Size Approximation: A closed fist is a good estimate of an individual's heart size.
Location: Mediastinum (the medial cavity of the thorax).
Location within the Thorax
The heart resides between the pleural cavities, each containing a lung.
The mediastinum also contains the esophagus, trachea, and large blood vessels.
The heart rests on the superior surface of the diaphragm.
Position relative to surrounding structures:
Posterior to the sternum.
Anterior to the vertebral column.
Clinical Significance: External chest compression can force blood out of the heart into circulation due to the heart being situated between the sternum and vertebral column; this principle underlies cardiopulmonary resuscitation (CPR).
Orientation
The heart is not centrally located in the thorax; approximately two-thirds of its mass lies to the left of the midline.
Base:
The superior portion of the heart.
In anatomy, the base is the widest part of an organ, not necessarily the bottom.
Attachment point for large blood vessels.
Lies posterior to the costal cartilage of ribs two and three.
Points towards the right shoulder.
Apex:
Formed by the tip of the left ventricle.
Typically located in the fifth intercostal space, just below the fifth rib.
Points inferiorly towards the left hip.
Pericardium
The heart is enclosed in a double-walled sac called the pericardium.
Peri means around, cardi refers to the heart.
Parietal Pericardium:
The outer layer.
Consists of an outer fibrous layer of tough fibrous tissue; this layer protects the heart and anchors it to surrounding tissues, such as the diaphragm.
An inner epithelial layer that produces pericardial fluid.
Epicardium:
The inner epithelial layer of the parietal pericardium turns to cover the outer surface of the heart.
Also known as the visceral pericardium.
Epi means upon (upon the heart).
Forms an integral part of the heart wall.
Analogy: A fist pushed into a partially filled water balloon illustrates the arrangement of the membranes:
The outer layer of the balloon = Parietal pericardium.
The inner layer of the balloon (contacting the heart) = Epicardium.
Pericardial Cavity
The space between the parietal pericardium and the epicardium.
Contains approximately 15 to 50 ml of pericardial fluid.
Function: Reduces friction between the opposing surfaces of the heart as it beats, preventing abrasive tissue damage.
Heart Wall Layers
Epicardium (Visceral Pericardium):
A thin layer that forms the outer layer of the heart wall.
Myocardium:
The majority of the heart wall.
Made up of cardiac muscle cells.
Also contains blood vessels and nerves.
Endocardium:
The innermost layer of the heart.
Made up of epithelial cells (flattened cells that look like pancakes).
When epithelial cells line the cardiovascular system, they are called endothelial cells.
Endothelial cells form a smooth lining to the heart, cover the valves, and line the inner surface of blood vessels.
Function: The smooth surface minimizes friction and allows blood to flow easily.