11. Control of Cardiac Output

Question 1

What is Cardiac Output?

Answer 1

The volume of blood ejected by each ventricle per minute.

CO = HR x SV

Question 2

What is heart rate?

Answer 2

Number of contractions per minute.

Question 3

What is Stroke Volume?

Answer 3

Volume of blood ejected by each ventricle per beat.

Question 4

Define chronotropic effect.

Answer 4

An influence upon the HR.

Question 5

What increases the HR?

Answer 5

• Noradrenaline and adrenaline have positive chronotropic effects i.e. increase HR
• Mechanism of Action? Stimulation of hyperpolarisation-activated ion channels

Question 6

Define Tachycardia

Answer 6

Under pathological conditions a fast HR is termed tachycardia

Question 7

What decreases the HR?

Answer 7

Parasympathetic nerve

Acetylcholine has a negative chronotropic effect i.e. slows HR

Mechanism of Action? (+) of muscarinic (ACh)-receptor gated K-channels; A component of ‘vagal tone’

Question 8

Define Bradycardia.

Answer 8

Under pathological conditions a slow HR is termed bradycardia

Question 9

What is End Diastolic Volume?

Answer 9

volume of blood in a ventricle at the end of loading or filling

Question 10

What is End Systolic Volume?

Answer 10

volume of blood in a ventricle at the end of contraction (systole).

Question 11

How does the autonomic nervous system innervate the heart?

Answer 11

Parasympathetic Nerve (Vagus Nerve) to SA node- Acetylcholine is released

Sympathetic Nerve to SA node and bottom of the heart releases noradrenaline/ adrenaline from the adrenal medulla.

Question 12

What regulates Blood Volume reflexes?

Answer 12

Sensory nerve fibres receive signals from;

• Baroreceptors in the wall of internal carotid artery
• Carotid Body Chemoreceptors
• Baroreceptors in the aorta.

Question 13

3 Key factors influencing Venous Return/ Central Venous Pressure

Answer 13

1. Posture (gravity) - Standing = ↓ venous return
2. Muscle Pump: Venomotor tone involving constriction of veins by skeletal muscles. Venous return is facilitated by valves which prevent backflow of blood.
3. Respiratory Pump: inspiration creates an internal pressure difference by lowering intra-thoracic pressure and increasing intra-abdominal pressure.

Question 14

What is the Frank-Starling Law of the Heart?

Answer 14

the force or tension developed in a muscle fibre depends on the extent to which the fibre is stretched
more in = more out

Question 15

How does preload volume determine EDV and ESV?

Answer 15

Ventricular filling relies on atrial filling which is determined by venous return of blood
↑ Rate of ventricular filling = ↑ SV (proportional)

↑ EDV increase the initial length (stretch) of cardiac muscle fibres ↑ SV and CO

Question 16

Define Contractility.

Answer 16

contractile ability of cardiac muscle at a given preload

Question 17

How does contractility determine EDV and ESV?

Answer 17

• ↑ force of ventricular contraction = ↑ stroke volume
• SNS & adrenaline exert positive inotropic effects
• SNS increases velocity of conduction of cardiac impulse
• PNS (ACh) has a negative inotropic influence; mainly exerted by on the atria as vagal nerve doesn’t innervate ventricles

Question 18

What are inotropic effects?

Answer 18

Substances that alter the contractility of the heart have inotropic effect.

Question 19

Define Afterload.

Answer 19

• the amount of tension that the contracting ventricle must generate to force open the aortic valve to eject blood.
• Ease of which the blood exits the ventricles and circulatory system.

Question 20

What influences Afterload?

Answer 20

Influenced by blood vessel tone (i.e. vasoconstriction, vasodilation)

Question 21

How does Afterload determine EDV and ESV?

Answer 21

• A high opposing pressure = greater isovolumetric contraction + a shortened ejection period + larger residual ESV.
• Increased afterload = decreased SV and decreased CO
• Sustained increases in afterload will weaken the myocardium = heart failure.
• Afterload is increased by factors that elevate blood pressure