Cardiovascular Circuits and Electrical Activity

The Two Cardiovascular Circuits

• The Pulmonary Circuit     * Definition: The pathway of blood flow between the heart and the lungs.     * Route: It travels from the right atrium (and right ventricle) to the lungs, where it picks up oxygen and gets rid of carbon dioxide, and then returns to the left side of the heart.     * Key Detail: The pulmonary system specifically refers to the lungs; the circuit is the loop to the lungs and back.

• The Systemic Circuit     * Definition: The pathway of blood flow between the heart and the various systems/tissues of the body.     * Route: It travels from the left side of the heart out to various tissues, including the lower body, upper body, kidneys, liver, gastrointestinal (GI) tract, arms, and head. After delivering oxygen, the blood returns to the right side of the heart.

• Textbook Discrepancy (Long and Baker Text)     * The instructor explicitly notes that the textbook Long and Baker describes these circuits incorrectly.     * The Textbook’s Error: It describes the circuits as moving from the lungs to the tissues and from the tissues to the lungs.     * The Correct Physiology: A circuit is defined as traveling from the heart and back to the heart. This is supported by 20 other physiology books on the instructor's shelf.     * Instructional Guideline: Students are instructed to learn the "heart-to-heart" definition for the pulmonary (heart-lungs-heart) and systemic (heart-tissues-heart) circuits, ignoring the textbook's version.

Coronary Circulation

• The Need for Coronary Circulation     * Blood in the left atrium and left ventricle is rich in oxygen because it was just picked up at the lungs.     * Important Caveat: None of the oxygen inside the heart chambers is absorbed by the heart tissue itself as it passes through.     * The heart is an organ made of tissue (the myocardium) that is working constantly and requires a massive amount of energy and oxygen to function.

• The Pathway of Coronary Blood Flow     * Coronary Arteries: These are the very first branch of the systemic circuit. They branch off the aorta immediately after the blood leaves the left ventricle and passes the aortic valve.     * Function: They deliver oxygenated blood directly to the myocardium.     * Branching and Return: Like other systems, the coronary arteries branch into smaller vessels to deliver oxygen. These then merge into cardiac capillaries, then into cardiac veins. The blood eventually collects in the coronary sinus and is returned to the right atrium.

• Clinical Significance: Heart Disease and Heart Attacks     * The coronary arteries are the primary site for heart disease.     * Plaque Buildup: Heart disease often involves the buildup of plaque in the coronary arteries, which restricts blood flow to the myocardium.     * Myocardial Oxygen Deprivation: If the myocardium does not receive the oxygen it needs for a prolonged period, the heart tissue will die.     * Heart Attack: This tissue death resulting from oxygen deprivation is what defines a heart attack.

Cardiac Muscle Contraction and Electrical Stimulation

• Intrinsic Electrical Stimulus     * The heart is a muscle, similar to the biceps or quadriceps, and requires electrical stimulation to contract.     * Key Difference: Unlike skeletal muscles, which receive signals from the brain, the heart generates its own electrical stimulus. This is known as an intrinsic stimulus.

• The Sinoatrial (SA) Node     * Location: Located at the top of the right atrium.     * Nickname: Known as the "pacemaker of the heart."     * Function: This is where the electrical stimulus is generated. The rate at which the SA node generates these signals determines the heart rate.

• Intercalated Discs and Syncytium     * Unique Feature of Cardiac Fibers: Cardiac muscle fibers branch and connect to one another, unlike skeletal or smooth muscle fibers.     * Intercalated Discs: These are the junctions where one cardiac muscle fiber (cell) connects to another.     * Value of Interconnection: When an action potential is generated at the SA node, it spreads across the sarcolemma. Because the fibers are interconnected at the intercalated discs, the electrical impulse spreads rapidly from fiber to fiber.     * Result: This allows all the fibers in a chamber to contract together simultaneously as a single unit.

Conduction Pathway of the Heart

• Atrial Contraction     * The signal starts at the SA node and spreads across the atria, causing them to contract together.     * This pumps the remaining blood from the atria into the ventricles (though much of the blood leaks down passively since the valves are usually open).

• The Atrioventricular (AV) Node     * The electrical signal does not spread directly from the atria to the ventricles.     * It must find the AV node (atrioventricular node), located at the base of the right atrium between the atria and the ventricles.

• Ventricular Conduction System     * AV Bundle (Atrioventricular Bundle): After the AV node, the signal travels down nervous tissue in the middle of the heart (the septum).     * Bundle Branches: The AV bundle splits into right and left bundle branches that travel down to the apex (the bottom) of the heart.     * Purkinje Fibers: At the apex, the bundle branches split into Purkinje fibers.     * Ventricular Contraction: The Purkinje fibers deliver the signal to the ventricles. The impulse spreads through the ventricular fibers via intercalated discs, causing the ventricles to contract together and pump blood out of the heart.

• Timing: The atria contract first, followed by the ventricles.

Abnormalities in Heart Rhythm

• Ectopic Pacemaker     * Occurs when a random area of the heart (other than the SA node) starts generating its own electrical signal.     * This causes different areas of the heart to fire out of sync, preventing the heart from pumping blood effectively.

• Medical Procedures     * Ablation: A cardiologist may perform this procedure to burn and kill the specific cells causing the inappropriate electrical firing.     * Artificial Pacemaker: If the SA node cannot maintain a regular rhythm, a mechanical pacemaker is implanted to monitor the heart and provide an electrical shock to ensure a regular beat.

The Electrocardiogram (ECG/EKG)

• Overview     * Action potentials can be tracked by placing electrodes on a person's chest and limbs.     * The ECG records the collective action potentials of all heart fibers at once.

• The Three Main Events of a Generic Wave     * $P$ Wave: Represents atrial depolarization. This occurs when the signal starts at the SA node and spreads across the atria.     * $QRS$ Complex: Represents ventricular depolarization. This is the major spike on the ECG and occurs as the impulse spreads across the ventricles.     * $T$ Wave: Represents ventricular repolarization.

• The Missing Wave: Atrial Repolarization     * Atrial repolarization does occur, but it is not visible on a standard ECG.     * Reason: It happens at the same time as the $QRS$ complex. Because the $QRS$ complex is so large, it electrically covers up/hides the smaller wave of atrial repolarization.