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Control of blood flow Flashcards

1
Q

What are the three main things that control Total Peripheral Resitance?

A
  1. Poiseuille’s law
  2. Myogenic response
  3. Blood viscosity
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2
Q

Darcy’s law: how is blood flow (Cardiac Output) calculated?

A

Cardiac output = (Arterial pressure - Central venous pressure) / Total periphral resistance

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3
Q

How is conductance calculated?

A

G= 1/TPR (reciprocal of total peripheral resistance)

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4
Q

How is blood flow calculated (in terms of pressure gradient and conductance)

A

Cardiac output = Arterial pressure - (Central venous pressure x conductance)

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5
Q

What is the mean arterial pressure?

A

90 mmHg

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6
Q

How does TPR control blood flow and blood pressure?

A

Increase in resistance results in the need to increase the pressure and keep the same flow

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7
Q

What is hypertension?

A
  • Over constriction of arterioles
  • Higher arterial blood pressure but less capillary flow
  • Increased peripheral resistance: can under perfuse vital organs with reduced blood flow even though the blood pressure is high
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8
Q

How does TPR affect both blood flow and blood pressure?

A

Blood flow (CO) = pressure gradient/TPR

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9
Q

What occurs when there is a pressure drop between arteries and arterioles?

A

Normal blood flow

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10
Q

What occurs when there is decreasd BP in the arteries and decreased TPR in the arterioles?

A

Vasodilation in the arteriole:

  • decreased blood pressure upstream
  • greater flow
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11
Q

What occurs when there is increased BP in the arteries and increased TPR in the arterioles?

A

Vasoconstriction in the arterioles:

  • Increased blood pressure upstream
  • Less flow
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12
Q

What are the changes in blood flow during the Sedentary state?

A
  • Superior mesenteric dilated, leading to increased flow to intestines
  • Common iliac constricted, leading to decreased flow to the legs
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13
Q

What are the changes in blood flow during Exercise?

A
  • Superior mesenteric constricted, leading to decreased flow to the intestines
  • Common iliac dilated, leading to increased flow to the legs
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14
Q

Darcy’s law

A

CO = Pa - CVP x G

CO: Blood flow
Pa: Arterial pressure
CVP: Central venous pressure
G: Conductance

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15
Q

Poiseuille’s Law (with resistance being the subject of the formula)

A

Resistance: (8 x N x L)/ pi x r4

r: radius of vessel
N: blood viscosity
L: vessel length
Blood vessel radius to power of 4 controls TPR

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16
Q

Poiseuille’s Law (with conductance being the subject of the formula)

A

Conductance (G): (pi x r^4) / 8 x N x L

r: radius of vessel
N: blood viscosity
L: vessel length
Blood vessel radius to the power of 4 controls TPR

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17
Q

What is Poiseille’s Law?

A

Describes the parameters that govern TPR

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18
Q

What is the combined Darcy and Poiseuille’s law?

A

CO= Pa - CVP x [ (pi x r^4) / (8 x N x L) ]

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19
Q

Describe the r^4 effect

A

With the same arterial blood pressure, doubling the radius vessel means that the change in r^4 is equal to 2^4 which is 16.

  • Vessel 2 therefore equals 1/16th of the resistance of vessel 1
  • There is 16 times greater flow in vessel 2 (as the flow is proportional to r^4)
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20
Q

What is the pressure drop in arterioles?

A

Arterioles have the largest pressure drop of 40-50 mmHg among vessels

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21
Q

What is the radius of arterioles controlled by?

A

Arteriole radius is tightly controlled by sympathetic nerves providing constant tone: we both dilate and constrict

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22
Q

What are the 3 main parameters that TPR is controlled by?

A
  1. Radius: r^4
  2. Pressure difference across vessels: P1-P2
  3. Length
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23
Q

What vessel is TPR not controlled by?

A

Capillaries

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24
Q

Why does pressure drop influence the capillaries’ inability to control TPR?

