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

Heart Structure and Circulation

Overview of Heart Function

The heart is a muscular organ responsible for pumping blood throughout the body via two main circuits: the pulmonary and systemic circuits. It ensures the delivery of oxygen and nutrients to tissues and the removal of waste products.

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

• Systemic circuit: Left side of the heart pumps oxygen-rich blood to body tissues.

Key terms: atrium (receiving chamber), ventricle (pumping chamber)

Heart Chambers

The heart consists of four chambers: two atria and two ventricles, each with distinct roles in blood circulation.

• Right atrium: Receives blood from systemic circuit (body)

• Left atrium: Receives blood from pulmonary circuit (lungs)

• Right ventricle: Pumps blood into pulmonary circuit

• Left ventricle: Pumps blood into systemic circuit

Heart Anatomy

The heart is approximately the size of a fist and is located in the mediastinum, between the second rib and fifth intercostal space, on the superior surface of the diaphragm. The base (posterior surface) leans toward the right shoulder, while the apex points toward the left hip.

• Base: Posterior surface, toward right shoulder

• Apex: Points toward left hip

• Apical impulse: Palpated between fifth and sixth ribs, just below left nipple

Coverings and Layers of the Heart

Pericardium

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: Outer layer, protects and anchors heart

• Serous pericardium: Inner layer, consists of parietal and visceral layers

• Pericardial cavity: Fluid-filled space between layers, decreases friction

Homeostatic Imbalances

• Pericarditis: Inflammation of pericardium, causes pericardial friction rub

• Cardiac tamponade: Excess fluid compresses heart, limits pumping ability

Layers of the Heart Wall

The heart wall consists of three layers, each with specific functions:

• Epicardium: Visceral layer of serous pericardium

• Myocardium: Middle layer, composed of cardiac muscle, responsible for contraction

• Endocardium: Inner layer, lines heart chambers and covers valves

Internal Heart Anatomy

Chambers and Associated Structures

• Atria: Thin-walled, receiving chambers

• Auricles: Appendages that increase atrial volume

• Pectinate muscles: Muscular ridges in atria

• Ventricles: Thick-walled, discharging chambers

• Trabeculae carneae: Irregular ridges of muscle on ventricular walls

• Papillary muscles: Anchor chordae tendineae

Heart Valves

Valves ensure unidirectional blood flow through the heart and prevent backflow.

• Atrioventricular (AV) valves: Between atria and ventricles

  • Tricuspid valve: Right AV valve

  • Mitral valve: Left AV valve (bicuspid)

  • Chordae tendineae: Anchor valve cusps to papillary muscles

• Semilunar (SL) valves: Between ventricles and major arteries

  • Pulmonary semilunar valve: Right ventricle to pulmonary trunk

  • Aortic semilunar valve: Left ventricle to aorta

Valve Disorders (Homeostatic Imbalances)

• Incompetent valve: Blood backflows, heart repumps same blood

• Valvular stenosis: Stiff flaps, constricted opening, heart works harder to pump blood

Pathway of Blood Through the Heart

Pulmonary Circuit

• Right atrium → tricuspid valve → right ventricle

• Right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs

• Lungs → pulmonary veins → left atrium

Systemic Circuit

• Left atrium → mitral valve → left ventricle

• Left ventricle → aortic semilunar valve → aorta → systemic circulation

Summary Table: Pathway of Blood Through the Heart

Circuit	Pathway	Type of Blood
Pulmonary	Right atrium → right ventricle → lungs → left atrium	Oxygen-poor to oxygen-rich
Systemic	Left atrium → left ventricle → body tissues → right atrium	Oxygen-rich to oxygen-poor

Coronary Circulation

Arteries

• Left coronary artery: Branches into anterior interventricular artery and circumflex artery

• Right coronary artery: Branches into right marginal artery and posterior interventricular artery

Coronary arteries supply blood to the myocardium. Arterial supply varies among individuals and contains anastomoses (junctions) for alternative blood delivery routes.

Veins

• Cardiac veins: Collect blood from capillary beds

• Coronary sinus: Empties blood into right atrium, formed by merging cardiac veins

• Great cardiac vein: In anterior interventricular sulcus

• Middle cardiac vein: In posterior interventricular sulcus

• Small cardiac vein: From inferior margin

Homeostatic Imbalances

• Angina pectoris: Thoracic pain due to fleeting deficiency in blood delivery to myocardium

• Myocardial infarction (heart attack): Prolonged coronary blockage, areas of cell death replaced with noncontractile scar tissue

Microscopic Anatomy of Cardiac Muscle

Cardiac Muscle Structure

Cardiac muscle cells are striated, short, branched, and interconnected, typically with one or two central nuclei. The connective tissue matrix (endomysium) connects to the cardiac skeleton, providing structural support.

• T tubules: Wide, less numerous than in skeletal muscle

• Sarcoplasmic reticulum (SR): Simpler than in skeletal muscle

• Mitochondria: Large and numerous (25–35% of cell volume)

Functional Syncytium

• Intercalated discs: Junctions between cells containing desmosomes and gap junctions

• Gap junctions: Allow ions to pass from cell to cell, electrically couple adjacent cells

• Functional syncytium: Heart behaves as a single coordinated unit

Key Equations and Concepts

• Cardiac Output: The amount of blood pumped by each ventricle in one minute. $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

• Stroke Volume: The amount of blood pumped by one ventricle with each beat. $\text{Stroke Volume} = \text{End Diastolic Volume} - \text{End Systolic Volume}$

Summary Table: Heart Wall Layers

Layer	Location	Function
Epicardium	Outer layer	Protects heart, contains blood vessels
Myocardium	Middle layer	Contracts to pump blood
Endocardium	Inner layer	Lines chambers, covers valves

Additional info:

• Cardiac muscle cells are interconnected by intercalated discs, which allow the heart to contract as a unit (functional syncytium).

• Coronary circulation is essential for providing oxygen and nutrients to the heart muscle itself.

• Valve disorders can lead to inefficient blood flow and increased cardiac workload.