3. Cardiac contractility and the events of the cardiac cycle

Question 1

What does an action potential result in cardiac muscle cells?

Answer 1

L-type dihydropyridine channels to open causing a large influx of Ca²⁺

Question 2

What is the difference between cardiac and skeletal muscle t-tubule?

Answer 2

Cardiac t-tubules are 5x greater in diamater and skeletal t-tubules are x25 more numerous

Question 3

What does DHP activation cause?

Answer 3

Release of Ca²⁺ from SR via ryanodine release channels

Question 4

At resting heart rates what is insufficient to cause maximal contractile force?

Answer 4

increase [Ca²⁺]_i due to influx and SR release

Question 5

What effect does sympathetic innervation have on contractility?

Answer 5

Effects the entire heart and has a positive inotropic (more tension) effect

Question 6

What effect does NA have on B1 receptors? And what effect does this have on contractility?

Answer 6

• Increase cAMP
• Enhances Ca²⁺ influx
• promotes storage and release of Ca²⁺ from sarcoplasmic stores

Increase contractility and increased speed of relaxation

Question 7

Where does parasympathetic innervation of the heart affect and how does it effect the heart?

Answer 7

SA node and atria

Decreases HR and has an indirect negative inotropic effect

Question 8

Indicate the position and timing of the ARP, RRP and SMP on a cardiac action potential

Answer 8

ARP = absolute refractory periord

RRP = relative refractory period

SNP = period of supranormal excitability

Question 9

Diastole

Answer 9

Period of relaxation

Question 10

Systole

Answer 10

period of contraction

Question 11

Give the blood pressure of systolic in the aorta

Answer 11

120mmHg

Question 12

Give the blood pressure of diastolic in the aorta

Answer 12

80mmHg

Question 13

Give the blood pressure of pulmonary systolic

Answer 13

30mmHg

Question 14

Give the blood pressure of pulmonary diastolic

Answer 14

12mmHg

Question 15

Why is the pressure in pulmonary circulation much lower?

Answer 15

Less resistance to flow. The right side of the heart needs to do less work and right ventricle walls contain less muscle mass

Question 16

End systolic volume (ESV)

Answer 16

Volume in ventricle at the end of systole

Question 17

End diastolic volume

Answer 17

Volume in ventricle at the end of diastole

Question 18

Stroke volume (SV) = EDV – ESV

Answer 18

Quantity of blood expelled per beat (L)

Question 19

Cardiac output (CO) = SV x HR

Answer 19

Volume of blood pumped by the heart (L/min)

Question 20

What is preload?

Answer 20

Stress on the walls of the ventricle at the end of diastole; full stretching of heart walls

Also refers to the volume inside the ventricle before it starts to contract

Question 21

What is the refractory period due to?

Answer 21

inactivation of Na⁺ channels. Na⁺ channels can’t reset until membrane potential is closer to resting

Question 22

What could occur during the RRP?

Answer 22

A greater than normal signal could trigger another AP to occur. This large signal is NOT normal.

Pathology could include ischaemia

Question 23

What is the difference in volume on either side of the heart?

Answer 23

None

Question 24

How much blood is expelled in the period of rapid ejection (isovolumic contraction)

Answer 24

70% of stroke volume

Question 25

What percentage of atrial filling is passive

Answer 25

80%

Question 26

What is HR influenced by?

Answer 26

SNS and PSNS

Question 27

What can SV be influenced by?

Answer 27

Preload