Anatomy & Physiology: Cardiovascular System I – The Heart

The Mediastinum

Definition and Location

The mediastinum is the central compartment of the thoracic cavity, located between the two pleural sacs that contain the lungs. It extends from the sternum in front to the vertebral column behind, and from the thoracic inlet above to the diaphragm below.

• Main Organs: The mediastinum contains the heart, thymus, portions of the esophagus and trachea, and major blood vessels such as the aorta and vena cava.

• Organ Systems: These organs are part of the cardiovascular, respiratory, digestive, and lymphatic systems.

Example: The heart is centrally located within the mediastinum, protected by the rib cage.

Thoracic Anatomy: Location and Orientation

Position of the Heart in the Thorax

The heart lies in the middle mediastinum, slightly to the left of the midline, and is oriented so that its apex points downward, forward, and to the left.

• Base: Directed upward, backward, and to the right.

• Apex: Directed downward, forward, and to the left.

• Surfaces: The heart has anterior (sternocostal), inferior (diaphragmatic), and posterior surfaces.

Example: The apex of the heart is typically found in the fifth intercostal space at the midclavicular line.

Coverings of the Heart

Pericardium: Structure and Function

The heart is enclosed by the pericardium, a double-walled sac that protects and anchors the heart within the mediastinum.

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

• Serous Pericardium: Composed of two layers:

  • Parietal Layer: Lines the inner surface of the fibrous pericardium.

  • Visceral Layer (Epicardium): Covers the heart surface.

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

Example: Pericardial effusion is a condition where excess fluid accumulates in the pericardial cavity, potentially leading to cardiac tamponade.

Layers of the Heart Wall

Structure and Function

The heart wall consists of three layers, each with distinct functions:

• Epicardium: The outermost layer, also known as the visceral pericardium.

• Myocardium: The thick, muscular middle layer responsible for contraction and pumping blood.

• Endocardium: The innermost layer lining the heart chambers and valves.

Example: The myocardium is much thicker in the left ventricle than the right, reflecting its role in pumping blood to the entire body.

Chambers and Valves of the Heart

Four Chambers

The heart is divided into four chambers:

• Right Atrium

• Right Ventricle

• Left Atrium

• Left Ventricle

Heart Valves: Structure and Function

• Atrioventricular (AV) Valves:

  • Tricuspid Valve: Between right atrium and right ventricle.

  • Mitral (Bicuspid) Valve: Between left atrium and left ventricle.

• Semilunar Valves:

  • Pulmonary Valve: Between right ventricle and pulmonary artery.

  • Aortic Valve: Between left ventricle and aorta.

Function: Valves ensure unidirectional blood flow and prevent backflow during the cardiac cycle.

Example: The mitral valve prevents blood from flowing back into the left atrium when the left ventricle contracts.

Path of Blood Flow Through the Heart

Pulmonary and Systemic Circuits

The heart pumps blood through two main circuits:

• Pulmonary Circuit: Carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.

• Systemic Circuit: Delivers oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.

Blood Flow Sequence:

1. Right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs

2. Lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body

3. Body → vena cava → right atrium

Example: Oxygen exchange occurs in the pulmonary capillaries of the lungs.

Coronary Circulation

Arterial Supply and Venous Drainage

The heart muscle (myocardium) receives its blood supply from the coronary arteries and is drained by cardiac veins.

• Coronary Arteries: Arise from the base of the aorta and branch into the right and left coronary arteries.

• Cardiac Veins: Drain deoxygenated blood from the myocardium into the coronary sinus, which empties into the right atrium.

Example: Blockage of a coronary artery can lead to myocardial infarction (heart attack).

Table: Heart Chambers and Valves

Chamber	Associated Valve	Function
Right Atrium	Tricuspid Valve	Receives deoxygenated blood from body; passes to right ventricle
Right Ventricle	Pulmonary Valve	Pumps blood to lungs via pulmonary artery
Left Atrium	Mitral (Bicuspid) Valve	Receives oxygenated blood from lungs; passes to left ventricle
Left Ventricle	Aortic Valve	Pumps blood to body via aorta

Additional info:

• The mediastinum is further subdivided into superior and inferior parts, with the heart located in the middle mediastinum.

• Pericardiocentesis is a clinical procedure to remove excess fluid from the pericardial cavity using a needle.

• Cardiac cycle refers to the sequence of events in one heartbeat, including systole (contraction) and diastole (relaxation).