Human Anatomy II: Detailed Study of the Pericardium and the Heart
The Pericardium and Great Vessel Roots
The pericardium is defined as a double-walled, fibrous sac that surrounds the human heart as well as the roots of the great vessels, which include the aorta, the pulmonary arteries, and the pulmonary veins. This structure acts as a protective container for the heart within the thoracic cavity.
Anatomy of the Pericardial Structure
The anatomy of the pericardium is characterized by a double-layered structure rather than a single membrane. It is composed of two main distinct layers with a potential space known as the pericardial cavity located between them. The two primary layers are the outer Fibrous Pericardium and the inner Serous Pericardium.
The Fibrous Pericardium
The fibrous pericardium constitutes the outer sac of the heart. It is a tough, thick, and inelastic layer composed of dense connective tissue. Morphologically, it is shaped like a blunt cone. Its primary physiological roles are to protect the heart from overfilling and to anchor the organ securely within the mediastinum.
This layer is physically attached to the diaphragm below and the sternum in front. At its apex, it is connected to the great vessels, a configuration that prevents excessive movement of the heart within the chest. Furthermore, it is connected specifically to the posterior aspect of the sternum by relatively weak structures known as the sternopericardial ligaments.
The Serous Pericardium
The serous pericardium is the inner lining of the pericardial sac and is notably thinner and more delicate than the fibrous layer. It is itself a double-layered membrane consisting of the parietal layer and the visceral layer. The parietal layer is fused directly to the inner surface of the fibrous pericardium. The visceral layer, also known as the epicardium, is intimately attached to the surface of the heart muscle, or myocardium, and serves as the heart's outermost layer.
The Pericardial Cavity and Fluid
The pericardial cavity is the potential space located between the parietal and visceral layers of the serous pericardium. Under normal physiological conditions, it contains a small volume of pericardial fluid, typically ranging from . This fluid is essential for the mechanical function of the heart.
Functions of the Pericardium
The pericardium serves several critical functions for cardiac health and stability. One primary function is lubrication and friction reduction; the pericardial fluid acts as a lubricant that allows the two layers of the serous pericardium to glide smoothly over one another during each heartbeat, minimizing friction during contraction and relaxation.
The structure also provides mechanical protection. The tough fibrous layer acts as a physical barrier that shields the heart from blunt trauma and prevents the spread of infection from adjacent organs, such as the lungs. Additionally, the pericardium prevents overfilling by ensuring that heart chambers—particularly the thin-walled right atrium and right ventricle—do not overstretch or dilate excessively during sudden increases in blood volume. Finally, it maintains cardiac position by anchoring the heart to the diaphragm, sternum, and great vessels, preventing displacement during bodily movement.
General Anatomy and Position of the Heart
The human heart is a cone-shaped muscular organ roughly the size of a clenched fist. It is situated within the middle mediastinum and serves as the central pump of the circulatory system, responsible for propelling blood to every tissue in the body. The heart possesses an apex directed anteriorly and to the left, a base facing mostly posteriorly, and four distinct sides.
Detailed Cardiac Anatomy: Base and Apex
The base of the heart is its posterior aspect, situated opposite the apex. It is formed primarily by the left atrium, with a lesser contribution from the right atrium. It faces posteriorly toward the bodies of the thoracic vertebrae .
The apex of the heart is formed by the inferolateral part of the left ventricle. In adults, it typically lies posterior to the left intercostal space. Anatomically, it is usually located approximately (roughly a hand's breadth) from the median plane.
Surfaces of the Heart
The heart has several distinct surfaces associated with adjacent anatomical structures. The Anterior (sternocostal) surface is formed mainly by the right ventricle. The Diaphragmatic (inferior) surface is formed mainly by the left ventricle and partly by the right ventricle; this surface is related primarily to the central tendon of the diaphragm.
The Right pulmonary surface is formed mainly by the right atrium. The Left pulmonary surface is formed mainly by the left ventricle and is responsible for creating the cardiac impression in the left lung.
Borders of the Heart
The heart's borders are defined as follows: The Right border is slightly convex and is formed by the right atrium, extending between the superior vena cava (SVC) and the inferior vena cava (IVC). The Inferior border is nearly horizontal and is formed mainly by the right ventricle, with a slight contribution from the left ventricle.
The Left border is oblique and nearly vertical, formed mainly by the left ventricle and slightly by the left auricle. The Superior border is formed by the right and left atria and their respective auricles when viewed from an anterior perspective.
