CVS S5 - Blood Flow

Question 0

Define flow

Answer 0

The volume of fluid passing a given point per unit time

Question 1

What drives flow?

Answer 1

A gradient of pressure

Question 2

Define velocity

Answer 2

Rate of movement of fluid particles along the tube

Question 3

What is laminar flow?

Answer 3

There is a gradient of flow from the middle to the edge of a blood vessel
Velocity is highest at the centre
Fluid is stationary at the edge
Most blood flow is laminar

Question 4

What is turbulent flow?

Answer 4

When layers of fluid try to move over one another faster than physics will allow
As velocity increases, eventually flow will become turbulent
Turbulence greatly increases resistance

Question 5

Define viscosity

Answer 5

The extent to which fluid layers resist flowing over one another

Question 6

What are the effects of viscosity on flow?

Answer 6

The higher the viscosity, the lower the difference between the velocity of flow in the middle and the edge of a vessel
The higher the viscosity, the lower the velocity

Question 7

What is the effect of tube diameter on velocity?

Answer 7

Velocity is proportional to cross sectional area of a tube

Question 8

How does viscosity affect resistance?

Answer 8

The greater the viscosity, the greater the resistance

Question 9

Describe the relationship between pressure and resistance if flow is fixed

Answer 9

If flow is fixed, then the higher the resistance, the greater the pressure change from one end of the vessel to the other.

Question 10

Describe the relationship between flow and resistance if pressure is fixed

Answer 10

If pressure is fixed then the higher the resistance, the lower the flow.

Question 11

What is the relationship between radius of vessel and resistance?

Answer 11

Resistance decreases with the 4th power of the radius

This means small changes in radius can have a great effect on resistance

Question 12

How is resistance combined for vessels in series?

Answer 12

The resistances for each vessel are added

Question 13

How is resistance combined for vessels in parallel?

Answer 13

The resistance for each is reduced as there is another path for the blood to go

Question 14

Describe flow over the systemic circulation

Answer 14

Flow remains the same at all points

Question 15

Describe resistance and pressure changes in arteries

Answer 15

Resistance is low in arteries

There is a low pressure drop from one end of an artery to the other

Question 16

Describe resistance and pressure changes in arterioles

Answer 16

Resistance is high

Pressure change from one end of an arteriole to the other is high

Question 17

Describe resistance in veins and venules

Answer 17

Resistance is low

Pressure change from one end to the other is low

Question 18

Why are arteries high pressure?

Answer 18

Because the arterioles directly after them are high resistance
It’s hard to push blood into them
Therefore pressure increases

Question 19

What is transmural pressure?

Answer 19

Stretches vessels

Generated by pressure in distensible vessels

Question 20

Describe blood flow through distensible vessels

Answer 20

The higher the pressure in the vessel, the more stretched the vessel so the wider the lumen so more laminar flow so lower the resistance so flow increases
If pressure falls to zero, vessel walls collapse and flow ceases

Question 21

Describe the capacitance property of distensible vessels

Answer 21

As vessels widen under increasing pressure, more blood transiently flows in than out
This allows distensible vessels to store blood - conferring capacitance
Veins are the most distensible (67% of their blood is at rest)

Question 22

Describe systolic arterial pressure and what factors affect it

Answer 22

Maximum arterial pressure
Usually ~120 mmHg
Affected by:
-How hard the heart pumps
-Total peripheral resistance (TPR)
-Compliance (stretchiness) of arteries

Question 23

Describe diastolic arterial pressure and what factors affect it

Answer 23

Minimum arterial pressure
Usually ~80mmHg
Affected by:
-Systolic pressure
-TPR

Question 24

What is pulse pressure?

Answer 24

The difference between systolic and diastolic pressure | Usually ~40mmHg

Question 25

What is the average arterial pressure?

Answer 25

Diastolic pressure plus 1/3rd of systolic pressure | Because systole is shorter than diastole

Question 26

Define TPR

Answer 26

Total peripheral resistance | The sum of the resistance of all the peripheral vasculature of the systemic circulation

Question 27

What is the pulse wave?

Answer 27

A pulse wave is brought about by contraction of the ventricles Travels faster than blood Can be felt at various locations where arteries may be pushed against a reasonably hard surface

Question 28

What is the dicrotic notch?

Answer 28

The slight dip in the pulse wave caused by slight back flow as blood flows back through the aortic valve as pressure in the LV goes below that in the aorta

Question 29

What is the dicrotic wave?

Answer 29

The slight rise seen in the pulse wave directly after the dicrotic notch Caused by the recoil of blood as the aortic valve closes

Question 30

Describe the importance of smooth muscle in arteriolar walls

Answer 30

Smooth muscle can be contracted or relaxed so can increase or decrease resistance and blood flow to tissues

Question 31

What is vasomotor tone?

Answer 31

The degree to which vascular smooth muscle is contracted If more contracted, vasoconstriction If less contracted, vasodilation/vasodilatation In all but maximum flow conditions, muscle is at least partially contracted

Question 32

What determines vasomotor tone?

Answer 32

Mainly a sympathetic branch of the ANS Alpha 1 adrenergic receptors Antagonised by vasodilator factors, so tone is determined by a balance

Question 33

What are vasodilator metabolites and what do they do?

Answer 33

Produced by metabolically active tissues Eg K, H, adenosine Cause relaxation of local smooth muscle So increase flow and decrease resistance

Question 34

What is reactive hyperaemia?

Answer 34

If a limb has blood flow cut off for a few minutes then when restored, the local arterioles will have dilated maximally (due to buildup of vasodilator metabolites) so blood flow will be very high

Question 35

What is auto regulation?

Answer 35

At most levels of metabolic activity, organs automatically take the amount of blood they require so long as the pressure of the arteries remains within a certain range

Question 36

Define central venous pressure

Answer 36

The pressure in the great veins

Question 37

Define venous return

Answer 37

The rate of flow of blood back to the heart (limits cardiac output)