 19

Why do veins retain majority of blood, not arteries? (think compliance)

Answer 19

Thick-walled arteries are less compliant than thin-walled veins

70% of blood volume stays in veins (need to exert the least pressure)

Question 20

What macromolecules compose vessel walls? Compare their compliance.

Answer 20

Elastin (high amount in arteries) is more compliant than collagen

Question 21

How does age affect wall compliance?

Answer 21

Ageing > wall composition changes > less elastin more collagen > overall less compliant

Question 22

How does neurological influence impact vessel final radius?

Answer 22

high sympathetic activity / high adrenaline > increase sympathetic tone > smooth muscle contracts in vessel wall > harder to stretch, less compliant than relaxed smooth muscle

Question 23

What are the 2 main pathological conditions that can change vessel wall compliance and affect final radius?

Answer 23

Hypertension

Atherosclerosis

Question 24

How does Hypertension change vessel compliance? (remember NO)

Answer 24

High BP damages endothelium > fails to
secrete factors (e.g. nitric oxide) to relax vascular
smooth muscle > low compliance > hypertension
progresses

+ high BP = deposit collagen = less compliant

Question 25

How does Atherosclerosis change vessel compliance?

Answer 25

1. Obstructing the lumen; and 2. Reducing wall compliance and passive stretch Atherosclerotic plaque deposit is hard to distend, very low compliance

Question 26

What are the 2 groups of factors under ACTIVE influences of vessel radius?

Answer 26

Extrinsic factors - from outside tissue Intrinsic factors - from inside tissue

Question 27

What are the 4 categories of active influences on vessel radius ?

Answer 27

Neural Hormonal Myogenic Metabolic

Question 28

Which categories of active influences on vessel radius are extrinsic, which are intrinsic?

Answer 28

``` Extrinsic = Neural, hormonal Intrinsic = myogenic, metabolic ```

Question 29

What is the role of extrinsic factors to modify vessel radius?

Answer 29

Enable ADAPTATION to different situations Modify blood flow to achieve appropriate distribution of blood flow between different organs and tissues

Question 30

What is the role of Intrinsic factors to modify vessel radius?

Answer 30

produce AUTOREGULATION Match supply of blood to demand/ needs of particular organ

Question 31

Using haemorrhage as an example, how do both extrinsic and intrinsic factors manage the situation?

Answer 31

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

Question 32

What are the individual roles of the extrinsic factors - neural (sympathetic and parasym. ) and hormonal?

Answer 32

``` Sympathetic = vasoconstrict Parasympathetic= vasodilate Hormonal= vasoconstrict ``` *Note parasym. innervates very few vessels*

Question 33

Name some hormones used in extrinsic influence over vessel radius?

Answer 33

Adrenaline Angiotensin Antidiuretic hormone

Question 34

Difference in overall effect between the neural and hormonal extrinsic influences on vessel radius?

Answer 34

Nerves change distribution between different organs Hormones produce global response with similar effects on most vessels

Question 35

Why are hormones constrictors and not dilators?

Answer 35

Dangerous to have circulating dilator hormones because low BP may lead to inadequate blood flow to brain

Question 36

Explain the action of sympathetic noradrenergic nerves on vessel raidus.

Answer 36

Sympathetic noradrenergic nerves supply ALL vascular SM > Release noradrenaline > act on a- adrenergic receptors in vascular SM > vasoconstriction

Question 37

How does withdrawal of sympathetic tone lead to vessel dilation?

Answer 37

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*

Question 38

How do parasympathetic cholinergic nerves lead to vasodilation?

Answer 38

Parasympathetic cholinergic nerve releases Ach > act on Muscarinic type cholinergic receptors > Increase in cholinergic tone > vasoconstrict

Question 39

What is the relationship between flow and pressure with and without autoregulation?

Answer 39

Without autoregulation: Flow proportional to pressure, linear With autoregulation: Flow maintained within a range of different pressures, non-linear

Question 40

What does autoregulation counter-act in vessels?

Answer 40

Passive response to change in pressure

Question 41

If pressure drops suddenly in vessel, what is the action of intrinsic influence on vessel radius?

Answer 41

Pressure drop > blood flow decreases > autoregulation induce vasoconstriction to maintain flow

Question 42

Explain myogenic autoregulation.

Answer 42

Vascular SM respond to stretch by depolarization and contracting back to normal size (negative feedback)

Question 43

Explain metabolic autoregulation.

Answer 43

Waste chemicals are released from tissue in proportion to rate of metabolic activity, produce vasodilation > keep flow to tissue constant

Question 44

Give some examples of metabolic autoregulation in intrinsic influence of vessel radius.

Answer 44

Lactate CO2 Adenosine

Question 45

Explain the action of metabolic autoregulation if blood flow drops?

Answer 45

low O2 supply to tissue > tissue produces more anaerobic respiration metabolites (e.g. lactate) > accumulation causes vasodilation to receive more blood for O2

Question 46

Explain the action of metabolic autoregulation during exercise? (Hyperaemia)

Answer 46

Exercise > tissue metabolic activity increase> Require more O2 and produce more metabolites > interstitial conc. of metabolite increases > vasodilation to increase flow

Question 47

Explain the action of metabolic autoregulation during INTERRUPTED blood flow? (e.g. sitting and compressing on leg vessels)(Reactive hyperaemia)

Answer 47

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

Question 48

Passive, active influences Instrinsic, extrinsic factors Categorize them.

Answer 48

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