Week 2: Cardiac function

Question 1

What are the 2 main cardiac cells? What are their main function?

Answer 1

• Autorythmic: Generate heart beat
• Myocardial: Mechanical Pumping

Question 2

What are the main components of cardiac myocytes? and breifly what do each of the components do?

Answer 2

Intercalated discs: Tightly connect cardiac myocytes together and keep them all in sync.

Desmosomes: Intercellular junctions that provide strong adhesion between cells

Gap Junctions: allow for depolarizing current to move quickly from cell to cell

Many Mitochondria: Required for high ammount of activity

Large T-Tubules: Allows for more Ca²⁺ from ECF to move into the cell.

Question 3

What is the “drift” event of the AR cells due to?

Answer 3

1. Cyclical decrease in normal passive K⁺ efflux from cells
2. Special voltage-gated Na²⁺ channels (I_f Channels) open when the membrane becomes hyperpolarised resulting in Na²⁺ influx and continued K⁺ Efflux.
3. Ca²⁺ moves through transient Ca²⁺ Channels (T-Type) which takes the membrane to threshold.

Question 4

How does the autonomic Nervous system effect AR cells?

Answer 4

SympNS: Speeds heart rate by increase calcium influx and I_f channel flow.

ParasympNS: Slows heart rate by Increasing pottasium Efflux and decreasing calcium influx

This is all controlled by type of neurotransmitter that is released.

Question 5

Why is the SA node the pacemaker of the heart?

Answer 5

Because it depolerizes the fastest.

Question 6

How does the electrical signal travel through the heart? what is the pathway?

Answer 6

1. AP originates from AR cells in SA node
2. Spreads via gap junctions down internodal pathways across atrial myocardial cells = Biggining atrial contraction
3. There is now an AV node delay (Pause)
4. The signal then continues through from the AV node to the bundle of his -> bundle branches -> purkinje fibres
5. As the signals transfer down through the ventricular walls, ventricular contraction will begin from the apex of the heart.

Question 7

Why do we have an AV nodal delay?

Answer 7

this 0.1 second delay allows for completion of atrial contraction = squeezing remaining blood into ventrical.

Question 8

Explain what happens during the excitation of contractile cardiac cells?

Answer 8

1. AP is brought about by a sudden increase in membrane permeability to Na⁺ .
2. Na⁺ Permeability then falls to a lower resting level
3. The membrane decreases permeability to potassium and increases permeability to calcium causing a maintained depolerisation phase that lasts around 700 milliseconds.

Question 9

Explain what is happening during steps 0-4? what charges are present at each step?

Answer 9

0. Sodium channels open
1. Sodium channels close (+20mv to +30mv)
2. Slow calcium channels open and fast K+ channels close
3. Calcium channels close and slow K+ channels open
4. Return to resting membrane potential (-90mv)

Question 10

why do we need a refractory period in contractile cardiac muscle but not skeletal muscle?

Answer 10

Contractile cardiac muscle acts as a pump for the heart and unlike skeletal muscle cannot go into tetatus. The reason for this is because the ‘pump” has to fill. The refractory period is the period that the heart refils itself (relaxing) and gets ready to pump blood back around the body (contraction).

Question 11

Are T-Tubules larger in Skeletal muscle cells or Myocardial cells? explain?

Answer 11

• T-Tubules of myocardial cells are larger than those in skeletal muscle cells and they branch inside of the cell
• Due to cardiac muscle having a relatively less sarcoplasmic reticulum within the cell, it relies on the larger T-Tubules to get Ca²⁺ from the ECF into the cell.

Question 12

What is happening in stages 1 through to 10?

Answer 12

1. AP enters from adjacent cell
2. Voltage gated Ca²⁺ channels open = Ca²⁺ enters cell
3. Ca²⁺ induces Ca²⁺ release (CICR) through Ryanodine receptor-channels (RyR)
4. Local release causes Ca²⁺ spark
5. Ca²⁺ sparks create a Ca²⁺ signal
6. Ca²⁺ ions bind to troponin to initiate contraction (Actin and Myosin complex)
7. Relaxation occurs when Ca²⁺ unbinds from troponin
8. Ca²⁺ is pumped back into the sarcoplasmic reticulum for storage.
9. Ca²⁺ is exchanged with Na⁺ by the NCX antiporter
10. Na⁺ gradient is maintained by the Na⁺-K⁺-ATPase

Question 13

How is the plateau phase established?

Answer 13

The extra Ca²⁺ from the ECF is largely responsible.

Question 14

What is the effect of phospholamban on Ca²⁺-ATPase activity when stimulated by Catecholamines (adrenalin)?

Answer 14

When phospholamaban is stimulated by adrenanlin it enhances Ca²⁺-ATPase activity on the SR membrane resulting in an increase to storage of Ca²⁺ so that when an AP comes, there will be a stronger contraction because of the greater ammount of Ca²⁺ that was stored.

