Anatomy and Physiology of the Heart: Structure, Function, and Electrophysiology

Module 17.1: Overview of the Heart

The Cardiovascular System

The cardiovascular system is composed of the heart, blood vessels, and blood. Its primary function is to transport oxygen, nutrients, and waste products throughout the body.

• Heart: A muscular organ that pumps blood.

• Blood vessels: Tubes (arteries, veins, capillaries) that carry blood throughout the body.

• Blood: The fluid that transports gases, nutrients, and wastes.

Location and Basic Structure of the Heart

• The heart is located in the thoracic cavity, posterior to the sternum, in the mediastinum, and rests on the diaphragm.

• Apex: The pointed end of the heart, directed toward the left hip.

• Base: The broad, superior portion facing the right shoulder.

• The heart has four chambers: right and left atria (superior), and right and left ventricles (inferior).

Functions of the Heart

• The heart pumps blood through two separate circuits:

  • Pulmonary circuit: Right side of the heart pumps deoxygenated blood to the lungs for oxygenation.

  • Systemic circuit: Left side of the heart pumps oxygenated blood to the rest of the body.

• The right side is sometimes called the "pulmonary pump" because it sends blood to the lungs.

• The left side is called the "systemic pump" because it sends blood to the body.

Gas Exchange and Circuits

• In the pulmonary capillaries, oxygen diffuses into the blood and carbon dioxide diffuses out to be exhaled.

• In the systemic capillaries, oxygen diffuses from blood into tissues, and carbon dioxide diffuses from tissues into blood.

• The pulmonary circuit is a low-pressure circuit; the systemic circuit is high-pressure.

Other Functions of the Heart

• The heart helps maintain blood pressure homeostasis.

• The atria produce atrial natriuretic peptide (ANP), a hormone that lowers blood pressure by promoting sodium and water excretion by the kidneys.

Module 17.2: Heart Anatomy and Blood Flow Pathway

The Pericardium, Heart Wall, and Heart Skeleton

• Pericardium: A double-walled sac surrounding the heart.

  • Fibrous pericardium: Outermost, tough connective tissue layer.

  • Serous pericardium: Thin inner membrane producing serous fluid.

    • Parietal layer: Fused to fibrous pericardium.

    • Visceral layer (epicardium): Fused to heart surface.

  • Pericardial cavity: Space between parietal and visceral layers, containing pericardial fluid to reduce friction.

• Heart wall: Composed of three layers:

  • Epicardium: Outer layer (visceral pericardium).

  • Myocardium: Middle, muscular layer responsible for contraction.

  • Endocardium: Inner endothelial lining.

• Fibrous skeleton: Dense connective tissue providing structural support, electrical insulation, and anchoring for heart valves.

Chambers and Valves of the Heart

• The heart has four chambers: two atria (receiving chambers) and two ventricles (pumping chambers).

• Valves ensure unidirectional blood flow:

  • Atrioventricular (AV) valves: Between atria and ventricles (tricuspid on right, bicuspid/mitral on left).

  • Semilunar valves: Between ventricles and arteries (pulmonary and aortic valves).

• Chordae tendineae and papillary muscles anchor AV valves, preventing prolapse.

Major Blood Vessels Associated with the Heart

• Systemic veins: Superior and inferior vena cava return deoxygenated blood to the right atrium.

• Pulmonary trunk: Receives blood from right ventricle, splits into right and left pulmonary arteries to the lungs.

• Pulmonary veins: Return oxygenated blood from lungs to left atrium (usually four veins).

• Aorta: Largest artery, carries oxygenated blood from left ventricle to the body.

Coronary Circulation

• The myocardium is supplied by coronary arteries, as it is too thick for diffusion from chambers.

• Coronary arteries: Branch from the ascending aorta (right and left coronary arteries).

• Coronary veins: Drain into the coronary sinus, which empties into the right atrium.

• Coronary artery disease (CAD): Caused by plaque buildup, can lead to myocardial infarction (heart attack).

Module 17.3: Cardiac Muscle Tissue Anatomy and Electrophysiology

Cardiac Muscle Tissue and Cells

• Cardiac muscle cells contract in response to electrical excitation (action potentials).

• They display autorhythmicity: the ability to generate their own rhythm without nervous input.

• Cardiac muscle cells have striations (alternating light and dark bands).

Cardiac Electrophysiology: Pacemaker Cells and the Cardiac Conduction System

• Pacemaker cells: Specialized cells (about 1% of cardiac cells) that spontaneously generate action potentials, setting the heart's pace.

• These cells form the cardiac conduction system, a network that coordinates heartbeats.

Pacemaker Cell Action Potential Phases

• Slow initial depolarization: Nonspecific cation channels open, allowing Na+ ions to enter and K+ ions to exit, slowly depolarizing the cell.

• Full depolarization: Voltage-gated Ca2+ channels open, causing rapid depolarization.

• Repolarization: Voltage-gated K+ channels open, K+ exits, repolarizing the cell.

• Minimum potential phase: K+ channels remain open until the minimum potential is reached, then the cycle repeats.

Anatomy of the Cardiac Conduction System

• Sinoatrial (SA) node: Located in the right atrium; fastest intrinsic rate (~60-100 bpm); primary pacemaker.

• Atrioventricular (AV) node: Located posterior and medial to the tricuspid valve; slower rate (~40 action potentials/min).

• Purkinje fiber system: Slowest pacemaker cells (~20 action potentials/min); includes AV bundle, right and left bundle branches, and terminal Purkinje fibers.

Pacing the Heart

• Sinus rhythm: Normal heart rhythm set by the SA node.

• Electrical impulses from the SA node spread through the atria, AV node, and ventricles, coordinating contraction.

Summary Table: Heart Chambers, Valves, and Major Vessels

Chamber	Receives Blood From	Pumps Blood To	Associated Valve
Right Atrium	Superior/Inferior Vena Cava, Coronary Sinus	Right Ventricle	Tricuspid (AV) Valve
Right Ventricle	Right Atrium	Pulmonary Trunk (to Lungs)	Pulmonary Semilunar Valve
Left Atrium	Pulmonary Veins	Left Ventricle	Bicuspid (Mitral) Valve
Left Ventricle	Left Atrium	Aorta (to Body)	Aortic Semilunar Valve

Key Equations

• Cardiac Output: The amount of blood pumped by each ventricle per minute. $\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

Additional info:

• Some details, such as the exact number of pulmonary veins (usually four), and the specific names of coronary arteries (left anterior descending, circumflex), were inferred for completeness.

• Clinical context, such as the definition and consequences of myocardial infarction, was expanded for clarity.