Heart Anatomy and Physiology

Overview

The human heart is a muscular organ essential for circulating blood throughout the body. Its structure and function are central to the cardiovascular system, ensuring the delivery of oxygen and nutrients while removing waste products.

Pulmonary and Systemic Circuits

Introduction to Circulatory Pathways

The heart acts as a dual pump, maintaining two separate circuits: the pulmonary circuit and the systemic circuit. Each circuit serves a distinct role in oxygenating blood and distributing it to tissues.

• Pulmonary Circuit: Carries deoxygenated blood from the right side of the heart to the lungs for gas exchange, then returns oxygenated blood to the left side of the heart.

• Systemic Circuit: Distributes oxygen-rich blood from the left side of the heart to all body tissues and returns deoxygenated blood to the right side.

• Key Structures: Right atrium, right ventricle, pulmonary arteries and veins, left atrium, left ventricle, aorta, venae cavae.

Example: Blood flows from the right ventricle through the pulmonary arteries to the lungs, returns via pulmonary veins to the left atrium, and is then pumped by the left ventricle through the aorta to the body.

Location, Size, and Orientation of the Heart

Physical Characteristics and Position

The heart is a compact, muscular organ located in the thoracic cavity, specifically within the mediastinum.

• Size: Approximately the size of a human fist.

• Weight: Less than 1 pound (about 250-350 grams).

• Location: Two-thirds of the heart lies to the left of the midsternal line, between the second rib and the fifth intercostal space, resting on the superior surface of the diaphragm.

• Orientation: The base is directed toward the right shoulder, and the apex points toward the left hip.

Example: The apical impulse can be felt at the fifth intercostal space, just below the left nipple.

Heart Coverings: The Pericardium

Structure and Function of the Pericardium

The heart is enclosed in a double-walled sac called the pericardium, which protects and anchors the heart while reducing friction during contractions.

• Fibrous Pericardium: The tough, superficial layer that protects, anchors, and prevents overfilling of the heart.

• Serous Pericardium: A two-layered membrane consisting of:

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

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

• The two serous layers are separated by the pericardial cavity, filled with serous fluid to decrease friction.

Homeostatic Imbalance: Pericarditis is inflammation of the pericardium, causing roughened membranes and a friction rub. Cardiac tamponade is the accumulation of excess fluid in the pericardial cavity, compressing the heart and impairing its function.

Layers of the Heart Wall

Structural Organization

The heart wall is composed of three distinct layers, each with specialized functions:

• Epicardium: The outermost layer, also known as the visceral layer of the serous pericardium.

• Myocardium: The thick, middle layer made of cardiac muscle cells arranged in circular or spiral bundles. Responsible for the contractile force of the heart.

• Cardiac Skeleton: A crisscrossing network of connective tissue within the myocardium that anchors muscle fibers, supports valves and vessels, and limits the spread of electrical impulses.

• Endocardium: The innermost layer, continuous with the endothelial lining of blood vessels. It lines the heart chambers and covers the cardiac skeleton of the valves.

Chambers and Associated Great Vessels

Heart Chambers

The heart contains four chambers: two atria (receiving chambers) and two ventricles (discharging chambers).

• Atria: Small, thin-walled chambers that receive blood returning to the heart.

  • Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava and the coronary sinus. The posterior wall is smooth, while the anterior wall contains pectinate muscles.

  • Left Atrium: Receives oxygenated blood from the lungs via four pulmonary veins. Pectinate muscles are found only in the auricles.

• Ventricles: Thick-walled chambers that pump blood out of the heart.

  • Right Ventricle: Forms most of the anterior surface and pumps blood into the pulmonary trunk.

  • Left Ventricle: Forms the apex and posteroinferior surface, pumping blood into the aorta for systemic circulation.

Example: The left ventricle has a thicker wall than the right ventricle due to the higher pressure required to pump blood throughout the systemic circuit.

Heart Valves

Types and Functions of Valves

Heart valves ensure unidirectional blood flow through the heart and open or close in response to pressure changes.

• Atrioventricular (AV) Valves: Located between the atria and ventricles.

  • Tricuspid Valve: Right AV valve with three cusps, between right atrium and right ventricle.

  • Mitral (Bicuspid) Valve: Left AV valve with two cusps, between left atrium and left ventricle.

  • Chordae Tendineae: Tendinous cords that anchor valve cusps to papillary muscles, preventing valve prolapse during ventricular contraction.

• Semilunar (SL) Valves: Located between the ventricles and major arteries.

  • Pulmonary Valve: Between right ventricle and pulmonary trunk.

  • Aortic Valve: Between left ventricle and aorta.

  • Open when ventricular pressure exceeds arterial pressure; close to prevent backflow when ventricles relax.

Valves between major veins and atria are not needed because the inertia of incoming blood and compression of venous openings during contraction prevent backflow.

Valve Disorders (Homeostatic Imbalances)

• Incompetent Valve: Fails to close properly, causing blood to backflow and requiring the heart to repump the same blood.

• Valvular Stenosis: Stiff valve flaps constrict the opening, forcing the heart to exert more force to pump blood. Defective valves may be replaced with mechanical, animal, or cadaver valves.

Summary Table: Heart Valves

Key Terms and Definitions

• Pericardium: Double-walled sac enclosing the heart.

• Myocardium: Muscular, contractile middle layer of the heart wall.

• Endocardium: Inner lining of the heart chambers and valves.

• Pectinate Muscles: Ridges of muscle in the atrial walls.

• Auricle: Ear-like appendage increasing atrial volume.

• Chordae Tendineae: Tendinous cords anchoring AV valve cusps to papillary muscles.

• Cardiac Skeleton: Connective tissue framework supporting the heart.

Additional info:

• The foramen ovale is a fetal shunt between the right and left atria, which closes after birth to become the fossa ovalis.

• Heart sounds ("lub-dub") are produced by the closing of the AV and SL valves, respectively.