Anatomy and Physiology of the Heart: Structure, Pressure, and Cardiac Function

Chapter 17: The Heart

Overview of Heart Location and Structure

The heart is a muscular organ located in the mediastinum of the thoracic cavity, flanked by the lungs and protected by the rib cage. It is responsible for pumping blood throughout the body via the circulatory system.

• Mediastinum: Central compartment of the thoracic cavity where the heart resides.

• Major anatomical landmarks: Superior vena cava, aorta, pulmonary trunk, ribs, diaphragm, right and left atrioventricular sulci, anterior interventricular sulcus.

• Orientation: The heart sits obliquely, with its base directed toward the right shoulder and apex toward the left hip.

• Protection: The heart is protected anteriorly by the sternum and laterally by the lungs.

Pressure and Flow in the Cardiovascular System

Definition and Principles of Pressure

Pressure in the context of the cardiovascular system refers to the force exerted by blood (a fluid) against the walls of blood vessels or heart chambers. This pressure is essential for driving blood flow throughout the body.

• Fluid Pressure: The cumulative force of all impacts of fluid molecules (such as blood or air) on the container wall (e.g., blood vessel or heart chamber).

• Relationship to Container Size:

  • Large container or low fluid volume: Fewer particles hit the wall, resulting in low pressure.

  • Small container or high fluid volume: More particles hit the wall, resulting in high pressure.

• Flow Direction: Fluids always flow from areas of high pressure to areas of low pressure.

• Effect of Container Changes: If the container (e.g., heart chamber) shrinks, pressure rises and fluid is pushed away from the area of higher pressure.

Resistance to Flow

Resistance is the opposition to fluid flow within a vessel, primarily due to friction between the fluid and the vessel wall.

• Factors Affecting Resistance:

  • Tube diameter: Smaller tubes have more wall area relative to volume, increasing friction and resistance.

  • Viscosity: Thicker fluids (higher viscosity) increase resistance.

  • Length of vessel: Longer vessels increase resistance.

• Blood as a Fluid: Blood is a liquid that exhibits these properties, and its flow is subject to resistance within the cardiovascular system.

Layers of the Heart Wall

Pericardium

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

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

• Serous Pericardium: A thin, two-layered membrane:

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

  • Visceral layer (Epicardium): Covers the external surface of the heart and is also considered the outermost layer of the heart wall proper.

• Pericardial Cavity: The space between the parietal and visceral layers, filled with serous fluid to reduce friction during heartbeats.

Heart Wall Proper

• Epicardium: The visceral layer of the serous pericardium; outermost layer of the heart wall.

• Myocardium: The thick, middle layer composed of cardiac muscle cells (cardiomyocytes) responsible for the heart's contractile function.

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

Summary Table: Layers of the Heart Wall

Layer	Location	Function
Fibrous Pericardium	Outermost sac	Protection, anchoring, prevents overfilling
Parietal Serous Pericardium	Lines fibrous pericardium	Secretes serous fluid
Visceral Serous Pericardium (Epicardium)	Outer heart wall	Protection, reduces friction
Myocardium	Middle layer	Cardiac muscle contraction
Endocardium	Inner lining	Lines chambers, continuous with vessels

Example:

During a heart attack (myocardial infarction), the myocardium is damaged due to lack of oxygen, impairing the heart's ability to contract and pump blood effectively.

Additional info: The pericardial cavity's serous fluid is essential for minimizing friction as the heart beats, and inflammation of this area (pericarditis) can cause chest pain and impaired heart function.