Haemodynamics And Peripheral Circulation

Question 1

What are the distribution vessels ?

Answer 1

Arteries > arterioles > capillaries > venules > veins

Question 2

What are the characteristics of the blood vessels ?

Answer 2

Arteries and veins have 3 layers :
1) tunica intima - innermost layer made of endothelial cells
2) tunica media - circular smooth muscle providing mechanical strength and sympathetic innervation
3) tunica adventitia - outermost layer made of connective tissue and it anchors the vessel

Question 3

What is the function of the arteries and arterioles ?

Answer 3

• transport oxygenated blood
• conduit (channel) for blood to the capillary beds
• act as pressure reservoirs to drive blood into the arterioles
• damp oscillations in pressure and flow
• control differential distribution to different organs and tissues

Question 4

What is the functions of veins and venules ?

Answer 4

• transport relatively deoxygenated blood
• conduit for blood to return to the heart
• act as a blood reservoir
• flow is influenced by external factors e.g. gravity

Question 5

Summarise blood flow

Answer 5

• blood flow is driven by pressure differences between the arteries and veins
• blood flow faces resistance offered by the blood vessels

Question 6

What is the equation for flow rate ?

Answer 6

Flow rate = driving force (change in pressure) / resistance

• higher flow rate = lower resistance
• higher driving force = higher flow rate

Question 7

What is equation for blood flow ?

Answer 7

Blood flow = perfusion pressure / vascular resistance

• increase in perfusion pressure = increase in blood flow
• increase vascular resistance = decrease in blood flow

Question 8

What is meant by perfusion pressure ?

Answer 8

The amount of pressure the heart uses to eject blood out of the ventricles

Question 9

Explain series resistance

Answer 9

• this theory states that the the vessels are one pipe
• total resistance = each individual resistance added together

Question 10

Explain parallel resistance

Answer 10

• this theory states that the vessels are branched
• the total resistance is the inverse of each individual resistance

Question 11

How does blood flow through vessels ?

Answer 11

• laminar flow in a normal resting person
• turbulent flow when the body is in greater demand

Question 12

Summarise laminar flow

Answer 12

• slow flow rate
• parallel stream lines
• silent
• flow rate is faster in the centre of the vessel

Question 13

Summarise turbulent flow

Answer 13

• high flow rate
• disorganised stream lines
• noisy

Question 14

Why do the terminal arterioles have lots of smooth muscle ?

Answer 14

Lots of smooth muscle is needed to damp the oscillations in pressure so when the blood moves into the capillaries they don’t burst

Question 15

What do changes in smooth muscle tone lead to ?

Answer 15

• vasoconstriction (muscle contracts) - resistance is increased so a decrease in flow rate
• vasodilation (muscle relaxes) - resistance is reduced so an increase in flow rate

Question 16

When there is an increased metabolic demand, blood is directed to those parts of the body that require that cardiac output. How is this achieved ?

Answer 16

This is achieved through :
- vasodilation of arterioles in the skeletal muscles
- this relaxation is achieved by the effects of circulating adrenaline and metabolic hyperaemia
- vasoconstriction of peripheral arterioles
- opening up, recruitment and distension of capillaries in muscle beds

Question 17

How is smooth muscle tone altered ?

Answer 17

Extrinsic control -
• neurotransmitters e.g. noradrenaline and acetylcholine
• hormones e.g. adrenaline

Intrinsic control -
• endothelium derived substances
• metabolites and related factors
• other locally produced factors e.g. histamine
• other factors e.g. pressure and heat

Question 18

What are the pressure values for systole and diastole ?

Answer 18

• systole = 120 mmHg
• diastole = 80 mmHg

Blood pressure = 120/80

Question 19

How is aortic pressure sustained ?

Answer 19

The closure of the aortic valve prevents the pressure in the aorta dropping to 0