Heart Anatomy and Circulation: Structure, Function, and Pathways

Heart Anatomy

Size, Location, and Orientation

The heart is a muscular organ located in the mediastinum, the central portion of the thoracic cavity. It is surrounded by the pericardial cavity and lies:

• Resting on the superior surface of the diaphragm

• About 2/3 of its mass is to the left of the midline

• Posterior to the sternum and anterior to the vertebral column

This anatomical placement is important for procedures such as CPR.

• Base: The flat superior surface of the heart.

• Apex: The inferior tip of the heart, located in the fifth intercostal space on the left. The apical pulse is often measured here.

The heart is surrounded by a double-layered membrane called the pericardium.

Layers of the Pericardium

• Fibrous pericardium: The superficial layer, composed of dense irregular connective tissue (DICT). Functions:

  • Allows the heart to move independently within the thoracic cavity

  • Anchors the heart to surrounding structures

  • Prevents the heart from overfilling with blood

• Serous pericardium: A thin, slippery membrane with two layers:

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

  • Visceral layer (epicardium): Attached to the external surface of the heart

  Pericardial cavity: The space between these layers, filled with pericardial fluid to reduce friction.

Layers of the Heart Wall

• Epicardium: Superficial layer, also called the visceral layer of serous pericardium. Produces pericardial fluid.

• Myocardium: Middle layer, composed primarily of cardiac muscle tissue. Responsible for contraction and generation of force/pressure for movement of blood.

• Endocardium: Deepest layer, a thin layer of areolar connective tissue continuous with the endothelium of blood vessels. Decreases resistance to blood flow and inhibits platelet activation.

Heart Chambers and Circulation

Right and Left Sides of the Heart

• Right side: Receives oxygen-poor blood from systemic veins and pumps it to the lungs via the pulmonary circuit.

• Left side: Receives oxygen-rich blood from the lungs and pumps it to the body via the systemic circuit.

The heart also contains the coronary circulation, which supplies blood to the heart muscle itself.

Structural and Functional Features of the Atria

• Thin-walled, dome-shaped chambers

• Smooth posterior section, muscular and ridged anterior section

• Interatrial septum: Separates the atria, prevents mixing of oxygenated and deoxygenated blood

• Fossa ovalis: Shallow depression, remnant of fetal foramen ovale

• Auricles: Small appendages that increase atrial volume

• Function: Receive blood from the body/lungs and pass it to the ventricles

Major veins entering the atria:

• Vena cava (superior and inferior)

• Coronary sinus

• Four pulmonary veins (into left atrium)

Structural and Functional Features of the Ventricles

• Thicker walls, more irregular, ridged by muscles

• Interventricular septum: Separates the ventricles, prevents mixing of blood

• Function: Pump blood out of the heart to the pulmonary and systemic circuits

Circulation Pathways

Pathway	Function	Main Structures
Pulmonary Circuit	Transports oxygen-poor blood to the lungs and returns oxygen-rich blood to the heart	Right ventricle, pulmonary trunk, pulmonary arteries, pulmonary veins, left atrium
Systemic Circuit	Transports oxygen-rich blood to the body and returns oxygen-poor blood to the heart	Left ventricle, aorta, systemic arteries, systemic veins, vena cavae, right atrium
Coronary Circulation	Supplies blood to the heart muscle itself	Coronary arteries and veins

Comparing the Ventricles

• Right ventricle: Pump for the pulmonary circuit; thinner wall, lower pressure

• Left ventricle: Pump for the systemic circuit; thicker wall (2-3x thicker), higher pressure

Key reason for thickness: The left ventricle must generate more force to pump blood throughout the entire body, while the right ventricle only pumps to the lungs.

Summary Table: Heart Wall Layers

Layer	Structure	Function
Epicardium	Visceral layer of serous pericardium	Produces pericardial fluid
Myocardium	Cardiac muscle tissue	Generates force for blood movement
Endocardium	Thin layer of areolar connective tissue	Decreases resistance to blood flow, inhibits platelet activation

Key Terms and Definitions

• Mediastinum: Central compartment of the thoracic cavity

• Pericardium: Double-layered membrane surrounding the heart

• Epicardium: Outer layer of the heart wall

• Myocardium: Muscular middle layer of the heart wall

• Endocardium: Inner layer lining the heart chambers

• Interatrial septum: Wall separating the right and left atria

• Interventricular septum: Wall separating the right and left ventricles

• Pulmonary circuit: Pathway from heart to lungs and back

• Systemic circuit: Pathway from heart to body and back

Relevant Equations

• Cardiac Output: The amount of blood pumped by the heart per minute. $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

Example Application

• During exercise, the heart rate and stroke volume both increase, resulting in a higher cardiac output to meet the body's increased oxygen demand.

Additional info: The notes have been expanded for clarity and completeness, including definitions, tables, and academic context for self-contained study.