Layers of the Heart Wall
The heart wall is comprised of three distinct layers. The Epicardium is the outermost thin, protective layer, which is identical to the visceral layer of the serous pericardium; it contains fat and the coronary blood vessels. The Myocardium is the middle and thickest layer, consisting of cardiac muscle tissue. Its cells, known as cardiomyocytes, are interconnected in a network that facilitates coordinated contraction. The thickness of this layer varies, being most pronounced in the left ventricle to support high-pressure systemic circulation. The Endocardium is the innermost layer, a smooth, thin, glistening membrane of endothelial cells that lines the chambers and covers the heart valves. Its smooth surface is critical for preventing blood clotting inside the heart.
Chambers and Septa of the Heart
The heart is divided into four chambers: the right and left atria and the right and left ventricles. The two sides of the heart are separated by a muscular wall known as a septum. The Interatrial Septum separates the two upper receiving chambers (atria), while the Interventricular Septum separates the two lower discharging chambers (ventricles).
The Atria and Ventricles
The atria are the upper receiving chambers. They have thinner, less muscular walls compared to the ventricles because they only need to pass blood down to the lower chambers. Each atrium features a small, ear-like pouch called an auricle. The Right Atrium (RA) receives deoxygenated blood from the body via the Superior Vena Cava (from the upper body), the Inferior Vena Cava (from the lower body), and the coronary sinus (from the heart muscle). The Left Atrium (LA) receives oxygenated blood from the lungs via four pulmonary veins.
The ventricles are the lower discharging chambers and possess much thicker, more muscular walls to generate the force required to propel blood out of the heart. The Right Ventricle (RV) has a moderately thick wall and pumps blood through the pulmonary valve into the pulmonary artery toward the lungs. The Left Ventricle (LV) has the thickest wall of all four chambers as it must generate enough force to pump blood through the aortic valve into the aorta for distribution throughout the entire systemic circuit.
Heart Valves and Mechanics
The heart contains four valves that ensure unidirectional blood flow and prevent backflow. These valves open and close passively in response to pressure changes.
The Atrioventricular (AV) Valves are located between the atria and ventricles. They are anchored by collagenous cords known as Chordae Tendineae, which are connected to papillary muscles in the ventricular walls. This apparatus prevents the valve flaps from prolapsing or blowing backward into the atria during ventricular contraction. The Tricuspid Valve has three cusps and is located between the right atrium and right ventricle. The Mitral (Bicuspid) Valve has two cusps and is located between the left atrium and left ventricle.
The Semilunar (SL) Valves are located between the ventricles and the great arteries. Each is composed of three cup-like cusps resembling half-moons. The Pulmonary Valve is located at the exit of the right ventricle leading into the pulmonary artery. The Aortic Valve is located at the exit of the left ventricle leading into the aorta.
Blood Supply to the Heart
Oxygenated blood is supplied to the heart by the coronary arteries, which branch from the base of the aorta. The Left Coronary Artery branches into the Left Anterior Descending Artery (supplying the left ventricle and interventricular septum) and the Circumflex Artery (supplying the left atrium and the side of the left ventricle). The Right Coronary Artery supplies the right atrium, the right ventricle, and often the bottom of the left ventricle and the sinoatrial node.
Deoxygenated blood from the heart muscle is collected by the cardiac veins, specifically the Great cardiac vein, Middle cardiac vein, and Small cardiac vein. These veins empty into the coronary sinus, which subsequently empties into the right atrium.
Innervation of the Heart
The heart is supplied by autonomic nerve fibers from the cardiac plexus, which is divided into superficial and deep portions. This plexus lies on the anterior surface of the bifurcation of the trachea. The sympathetic supply originates from presynaptic fibers. Postsynaptic fibers traverse the cardiopulmonary splanchnic nerves and the cardiac plexus to terminate in the sinoatrial (SA) and atrioventricular (AV) nodes.
The Electrical Conduction System
The rhythmic beating of the heart is controlled by an intrinsic electrical system. The Sinoatrial (SA) Node, located in the right atrium, is the natural pacemaker that initiates electrical impulses, causing the atria to contract. The Atrioventricular (AV) Node is located at the junction of the atria and ventricles; it delays the impulse briefly to allow the atria to finish contracting and filling the ventricles. The Bundle of His carries the impulse from the AV node into the interventricular septum. Finally, the Purkinje Fibers spread the impulse rapidly through the muscular walls of the ventricles, causing them to contract from the apex upward to efficiently eject blood into the arteries.