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The Cardiovascular system > Peripheral circulation, pressure, flow and resistance > Flashcards

Peripheral circulation, pressure, flow and resistance Flashcards

1
Q

What is the relationship between flow of blood and pressure difference of vessels?

A

•Flow of blood through blood vessels driven by
gradient of pressure
• Flow is proportional to the pressure difference between the ends of a vessel
• The higher the pressure difference the greater the flow

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2
Q

At a given pressure gradient, what determines the flow rate of blood?

A

• The flow for a given pressure gradient is
determined by the resistance of the vessel
• Resistance is determined by the nature of
the fluid and the vessel

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3
Q

Define flow.

A

The volume of fluid passing a given point per unit time

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4
Q

Define velocity.

A

The rate of linear movement of fluid particles along the tube

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5
Q

What is the relationship between flow and velocity?

A

•To investigate this, flow must be the same at
all points along a vessel
• Velocity can vary along the length if the radius of the tube changes
• At a given flow velocity is inversely proportional to cross sectional area:
• vessels with small cross sectional area
have a high velocity e.g. aorta
• vessels with large overall cross sectional
area have a low velocity e.g. capillaries

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6
Q

Describe laminar flow.

A
  • In most blood vessels flow is laminar
  • There is a gradient of velocity from the middle to the edge of the vessel
  • Velocity is highest in the centre
  • Fluid is stationary/slower at the edge
  • Fluid layers must slide over one another
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7
Q

Describe turbulent flow.

A
  • As the mean velocity increases flow eventually becomes turbulent
  • The velocity gradient breaks down
  • Fluid tumbles over
  • Flow resistance greatly increased
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8
Q

In a vessel with constant pressure, what determines the flow rate?

A 
• The flow will be determined by the mean
velocity
• The mean velocity depends upon:
– the viscosity of the fluid
– the radius of the tube
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9
Q

What is viscosity, and how does it affect velocity?

A
  • The extent to which fluid layers resist sliding over one another
  • The higher the viscosity the slower the central layers will flow, and the lower the average velocity
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10
Q

What determines cardiac output?

A

Stroke volume and heart rate

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11
Q

What is stroke volume?

A

The amount of blood pumped out of the heart per heartbeat. Usually about 80ml.

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12
Q

What is heart rate measured in?

A

Beats per minute.

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13
Q

What is the average resting heart rate, and why?

A

Around 60 bpm. Intrinsic heart rate is around 100bpm, however at rest parasympathetic tone from the vagus nerve slows it down.

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14
Q

What happens at each heart beat?

A

The stroke volume is pushed into the arteries.

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15
Q

Describe the resistance and pressure in arteries.

A

Low resistance vessels

Contain high pressure

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16
Q

Why is the pressure in arteries high?

A

In order to be able to drive the cardiac output through the resistance of the arterioles.

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17
Q

What is the total peripheral resistance?

A

The combined resistance of the arterioles.

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18
Q

What is pulsatile flow?

A
  • The heart ejects blood intermittently
  • In systole blood flows into arteries
  • In diastole it does not
19
Q

What would happen if arteries had rigid walls?

A
  • The pressure would rise really high in systole to force the whole stroke volume through the total peripheral resistance
  • And fall to zero in diastole
20
Q

What does capacitance mean?

A

Not all blood has to leave during systole, but can leave the vessel in diastole- a sudden increase in flow rate does not mean that it has to flow out other end immediately. This happens in vessels with distensible walls (especially veins).

21
Q

Why is it important for arteries to have distensible walls?

A
  • In systole, arteries stretch
  • More blood flows in than out
  • So pressure does not rise so much
  • As arteries recoil in diastole, flow continues through the arterioles
22
Q

What is systolic pressure?

A

The maximum pressure in an artery during systole (typically 120mmHg)

23
Q

What is diastolic pressure?

A

The minimum pressure in an artery during diastole (typically 80 mmHg)

24
Q

Which factors affect systolic pressure?

