20. The Heart

Question 1

pericardium

Answer 1

Fibrous pericardium (outer part)-
Serous pericardium - double-walled sac
> parietal pericardium is fused to fibrous pericardium
> visceral pericardium adheres to heart surface

Question 2

Wall of the heart consists of three layers:

Answer 2

1) Outer epicardium (= visceral pericardium)
2) Middle myocardium
3) Inner endocardium

Question 3

Are three main circuits of blood flow:

Answer 3

1) Pulmonary circulation
- functions to bring blood to the lungs for gas exchange

2) Systemic circulation
- functions to bring blood to other body organs

3) Coronary circulation
- functions to supply blood to the myocardium

Question 4

Heart Attacks

Answer 4

• Narrowing of coronary vessels decreases O2 supply of myocardium
• O2 deprived myocardium switches to anaerobic metabolism which produces lactic acid
• This is accompanied by pain in heart that radiates down left arm (angina pectoris)
• This is forewarning of impending heart attack
• Prolonged coronary blockage can lead to the sudden death of a patch of myocardium (myocardial infarction or heart attack)

Question 5

intercalated discs consist of 2

Answer 5

1) desmosomes (physical connection) that will anchor cells so they don’t separate when contract
2) gap junctions (electrical connection) which allow ions to pass freely from cell to cell so cells contract as a coordinated unit

Question 6

autorhythmic fibers

Answer 6

• make up 1% of cardiac muscle fibers
• are more like nerve cells
• are self-excitable and initiate an electrical impulse
• form an impulse conduction system through-out the heart

Question 7

Pacemaker Cell depolarization

Answer 7

fibers spontaneously depolarize to threshold and generate an action potential

1) starts at -60mV and slowly drifts upwards due to slow inflow of Na+. This gradual depolarization is called pacemaker potential
2) reaches a threshold of -40mV and action potential spike occurs due to Ca2+ influx through Ca2+ channels
3) action potential spreads through atria and then both atria contract
4) repolarization occurs due to the closing of Na+ and Ca2+ channels and the opening of K+ channels

Question 8

Non-contractile Cardiac Cells Pathway

Answer 8

SA node (pacemaker)–> AV node–> Bundle of His –> R/L. Bundle Branches –> Purkinje Fibers –>ventricular myocardium–>ventricle contracts

Question 9

Contractile Cardiac Cells Cardiac Muscle Action Potential:

Answer 9

• action potential spreads from autorhythmic (NONcontractile) fibers to CONTRACTILE fibers of myocardium
Steps:
• Depolarization: Na+ channels open (fast voltage-gated channels) and depolarize membrane
• Plateau: slow Ca2+ channels prolongs depolarization
• Repolarization: Ca2+ closes, K+ channels open

Question 10

Electrocardiogram (ECG):

Answer 10

• A recording of the electrical activities of the heart
• Can analyze the size of the waves and the time intervals between waves to determine heart abnormalities

P wave, depolarization of the atria
QRS complex, depolarization of the ventricles
T wave, repolarization of the ventricles

Question 11

Average Heart rate

Average length of cardiac cycle

Answer 11

Heart rate is about 75 beats/min, so one cardiac cycle (or heart beat) requires 0.8 sec.

Question 12

One cardiac cycle consists of:

Answer 12

• a contraction and relaxation of both atria and contraction and relaxation of both ventricles
> systole (S) = contraction
> diastole (D) = relaxation
> 1 cardiac cycle = (S + D of atria) + (S + D of ventricle)

Question 13

Heart Sounds

Answer 13

• Heart sounds are caused by closure of valves during the cardiac cycle
> Lubb: created by closure of the atrioventricular valves at the start of ventricular contraction. loader and longer sound
> Dupp: created by closure of the semilunar valves during ventricular relaxation. shorter and higher pitch sound

Question 14

Summary of Events During Cardiac Cycle

Answer 14

1. Opening of atrioventricular values, blood flows passively from atria into ventricle until about 80% of blood filled the ventricle, then atria would contract and squeeze remain blood into ventricle
2. Atria relaxes and ventricle begins contracting, this gives increased pressure in ventricle so AV valves snap shut
3. When the ventricle contracs, the semilunar valuves open so blood can move into pulmonary trunk/aorta
4. Ventricle relaxes and decereased pressure in ventricle causes semilunar valives to close
5. Back to step 1, repeat

Question 15

Features of the Cardiac Conduction System

Answer 15

• uses aerobic metabolism for ATP
• rich in myoglobin = O2 binding pigment in muscle
• more mitochondria (larger/resistant to fatigue)
• muscle cells = branched = added strength
• smaller sarcoplasmic reticulum and less reserve of intracellular Ca+2