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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
What are the 3 parameters regulating TPR?
- Radius r⁴ - Pressure difference across vessels - length L
26
What is the role of capillaries in TPR?
TPR is not controlled by capillaries despite their smaller radius than arterioles
27
Why do capillaries not affect TPR?
- 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
Why is total resistance of capillaries so low?
Capillaries are arranged in parallel, so have a low total resistance
29
What causes arterioles to have a higher total resistance?
Arterioles are arranged in series
30
How is local blood flow controlled?
Local blood flow through individual organs / tissues is mainly controlled by changes in radius of arterioles supplying that given organ / tissue
31
What are intrinsic controls?
Factors entirely within an organ or tissue, allowing responses to local factors
32
What are extrinsic control mechanisms?
Factors outside the organ or tissue
33
Give examples of intrinsic control mechanisms
- local hormones - tissue metabolites - myogenic properties - endothelial factors
34
What are some of the extrinsic controls in our body?
Hormonal - e.g. adrenaline Neural - e.g. sympathetic nervous system
35
What causes a vessel to constrict?
increased distension of a vessel
36
What causes dilation?
Decreased distension of a vessel causes dilation
37
What is the Bayliss myogenic response?
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
What is the function of the bayliss myogenic response?
Maintains blood pressure during changing arterial pressures | - important in renal, coronary and cerebral circulation
39
What factors does blood flow depend on?
- Blood viscosity - Vessel diameter - Haematocrit
40
What is velocity in blood flow?
velocity is a measure of internal friction opposing the separation of the lamina
41
How does vessel diameter affect blood viscosity?
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
What is the normal haematocrit?
(45%)
43
What is haematocrit?
ratio of the volume of red blood cells compared to the total volume of blood
44
What is the consequences high viscosity?
Polycythaemia (high ɳ) - increased TPR - Increased BP - Decreased blood flow
45
What is low ɳ lead to?
Anaemia (low ɳ) - Decreased TPR - Decreased BP - Increased blood flow
46
How does red cell deformability affect viscosity?
increases ɳ decreases blood flow sickle cell anaemia arises
47
How does remaining sedentary affect velocity of blood?
Slow venous flow in immobile legs | increased velocity due to partial clotting
48
What is the role of veins / venules?
functions as blood reservoirs | 60% of blood volume at rest is held in systemic veins and venules
49
What is the use of the blood reservoir in venous system?
blood can be diverted from the reservoir in times of need | e.g. exercise, haemorrhage etc.
50
Describe the structure of veins
- 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
What is the significance of vessel contraction?
Contraction of vessels expels blood into central veins - > increases venous return / CVP /EDV - > increases SV (Starling's Law)
52
What are the typical venous pressures in the body?
Limb veins, heart level: 5 - 10 mmHg CVP entering heart: 0 - 7 mmHg Foot vein while standing: 90 mmHg
53
Explain the venous return pressure - volume loop
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
State Bernoulli's Law
mechanical energy of flow os determined by pressure, kinetic and potential energies
55
Outline the equation for Bernoulli's Law
Energy = Pressure (PV) + kinetic (ρV2/2) + potential (ρgh)
56
Using bernoulli's law describe blood flow during orthostasis
-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
How does blood flow back to the heart from the feet despite the pressure gradient ?
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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