CVS S5 - Blood flow

Question 1

What is the relationship between flow and velocity?

Answer 1

At fixed flow, the velocity is inversely proportional to cross sectional area

Question 2

Give an example of a vessel through which blood would travel at high velocity at a given flow rate

Answer 2

Artery

Question 3

How does velocity alter across a vessel with laminar flow?

Answer 3

Velocity gradient from middle to edges of the vessel with blood travelling faster at the centre and stationary at the edges

Question 4

What is turbulent flow?

Answer 4

As mean velocity increases turbulent flow begins whereby the velocity gradient breaks down, fluid tumbles over and resistance increases

Question 5

List the factors effecting mean velocity

Answer 5

Viscosity and radius of vessel

Question 6

What is the relationship between mean velocity, and viscosity and radius?

Answer 6

• Mean velocity is inversely proportional to viscosity

- Mean velocity is proportional to cross-sectional area

Question 7

List the factors effecting flow resistance

Answer 7

Resistance increases as viscosity increases

Resistance decreases with fourth power of radius

Question 8

Outline the key relationships between pressure, resistance and flow

Answer 8

Fixed pressure- increasing resistance decreases flow

Fixed flow- increasing resistance increases pressure

Question 9

How does resistance differ between vessels in series and those in parallel?

Answer 9

Series - resistance of vessels adds

Parallel - effective resistance is less (RT = R1 x R2/ R1 + R2)

Question 10

At which points of the systemic circulation will resistance be low and why?

Answer 10

• Arteries and veins due ti small drop in pressure

- Capillaries as despite high individual resistance, they are connected in parallel creating low overall resistance

Question 11

Why is pressure within arteries so high?

Answer 11

Pressure must be high enough to pump cardiac output into arterioles and overcome total peripheral resistance

Question 12

What is transmural pressure?

Answer 12

Pressure gradient between the inside and outside of a vessel that stretches walls of distensible vessels

Question 13

How would a decrease in pressure effect blood flow through distensible vessels?

Answer 13

Walls of the vessel would collapse causing blood flow to cease before driving pressure reaches zero

Question 14

How are distensible vessels capable of capacitance?

Answer 14

As vessels widen due to an increase in pressure, more blood transiently flows in than flows out which is stored

Question 15

How is mean arterial pressure calculated and why are these proportions used?

Answer 15

1/3 systolic pressure + 2/3 diastolic pressure

Diastole is longer in duration than systole

Question 16

What is pulse pressure?

Answer 16

Difference between systolic and diastolic pressure, approximately 40 mmHg

Question 17

Define total peripheral resistance

Answer 17

Resistance of vessels of the systemic circulation that opposes blood flow

Question 18

How does the elastic nature of arteries reduce the difference in pressure between systole and diastole

Answer 18

• Walls stretch in systole allowing more blood to flow in than out so pressure doesn’t rise as much
• Arteries then recoil in diastole

Question 19

How would arterial pressure differ during systole and diastole if artery walls were rigid opposed to elastic?

Answer 19

Pressure would rise to a maximum in systole to force the stroke volume through the total peripheral resistance and would fall to zero in diastole

Question 20

List the factors effecting systole and how they differ from diastole

Answer 20

How hard heart pumps, total peripheral resistance, stretchiness of arteries

Diastolic pressure is unaffected by stretchiness of arteries

Question 21

What is a pulse wave?

Answer 21

Wave produced from contraction of ventricles which propagates along the arteries faster than blood

Question 22

How do arterioles act as resistance vessels?

Answer 22

Narrow lumen, narrowed by tonic contraction of smooth muscle, generates high resistance

Question 23

What would be the resistance vessels found in skeletal muscle?

Answer 23

Pre-capillary sphincters

Question 24

Define vasomotor tone

Answer 24

Tonic contraction of smooth muscle in the walls of vasculature

Question 25

What is reactive hyperaemia?

Answer 25

• Removal of blood supply
• Build up of local vasodilator metabolites
• Maximum dilation of arterioles
• Blood flow returns
• Enormous increase initially due to low resistance generating high flow
• High flow washes away metabolites
• Smooth muscle constricts again

Question 26

How does vasodilatation differ from vasodilation?

Answer 26

• Vasodilation is when blood vessels become fully relaxed

- Vasodilatation is reduced vasoconstriction

Question 27

Outline the process of blood flow autoregulation

Answer 27

• Change in supply pressure
• Blood flow changes
• Metabolite concentration alters
• Alter resistance of arterioles
• Blood flow returns to appropriate level in line with metabolism

Question 28

How is pressure in the veins determined?

Answer 28

Determined by volume of blood they contain which is dependent on balance between the inflow and outflow of the heart

Question 29

What is central venous pressure and what does it depend on?

Answer 29

Pressure in the great veins

Depends on return of blood from the body, pumping of the heart, gravity and ‘muscle pumping’