Cardiac cycle and its control

Question 1

Function of the cardiac cycle

Answer 1

• Transport nutrients, oxygen and waste products around body
• Transport hormones (e.g. adrenaline from adrenals)
• Transfer heat, usually from core to skin
• Buffer body pH
• Assist in response to infection
• Assist in formation of urine-filtration and circulation

Question 2

Sequences of cardiac cycle:

Answer 2

1. Diastole: AV valves open, blood pools into atria from SVC, IVC and coronary sinus (right) and pulmonary veins (left), flows directly into ventricles
2. Atrial systole: SAN initiates action potential causing contraction of atria forcing residual blood into ventricles, increases ventricular pressure so AV valves shut
3. Ventricular contraction: AV and semilunar valves shut as ventricles contract (isovolumetric contraction), rise in pressure causes SL valves to open and blood forced out outflow tract
4. Both valves close again, ventricles relax reducing pressure causing AV valves to open and filling stage begins again

Question 3

Define end-diastolic volume or preload

Answer 3

Total volume of blood present in ventricles at end of diastole

Question 4

Define isovolumetric contraction

Answer 4

Contraction of ventricle when both valves are shut causing a rise in pressure

Question 5

Define end-systolic volume (how much is it?)

Answer 5

Blood remaining in ventricles at end of ventricular systole
Around 40-50ml

Question 6

Define stroke volume output

Answer 6

Volume of blood ejected from ventricles during ventricular systole

Question 7

What is the ejection fraction?

Answer 7

Ratio of stroke volume output to end-diastolic volume

Question 8

Define isovolumetric relaxation

Answer 8

Both sets of valves closed but area in ventricles increases during diastole causing pressure to decrease, allows AV valves to open

Question 9

Role of chordae tenidiae and papillary muscles

Answer 9

Orientate the AV valves and prevents them from becoming inverted during ventricular systole

Question 10

What determines opening and closing of valves?

Answer 10

Pressure gradient across the valves

Question 11

Sequence of intitation and conduction of impulse through the heart:

Answer 11

SAN -> AVN -> bundle of his -> left/ right bundle branch -> purkinje fibres

Question 12

What are the pacemaker cells of the heart?

Answer 12

SAN = primary pacemaker as has fastest intrinsic rate so determines heart rate
AVN = slows condution and acts as a secondary pacemaker

Question 13

Describe the sequence of events involved in contraction of cardiac muscle:

Answer 13

• Calcium enters into cardiac muscle from exterior by voltage gated channels which have been opened by depolarising impuse of purkinje fibres
• Caclium binds to receptors on SR causing more calcium to be release (calcium-induced calcium release) - amplified calcium
• Calcium binds to troponin C and cardiac muscle sarcomeres contract in same mechanism as skeletal muscle

Question 14

Describe the phases of an action potential in the heart:

Answer 14

Phase 4 (baseline):
- K+ channels open at rest so K+ moves inwards and membrane potential is negative

Phase 0 (fast depolarisation):
- Voltage gated Na+ channels open in response to depolarisation from gap junctions
- Influx of Na+ depolarises cell further causing more Na+ channels to open = steep depolarisation
- Na+ channels inactivated almost immediately and can’t reopen

Phase 1 (notch):
- Reopening of K+ ion channels causes some rapid repolarisation

Phase 2 (plateau):
- Inwards movement of Ca2+ balances outward movement of K+ so no change in membrane potential

Phase 3 (repolarisation):
- Ca2+ channels close, K+ currents repolarise
- Na+ channels recover as membrane potential becomes more negative
- Cycle can now restart

Question 15

How can heart rate be altered?

Answer 15

Action of ACh from vagus nerve increases K+ permeability so takes longer time to reach threshold potential = less BPM

Action of NA: increases Ca2+ permeability, shorter time to threshold = more BPM

Question 16

What do the different parts of an ECG represent?

Answer 16

P wave: atrial depolarisation (contraction of Atria)
QRS complex: ventricular depolarisation (contraction of V)
T wave: ventricular repolarisation (start of ventricular diastole)
P-R interval: delay through AVN
ST interval: plateau pahse of AP, important at allowing tissues to relax and not be reactivated

Question 17

What abnormalities are visible on ECG?

Answer 17

Sinus bradycardia
Atrial fibrillation
Ventricular fibrilation
STEMI

Question 18

What determines cardiac output?

Answer 18

Heart rate x stroke volume

Question 19

What is Startling’s law?

Answer 19

Force of contraction is increases as muscle is stretched in response to filling of chambers
(more blood returning to heart -> more full atria -> increased cardiac output)

Question 20

What adrenoreceptors are mainly found on heart? What binds to them? What are the effects?

Answer 20

Mainly B1 receptors which are found on nodal tissue, conduting system and myocardium
Noradrenaline (released by sympathetic nervous system) and adrenaline bind to them
Effects:
- Positive inotropy (force)
- Positive chronotropy (rate)
- Positive lusitroy (speed of relaxation)
- Positive dromotropy (speed of contraction)

Question 21

Role of vagus nerve in altering cardiac output?

Answer 21

Terminates on nodal tissue (right vagus on SAN and L vagus on AVN)
Releases ACh which activates M2 receptors which reduce heart rate due to increased K+ permeability.