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The Heart: Structure, Function, and Physiology

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The Heart

Introduction

The heart is a muscular organ essential for pumping blood throughout the body, supplying tissues with oxygen and nutrients while removing waste products. This study guide covers the anatomy, physiology, and key mechanisms of the heart, as relevant to a college-level Anatomy & Physiology II course.

Objectives

Learning Goals

  • Describe the size, location, and orientation of the heart.

  • Identify the structures of the heart, including valves, chambers, and associated great vessels.

  • Diagram the route of blood flow through the heart, including the pulmonary and systemic circuits.

  • Compare the structural and functional differences between cardiac and skeletal muscle.

  • Explain the mechanism and events of cardiac muscle contraction.

  • Name the components of the conduction system of the heart and trace the conduction pathway.

  • Interpret a normal electrocardiogram (ECG) tracing.

The Pulmonary and Systemic Circuits

Overview

The heart functions as a dual pump, maintaining two major circulatory pathways: the pulmonary and systemic circuits.

  • Pulmonary Circuit: The right side of the heart receives oxygen-poor blood from body tissues and pumps it to the lungs to eliminate carbon dioxide (CO2) and absorb oxygen (O2).

  • Systemic Circuit: The left side of the heart receives oxygen-rich blood from the lungs and pumps it to the rest of the body to supply tissues with oxygen and nutrients.

Key Points

  • Right side: Receives deoxygenated blood → lungs (via pulmonary circuit).

  • Left side: Receives oxygenated blood → body tissues (via systemic circuit).

Example

Blood flows from the right atrium to the right ventricle, then to the lungs. After oxygenation, it returns to the left atrium, moves to the left ventricle, and is pumped to the body.

Heart Anatomy

Size, Location, and Orientation

The heart is approximately the size of a closed fist and is located in the mediastinum, the central compartment of the thoracic cavity, between the second rib and the fifth intercostal space.

  • Mediastinum: Area between the lungs, behind the sternum.

  • Orientation: The heart is slightly tilted, with the apex pointing toward the left hip.

Example

On a chest diagram, the heart sits just left of the midline, protected by the rib cage.

Coverings of the Heart: Pericardium

Structure and Function

The heart is enclosed in a double-walled sac called the pericardium, which protects and anchors the heart, prevents overfilling, and reduces friction.

  • Fibrous Pericardium: Tough outer layer for protection and anchoring.

  • Serous Pericardium: Thin, inner layer with two parts:

    • Parietal Layer: Lines the internal surface of the fibrous pericardium.

    • Visceral Layer (Epicardium): Covers the heart surface.

  • Pericardial Cavity: Space between layers, filled with serous fluid to reduce friction.

Layers of the Heart Wall

  • Epicardium: Outer layer (visceral pericardium).

  • Myocardium: Middle, muscular layer responsible for contraction.

  • Endocardium: Inner lining of heart chambers and valves.

Chambers of the Heart

Structure

The heart contains four chambers:

  • Two atria: Superior, thin-walled chambers that receive blood.

  • Two ventricles: Inferior, thick-walled chambers that pump blood out of the heart.

Key Points

  • Interatrial septum: Separates the right and left atria.

  • Interventricular septum: Separates the right and left ventricles.

Example

Blood enters the right atrium from the superior and inferior vena cava and the coronary sinus; it enters the left atrium from the pulmonary veins.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow and prevent backflow.

  • Atrioventricular (AV) Valves:

    • Tricuspid Valve: Between rig ht atrium and right ventricle.

    • Mitral (Bicuspid) Valve: Between left atrium and left ventricle.

  • Semilunar (SL) Valves:

    • Pulmonary Valve: Between right ventricle and pulmonary trunk.

    • Aortic Valve: Between left ventricle and aorta.

Key Points

  • Valves open and close in response to pressure changes.

  • Chordae tendineae and papillary muscles prevent valve prolapse.

Blood Flow Through the Heart

Pathway

Blood flows through the heart in a specific sequence, passing through chambers and valves.

  1. Right atrium → tricuspid valve → right ventricle

  2. Right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs

  3. Lungs → pulmonary veins → left atrium

  4. Left atrium → mitral valve → left ventricle

  5. Left ventricle → aortic semilunar valve → aorta → systemic circulation

Example

Deoxygenated blood enters the right atrium, is pumped to the lungs, returns oxygenated to the left atrium, and is then pumped to the body.

Coronary Circulation

Overview

Coronary circulation provides the heart muscle (myocardium) with its own blood supply, delivered primarily when the heart is relaxed.

  • Coronary arteries: Branch from the aorta to supply oxygen-rich blood.

