Anatomy & Physiology of the Heart: Structure, Coverings, Vessels, and Function

Gross Anatomy of the Heart

Size, Shape, and Location

The heart is a muscular organ responsible for pumping blood throughout the body. It is approximately the size of a closed fist, hollow, and cone-shaped, weighing just less than one pound.

• Location: The heart extends obliquely from the 2nd to the 5th intercostal space, flanked laterally by the lungs and superiorly by the diaphragm.

• Orientation: The apex points to the left of the midclavicular line, anterior to the vertebral column, posterior to the sternum.

Heart Coverings

Pericardial Physiology

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

• Fibrous Pericardium: The loose-fitting superficial portion that protects and anchors the heart.

• Serous Pericardium: Composed of parietal and visceral layers separated by the pericardial cavity.

• Epicardium: The visceral layer of the serous pericardium, in intimate contact with the heart.

• Histology: The epicardium is made of simple squamous epithelium.

• Pericardial Effusion: In severe conditions, fluid may seep into the pericardial cavity, compressing the heart and causing cardiac tamponade. Treatment involves draining the fluid.

Heart Wall Structure

Layers of the Heart Wall

• Epicardium: Visceral layer of the serous pericardium.

• Myocardium: Cardiac muscle layer forming the bulk of the heart, arranged in a spiraling pattern for efficient contraction.

• Endocardium: Thin endothelial layer lining the inner myocardial surface.

• Fibrous Skeleton: Crisscrossing, interlacing layer of connective tissue that anchors cardiac muscle fibers and supports the heart valves.

External Heart: Major Vessels

Anterior View

The heart is connected to major vessels that transport blood to and from the body and lungs.

• Veins: Right and left pulmonary veins (deliver oxygen-rich blood to the left atrium), superior and inferior vena cava (return blood to the heart).

• Arteries: Pulmonary trunk (splits into right and left pulmonary arteries, delivering oxygen-poor blood to the lungs), ascending aorta (branches: brachiocephalic, left common carotid, left subclavian).

Vessels that Supply/Drain the Heart (Anterior View)

• Arteries: Right and left coronary arteries (in atrioventricular groove or coronary sulcus), right marginal and posterior interventricular arteries.

• Veins: Small cardiac vein, anterior cardiac vein, great cardiac vein.

• Anastomoses: Anatomically, vessels may merge or form junctions to ensure blood supply even if one route is blocked.

Posterior View

• Veins: Great cardiac vein, posterior vein to left ventricle, coronary sinus, middle cardiac vein.

• Arteries: Right coronary artery, posterior interventricular artery.

• Coronary Sinus: Has three tributaries: Great, Middle, and Small cardiac veins.

Gross Anatomy of Heart: Frontal Section

Chambers and Valves

The heart contains four chambers: two atria and two ventricles, separated by valves that regulate blood flow.

• Atria: Receiving chambers; right and left atria. The right atrium has pectinate muscles (comb-like ridges), and a smooth posterior wall. The left atrium is more prominent.

• Crista Terminalis: C-shaped ridge separating anterior and posterior portions of the right atrium.

• Fossa Ovalis: Located in the interatrial septum, a remnant of the fetal foramen ovale.

Ventricles of the Heart

• Ventricles: Discharging chambers; right and left ventricles.

• Papillary Muscles: Play a role in valve function by anchoring chordae tendineae.

• Trabeculae Carneae: Crossbars of flesh in the ventricular walls.

• Right Ventricle: Pumps blood into the pulmonary trunk (short, low-pressure circuit).

• Left Ventricle: Pumps blood into the aorta (long, high-pressure circuit).

Pathway of Blood Through the Heart and Lungs

Blood flows through the heart in a specific sequence, ensuring oxygenation and systemic circulation.

• Right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary arteries → lungs

• Lungs → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aortic semilunar valve → aorta

Heart Valves

Types and Functions

• Atrioventricular (AV) Valves: Lie between the atria and ventricles; prevent backflow into the atria when ventricles contract.

• Chordae Tendineae: Anchor AV valve flaps to papillary muscles, securing them in closed position.

• Semilunar Valves: Aortic and pulmonary valves prevent backflow into the ventricles.

• Valve Stenosis: Narrowing of the valve due to stiffening from calcium salt deposits or scar tissue, causing the heart to work harder.

• Mitral Valve: Most often replaced due to its high pressure exposure.

Microscopic Heart Muscle Anatomy

Cardiac Muscle Structure

• Striated, short, branched, and interconnected cells.

• Intercalated discs: Specialized connections between cells for synchronized contraction.

• Mitochondria: Account for 25-35% of cell volume, allowing for high fatigue resistance.

• T-tubules: Wider and fewer than in skeletal muscle, facilitating rapid transmission of action potentials.

Coronary Circulation

Blood Supply to the Heart

• Coronary arteries: Provide intermittent, pulsating blood flow to the myocardium.

• Collateral routes: Ensure blood delivery even if major vessels are blocked.

• Angina Pectoris: Chest pain caused by transient deficiency of blood delivery to the myocardium.

• Myocardial Infarction: Prolonged coronary blockage leading to heart muscle death.

Summary Table: Heart Chambers and Major Vessels

Chamber	Main Vessel(s) In	Main Vessel(s) Out	Oxygenation
Right Atrium	Superior/Inferior Vena Cava	Right Ventricle (via tricuspid valve)	Oxygen-poor
Right Ventricle	Right Atrium	Pulmonary Arteries	Oxygen-poor
Left Atrium	Pulmonary Veins	Left Ventricle (via mitral valve)	Oxygen-rich
Left Ventricle	Left Atrium	Aorta	Oxygen-rich

Key Terms and Definitions

• Pericardium: Double-walled sac enclosing the heart.

• Epicardium: Visceral layer of the serous pericardium.

• Myocardium: Muscular middle layer of the heart wall.

• Endocardium: Inner endothelial lining of the heart chambers.

• Chordae Tendineae: Tendinous cords anchoring AV valve flaps.

• Trabeculae Carneae: Irregular muscular columns in the ventricles.

• Angina Pectoris: Chest pain due to transient myocardial ischemia.

• Myocardial Infarction: Heart attack; death of heart muscle due to prolonged ischemia.

Formulas and Equations

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

$CO = HR \times SV$

• HR: Heart Rate (beats per minute)

• SV: Stroke Volume (amount of blood pumped per beat)

Additional info: Some context and terminology have been expanded for clarity and completeness, including definitions and the summary table.