Week 5 - Cardiac structure and Function

Question 1

Brief outline of the journey of blood in the heart.

Answer 1

Right atrium receives blood from super and inferior vena cava.
Travels to right ventricles via tricuspid valve.
Goes into pulmonary circulation via the pulmonary artery and returns to the left atrium via the pulmonary vein.
Then into left ventricle through the mitral valve and ejected through aorta into systemic circulation.

Question 2

What are the 3 structures of the heart wall?

Answer 2

1) Epicardium
2) Myocardium
3) Endocardium

Question 3

Describe the characteristics and function of the 3 structures that make up the heart wall.

Answer 3

Epicardium
- Characteristics: serous membrane including blood capillaries, lymph capillaries and nerve fibers
- Function: serves as a lubricative outer cover.

Myocardium:
- Characteristics: cardiac muscle tissue separated by connective tissues and includes blood capillaries, lymph capillaries and nerve fibers
- Function: provides muscular contractions that eject blood from the heart chambers.

Endocardium
- Characteristics: endothelial tissue and a thick subendothelial layer of elastic and collagenous fibers
- Function: serves as protective inner lining of the chambers and valves.

Question 4

The heart receives blood supply (oxygen and nutrients) via what arteries?

Answer 4

coronary arteries - it has a high demand for oxygen and nutrients

Question 5

Myocardial infarction (MI) and influence of exercise.

Answer 5

Blockage in coronary blood flow which results in cell damage.

Exercise training protects against heart damage during an MI.

Question 6

Structural differences between heart muscle and skeletal muscle.
1) Nuclei
2) Neural control
3) Calcium store
4) Shape of muscle fibers
5) Regeneration potential
6) Energy production

Answer 6

1) Nuclei: heart muscle has single, whereas skeletal has multiple
2) Neural control: heart is involuntary and skeletal voluntary
3) Calcium store: heart sources are SR and extracellular calcium, skeletal is SR
4) Shape of muscle fibers: heart has shorter muscle fibers and have branching, skeletal is elongated - no branching.
5) Regeneration potential: heart has none (no satellite cells) whereas skeletal has some possibilities via satellite cells.
6) Energy production: heart is aerobic (primary) whereas skeletal muscle is aerobic and anaerobic

Question 7

The contraction of the heart depends on the electrical stimulation of…
a) Epicardium
b) Myocardium
c) Endocardium

Answer 7

Myocardium

Question 8

Cardiac automaticity

Answer 8

when the heart can generate its own action potentials

Question 9

What are the two types of cells in the heart?

Answer 9

Nodal cells (e.g. SA node, AV node - makes up 1%) and muscular cells (actin, myosin)

Question 10

What is the intrinsic regulation of the heart controlled by?

Answer 10

SA node - pacemaker and initiates depolarization by generating AP.

Question 11

Atrioventricular node (AV node)

Answer 11

Passes depolarization to ventricles and has a brief delay to allow for ventricular filling.

Question 12

What are the two mechanistic reasons for AV node having a brief delay for ventricular filling?

Answer 12

1) There are less gap junctions at the AV node so conduction moves slower through.
2) Diameter of the cells are smaller in the AV node so slows conduction.

Question 13

Bundles branches / Bundle of His

Answer 13

branch that connects the atria to left and right ventricle - travels down the septum

Question 14

Purkinje fibers

Answer 14

spread wave of depolarization throughout the ventricles

Question 15

Describe the 4 steps of the conduction system of the heart.

Answer 15

1) Action potentials originate in the sinoatrial (SA) node and travel across the wall of the atrium from the SA node to the atrioventricular (AV) node.

2) There is a slight pause in the AV node to allow for ventricular filling. APs then pass through the AV node and along the AV bundle, which extends from the AV node, through the fibrous skeleton, into the interventricular septum.

3) The Atrioventricular (AV) bundle divides into right and left bundle branches, and APs descend to the apex of each ventricle along the bundle branches.

4) APs are carried by the Purkinje fibers from the bundle branches to the ventricular walls.

Bundle branches become depolarized, the ventricles contract, the ventricular pressure increases, and blood exits the ventricles.

Question 16

In electrocardiograms (ECG), what do the following represent:
a) P wave
b) QRS complex
c) T wave

Answer 16

a) atrial depolarization
b) ventricular depolarization and atrial repolarization
c) ventricular repolarization

Question 17

What is the rate of the intrinsic pacemaker?

Answer 17

100bpm

Question 18

Describe the cardiac cycle.

