Cardiac output

Question 1

Factors that effect cardiac output

Answer 1

Afterload
Preload
HR
Contractility

Question 2

What is preload?

Answer 2

The force with which blood enters the heart

Question 3

What is afterload?

Answer 3

The force against which the heart has to pump. Dependant on the TPR and the stiffness of the aorta

Question 4

What is the Frank-Starling relationship?

Answer 4

The relationship between end diastolic volume and cardiac output/stroke volume

EDV increases the stretch of the muscle, therefore the length of the sarcomeres and therefore the force with which the ventricles will contract so increasing stroke volume

Question 5

Why does the Frank-Starling relationship mean that if output in one ventricle increases, it will increase in the other ventricle?

Answer 5

Increased output/SV in one ventricle causes an increased EDP of the other which causes its SV to increase so they match

Question 6

Describe compensation that occurs in compensated heart failure

Answer 6

If there is a decreased BP then there will be decreased Na and water excretion renally therefore an increased blood vol and so an increased EDP therefore increasing SV, increasing the BP back towards normal levels.
Decreased Na and water excretion due to activation of the renin-angiotensin-aldosterone system via the sympathetic nervous system.
Sympathetic nervous system also causes increased cardiac contractility and venoconstriction (increasing EDP) helping to compensate for HF

Question 7

How does afterload effect cardiac output?

Answer 7

The blood in the ventricle is only ejected into the aorta when the ventricular pressure is greater than the aortic pressure therefore causing the aortic valve to open. If the aortic pressure is higher/increased afterload then the time during which there is ventricular ejection decreases. This means that less blood is ejected so cardiac output decreases.

Question 8

How is the effect of afterload on cardiac output compensated for?

Answer 8

Decreased ejection from the ventricle means that there is blood left over after systole therefore the end diastolic pressure will be higher so stroke volume will be greater, compensating for increased afterload.

There is also an ANREP response which is a release of substances which can increase Ca2+ conc in the myocytes therefore increasing contractility.

However, baroreceptors can detect high blood pressure and so will reduce cardiac output to reduce this pressure.

Question 9

How is cardiac contractility managed?

Answer 9

Via changing Ca2+ conc. which is under control of the sympathetic nervous system. NA increases contractility by stimulating beta1 and to a lesser extent beta2 receptors.

Question 10

How does sympathetic stimulation effect the activity of the SAN?

Answer 10

It increases the funny current so that it takes less time to reach the threshold (increased rate of diastolic depolarisation) for contraction so heart rate increases.

This happens via adrenaline and noradrenaline

Question 11

How does parasympathetic stimulation effect the activity of the SAN?

Answer 11

It decreases the funny current so that it takes less time to reach the threshold (decreases rate of diastolic depolarisation) for contraction so heart rate decreases.

This happens via acetlycholine