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Cardiovascular System > Control of Blood Flow > Flashcards

Control of Blood Flow Flashcards

1
Q

What is TPR?

A

Total peripheral resistance

- controls blood flow and blood pressure

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2
Q

What is TPR controlled by?

A
  • Poiseuille’s Law (r⁴)
  • Role of arteries / capillaries
  • Myogenic response
  • Blood viscosity
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3
Q

What is Venous return?

A

Rate of blood flow back to the heart

Venous return describes central venous pressure and the major mechanisms of venous return

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4
Q

What is venous return determined by?

A

Determined by factors affecting:

  • volume
  • pressure
  • distribution of blood in veins
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5
Q

Outline Darcy’s Law

A

Blood flow (CO) = (Pa - CVP) x G

(Pa - CVP) = pressure gradient
G = conductance (reciprocal of TPR (1/tpr))

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6
Q

How would an increase in TPR affect flow and pressure of blood?

A

Increase in resistance means need to increase pressure to maintain flow

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7
Q

Explain the relationship between Blood Flow (CO) and resistance

A

CO is equivalent to the pressure difference across blood vessels
CO is inversely proportional to TPR - if TPR increases, CO decreases

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8
Q

Explain the relationship of CO and conductance (G) with TPR

A

TPR is resistance to flow
Reciprocal of resistance (1/tpt) = conductance (G)

when conductance increases, CO also increases

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9
Q

What causes blood flow?

A

The pressure drop between arteries and arterioles cause blood flow

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10
Q

How does vasodilation affect blood flow?

A

Decreased BP upstream leads to decreased TPR, but greater flow

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11
Q

How does vasoconstriction affect blood flow?

A

Increased BP upstream, causes an increase in TPR but less flow

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12
Q

What is hypertension?

A

The over-constriction of arterioles, causing higher arteriole BP but less capillary flow –> under perfusion

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13
Q

How is blood flow maintained?

A

Blood flow changes in response to changes of need

The brain stem areas control sympathetic nervous activity to various areas of the body

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14
Q

What occurs to blood flow when a person is sedentary?

A

Superior mesenteric dilated
- increased flow to intestines

Common iliac constricted
- decreased flow to legs

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15
Q

What happens to blood flow during exercise?

A

Superior mesenteric constricted
- decreased flow to intestines

Common iliac dilated
- increased flow to legs

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16
Q

What is Poiseuile’s Law?

A

Describes the parameters governing TPR

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17
Q

Outline the equation for Poiseuille’s Law (resistance)?

A

Resistance = 8ɳL / πr⁴

ɳ - blood viscosity
L - vessel length
r⁴ - radius (controls TPR)

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18
Q

Outline the equation for Conductance and Poiseuille’s Law

A

G = πr⁴ / 8ɳL

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19
Q

What is the combined equation of Darcy and Poiseuille’s Law?

A

CO = (Pa - CVP) X (πr⁴ / 8ɳL)

these parameters illustrate why vessel radius is a significant determinant in changing blood flow

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20
Q

Describe the r⁴ effect on vessels

A

Doubling the radius of a vessel , causes a change in r⁴ by x16 ∴ there is 16x greater flow as flow is proportional to r⁴

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21
Q

What is the effect of vasodilators / vasoconstrictors on vessel radius?

A

Vasoconstrictors and vasodilators produce small change sin vessel radius, by affecting smooth muscle, producing larger affects on blood flow

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22
Q

Which are the main vessels involved in TPR?

A

arterioles

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23
Q

Which vessels have the largest pressure drop?

A

Arterioles have the largest pressure drop of 40-50 mmHG amongst all vessels.

24
Q

How is arteriole radius regulated?

A

Arteriole radius is tightly controlled by the sympathetic nerves providing constant tone - both dilate and constrict

25
Q

What are the 3 parameters regulating TPR?

A
  • Radius r⁴
  • Pressure difference across vessels
  • length L
26
Q

What is the role of capillaries in TPR?

A

TPR is not controlled by capillaries despite their smaller radius than arterioles

27
Q

Why do capillaries not affect TPR?

A
  • Small pressure drop in capillaries as less resistance to
    blood flow
  • less resistance in capillaries as bolus flow reduces
    viscosity
  • individual capillaries are short
  • no sympathetic innervation or smooth muscle, so
    capillary radius isn’t altered
28
Q

Why is total resistance of capillaries so low?

A

Capillaries are arranged in parallel, so have a low total resistance

29
Q

What causes arterioles to have a higher total resistance?

A

Arterioles are arranged in series

30
Q

How is local blood flow controlled?

