Peripheral circulation, pressure, flow and resistance

Question 1

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

Answer 1

•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

Question 2

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

Answer 2

• 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

Question 3

Define flow.

Answer 3

The volume of fluid passing a given point per unit time

Question 4

Define velocity.

Answer 4

The rate of linear movement of fluid particles along the tube

Question 5

What is the relationship between flow and velocity?

Answer 5

•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

Question 6

Describe laminar flow.

Answer 6

• 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

Question 7

Describe turbulent flow.

Answer 7

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

Question 8

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

Answer 8

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

Question 9

What is viscosity, and how does it affect velocity?

Answer 9

• 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

Question 10

What determines cardiac output?

Answer 10

Stroke volume and heart rate

Question 11

What is stroke volume?

Answer 11

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

Question 12

What is heart rate measured in?

Answer 12

Beats per minute.

Question 13

What is the average resting heart rate, and why?

Answer 13

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

Question 14

What happens at each heart beat?

Answer 14

The stroke volume is pushed into the arteries.

Question 15

Describe the resistance and pressure in arteries.

Answer 15

Low resistance vessels

Contain high pressure

Question 16

Why is the pressure in arteries high?

Answer 16

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

Question 17

What is the total peripheral resistance?

Answer 17

The combined resistance of the arterioles.

Question 18

What is pulsatile flow?

Answer 18

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

Question 19

What would happen if arteries had rigid walls?

Answer 19

• 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

Question 20

What does capacitance mean?

Answer 20

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).

Question 21

Why is it important for arteries to have distensible walls?

Answer 21

• 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

Question 22

What is systolic pressure?

Answer 22

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

Question 23

What is diastolic pressure?

Answer 23

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

Question 24

Which factors affect systolic pressure?

Answer 24

• 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).

Question 25

Which factors affect diastolic pressure?

Answer 25

Systolic pressure and total peripheral resistance.

Question 26

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

Answer 26

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

Question 27

What is pulse pressure?

Answer 27

* 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

Question 28

How is average pressure calculated, and why?

Answer 28

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

Question 29

Name the resistance vessels.

Answer 29

Arterioles and precapillary sphincters

Question 30

What are precapillary sphincters?

Answer 30

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

Question 31

Why do arterioles have high resistance?

Answer 31

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

Question 32

What is vasomotor tone?

Answer 32

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

Question 33

Which factors affect vasomotor tone?

Answer 33

* 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

Question 34

What is reactive hyperaemia?

Answer 34

* 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

Question 35

What are vasodilator metabolites?

Answer 35

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.

Question 36

Which factors affect the action of vasodilator metabolites?

Answer 36

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.

Question 37

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

Answer 37

* 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

Question 38

How is blood flow matched to metabolism?

Answer 38

* If metabolism increases, more metabolites are produced * So concentration increases, vasodilatation occurs, and * Washes away the metabolites * More metabolism, more blood flow

Question 39

What is autoregulation of blood flow?

Answer 39

* 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

Question 40

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

Answer 40

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

Question 41

How is the pressure in the veins determined?

Answer 41

* 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

Question 42

What is central venous pressure?

Answer 42

``` • 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’ ```

Question 43

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

Answer 43

* 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