Answer 18

1) Atrial diastole: fill with blood from vena cava/pulmonary vein.
2) Ventricular diastole: AV valves (tricuspid and mitral valves) opens and fills with blood.
3) Atrial systole: atria contracts squeezing any leftover blood into ventricles (P wave).
4) Ventricular diastole: AV valve shuts. As the ventricles contract, ventricular pressure increases causing the aortic valve (semilunar valve) to open and blood to be ejected out the aorta into the systemic circulation.
5) Isovolumetric relaxation of the ventricles (all valves closed).

Question 19

What is the relationship between intraventricular pressure and ECG?

Answer 19

Intraventricular pressure rises as ventricles contract - Q to R

Intraventricular pressure falls as ventricles relax - R to S

Question 20

What does S-T segment depression on ECG during exercise suggest?

Answer 20

myocardial ischemia - lack of oxygen supplying cardiac muscle which means calcium ions aren’t taken up into SR.

Question 21

What is the graded exercise test?

Answer 21

the diagnostic use of ECG during exercise to measure cardiac function and changes in bp (e.g. coronary artery disease)

Question 22

What is a potential cause of myocardial ischemia?

Answer 22

atherosclerosis - fatty plaque that narrows coronary arteries which reduces blood flow and oxygen to myocardium

Question 23

How is regular exercise cardioprotective?

Answer 23

• Reduces incidence of heart attacks.
• Improves survival from a heart attack.

Question 24

How does exercise reduce the amount of myocardial damage from heart attack?

Answer 24

• Improves heart’s antioxidant capacity.
• Improved function of ATP-sensitive potassium channels.

Question 25

Identify the 2 nervous systems that extrinsically regulate HR and explain how they increase/decrease HR?

Answer 25

Autonomic NS:

1) PNS via vagus nerve which slows HR by inhibiting SA and AV node (via acetylcholine which binds to muscarinic 2 receptor. This activates G-inhibiting protein which gets potassium to exit the cell and this reduces AP and HR.)

2) SNS via cardiac accelerator nerves which increases HR by stimulating SA and AV node (via norepinephrine which binds to beta-1 adrenergic receptor which causes more calcium to come into the cell which increase the AP and subsequent HR. Also increases force of contraction).

Although the heart can generate AP by itself, when it needs to slow down or increase HR a bit more, the SNS and PNS influence its regulation.

Question 26

How does heart rate increase as exercise intensity increases?

Answer 26

Initially there is parasympathetic withdrawal that increases HR. Then we get sympathetic outflow for the further increase in HR.

Question 27

What are two key characteristics of the cardiac muscle (myocardium)?

Answer 27

1. Highly oxidative
2. Very dense network of capillaries

Question 28

At rest, what % does the heart receive of 5L-min of cardiac output?

Answer 28

5% = 0.25L-min

Question 29

In exercise, what % does the heart take of 25L-min of cardiac output?

Answer 29

5% = 1.25L-min

Question 30

In the heart, what is the % of oxygen extraction at rest and during exercise?

Answer 30

At rest = 65-75%
Exercise = +90%

Question 31

Although oxygen extraction increases during exercise, what is increased oxygen delivery to the heart mainly reliant upon?

Answer 31

an increase in coronary blood flow - important in delivery of oxygen and nutrients to the heart muscle

Question 32

What is oxygen consumption by the heart principally required for?

Answer 32

Contraction
- requirements for maintaining basal metabolism comprises of only 10-20% of total oxygen consumption.

Question 33

Explain the metabolic factors that control coronary circulation/blood flow.

Answer 33

The main metabolic vasodilator of coronary blood vessels is adenosine (from ATP breakdown), there is also some b-adrenergic vasodilation via ANS.

Question 34

What % of coronary flow takes place during diastole? Why?

Answer 34

80% - because of vessel compression (contraction) during systole which reduces blood flow

Question 35

During heavy exercise, what % of coronary flow occurs during systole? Why?

Answer 35

40-50% - this is because we have less time in diastole (filling) due to a greater HR

Question 36

Which circulation requires a greater mean pressure to deliver blood and return it to the heart? Why?

Answer 36

Systemic circulation - it has a greater resistance to flow therefore to achieve the same blood flow through the body tissues as through the lungs (pulmonary circulation), it requires a mean pressure that is 4x greater.

Question 37

During exercise, what happens to the time spent in diastole and systole?

Answer 37

Its decreases, with the greatest decrease occurring in diastole

Question 38

Stroke volume continues to increase during incremental exercise up to what % of V02max?

Answer 38

40% of V02max