A
  • How hard the heart pumps
  • The total peripheral resistance
  • Stretchiness (‘compliance’) of the arteries (As compliance increases, pressure decreases, as not all blood flows straight out of the vessel. As we age, arteries become less compliant, systolic pressure rises).
25
Q

Which factors affect diastolic pressure?

A

Systolic pressure and total peripheral resistance.

26
Q

Is systolic or diastolic pressure a better indicator of total peripheral resistance and why?

A

Diastolic pressure, as it is not affected by compliance of arteries.

27
Q

What is pulse pressure?

A
  • The difference between systolic and diastolic
  • Typically about 40 mmHg
  • It gives information about force of contraction of heart- may increase e.g. in exercise
28
Q

How is average pressure calculated, and why?

A

It’s calculated as diastolic plus 1/3 pulse pressure, because systole is shorter than diastole.

29
Q

Name the resistance vessels.

A

Arterioles and precapillary sphincters

30
Q

What are precapillary sphincters?

A

Rings of smooth muscle on arterial end of capillaries. They control blood flow to skeletal muscle. The open more to increase blood flow.

31
Q

Why do arterioles have high resistance?

A

Due to a narrow lumen. Lumen is narrowed by tonic contraction of smooth muscle in the arteriole walls.

32
Q

What is vasomotor tone?

A
  • Tonic contraction of smooth muscle is known as vasomotor tone
  • Increases- known as vasoconstriction (increases resistance to flow)
  • Decreases- vasodilatation (decreases resistance to flow)
33
Q

Which factors affect vasomotor tone?

A
  • Vasomotor tone mostly produced by the sympathetic branch of the autonomic nervous system (alpha receptors innervated by noradrenaline)
  • This tone is antagonised by vasodilator factors
  • The actual resistance determined by balance between the two
34
Q

What is reactive hyperaemia?

A
  • If the circulation to, say an arm is cut off for a minute or two
  • When blood flow is restored
  • There is an enormous increase for a short while
35
Q

What are vasodilator metabolites?

A

Produced by metabolically active tissues.
These act to relax vascular smooth muscle, thereby increasing blood flow to metabolically active tissues.
E.g. H+ (lactic acid- signal to increase blood flow), K+ (signal for not enough oxygen) and adenosine.

36
Q

Which factors affect the action of vasodilator metabolites?

A

Their effect depends on the balance between the rate at
which they are produced, and the rate at which the blood
flow washes them away.

37
Q

How does reactive hyperaemia work? Why does it only occur for a short time?

A
  • When there is no blood flow, metabolites accumulate
  • This causes arterioles to dilate maximally
  • When flow is returned, resistance is very low
  • This means that flow is very high
  • The high flow washes away the metabolites
  • Which makes the smooth muscle constrict again
38
Q

How is blood flow matched to metabolism?

A
  • If metabolism increases, more metabolites are produced
  • So concentration increases, vasodilatation occurs, and
  • Washes away the metabolites
  • More metabolism, more blood flow
39
Q

What is autoregulation of blood flow?

A
  • If supply pressure changes, blood flow to a tissue will change
  • Which will change metabolite concentration
  • And alter the resistance of arterioles so blood flow returns to an appropriate level for metabolism
40
Q

What is the relationship between total peripheral resistance and the bodys need for blood flow?

A

TPR is inversely proportional to the bodys need for blood flow- when need for blood flow is high, TPR is low (and vice versa).

41
Q

How is the pressure in the veins determined?

A
  • Veins are very stretchy
  • The pressure in the veins is determined by the volume of blood they contain, and how stretched they are
  • Which depends on the balance between blood flowing in from the body and out via the heart
  • The easier for blood to flow through resistance vessels, the more will be in the veins
42
Q

What is central venous pressure?

A 
• The pressure in the great veins, which fills the heart in diastole
• Depends on
– return of blood from the body
– pumping of the heart
– gravity & ‘muscle pumping’
43
Q

Briefly state what arterial pressure, TPR and venous pressure do.

A
  • Arterial pressure must be high enough to ensure tissues get what blood they need
  • Total peripheral resistance falls as more blood is needed
  • Central venous pressure fills the heart

The Cardiovascular system (6 decks)

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