Pressure and Flow in the Systemic Circulation -- 5.2

Question 1

What is cardiac output the product of?

Answer 1

The product of stroke volume and heart rate

Question 2

What type of vessels are arteries?

Answer 2

Low resistance vessels with a high pressure in them

Question 3

Why must arterial pressure be high?

Answer 3

Has to drive the cardiac output through the arterioles in order to push through the total peripheral resistance

Question 4

What kind of walls do arteries have?

Answer 4

Distensible walls

Question 5

Describe the effects of distensible walls in arteries

Answer 5

• • arteries stretch during systole so more blood flows in than out & pressure does not rise much
• • in diastole, the walls recoil so flow continues

Question 6

What are the typical systolic and diastolic pressures?

Answer 6

systolic ~ 120 mmHg – max. pressure

diastolic ~ 80 mmHg – min. pressure

Question 7

What factos have an effect on the systolic pressure?

Answer 7

• how hard the heart pumps
• the total peripheral resistance
• stretchiness (‘compliance’) of the arteries

Question 8

What factors effect diastolic pressure?

Answer 8

• systolic pressure

* total peripheral resistance

Question 9

Define pulse pressure

Answer 9

• the difference between systolic and diastolic

* typically about 40 mmHg

Question 10

How do you calculate the average pressure

Answer 10

iastolic plus one third pulse pressure as systole is shorter than diastole

Question 11

Where do you find pre-capillary spinchters?

Answer 11

ring of muscle found at the arterial end of capillaries

Question 12

What vessels make up resistance vessels?

Why are they high resistance?

What can change the resistance?

Answer 12

Arterioles and pre-capillary sphincters

They have a narrow lumen

Tonic contraction of smooth muscles in the walls

Question 13

What is vasomotor tone?

Answer 13

The tonic contraction of smooth muscle

• • vasoconstriction is an increase in contraction
• • vasodilatation is a decrease in contraction

Question 14

How does vasoconstriction and vasodilatation affect resistance?

Answer 14

Vasoconstriction = increase resistance
Vasodilatation = decrease resistance

Question 15

What affects the contraction of vascular smooth muscle?

Answer 15

Vasomotor tone in produced by the sympathetic branch of the ANS and antagonised by vasodilator factors
– the resistance is determined by the balance between the two

Question 16

What is reactive hyperaemia?

Answer 16

A short increase in organ blood flow following a short period of
–VERY high pressure is present when you restore the blood supply after ischaemia

Question 17

Why is there such a high pressure present in reactive hyperaemia?

Answer 17

Smooth muscle relaxes due to accumulation of vasodilator metabolites.
When the blood supply returns it meets little resistance (high pressure vs low resistance)
Causes redness
– metabolites are washed away, which causes constriction

Question 18

What are produced by metabolically active metabolites?

Answer 18

Vasodilator metabolites

e. g. H+, K+, Adenosine
- - they relax vascular smooth muscle
- - K+ leak from muscles during exercise

Effect is determined by the rate of production and the rate at which they are removed.

Question 19

How is blood flow matched to metabolism?

Answer 19

When metabolism increases, more metabolites are produced
So, conc increases and vasodilatation occurs and washes them away
More metabolism = more blood flow

Question 20

When bloos flow returns during reactive hyperaemia, how much blood passes through the vessels?

Answer 20

Same volume will pass though as if the flow was at the normal rate
– flow is unaffected

Question 21

What is auto regulation?

Answer 21

When supply to a tissue changes, blood flow to that tissue will change

• • changes metabolite concentration
• • arteriole resistance changes

Question 22

What is a basic rule of autoregulation?

Answer 22

Provided supply pressure remains within certain limit, tissues will automatically take what blood they need

Question 23

Define the relationship between TPR and the body’s need for blood flow

Answer 23

total peripheral resistance is inversely proportional to the body’s need for blood flow

Lower resistance = less need for blood
Higher resistance = more need for blood

Question 24

What is the pressure in veins determined by?

Answer 24

The volume of blood they contain which depends on the balance between flow in from the body and out via the heart

Question 25

What is the central venous pressure?

Answer 25

The pressure in teh great veins which fill teh heart in diastole.

Question 26

What does the central venous pressure depend on?

Answer 26

– return of blood from the body
– pumping of the heart
– gravity & ‘muscle pumping’

Question 27

What are the main things to remember?

Answer 27

• 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