Question 15

What is an ECG?

Answer 15

A graphic recording of the hearts electrical activity. It indirectly measures the depolerisation and repolarisation of heart muscle cells.

Question 16

Fill in what is happening between A-E and 1-5?

Answer 16

A. Atrial Depolarization 1. P-Wave

B. Delay at AV node 2. P-R or P-Q segment

C. Ventricular Depolarization 3. QRS complex

D. Ventricular Repolarization 4. T-Wave

E. No Electrical Activity (Ventricles are relaxing and filling 5. Isoelectric line (TP Interval)

Question 17

How do leads aVR, a VL, and aVF view the heart?

Answer 17

aVR = From right shoulder

aVL = From left shoulder

aVF = Directly upward from feet

Question 18

What is happening in events 1 to 8 in relation to the Electrical events of the cardiac cycle?

Answer 18

1. P-Wave (atrial Depolarization)
2. PQ or PR segment: Atria contract

3, 4, and 5. Ventricular depolarization and atrial repolazing simultaneously

6. ST segment: Ventricles contract
7. T-Wave: Ventricular Repolarization
8. End: Ventricular Filling

Question 19

In detail, What is happening during Ventricular systole?

Answer 19

• The wave of depolarization passes through the ventricular muscle mass and causes ventricular contraction.
• The ventricular pressure rises and almost immediately closes the AV valve.
• Ventricular pressure increases above aortic pressure and Aortic semilunar valve opens and ventricular ejection occurs
• The Ventrical does not empty completely, the remaining blood is called end systolic volume (ESV)
• As blood flows into the aorta, aortic pressure rises along with ventricular pressure however pressure begins to fall due to blood flow tapering off.

Question 20

What is diastole?

Answer 20

Period of ventricular relaxation

Question 21

What is happening during Mid to Late diastole?

Answer 21

• Left atrium and left ventricle are both relaxed
• AV valve is open and blood enters from pulmonary veins into atrium then about 70-80% of blood passes through open AV valve to ventricle before atrial contraction.
• At the very end of diastole, SA node fires and atrium depolarizes, contracts and the remaining blood is added to the ventricle. (EDV)

Question 22

What is the EDV?

Answer 22

The ammount of blood in the ventricle just prior to systole.

Question 23

What is happening to cause the heart sounds in labels 1, 2, 3, and 4?

Answer 23

1. Semilunar valve opens
2. Semilunar Valve closes
3. AV valve closes
4. AV valve opens

Question 24

Explain what happens during Late Diatstole (mechanically)?

Answer 24

Both sets of chambers are relaxed and ventricles fill passively

Question 25

What happens during Atrial systole (mechanically)?

Answer 25

Atrial contraction forces a small ammount of additional blood into the ventricles

Question 26

What happens during isovolumetric ventricular contraction (mechanically)?

Answer 26

First phase of ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves

Question 27

What happens during ventricular ejection (mechanically)?

Answer 27

as ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected

Question 28

What hapens during isovolumetric ventricular relaxation (mechanically)?

Answer 28

As ventricles relax, pressure in ventricles falls, blood flows back into cusps of semilunar valves and snaps them closed

Question 29

Why is it important that majority of ventricular filling takes place in early diastole?

Answer 29

It ensures that ventricular filling is not seriously impaired during periods of increased heart rate as early diastole is shortened during an increase of HR.

Question 30

What 2 vessels branches off of the Aorta to supply the heart with blood?

Answer 30

Left and Right coronary artery

Question 31

During what mechanical event do coronary arteries recieve most of their blood flow? why?

Answer 31

Most of their blood flow occurs during diastole because during systole, the corronary arteries are compressed by the contracting myocardial mass and thier opening are partially blocked by the open aortic valve (70% blood flow during diastole, 30% during systole)

Question 32

During excersise, explain what happens to the blood flow in the coronary arteries and explain the efficiency of haemoglobin?

Answer 32

During excersise: - The heart uses almost all the oxygen (haemoglobin) brought to it by the corronary arteries (about 65%) - Only way to increase O₂ Is by increase blood flow. - Matching the O₂ requirement is critical because the heart cannot generate sufficient ATP through anaerobic processes.

Question 33

Explain the basic pathway of what happens regarding cardiac muscle cells requiring oxygen and its relation to ATP?

Answer 33

Metabolic Activity of cardiac muscle cells increase causing ATP use to increase; More oxygen is required -\> = Increase in Adenosine released -\> = Vasodilation of coronary vessels -\> = Increase in blood flow to cardiac muscle -\> = More oxygen availible to meet requirement

Question 34

What does the term anaerobic mean in relation to exercise?

Answer 34

An activity that cannot be sustained from the energy of oxygen alone, requires ATP (or another source of energy)

Question 35

What does the term aerobic mean in relation to exercise?

Answer 35

Activity that does not require any other energy source apart from Oxygen.