Cardiac contractility and the events of the cardiac cycle

Question 1

what causes the opening of the L-type dihydropyridine channels

Answer 1

an AP:

• there is a large influx of [Ca2+]e (however this only 10% contributes to the contraction
• the cardiac muscle t-tubules are 5x greater in diameter than the sk. muscle t-tubules and therefore have 25x the volume
• the cardiac t-tubule mucopolysaccharides sequester Ca2+

Question 2

what does DHP activation cause

Answer 2

release of Ca2+ from sarcoplasmic

reticulum via ryanodine release channels

Question 3

what happens at resting heart rate

Answer 3

At res4ng heart rates, ↑[Ca2+]i due to influx and sarcoplasmic release is insufficient to cause maximal contractile force.

Question 4

what role does sympathetic innervation play

Answer 4

– Throughout entire heart

– Positive inotropic effect

Question 5

what is the tole of noradrenaline on Beta 1 receptors

Answer 5

– ↑[cAMP]i
– Enhances Ca2+ influx
- Promotes storage and release of Ca2+ from sarcoplasmic stores
– ↑contractility
– ↑speed of relaxation

Question 6

what role does parasympathetic innervation play

Answer 6

– MostlytoSAnode
– Innervatesatria
– Main effect is↓rate
– Indirect -ve inotropic effect

Question 7

Refractory period of the heart

Answer 7

Cardiac twitches involve all fibers of the myocardium
Can not significantly summate contractions of cardiac muscle
Refractory period due to inactivation of Na+ channels

Question 8

skeletal muscle refractory period

Answer 8

– Absolute refractory period 1-2ms

– Period of contraction 20-100ms

Question 9

cardiac muscle refractory period

Answer 9

– Absolute refractory period (ARP) ~245ms
– Relative refractory period (RRP)
– Period of supranormal excitability (SNP)
– Period of contraction 250ms

Question 10

Diastole

Answer 10

– Period of relaxation

Question 11

Systole

Answer 11

– Period of contraction

Question 12

Atria as primer pumps

Answer 12

– ~80% of ventricular filling is passive due to normal blood flow
– Atrial contraction ‘tops up’ remaining ~20% volume

Question 13

Ventricles as pumps

Answer 13

– Isovolumic (isometric) period of contraction
– Period of rapid ejection (1/3) when 70% of stroke volume
ejected
– Period of slow ejection (2/3) when remaining 30% ejected
– Isovolumic (isometric) period of relaxation

Question 14

Systolic blood pressure in the aorta and Diastolic blood pressure in the aorta

Answer 14

– ~120 mmHg

– ~80 mmHg

Question 15

Pressure in pulmonary circulation is much lower

Answer 15

– Much less resistance to flow
– Right side of heart needs to do less work
– Right ventricle walls contain less muscle mass

Question 16

Pulmonary systolic pressure and diastolic pressure

Answer 16

– ~30 mmHg

– ~12 mmHg

Question 17

End systolic volume (ESV)

Answer 17

Volume in ventricle at the end of systole

Question 18

End diastolic volume (EDV)

Answer 18

Volume in ventricle at the end of diastole

Question 19

Stroke volume (SV)

Answer 19

EDV-ESV. Quantity of blood expelled per beat (L)

Question 20

Cardiac output (CO)

Answer 20

SV x HR. Volume of blood pumped by the heart (L/min)