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

Module 17.1 Overview of the Heart

The Cardiovascular System

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

• Heart: Pumps blood through the vessels.

• Blood vessels: Tubes that distribute blood throughout the cardiovascular system.

• Blood: The liquid medium carrying oxygen and nutrients.

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, flat part opposite the apex, facing the posterior thoracic cavity.

• Chambers: 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 sets of vessels:

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

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

Why is the right side of the heart sometimes called the pulmonary pump? The right side pumps deoxygenated blood to the lungs via the pulmonary circuit.

Blood Flow Through the 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 the blood into tissues, and carbon dioxide diffuses from tissues into the blood.

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

Other Functions of the Heart

• Maintains homeostasis of blood pressure.

• Produces atrial natriuretic peptide (ANP) in the atria, which lowers blood pressure by promoting sodium and water excretion.

Module 17.2 Heart Anatomy and Blood Flow Pathway

The Pericardium, Heart Wall, and Heart Skeleton

The heart is surrounded by membranes that protect and anchor it.

• Fibrous pericardium: Outermost layer, tough and protective.

• Serous pericardium: Thin inner membrane producing serous fluid. It has two layers:

  • Parietal pericardium: Fused to the fibrous pericardium.

  • Visceral pericardium (epicardium): Superficial layer of the heart wall.

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

Heart Wall Structure

• Myocardium: Muscular middle layer, responsible for contraction.

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

Heart Chambers and Valves

• Four chambers: two atria (receiving blood from veins) and two ventricles (pumping blood into arteries).

• Valves: Prevent backflow of blood.

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

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

Major Vessels of the Heart

• Systemic veins: Superior and inferior vena cava drain deoxygenated blood into the right atrium.

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

• Pulmonary veins: Return oxygenated blood from lungs to the left atrium.

• Aorta: Supplies oxygenated blood to the systemic circuit from the left ventricle.

Coronary Circulation

The heart muscle (myocardium) is supplied by its own network of blood vessels called the coronary circulation.

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

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

Coronary Artery Disease and Myocardial Infarction

• Coronary artery disease (CAD): Buildup of fatty plaques in coronary arteries, leading to reduced blood flow.

• Myocardial infarction (MI): Heart attack caused by blockage of coronary arteries, resulting in death of heart muscle tissue.

Module 17.3 Cardiac Muscle Tissue Anatomy and Electrophysiology

Cardiac Muscle Tissue

• Cardiac muscle cells: Striated, involuntary, and capable of autorhythmicity (self-generated rhythm).

• Action potentials: Electrical impulses that trigger contraction.

Histology of Cardiac Muscle Tissue

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

Cardiac Electrophysiology: Pacemaker Cells and the Cardiac Conduction System

• Pacemaker cells: Specialized cardiac cells that spontaneously generate action potentials, setting the heart's pace.

• Cardiac conduction system: Network of pacemaker cells and conducting fibers that coordinate heart contractions.

Pacemaker Cell Action Potential Phases

• Slow initial depolarization phase: Nonspecific cation channels open, allowing sodium ions to enter and potassium ions to leak out.

• Full depolarization phase: Voltage-gated calcium channels open, causing rapid influx of calcium ions.

• Repolarization phase: Voltage-gated potassium channels open, potassium ions exit, restoring resting potential.

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

Anatomy of the Cardiac Conduction System

• Sinoatrial (SA) node: Located in the right atrium; fastest intrinsic rate (~60-100 action potentials/min).

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

• Purkinje fiber system: Depolarizes only about 20 times per minute; includes AV bundle, right and left bundle branches, and terminal Purkinje fibers.

Pacing the Heart: Sinus Rhythm

• The SA node is the normal pacemaker; its electrical signals maintain the heart's rhythm.

Key Terms and Definitions

• Apex: Pointed tip of the heart.

• Base: Broad, flat part of the heart opposite the apex.

• Atria: Upper chambers of the heart.

• Ventricles: Lower chambers of the heart.

• Valve: Structure that prevents backflow of blood.

• Autorhythmicity: Ability of cardiac cells to generate their own rhythm.

• Action potential: Electrical impulse that triggers muscle contraction.

• Coronary circulation: Blood supply to the heart muscle itself.

Table: Comparison of Pulmonary and Systemic Circuits

Feature	Pulmonary Circuit	Systemic Circuit
Side of Heart	Right	Left
Destination	Lungs	Body tissues
Blood Oxygenation	Deoxygenated to oxygenated	Oxygenated to deoxygenated
Pressure	Low	High

Key Equations

• Cardiac Output: The amount of blood pumped by the heart per minute. $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

• Blood Pressure: $\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Example: Pathway of Blood Through the Heart

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava.

2. Passes through the tricuspid valve into the right ventricle.

3. Pumped through the pulmonary valve into the pulmonary trunk and arteries to the lungs.

4. Oxygenated blood returns via pulmonary veins to the left atrium.

5. Passes through the bicuspid (mitral) valve into the left ventricle.

6. Pumped through the aortic valve into the aorta and systemic circulation.

Additional info: Some details, such as specific ion movements and the names of conduction system components, were inferred from standard anatomy and physiology knowledge to ensure completeness.