Control Of Cardiac Cycle

Question 1

Define afterload

Answer 1

The load the heart must eject blood against (roughly equivalent to aortic pressure)

Question 2

Define preload

Answer 2

Amount the ventricles are stretched in diastole

Question 3

Define total peripheral resistance

Answer 3

Resistance to blood flow offered by total systemic vasculature

Question 4

Define contractility

Answer 4

Force of contraction for a given fibre length

Question 5

What happens to pressure downstream if fluid counters resistance

Answer 5

Decrease

Question 6

What happens to venous and arterial pressure if TPR falls and cardiac output unchanged

Answer 6

Arteriole pressure will fall and venous pressure will rise

Question 7

In relation to pressure and resistance, how does the body cope with changes in blood demand

Answer 7

If the tissues need more blood, the arterioles and precapillary sphincters will dilate, which causes peripheral resistance to fall
Heart needs to pump more so that arterial pressure does not fall and venous pressure does not rise
The heart sees changes in this demand as changes in arterial blood pressure and central venous pressure

Question 8

Give the equation of cardiac output and its normal value in a 70kg person

Answer 8

Cardiac output = stroke volume x heart rate

For a 70kg person, cardiac output is 5L

Question 9

Give the equation for stroke volume

Answer 9

Stroke volume = end diastolic volume - end systolic volume

Typically 70ml at rest

Question 10

What factors determine cardiac output

Answer 10

How hard heart contracts
Determined by the end diastolic volume and contractility
How hard it is to eject blood
Determined by aortic impedance

Question 11

Explain how cardiac output is controlled when metabolism increases, when you stand up and during exercise

Answer 11

Contractility and heart rate are controlled by the autonomic nervous system
A decrease in arterial blood pressure will reduce parasympathetic nervous system activity and stimulate sympathetic nervous system to increase heart rate and increase contractility
If the metabolism of the body increases, total peripheral resistance will fall to increase blood supply (vasodilation)
Reduction in arterial pressure will be sensed
Increase in venous pressure increases stroke volume through starlings law and increase cardiac output
Increasing cardiac output increases arterial pressure and decreases venous pressure to return body back to normal
When your stand up, venous pressure decreases to lower cardiac output and thus arterial pressure decreases
Baroreceptor reflex and autonomic nervous system increase heart rate and increase TPR
During exercise, venous pressure, heart rate and contractility all increase to increase cardiac output

Question 12

Explain ventricular filling

Answer 12

The ventricle fills until the walls stretch enough to produce an intraventricular pressure equal to the venous pressure
The higher the venous pressure, the more the heart fills
Dilated ventricles will have increase compliance
Normal heart has 120mL end diastolic volume

Question 13

Explain the Frank Starling law of the heart

Answer 13

If you stretch the fibres of the heart before contraction, the more it will contract
The more the heart fills, the harder it contracts (up to a limit)
The harder the heart contracts, the bigger the stroke volume
An increase in venous pressure will fill the heart more
Depends on the compliance of the ventricle
Starling curve - states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (end diastolic volume) when all other factors are kept constant
The force developed in a muscle fibre depends on the degree to which the fibre is stretched

Question 14

Highlight what occurs in overcontraction of heart

Answer 14

If sarcomere length is too short, filament overlap interferes with contraction
Overcontraction at high venous pressure can lead to non-overlap of actin and myosin filaments
In cardiac muscle, also get an increase in calcium sensitivity as the muscle fibres are stretched

Question 15

How does a change in contractility affect Starling’s Curve

Answer 15

A change in contractility is seen as a change in the slope of the starling curve
An increase in contractility leads to a steeper curve and thus increased stroke volume for the same diastolic pressure
An increase in contractility will increase the force of contraction for a given EDP

Question 16

What is the effect of increasing arterial pressure on stroke volume

Answer 16

Arterial (aortic) pressure is increased with peripheral resistance
Total peripheral resistance also reduce venous pressure and therefore reduces filling of the heart
Over time, you can get an inappropriate increase in arterial pressure (heart will have to work harder)