The Cardiovascular System: Heart Anatomy and Physiology

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Cardiovascular System Overview

Introduction to the Heart

The heart is a muscular organ that functions as a double pump, circulating blood through two main circuits: the pulmonary circuit and the systemic circuit. It is essential for delivering oxygen and nutrients to tissues and removing 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.

Diagram of pulmonary and systemic circuits

Heart Anatomy

External Structure and Orientation

The heart is located in the mediastinum between the lungs, with its apex pointing to the left. It typically weighs 250–350 grams and is the largest organ of the mediastinum.

Apex: Inferior, pointed end directed leftward.
Base: Broad, superior surface facing posteriorly.

Heart location and orientation in the thorax

Gross Anatomy: External Features

The heart has several external grooves (sulci) that mark the boundaries between its chambers:

Coronary sulcus: Encircles the heart, separating atria from ventricles.
Anterior and posterior interventricular sulci: Separate the right and left ventricles on the anterior and posterior surfaces, respectively.

Anterior view of the heart with labeled vessels and sulci Posterior view of the heart with labeled vessels and sulci

Heart Chambers

The heart consists of four chambers:

Right atrium: Receives oxygen-poor blood from the body via the superior and inferior venae cavae and the coronary sinus.
Right ventricle: Pumps blood into the pulmonary trunk toward the lungs.
Left atrium: Receives oxygen-rich blood from the lungs via the pulmonary veins.
Left ventricle: Pumps blood into the aorta for systemic circulation.

Frontal section of the heart showing chambers and valves

Internal Structures and Valves

Valves ensure unidirectional blood flow through the heart:

Atrioventricular (AV) valves: Tricuspid (right) and bicuspid/mitral (left) valves between atria and ventricles.
Semilunar valves: Pulmonary valve (right ventricle to pulmonary trunk) and aortic valve (left ventricle to aorta).

Heart valves and internal anatomy

Heart Wall and Coverings

Pericardium

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

Fibrous pericardium: Tough, outer layer of dense connective tissue.
Serous pericardium: Thin, double-layered membrane (parietal and visceral layers) with a pericardial cavity filled with fluid to reduce friction.

Layers of the pericardium Close-up of pericardial layers

Layers of the Heart Wall

Epicardium: Visceral layer of the serous pericardium (outermost layer).
Myocardium: Thick, middle layer composed of cardiac muscle arranged in circular and spiral bundles.
Endocardium: Thin, inner lining of endothelium and connective tissue.

Layers of the heart wall Arrangement of cardiac muscle bundles

Microscopic Structure of Cardiac Muscle

Cardiac Muscle Tissue

Cardiac muscle tissue forms the myocardium and is responsible for the heart's contractile function.

Striated and contracts by the sliding filament mechanism.
Cells are short, branching, and typically have one or two nuclei.
Cells are joined by intercalated discs containing gap junctions and fasciae adherens for electrical and mechanical coupling.

Cardiac muscle cell with intercalated discs

Pathway of Blood Through the Heart

Sequential Flow

Blood flows through the heart in a specific sequence, ensuring separation of oxygen-poor and oxygen-rich blood:

Oxygen-poor blood enters the right atrium from the superior/inferior vena cava and coronary sinus.
Passes through the tricuspid valve into the right ventricle.
Pumped through the pulmonary semilunar valve into the pulmonary trunk and arteries to the lungs.
Oxygen-rich blood returns via pulmonary veins to the left atrium.
Passes through the mitral valve into the left ventricle.
Pumped through the aortic semilunar valve into the aorta and systemic circulation.

Pathway of blood through the heart

Heart Valves: Structure and Function

Valve Structure

Each valve consists of endocardium with a connective tissue core. The AV valves are supported by chordae tendineae and papillary muscles to prevent backflow during ventricular contraction.

Valve Function

AV valves open when atrial pressure exceeds ventricular pressure, allowing blood to flow into the ventricles.
During ventricular contraction, AV valves close to prevent regurgitation, and semilunar valves open to allow ejection of blood into arteries.

Heart valves and cardiac skeleton

Conducting System of the Heart

Intrinsic Conduction System

The heart has an intrinsic ability to generate and conduct electrical impulses, coordinating rhythmic contractions:

Sinoatrial (SA) node: Pacemaker of the heart, initiates impulse.
Atrioventricular (AV) node: Delays impulse before passing to ventricles.
AV bundle (Bundle of His): Conducts impulse to bundle branches.
Bundle branches: Carry impulse through interventricular septum.
Purkinje fibers: Distribute impulse to ventricular myocardium.

Pathway and components of the electrical conducting system

Blood Supply to the Heart

Coronary Circulation

The heart muscle receives its blood supply from the right and left coronary arteries, which branch from the base of the aorta and run in the coronary sulcus. Major branches include:

Left coronary artery (LCA): Branches into the anterior interventricular artery (LAD) and circumflex artery.
Right coronary artery (RCA): Branches into the marginal artery and posterior interventricular artery (PDA).

Anterior view of the heart with labeled coronary arteries

Cardiac Veins

Deoxygenated blood from the heart wall is collected by cardiac veins and returned to the right atrium via the coronary sinus. Major veins include the great cardiac vein, middle cardiac vein, and small cardiac vein.

Heart Sounds

Valvular Closure

The "lub-dup" sounds of the heartbeat are produced by the closing of heart valves:

First sound (lub): Closure of AV valves.
Second sound (dup): Closure of semilunar valves.

Disorders of the Heart

Coronary Artery Disease

Atherosclerosis: Fatty deposits in coronary arteries.
Angina pectoris: Chest pain due to reduced blood flow.
Myocardial infarction: Heart attack caused by blocked coronary artery.

Heart Failure

Progressive weakening of the heart, leading to inadequate blood supply to tissues.
Congestive heart failure (CHF): Heart enlarges, pumping efficiency declines.

Disorders of the Conduction System

Arrhythmias: Abnormal heart rhythms.
Ventricular fibrillation: Rapid, uncoordinated contractions; can cause cardiac arrest.
Atrial fibrillation: Disorganized atrial impulses; increases risk of stroke.

Congenital Heart Defects

Most common defect: Ventricular septal defect (hole in the interventricular septum).
Other defects: Transposition of the great vessels, coarctation of the aorta, Tetralogy of Fallot.

Summary Table: Heart Chambers and Valves

Chamber	Receives Blood From	Pumps Blood To	Valve
Right Atrium	Systemic veins (SVC, IVC, coronary sinus)	Right ventricle	Tricuspid (right AV) valve
Right Ventricle	Right atrium	Pulmonary trunk	Pulmonary semilunar valve
Left Atrium	Pulmonary veins	Left ventricle	Bicuspid (mitral, left AV) valve
Left Ventricle	Left atrium	Aorta	Aortic semilunar valve