A

Local blood flow through individual organs / tissues is mainly controlled by changes in radius of arterioles supplying that given organ / tissue

31
Q

What are intrinsic controls?

A

Factors entirely within an organ or tissue, allowing responses to local factors

32
Q

What are extrinsic control mechanisms?

A

Factors outside the organ or tissue

33
Q

Give examples of intrinsic control mechanisms

A
  • local hormones
  • tissue metabolites
  • myogenic properties
  • endothelial factors
34
Q

What are some of the extrinsic controls in our body?

A

Hormonal
- e.g. adrenaline

Neural
- e.g. sympathetic nervous system

35
Q

What causes a vessel to constrict?

A

increased distension of a vessel

36
Q

What causes dilation?

A

Decreased distension of a vessel causes dilation

37
Q

What is the Bayliss myogenic response?

A

pressure-induced vasoconstriction

  • at higher arterial pressures, when arteries are stretched,
    it contracts to reduce blood flow
  • stretching the muscle causes ion channels to open,
    which then depolarise, leading to muscle contraction
38
Q

What is the function of the bayliss myogenic response?

A

Maintains blood pressure during changing arterial pressures

- important in renal, coronary and cerebral circulation

39
Q

What factors does blood flow depend on?

A
  • Blood viscosity
  • Vessel diameter
  • Haematocrit
40
Q

What is velocity in blood flow?

A

velocity is a measure of internal friction opposing the separation of the lamina

41
Q

How does vessel diameter affect blood viscosity?

A

Blood viscosity falls in narrower tubes ( >100 um vessels)
cells move to the centre, reducing friction
- decreases Resistance
- increases blood flow in micro vessels

e.g. capillaries

42
Q

What is the normal haematocrit?

A

(45%)

43
Q

What is haematocrit?

A

ratio of the volume of red blood cells compared to the total volume of blood

44
Q

What is the consequences high viscosity?

A

Polycythaemia (high ɳ)

  • increased TPR
  • Increased BP
  • Decreased blood flow
45
Q

What is low ɳ lead to?

A

Anaemia (low ɳ)

  • Decreased TPR
  • Decreased BP
  • Increased blood flow
46
Q

How does red cell deformability affect viscosity?

A

increases ɳ
decreases blood flow
sickle cell anaemia arises

47
Q

How does remaining sedentary affect velocity of blood?

A

Slow venous flow in immobile legs

increased velocity due to partial clotting

48
Q

What is the role of veins / venules?

A

functions as blood reservoirs

60% of blood volume at rest is held in systemic veins and venules

49
Q

What is the use of the blood reservoir in venous system?

A

blood can be diverted from the reservoir in times of need

e.g. exercise, haemorrhage etc.

50
Q

Describe the structure of veins

A
  • Thin walled, collapsible voluminous vessels
  • Contains 2/3 of blood volume
  • Contractile: contains smooth muscle innervated by
    sympathetic nerves
  • Thinner than arterial muscle and more compliant
    forming blood reservoir
51
Q

What is the significance of vessel contraction?

A

Contraction of vessels expels blood into central veins

  • > increases venous return / CVP /EDV
  • > increases SV (Starling’s Law)
52
Q

What are the typical venous pressures in the body?

A

Limb veins, heart level: 5 - 10 mmHg
CVP entering heart: 0 - 7 mmHg
Foot vein while standing: 90 mmHg

53
Q

Explain the venous return pressure - volume loop

A
  1. High Venous pressure at the feet
  2. So pressure returns to the heart - aided by thoracic
    pump and skeletal muscle
  3. Sympathetic nerve stimulation causes venconstriction
    • blood shifted centrally
  4. increased venous return, CVP and EDV
    - > increased CVP = ↑preload = ↑SV
54
Q

State Bernoulli’s Law

A

mechanical energy of flow os determined by pressure, kinetic and potential energies

55
Q

Outline the equation for Bernoulli’s Law

A

Energy = Pressure (PV) + kinetic (ρV2/2) + potential (ρgh)

56
Q

Using bernoulli’s law describe blood flow during orthostasis

A

-90 mmHg pressure gradient against blood flow back to the heart from feet
Ejected blood at the heart has a greater KE (than at feet)
due to more velocity
Also greater potential energy at heart due to more height

57
Q

How does blood flow back to the heart from the feet despite the pressure gradient ?

A

Greater kinetic and potential energies overcome the pressure gradient to maintain flow
- returning blood to heart has no pressure gradient
(only KE)

Cardiovascular System (16 decks)

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