Anatomy and Physiology of the Heart: Structure, Location, and Layers

Section 4.2 The Heart

Introduction

This section provides an overview of the heart's anatomy, including its size, location, orientation, and the structural layers that compose its wall. Understanding these foundational aspects is essential for further study of cardiovascular physiology and pathology.

Heart Anatomy

Size and General Structure

• Size: The heart is approximately the size of a human fist.

• Structure: It functions as two side-by-side pumps, each serving a different circulatory pathway (pulmonary and systemic).

Location and Orientation

• Location: The heart is located on the superior surface of the diaphragm, left of the midline, extending from the 2nd rib to the 5th intercostal space.

• Position: It lies anterior to the vertebral column and posterior to the sternum, within the mediastinum (the central compartment of the thoracic cavity).

• Orientation: The broad, flat base of the heart points toward the right shoulder, while the apex points toward the left hip.

• Example: In anatomical diagrams, the heart is often shown with its apex directed inferiorly and to the left, emphasizing its oblique position in the thorax.

Layers of the Heart

Overview of Heart Wall Layers

• Pericardium: The outermost layer, forming a protective sac around the heart.

• Myocardium: The middle, muscular layer, responsible for the contractile force of the heart.

• Endocardium: The innermost layer, lining the heart chambers and continuous with the endothelium of blood vessels.

Pericardium: Structure and Function

• Definition: The pericardium is a double-walled sac that surrounds and protects the heart.

• Components:

  • Superficial fibrous pericardium: Tough, dense connective tissue that protects, anchors, and prevents overfilling of the heart.

  • Deep serous pericardium: Composed of two layers:

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

    • Visceral layer (epicardium): Covers the external surface of the heart.

  • The parietal and visceral layers are separated by the pericardial cavity, which contains a lubricating fluid to reduce friction during heart movements.

Myocardium

• Definition: The myocardium is the thick, contractile middle layer of the heart wall, composed primarily of cardiac muscle cells.

• Function: Responsible for the pumping action of the heart.

• Additional info: The myocardium contains a network of connective tissue fibers (the cardiac skeleton) that supports the heart's structure and helps coordinate contraction.

Endocardium

• Definition: The endocardium is a thin, smooth layer of endothelial cells lining the inner surfaces of the heart chambers and valves.

• Function: Provides a smooth surface for blood flow and is continuous with the endothelium of the great vessels.