Structure and Functions of the Heart

Complete study notes covering macroscopic and microscopic anatomy, the cardiac cycle, neurovascular supply, and physical examination correlations.

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1. Introduction

The heart is a hollow, muscular, cone-shaped organ located in the mediastinum, between the lungs. Approximately two-thirds of its mass lies to the left of the midline. It functions as a double pump maintaining continuous blood circulation through the pulmonary and systemic circuits.

Key Parameters:

• Size: Approximately 12 cm long, 9 cm wide, 6 cm thick.
• Weight: Approximately 250-350 g in adult males; slightly less in females.
• Location: Posterior to the sternum, anterior to the vertebral column at levels T5-T8.
• Apex: Directed anteriorly, inferiorly, and to the left (located at the 5th intercostal space, midclavicular line).
• Base: Directed posteriorly, superiorly, and to the right.

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2. Structure of the Heart

2.1 Pericardium

The heart is enclosed within the pericardium, a double-walled sac consisting of two main layers:

• Fibrous pericardium: A tough, outer layer of dense connective tissue that prevents overfilling and anchors the heart to surrounding structures.
• Serous pericardium:
  • Parietal layer: Lines the inner surface of the fibrous pericardium.
  • Visceral layer (epicardium): Covers the external surface of the heart.
  • Pericardial cavity: The potential space between the parietal and visceral layers containing 10-20 mL of serous fluid to reduce friction.

2.2 Heart Wall

The heart wall consists of three distinct layers:

2.3 Chambers of the Heart

The heart has four chambers: two atria (upper) and two ventricles (lower).

Right Atrium:

• Receives deoxygenated blood from systemic circulation via the superior vena cava (head, neck, limbs), inferior vena cava (abdomen, pelvis, lower limbs), and coronary sinus (heart wall).
• Contains the fossa ovalis (remnant of fetal foramen ovale).
• Houses the sinoatrial (SA) node — the natural pacemaker.

Right Ventricle:

• Receives blood from the right atrium via the tricuspid valve.
• Pumps blood through the pulmonary valve into the pulmonary trunk.
• Wall is thinner than the left ventricle as it pumps against low pulmonary resistance (15-30 mmHg).
• Contains the moderator band (septomarginal trabecula) carrying the right bundle branch.

Left Atrium:

• Receives oxygenated blood from the lungs via four pulmonary veins (two from each lung).
• Smaller but thicker-walled than the right atrium.

Left Ventricle:

• Receives oxygenated blood from the left atrium via the mitral (bicuspid) valve.
• Pumps blood through the aortic valve into the ascending aorta for systemic circulation.
• Wall is 2-3 times thicker than the right ventricle to pump against high systemic pressure (80-120 mmHg).
• Forms the apex of the heart.

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3. Valves and Blood Flow

3.1 Atrioventricular (AV) Valves

• Tricuspid Valve (Right AV): Three cusps (anterior, posterior, septal). Prevents backflow to RA during ventricular systole.
• Mitral (Bicuspid) Valve (Left AV): Two cusps (anterior, posterior). Most commonly affected by rheumatic heart disease and mitral valve prolapse.
• Both are anchored by chordae tendineae to papillary muscles of the ventricles.

3.2 Semilunar Valves

• Pulmonary Valve: Three semilunar cusps. Located between RV and pulmonary trunk.
• Aortic Valve: Three semilunar cusps (right, left, non-coronary). Right and left coronary cusps give rise to the coronary arteries.

3.3 Fibrous Skeleton of the Heart

A dense connective tissue framework that:

• Provides attachment for valves and cardiac muscle fibers.
• Electrically insulates atria from ventricles.
• The AV bundle (Bundle of His) is the only normal electrical connection between atria and ventricles.

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4. Coronary Circulation

4.1 Arterial Supply

• Right Coronary Artery (RCA): Arises from the right aortic sinus. Supplies the RA, most of the RV, SA node (in 60%), AV node (in 80%), and posterior 1/3 of the septum.
• Left Coronary Artery (LCA): Arises from the left aortic sinus. Divides into:
  • Left Anterior Descending (LAD): Supplies anterior 2/3 of septum, apex; known as the "widow maker" when occluded.
  • Circumflex artery: Supplies LA and lateral/posterior walls of the LV.

4.2 Venous Drainage

• Great cardiac vein: Accompanies LAD; drains into coronary sinus.
• Middle cardiac vein: Accompanies posterior interventricular artery.
• Coronary sinus: Drains into the right atrium (main venous return).

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5. Functions and Physiology

5.1 The Cardiac Cycle

Sequence of events in one heartbeat (~0.8 seconds at 75 bpm):

1. Atrial Systole (0.1 sec): SA node fires; atria contract, forcing 20-30% of blood into ventricles ("atrial kick").
2. Isovolumetric Ventricular Contraction (0.05 sec): Ventricular pressure rises; AV valves close (S1 sound). All valves closed.
3. Ventricular Ejection (0.25 sec): Pressure exceeds arterial pressure; semilunar valves open. SV is ~70 mL.
4. Isovolumetric Ventricular Relaxation (0.08 sec): Semilunar valves close (S2 sound). All valves closed.
5. Ventricular Filling (0.4 sec): AV valves open; blood flows passively from atria.

5.2 Heart Sounds

5.3 Cardiac Output and Stroke Volume

Cardiac Output (CO): Volume of blood pumped per minute (SV x HR). Normal: 4.0-8.0 L/min.

Ejection Fraction (EF): Percentage of end-diastolic volume ejected. Normal: 55-70%.

Factors Affecting Stroke Volume:

• Preload: Frank-Starling mechanism (increased venous return stretches muscle, increasing force).
• Contractility: Force of contraction (increased by Sympathetics, Digoxin).
• Afterload: Systemic vascular resistance (pressure the heart must pump against).

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6. Conduction System

1. Sinoatrial (SA) Node: Pace-maker; generates impulses at 60-100 bpm.
2. Atrioventricular (AV) Node: Located in interatrial septum; delays impulse (0.1 sec) to allow ventricular filling.
3. AV Bundle (His): Passes through fibrous skeleton.
4. Bundle Branches: Conduct impulses to respective ventricles.
5. Purkinje Fibers: Rapidly conduct impulses to ventricular myocardium (20-40 bpm escape rhythm).

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7. Clinical Correlation: Common Cardiac Examinations