Pressure and Flow in the Systemic Circulation -- 5.2 Flashcards
What is cardiac output the product of?
The product of stroke volume and heart rate
What type of vessels are arteries?
Low resistance vessels with a high pressure in them
Why must arterial pressure be high?
Has to drive the cardiac output through the arterioles in order to push through the total peripheral resistance
What kind of walls do arteries have?
Distensible walls
Describe the effects of distensible walls in arteries
- arteries stretch during systole so more blood flows in than out & pressure does not rise much
- in diastole, the walls recoil so flow continues
What are the typical systolic and diastolic pressures?
systolic ~ 120 mmHg – max. pressure
diastolic ~ 80 mmHg – min. pressure
What factos have an effect on the systolic pressure?
- how hard the heart pumps
- the total peripheral resistance
- stretchiness (‘compliance’) of the arteries
What factors effect diastolic pressure?
- systolic pressure
* total peripheral resistance
Define pulse pressure
- the difference between systolic and diastolic
* typically about 40 mmHg
How do you calculate the average pressure
iastolic plus one third pulse pressure as systole is shorter than diastole
Where do you find pre-capillary spinchters?
ring of muscle found at the arterial end of capillaries
What vessels make up resistance vessels?
Why are they high resistance?
What can change the resistance?
Arterioles and pre-capillary sphincters
They have a narrow lumen
Tonic contraction of smooth muscles in the walls
What is vasomotor tone?
The tonic contraction of smooth muscle
- vasoconstriction is an increase in contraction
- vasodilatation is a decrease in contraction
How does vasoconstriction and vasodilatation affect resistance?
Vasoconstriction = increase resistance Vasodilatation = decrease resistance
What affects the contraction of vascular smooth muscle?
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
What is reactive hyperaemia?
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
Why is there such a high pressure present in reactive hyperaemia?
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
What are produced by metabolically active metabolites?
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.
How is blood flow matched to metabolism?
When metabolism increases, more metabolites are produced
So, conc increases and vasodilatation occurs and washes them away
More metabolism = more blood flow
When bloos flow returns during reactive hyperaemia, how much blood passes through the vessels?
Same volume will pass though as if the flow was at the normal rate
– flow is unaffected
What is auto regulation?
When supply to a tissue changes, blood flow to that tissue will change
- changes metabolite concentration
- arteriole resistance changes
What is a basic rule of autoregulation?
Provided supply pressure remains within certain limit, tissues will automatically take what blood they need
Define the relationship between TPR and the body’s need for blood flow
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
What is the pressure in veins determined by?
The volume of blood they contain which depends on the balance between flow in from the body and out via the heart
What is the central venous pressure?
The pressure in teh great veins which fill teh heart in diastole.
What does the central venous pressure depend on?
– return of blood from the body
– pumping of the heart
– gravity & ‘muscle pumping’
What are the main things to remember?
- 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
