LO6: Cardiac System: Circulation, Muscle, and Conduction

The heart muscle receives its own oxygenated blood supply through the coronary arteries and deoxygenated blood is collected by cardiac veins.
The two main coronary arteries, the right coronary artery (RCA) and left coronary artery (LCA), originate from the ascending aorta.

Supplies blood to: the right atrium, part of the left atrium, most of the right ventricle, and the inferior portion of the left ventricle.

Branches into the anterior descending artery (LAD) and the circumflex artery.
Supplies blood to: the left atrium, most of the left ventricle, and most of the interventricular septum.

LAD Artery Blockage: A blockage in the LAD (also known as the "widowmaker") can be severe due to its supply to the left ventricle's front wall.
Left Ventricle's High Demand: The left ventricle's myocardium requires the most blood due to its demanding work.

Coronary arteries mainly receive blood when the ventricles relax (diastole).
During contraction, blood flow is restricted by myocardial compression and the aortic valve cusps covering the coronary artery openings.
Sustained high heart rate is detrimental as it reduces ventricular relaxation time, decreasing coronary artery filling.

The leading cause of death in America ( 600,000 annually).
Occurs when coronary arteries narrow or block due to atherosclerosis (cholesterol and fatty deposits).
Ischemia: Oxygen deprivation to heart tissue, causing pain, due to interrupted blood supply.
Necrosis: Cell death in the heart tissue if ischemia persists.

A temporary interruption of blood supply to the heart, causing chest pain (ischemia).
Often due to a partially blocked vessel or increased oxygen demand exceeding supply.
Usually subsides with rest as circulation resumes, averting permanent damage.

A complete blockage of blood flow (e.g., by a clot or fatty deposit) leading to the death of myocardial cells (necrosis).

Men: Common symptoms include chest pain/pressure, upper body discomfort (arms, back, neck, jaw, stomach), shortness of breath, nausea, sweating, anxiety.
Women: More likely to report unusual symptoms like extreme fatigue, abdominal pain, dizziness, weakness.

Most cardiac veins drain into the coronary sinus on the heart's posterior surface, which then returns deoxygenated blood to the right atrium.
Anterior cardiac veins are an exception, emptying directly into the right atrium.

Similarities to Skeletal Muscle:
- Striated.
- Contraction initiated by action potential triggering calcium release from sarcoplasmic reticulum.
- Calcium binds to troponin, forming cross-bridges.
Unique Characteristics:
- Found only in the heart; cells are short, fat, branched, with usually a single nucleus.
- Branched cells connect with 3 or 4 others, forming an interconnected network.
- Intercalated discs: Thick connections joining cells end-to-end, with finger-like projections preventing separation.
- Gap junctions: Small channels in intercalated discs that rapidly transmit electrical impulses, allowing coordinated, wave-like contraction.
- Large mitochondria indicate near-exclusive reliance on aerobic respiration for energy, meaning it cannot function long without oxygen.

Unlike skeletal muscle, cardiac muscle lacks regenerative cells.
Damage is repaired primarily via fibrosis (scarring), not new muscle tissue regeneration.

Cardiac muscle contraction is independent of external nerves due to internal pacemaker cells that generate action potentials (automaticity).
The heart exhibits rhythmicity (regular beat).
The nervous system and hormones can modulate (but not initiate) heart rate and rhythm.

Sinoatrial (SA) Node: Primary pacemaker in the right atrium; originates normal cardiac impulses.
Internodal/Interatrial Bundles: Impulse travels to the AV node and left atrium, causing atrial contraction.
Atrioventricular (AV) Node: Impulse slows here, allowing ventricles to fill. The heart's fibrous skeleton acts as an insulator, directing impulses through the AV node.
Bundle of His (AV Bundle): Impulse accelerates after exiting the AV node.
Right and Left Bundle Branches: The AV bundle divides.
Purkinje Fibers: Distribute impulses to ventricular muscle cells for simultaneous contraction.

Pacemakers other than the SA node (e.g., AV node, Purkinje fibers) can initiate impulses at slower rates if the SA node fails.
Resting Firing Rates:
- SA Node: 60 to 80 beats/minute.
- AV Node: 40 to 60 beats/minute.
- Purkinje Fibers: 20 to 40 beats/minute.

Due to internal signal generation, a heart removed from the body can continue to beat for hours in a protective environment.

Definition: A visual record of electrical currents from cardiac impulses, detected by electrodes on the body's surface.
A composite recording of all action potentials from nodal and myocardial cells.
Normal sinus rhythm: A normal-appearing ECG with impulses from the SA node.
Arrhythmia: An irregular heartbeat.

P wave: Represents atrial depolarization (SA node impulse spread through atria), occurring before atrial contraction.
PR interval: Time for impulse to travel from atria to ventricles.
QRS complex: Represents ventricular depolarization (impulse spread through ventricular muscle).
ST segment: Period from end of ventricular depolarization to start of ventricular repolarization.
T wave: Represents ventricular repolarization (electrical recovery of ventricles).