The Cardiovascular System: Heart Anatomy and Physiology

The Cardiovascular System: The Heart

Overview of Heart Function

The heart is a muscular organ responsible for pumping blood throughout the body via two main circuits: the pulmonary circuit and the systemic circuit. It ensures the delivery of oxygen and nutrients to tissues and the removal of carbon dioxide and waste products.

• Pulmonary circuit: Right side of the heart pumps oxygen-poor blood to the lungs for gas exchange.

• Systemic circuit: Left side of the heart pumps oxygen-rich blood to the rest of the body.

Heart Anatomy

Size, Location, and Orientation

The heart is approximately the size of a fist and weighs less than one pound. It is located in the mediastinum, between the second rib and fifth intercostal space, on the superior surface of the diaphragm, and two-thirds of its mass lies to the left of the midsternal line. The base leans toward the right shoulder, and the apex points toward the left hip.

• Base: Posterior surface, leans toward right shoulder.

• Apex: Points toward left hip; apical impulse can be palpated between the fifth and sixth ribs.

Heart Chambers and Associated Great Vessels

The heart consists of four chambers: two superior atria and two inferior ventricles. The interatrial septum separates the atria, while the interventricular septum separates the ventricles.

• Right atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava and the coronary sinus.

• Left atrium: Receives oxygenated blood from the lungs via four pulmonary veins.

• Right ventricle: Pumps blood into the pulmonary trunk toward the lungs.

• Left ventricle: Pumps blood into the aorta for systemic circulation.

Surface Features

• Coronary sulcus (atrioventricular groove): Encircles the junction of the atria and ventricles.

• Anterior and posterior interventricular sulci: Mark the position of the interventricular septum on the heart's surface.

Heart Wall and Coverings

Pericardium

The heart is enclosed in a double-walled sac called the pericardium, which consists of:

• Fibrous pericardium: Tough, dense connective tissue that protects, anchors, and prevents overfilling.

• Serous pericardium: Thin, two-layered membrane (parietal and visceral layers) with a fluid-filled pericardial cavity to reduce friction.

Layers of the Heart Wall

• Epicardium: Outer layer, also known as the visceral layer of the serous pericardium.

• Myocardium: Middle, muscular layer composed of cardiac muscle cells arranged in circular and spiral bundles. Responsible for contraction.

• Endocardium: Inner layer, continuous with the endothelial lining of blood vessels, lines heart chambers and covers valves.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow through the heart and open/close in response to pressure changes.

• Atrioventricular (AV) valves: Located between atria and ventricles. Prevent backflow into atria when ventricles contract.

• Semilunar (SL) valves: Located between ventricles and major arteries. Prevent backflow into ventricles.

Atrioventricular (AV) Valves

• Tricuspid valve: Right AV valve, three cusps, between right atrium and ventricle.

• Mitral (bicuspid) valve: Left AV valve, two cusps, between left atrium and ventricle.

• Chordae tendineae: Anchor valve cusps to papillary muscles, preventing eversion.

Semilunar (SL) Valves

• Pulmonary semilunar valve: Between right ventricle and pulmonary trunk.

• Aortic semilunar valve: Between left ventricle and aorta.

• Each SL valve has three half-moon-shaped cusps.

Pathway of Blood Through the Heart

Right Side (Pulmonary Circuit)

1. Blood enters right atrium from superior/inferior vena cava and coronary sinus.

2. Passes through tricuspid valve to right ventricle.

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

Left Side (Systemic Circuit)

1. Oxygenated blood returns from lungs via pulmonary veins to left atrium.

2. Passes through mitral valve to left ventricle.

3. Pumped through aortic semilunar valve into aorta and systemic circulation.

Comparative Anatomy of Ventricles

• Left ventricle: Thicker walls, pumps with greater pressure to systemic circuit.

• Right ventricle: Thinner walls, pumps to the shorter, low-pressure pulmonary circuit.

Coronary Circulation

Arterial Supply

• Left coronary artery: Branches into anterior interventricular and circumflex arteries; supplies left heart.

• Right coronary artery: Branches into right marginal and posterior interventricular arteries; supplies right heart.

• Coronary arteries arise from the base of the aorta and encircle the heart in the coronary sulcus.

Venous Drainage

• Cardiac veins collect deoxygenated blood from myocardium and drain into the coronary sinus, which empties into the right atrium.

• Major veins: great cardiac vein, middle cardiac vein, small cardiac vein, and anterior cardiac veins.

Microscopic Anatomy of Cardiac Muscle

Cardiac Muscle Cell Structure

• Striated, short, branched, interconnected cells with one or two central nuclei.

• Abundant mitochondria (25–35% of cell volume) for resistance to fatigue.

• Intercalated discs connect cells via desmosomes (mechanical strength) and gap junctions (electrical coupling).

• Cardiac muscle acts as a functional syncytium (coordinated contraction).

Comparison: Cardiac vs. Skeletal Muscle

Feature	Cardiac Muscle	Skeletal Muscle
Cell shape	Short, branched	Long, cylindrical
Nuclei	1–2, central	Multiple, peripheral
Intercalated discs	Present	Absent
Contraction	Involuntary, rhythmic	Voluntary
Source of Ca2+	SR and extracellular	SR only
Refractory period	Long (prevents tetanus)	Short

Clinical Correlations

• Pericarditis: Inflammation of the pericardium, causing friction rub and possible cardiac tamponade (compression by fluid).

• Valve disorders: Incompetent valves (backflow), valvular stenosis (stiff, narrowed valves), may require replacement.

• Angina pectoris: Chest pain from transient myocardial ischemia.

• Myocardial infarction: Heart attack due to prolonged coronary blockage, resulting in noncontractile scar tissue.