Cardiac Contractility and the Events of the Cardiac Cycle

Question 1

What 2 things can’t you do in cardiac muscle in order to increase the force of contraction?

Answer 1

• recruit more muscle fibres

- summate action potentials in order to get a tetanic contraction

Question 2

What channels does a cardiac action potential open? What occurs as a result of this?

Answer 2

• L-type dihydropyridine DHP channels open

- large influx of extracellular calcium

Question 3

What occurs when cardiac DHP channels are activated?

Answer 3

calcium is released from sarcoplasmic reticulum through ryanodine release channels

Question 4

How do cardiac T-tubules differ from skeletal T tubules?

Answer 4

• 5x greater in diameter

- contain mucopolysaccharides which sequester calcium

Question 5

What does increased intracellular concentration of calcium in heart cells mean?

Answer 5

increase the recruitment of cardiac actin and myosin and hence increase the force of contraction

Question 6

What 2 things can happen to intracellular calcium when heart cells relax?

Answer 6

• retaken up into sarcoplasmic reticulum by calcium ATPase

- pumped out of the cell by the sodium calcium pump

Question 7

How can calcium ATPase be modified so as to lead to a greater force of cardiac contraction?

Answer 7

activity can be increased so more calcium is stored in sarcoplasmic reticulum, hence on next cycle of contraction more calcium is released and a greater force of contraction can occur

Question 8

How do extrinsic sympathetic nerves increase force production by direct effects on calcium availability?

Answer 8

• Noradrenaline acts on B1 receptors which activates protein kinases
• increases intracellular cAMP

Question 9

What does an increase of intracellular cAMP concentration mean on intracellular calcium levels?

Answer 9

• enhances calcium influx
• promotes its storage and release from calcium sarcoplasmic stores
• increased contractility and increased speed of relaxation

Question 10

How does parasympathetic Innervation affect the heart? Which structure of the heart do parasympathetic nerves mainly affect?

Answer 10

• effects mostly the SA node
• innervates atria
• reduce the rate of contraction
• indirect negative inotropic effect

Question 11

Why can’t cardiac muscle be tetanised?

Answer 11

• cardiac twitches involve all fibres of the myocardium

- cant summate contractions because of the refractory period of cardiac cells due to inactive sodium channels

Question 12

What are the absolute and relative refractory periods of the heart?

Answer 12

• relative is where we could stimulate muscle to contract again
• absolute is time interval during which a normal cardiac impulse can’t re-excite=can’t be a tetanic contraction

Question 13

What is responsible for the first heart sound?

Answer 13

Atrioventricular valve snapping shut due to pressure being greater in ventricle than atrium

Question 14

What makes the second heart sound?

Answer 14

aortic valve snapping shut due to increased pressure in the aorta over the ventricle

Question 15

Why does ventricular volume never decrease below 50ml?

Answer 15

needs to be a reservoir of blood so that ventricular pressure does not decrease

Question 16

How are the ventricles filled?

Answer 16

• 80% passive due to normal blood flow

- 20% topped up by atrial contraction

Question 17

What is isovolumic contraction?

Answer 17

period when ventricles contract yet there is no change in volume because the Atrioventricular valve has just closed and the semi lunar valve has yet to open due to increased pressure

Question 18

Explain how blood is ejected from the ventricles

Answer 18

• rapid ejection of 70% of stroke volume

- slow ejection taking up remaining 2/3 of time of ventricular contraction to eject final 30% of stroke volume

Question 19

What is the main difference between the right and left side of the heart?

Answer 19

pressure in the pulmonary circulation (right) is much lower than the systemic (left)

Question 20

why is pressure in pulmonary circulation lower? 3 reasons

Answer 20

• less resistance to flow
• right side of heart needs to do less work
• right ventricle wall contains far less muscle mass than left

Question 21

What is the average systemic pressure compared to the pulmonary pressure?

Answer 21

• systemic 120/80

- pulmonary 30/12

Question 22

What is cardiac output?

Answer 22

cardiac output= stroke volume x heart rate

Question 23

what is stroke volume?

Answer 23

-volume of blood expelled per beat
-End diastolic volume-end systolic volume
( blood in ventricle after diastole, filling, - volume of blood in ventricle after systole, expulsion,)