CVS 5.1 - Blood flow

Question 1

What is the definition of flow?

Answer 1

• The volume of fluid passing a given point per unit time

- Flow = volume/time

Question 2

What are the determinants of flow?

Answer 2

• Fluid viscosity

- Tube radius

Question 3

What is velocity?

Answer 3

• The rate of movement of fluid particles along a tube

- Velocity = Distance/Time

Question 4

What is velocity proportional to?

Answer 4

• 1/cross sectional area e.g. small CSA (aorta) = high velocity
• If flow rate is constant*

Question 5

What is flow proportional to?

Answer 5

• Pressure gradient

- Higher pressure difference = Greater flow

Question 6

What is resistance to flow?

Answer 6

The determinant of flow for a specific pressure gradient depending on the nature of the fluid and type of vessel

Question 7

What is laminar flow?

Answer 7

• Flow where a gradient builds up from the middle to the peripheries
• Highest velocity = central, lowest = edges (stationary)

Question 8

What is turbulent flow?

Answer 8

• Increased turbulence due to increased velocity

- Breaks down gradient and increases flow resistance

Question 9

What causes turbulent flow?

Answer 9

• High velocity
• Low viscosity
• Irregular lumen of the vessel

Question 10

What is viscosity?

Answer 10

• The extent to which fluid layers resist sliding over one another therefore determining the gradient
• Higher viscosity = lower average velocity

Question 11

What is mean velocity proportional to?

Answer 11

• 1/viscosity

- Cross sectional area (wider tube = middle layers move faster)

Question 12

What is Poiseuille’s Law?

Answer 12

At a steady, laminar flow and in a vessel that is larger than an arteriole, flow is:

• Proportional to the pressure gradient
• Proportional to the 4th power of the radius
• Inversely proportional to the length of the vessel
• Inversely proportional to the blood viscosity

Question 13

Describe the effects of combining flow resistances in series

Answer 13

• Sum of individual resistances = higher resistance

Question 14

Describe the effects of combining flow resistances in parallel

Answer 14

• Add 1/resistance etc.

- Gives a lower resistance as there are multiple paths for the blood to travel down

Question 15

Describe the relationship between resistance and pressures when flow is fixed

Answer 15

• Proportional

- Higher resistance = Greater change in pressure from end to end

Question 16

Describe the relationship between resistance and flow when pressure is fixed

Answer 16

• Inversely proportional

- Higher resistance = lower flow

Question 17

Describe the resistance and pressure in the arteries

Answer 17

• Low resistance (pressure drop is small)

- High pressure

Question 18

Describe the resistance and pressure in the arterioles

Answer 18

• High resistance (pressure drop is large)

- Low pressure

Question 19

Describe the resistance and pressure in the veins and venules

Answer 19

• Low resistance (pressure drop is small)

- Low pressure

Question 20

Why is the pressure in the arteries high?

Answer 20

• Due to the high resistance of the arterioles

- Difficult to push blood through

Question 21

What happens to arterial pressure if cardiac output increases but arteriolar resistance remains the same?

Answer 21

Arterial pressure rises

Question 22

What happens when blood vessels distend?

Answer 22

• Become larger and rounder due to transmural pressure, increaseing flow
• Walls stretch and store elastic energy (decreases resistance)

Question 23

What is the significance of distensibility?

Answer 23

• Enables vessels to ‘store’ blood i.e. they have a capacitance