The Heart: Structure, Function, and Physiology

Introduction

The heart is a muscular organ that serves as the central component of the circulatory system, functioning as a pump to deliver blood throughout the body. Understanding its anatomy and physiology is essential for comprehending cardiovascular function and health.

Internal and External Anatomy of the Heart

Location and Orientation

• Location: The heart is situated within the mediastinum of the thorax, positioned obliquely from the 2nd rib to the 5th intercostal space.

• Orientation: The base of the heart is directed toward the right shoulder, while the apex points toward the left hip. The heart rests on the diaphragm, with the left side facing anteriorly.

Heart Wall Layers

• Pericardium: A double-walled, fibro-serous sac that encloses the heart.

• Fibrous Pericardium: The tough outer layer that protects and anchors the heart, preventing overfilling.

• Serous Pericardium: Composed of two layers:

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

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

• Pericardial Cavity: The space between the parietal and visceral layers, filled with serous fluid to reduce friction.

• Myocardium: The thick, muscular middle layer composed of cardiac muscle cells arranged in bundles, responsible for contraction.

• Endocardium: The inner lining of the heart, consisting of endothelium (simple squamous epithelium) that is continuous with the lining of blood vessels.

Chambers and Major Vessels of the Heart

Chambers

• Atria: The two upper chambers (right and left atria) are thin-walled and serve as receiving chambers for blood returning to the heart.

• Ventricles: The two lower chambers (right and left ventricles) are thick-walled and function as the main pumping chambers.

Major Vessels

• Superior and Inferior Vena Cava: Return deoxygenated blood from the body to the right atrium.

• Pulmonary Trunk and Arteries: Carry deoxygenated blood from the right ventricle to the lungs.

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

• Aorta: Distributes oxygenated blood from the left ventricle to the systemic circulation.

Heart Valves

Types and Functions

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Between right atrium and right ventricle.

  • Bicuspid (Mitral) 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.

• Function: Ensure unidirectional blood flow and prevent backflow.

• Chordae Tendineae and Papillary Muscles: Anchor AV valves and prevent prolapse during ventricular contraction.

Coronary Circulation

Overview

• Coronary Arteries: Supply oxygenated blood to the myocardium. The right and left coronary arteries branch from the base of the aorta and encircle the heart in the atrioventricular groove.

• Coronary Veins: Collect deoxygenated blood from the myocardium and return it to the right atrium via the coronary sinus.

• Anastomoses: Junctions between vessels that provide alternate routes for blood flow.

Histology and Physiology of Cardiac Muscle

Cardiac Muscle Structure

• Cardiac Muscle Cells: Short, branched, and interconnected by intercalated discs.

• Intercalated Discs: Specialized junctions containing desmosomes (for mechanical strength) and gap junctions (for electrical coupling), allowing the heart to function as a functional syncytium.

• Comparison with Skeletal Muscle: Cardiac muscle cells are shorter, have one or two nuclei, more mitochondria, and rely exclusively on aerobic metabolism.

Table: Comparison of Cardiac and Skeletal Muscle

Electrical Properties and Conduction System

Autorhythmic Cells and Pacemaker Activity

• Autorhythmic Cells: Specialized cardiac cells (about 1%) that can spontaneously depolarize and generate action potentials.

• Sinus Rhythm: The normal rhythm of the heart set by the sinoatrial (SA) node, the natural pacemaker.

• Pacemaker Potentials: Unstable resting membrane potentials that gradually depolarize to threshold, triggering action potentials.

Intrinsic Conduction System

• Pathway: SA node → AV node (delay) → AV bundle (Bundle of His) → Right and left bundle branches → Purkinje fibers.

• Function: Coordinates the timing of atrial and ventricular contractions, ensuring efficient pumping.

Electrocardiogram (ECG)

Principles and Interpretation

• ECG: A recording of the electrical activity of the heart as detected by electrodes on the skin.

• Waves:

  • P wave: Atrial depolarization

  • QRS complex: Ventricular depolarization

  • T wave: Ventricular repolarization

• Clinical Use: Diagnosing arrhythmias, heart blocks, and myocardial infarction.

The Cardiac Cycle

Phases and Events

• Systole: Contraction phase; blood is pumped out of the chambers.

• Diastole: Relaxation phase; chambers fill with blood.

• Sequence: Atrial systole → Ventricular systole → Quiescent period (all chambers relaxed).

• Valve Actions: Pressure changes drive the opening and closing of AV and semilunar valves.

Key Terms

• Isovolumetric Contraction: Ventricles contract with all valves closed; volume remains constant.

• Isovolumetric Relaxation: Ventricles relax with all valves closed.

• Dicrotic Notch: Brief rise in aortic pressure following closure of the aortic valve.

Heart Sounds and Murmurs

Physiological Significance

• First Heart Sound (S1): Closure of AV valves; marks the beginning of systole.

• Second Heart Sound (S2): Closure of semilunar valves; marks the end of systole.

• Murmurs: Abnormal heart sounds due to turbulent blood flow, often caused by valve defects (stenosis or insufficiency).

Cardiac Output and Regulation

Definition and Calculation

• Cardiac Output (CO): The amount of blood pumped by each ventricle per minute.

• Formula:

• Where HR = heart rate (beats per minute), SV = stroke volume (mL per beat).

• Typical resting CO: 5 L/min.

Regulation of Heart Rate and Stroke Volume

• Autonomic Nervous System:

  • Sympathetic Stimulation: Increases heart rate and contractility (norepinephrine).

  • Parasympathetic Stimulation: Decreases heart rate (acetylcholine).

• Hormones: Epinephrine increases heart rate and contractility.

• Electrolytes: Imbalances in Ca2+, Na+, and K+ affect heart function.

• Temperature: Increased body temperature raises heart rate.

Frank-Starling Law of the Heart

• Within physiological limits, the heart pumps all the blood it receives.

• Increased venous return stretches cardiac muscle, increasing force of contraction and stroke volume.

• Each ventricle regulates its output to match input, maintaining balance between pulmonary and systemic circuits.

Afterload and Contractility

• Afterload: The pressure the ventricles must overcome to eject blood (mainly determined by arterial pressure).

• Increased afterload (e.g., hypertension) reduces stroke volume.

• Contractility: The strength of contraction at a given muscle length, influenced by sympathetic stimulation and certain drugs (e.g., digoxin).

Summary Table: Key Heart Structures and Functions

Additional info: Some details, such as the precise mechanisms of action potential generation and the effects of chronic hypertension on cardiac muscle, have been expanded for academic completeness.