  • Cardiac veins: Collect deoxygenated blood and drain into the coronary sinus, which empties into the right atrium.

  • Anastomoses: Junctions that provide alternate routes for blood delivery.

Microscopic Anatomy of Cardiac Muscle

Structure

Cardiac muscle cells are striated, short, branched, and interconnected by intercalated discs.

  • Intercalated discs: Specialized junctions containing desmosomes and gap junctions for mechanical and electrical coupling.

  • Functional syncytium: Cardiac cells contract as a unit due to electrical coupling.

  • Mitochondria: Abundant, supporting aerobic metabolism.

Cardiac Muscle Contraction

Mechanism

  • Some cardiac muscle cells are self-excitable (pacemaker cells).

  • Depolarization opens slow Ca2+ channels, allowing extracellular Ca2+ to enter and trigger further Ca2+ release from the sarcoplasmic reticulum.

  • Cardiac muscle contracts as a unit (functional syncytium), unlike skeletal muscle.

  • No tetanic contractions; longer refractory period allows relaxation and filling.

  • Relies almost exclusively on aerobic respiration.

Comparison: Skeletal vs. Cardiac Muscle

Key Differences

Feature

Skeletal Muscle

Cardiac Muscle

Cell Shape

Long, cylindrical, multinucleate

Short, branched, one or two nuclei

Intercalated Discs

No

Yes

Contraction as Unit

No, motor units stimulated individually

Yes, functional syncytium

Sarcoplasmic Reticulum

Elaborate, terminal cisterns

Less elaborate, no terminal cisterns

Ca2+ Source

Sarcoplasmic reticulum only

Sarcoplasmic reticulum and extracellular fluid

Pacemaker Cells

No

Yes

Tetanic Contractions

Possible

No

ATP Source

Aerobic and anaerobic

Aerobic only (more mitochondria)

Heart Physiology: Electrical Events

Intrinsic Conduction System

The heart's rhythm is maintained by a network of pacemaker cells that initiate and distribute electrical impulses for coordinated contraction.

  • Sinoatrial (SA) node: Pacemaker in right atrial wall; generates impulses (~75/min).

  • Atrioventricular (AV) node: Delays impulse (~0.1 sec) to allow atrial contraction before ventricular contraction.

  • AV bundle (Bundle of His): Only electrical connection between atria and ventricles.

  • Right and left bundle branches: Carry impulses through interventricular septum.

  • Subendocardial conducting network (Purkinje fibers): Spread impulse through ventricular walls.

Sequence of Excitation

  1. SA node generates impulse.

  2. Impulse spreads across atria to AV node.

  3. AV node delays impulse.

  4. Impulse travels through AV bundle, bundle branches, and Purkinje fibers.

  5. Ventricular contraction follows, from apex toward atria.

Electrocardiography (ECG/EKG)

Overview

An electrocardiogram (ECG) records the electrical activity of the heart, providing information about heart rhythm and conduction.

  • P wave: Atrial depolarization (SA node to atria).

  • QRS complex: Ventricular depolarization and atrial repolarization.

  • T wave: Ventricular repolarization.

Example

A normal ECG tracing shows a regular sequence of P wave, QRS complex, and T wave, corresponding to the electrical events of one heartbeat.

Homeostatic Imbalances

Common Disorders

  • Incompetent valve: Blood backflows, requiring the heart to repump the same blood.

  • Valvular stenosis: Stiff valve flaps constrict opening, increasing workload.

  • Myocardial infarction (heart attack): Prolonged coronary blockage leads to cell death and scar tissue formation.

  • Myocardial infarction (heart attack): Irregular heart rhythms due to conduction system defects.

  • Fibrillation: Rapid, irregular contractions; heart cannot pump effectively.

  • Ectopic focus: Abnormal pacemaker activity, often due to excessive caffeine or nicotine.

Key Equations

Cardiac Output

  • Cardiac Output (CO): The amount of blood pumped by each ventricle per minute. where = heart rate (beats/min), = stroke volume (mL/beat)

Summary Table: Heart Structures and Functions

Structure

Function

Right Atrium

Receives deoxygenated blood from body

Right Ventricle

Pumps blood to lungs

Left Atrium

Receives oxygenated blood from lungs

Left Ventricle

Pumps blood to body

Tricuspid Valve

Prevents backflow into right atrium

Mitral Valve

Prevents backflow into left atrium

Pulmonary Valve

Prevents backflow into right ventricle

Aortic Valve

Prevents backflow into left ventricle

Additional info: Some details, such as the full conduction pathway and ECG interpretation, were expanded for academic completeness.

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