A

Less pressure drop across capillaries due to less resistance to blood flow in capillaries

25
Why does Radius influence the capillaries' inability to control TPR?
No sympathetic innervation/smooth muscle in capillaries so cannot alter radius
26
Why does length influence the capillaries' inability to control TPR?
Individual capillaries are short
27
Why does blood viscosity influence the capillaries' inability to control TPR?
Less resistance in capillaries because blood flow reduces viscosity
28
What is the difference in arrangement in capillaries and arterioles and how does this influence resistance?
Capillaries are arranged in parallel, so have a low total resistance as Rtotal= 1/R1 + 1/R2. Whereas, arterioles are arranged in series so the total resistance is greater as Rtotal = R1 + R2
29
What is local blood flow through individual/organs and tissues mainly controlled by?
Changes in radius of arterioles supplying a given tissue/organ
30
What is the meaning of intrinsic control mechanisms of arteriole radius?
Factors that are entirely within an organ or tissue (allow response to local factors)
31
What is the meaning of extrinsic control mechanisms of arteriole radius?
Factors outside the organ or tissue (nervous and hormonal control)
32
List examples of intrinsic control mechanisms of arteriole radius
- Local hormones: e.g. in the event of a bee sting - Tissue metabolites - Myogenic properties of the muscle - Endothelial factors: the endothelium is constantly producing low amounts of Nitrous Oxide which has the tendency to dilate blood vessels
33
List examples of extrinsic control mechanisms of arteriole radius
- Neural: e.g. sympathetic nervous system | - Hormonal: e.g. adrenaline
34
What is muscle tone?
Situation in between contraction and relaxation
35
What is the effect of increased distension of a vessel?
It constricts
36
What is the effect of decreased distension of a vessel?
It dilates
37
Why is the expected linear resistance (for flow vs pressure) different to the true curve?
BAYLISS MYOGENIC RESPONSE: - Having a linear relationship would entail very large differences in blood flow with differences in pressure - Maintains blood flow at the same level during changes in arterial pressure - At higher pressures, when the vessel is stretched it contracts to reduce flow (which is not linear)
38
What are the three factors that blood flow depends on?
1. Viscosity of blood 2. Vessel diameter 3. Haematocrit
39
What is viscosity a measure of?
Internal friction opposing the separation of the lamina
40
What are the clinical implications of: - Haematocrit (45%) - Typical blood viscosity: 4-5
- Polycythaemia (high blood viscosity), which increases TPR, increases blood pressure and decreases blood flow - Anaemia (low blood viscosity): decreases TPR, decreases blood pressure and increases heart rate (as a result of baroreceptor reflex)
41
What is Haematocrit equal to?
Red Blood Cell number
42
What are the clinical implications of Tube diameter (Fahraeus-Lindqvuist effect)?
- Blood viscosity falls in narrow tubes (<100 um vessels) | - decrease in resistance and increase in blood flow in microvessels like capillaries
43
What are the clinical implications of Red cell deformability?
- Increase in blood viscosity - Decrease in blood flow - Sickle cell anaemia crises
44
What are the clinical implications of Velocity of blood?
Slow venous flow in immobile legs: increased viscosity due to partial clotting.
45
% of blood volume at rest in pulmonary vessels?
12%
46
% of blood volume at rest in heart?
8%
47
% of blood volume at rest in systemic arteries and arterioles?
15%
48
% of blood volume at rest in systemic capillaries?
5%
49
% of blood volume at rest in systemic veins and venules? what is the significance of this?
60% - the systemic veins and venules function as blood reservoirs - blood can be diverted from them in times of need, e.g. exercise, haemorrhage
50
What are the properties of veins that means they form blood reservoirs?
- Thin-walled, collapsible, voluminous vessels - Contain 2/3rds of blood volume - Contractile: contain smooth muscle - Innervated by sympathetic nerves - Thinner and more compliant than arterial muscles Therefore form blood reservoirs
51
How does contractility influence the volume of blood in veins?
- The contraction of the vessels: expels blood into central veins - Increased venous return/CVP/end-diastolic volume - Increases stroke volume (Starling's law)
52
Typical venous pressures in the limb vein, heart level
5-10 mmHg
53
Typical venous pressures in the central venous
0-7 mmHg
54
Typical venous pressures in the foot vein, while standing
90 mmHg
55
What happens to veins under low pressure?
Veins collapse
56
What happens to veins under high pressure?
Veins distend
57
Describe the pressure-volume curve of veins:
- Stimulation of sympathetic nerves causing vasoconstriction shifts blood centrally - Increases venous return, CVP and end-diastolic pressure - Increased CVP increases preload and so increases stroke volume (Starling's law)
58
How does Bernoulli's law explain blood flow?
Mechanical energy of flow is determined by: pressure, kinetic, potential energy, (p= fluid mass) ENERGY= PRESSURE (PV) + KINETIC (PV^2/2) + POTENTIAL (PGH)

Cardiovascular system (16 decks)

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