The Heart and Cardiovascular System: Structure and Function

Heart Anatomy and Position

Overview and Location

The heart is a muscular organ located in the thoracic cavity, between the lungs, and slightly to the left of the midline. It is protected by the rib cage and rests on the diaphragm. The base of the heart is positioned superiorly, while the apex points inferiorly and to the left.

• Base: Superior, broad portion of the heart.

• Apex: Inferior, pointed end of the heart.

Cardiovascular System Circuits

Pulmonary and Systemic Circuits

The cardiovascular system is divided into two main circuits that transport blood throughout the body:

• Pulmonary Circuit: Carries blood to and from the lungs for gas exchange.

• Systemic Circuit: Delivers blood to and from the rest of the body, supplying tissues with oxygen and nutrients.

Example: Deoxygenated blood travels from the right ventricle to the lungs via the pulmonary arteries, returns oxygenated to the left atrium, and is then pumped to the body by the left ventricle.

Blood Vessels

Types and Functions

• Arteries: Carry blood away from the heart (usually oxygenated, except pulmonary arteries).

• Veins: Carry blood toward the heart (usually deoxygenated, except pulmonary veins).

• Capillaries: Permit exchange of gases, nutrients, and wastes between blood and tissues.

Pericardium

Structure and Function

The pericardium is a double-walled sac that surrounds and protects the heart. It consists of two main layers:

• Visceral Pericardium (Epicardium): Inner layer that touches the heart surface.

• Parietal Pericardium: Outer layer forming the pericardial sac.

• Pericardial Fluid: Lubricates the heart, reducing friction during contractions.

Additional info: The pericardium anchors the heart to surrounding structures and prevents overfilling.

Pericarditis and Cardiac Tamponade

Clinical Correlation

• Pericarditis: Inflammation or infection of the pericardium, often treated with anti-inflammatory drugs or antibiotics.

• May cause increased pericardial fluid, restricting heart movement and leading to cardiac tamponade (compression of the heart).

• Cardiac tamponade is a medical emergency requiring fluid drainage.

Heart Chambers and Contraction Sequence

Chambers

• Right Atrium

• Right Ventricle

• Left Atrium

• Left Ventricle

Contraction Sequence

• First, both atria contract to fill the ventricles.

• Then, both ventricles contract to pump blood into the pulmonary and systemic circuits.

Heart Wall Structure

Layers of the Heart Wall

• Epicardium: Outer layer (visceral pericardium).

• Myocardium: Middle, muscular layer responsible for contraction.

• Endocardium: Inner endothelial lining of the heart chambers.

Cardiac Muscle Cells and Intercalated Discs

Microscopic Structure

• Cardiac Muscle Cells: Striated, branched cells with a single nucleus.

• Intercalated Discs: Specialized junctions that connect cardiac muscle cells, allowing rapid transmission of electrical impulses and synchronized contraction.

Myocardial Infarction (Heart Attack)

Pathology

• Myocardial Infarction: Irreversible damage to heart muscle due to prolonged lack of blood supply.

• White blood cells (WBCs) remove necrotic tissue; dying cells are replaced with non-contractile scar tissue.

Major Arteries and Veins of the Heart

Key Vessels

• Brachiocephalic Trunk

• Left Common Carotid Artery

• Left Subclavian Artery

• Coronary Arteries and Veins: Supply and drain the heart muscle itself.

Coronary Circulation

Anterior and Posterior Views

• Left Coronary Artery (LAD): Supplies anterior interventricular septum and anterior walls of both ventricles.

• Right Coronary Artery: Supplies right atrium and most of right ventricle.

• Great Cardiac Vein, Middle Cardiac Vein, Coronary Sinus: Drain deoxygenated blood from myocardium into right atrium.

Heart Valves

Types and Functions

• Atrioventricular (AV) Valves: Prevent backflow from ventricles to atria (tricuspid on right, bicuspid/mitral on left).

• Semilunar Valves: Prevent backflow into ventricles from pulmonary trunk (pulmonary valve) and aorta (aortic valve).

Vena Cava

Major Veins Returning Blood to the Heart

• Superior Vena Cava (SVC): Receives blood from head, neck, upper limbs, and chest.

• Inferior Vena Cava (IVC): Receives blood from trunk, viscera, and lower limbs.

Pathway of Blood Through the Heart

Stepwise Flow

1. Blood enters right atrium via SVC and IVC.

2. Passes through tricuspid valve into right ventricle.

3. Pumped through pulmonary valve into pulmonary trunk and arteries to lungs.

4. Returns oxygenated via pulmonary veins to left atrium.

5. Passes through bicuspid (mitral) valve into left ventricle.

6. Pumped through aortic valve into aorta and systemic circulation.

Ligamentum Arteriosum and Fetal Circulation Remnants

Special Structures

• Ligamentum Arteriosum: Fibrous band connecting pulmonary trunk to aortic arch; remnant of ductus arteriosus (fetal vessel bypassing lungs).

• Fossa Ovalis: Shallow depression in right atrium; remnant of foramen ovale (fetal opening between atria).

Clinical Note: Patent foramen ovale is a condition where the foramen ovale fails to close after birth, allowing abnormal blood flow between atria.

Opening of Coronary Sinus

Venous Drainage of the Heart

• Coronary Sinus: Large vein draining blood from the heart wall into the right atrium; guarded by a rudimentary, nonfunctioning valve.