Introduction to Cardiac Anatomy

Why Study the Heart

The human heart is a small but incredibly powerful organ that is absolutely essential for life. It is a muscular pump that works tirelessly 24 hours a day, 365 days a year, without taking a break. Every minute it circulates oxygenated blood throughout the entire body and returns deoxygenated blood back to the lungs. If the heart stops working even for a few seconds, oxygen delivery to organs is interrupted, and within minutes, this can cause brain damage or even death.

Cardiac anatomy means studying the structure of the heart, while cardiac physiology means understanding how the heart functions. Together, these form one of the most important foundation topics for every medical student, nursing student, physiotherapist, or healthcare professional. Without a clear understanding of normal cardiac anatomy and physiology, it becomes very difficult to understand diseases of the heart, their symptoms, and their treatment.

Location and Size of the Heart

The heart is roughly the size of a closed fist, cone-shaped, and made up of specialized cardiac muscle. It is located in the thoracic cavity, specifically in the mediastinum — the space between the two lungs. The average adult male heart weighs about 280–340 grams, while the average female heart weighs about 230–280 grams. The apex points downwards and slightly to the left, while the base is directed upwards. The primary job of the heart is to pump blood so that every cell in the body receives oxygen and nutrients while carbon dioxide and waste products are carried back for removal.

The apex is located at the fifth intercostal space, along the midclavicular line, where the apical impulse can be felt. The right border is formed by the right atrium, the left border by the left ventricle, the inferior border by the right ventricle, and the superior border by the atria. This anatomical knowledge is essential for interpreting chest X-rays, placing ECG leads, and performing clinical examination.

Layers of the Heart Wall

The wall of the heart is made up of three main layers:

Endocardium: The innermost layer that provides a smooth lining for blood to flow without friction. The heart valves are lined by this layer.
Myocardium: The thick, muscular middle layer that performs the actual pumping work. The left ventricle has the thickest myocardium because it must generate the highest pressure.
Epicardium: The outermost layer, also called the visceral pericardium.

Surrounding the heart is the pericardium, a double-walled sac that contains a small amount of pericardial fluid. This fluid lubricates the movements of the heart and reduces friction during contraction and relaxation.

Chambers of the Heart

The heart has four chambers:

Right Atrium: Receives deoxygenated blood from the body through the superior and inferior vena cava. Contains the SA node, the natural pacemaker.
Right Ventricle: Pumps deoxygenated blood to the lungs through the pulmonary artery.
Left Atrium: Receives oxygenated blood from the lungs through the pulmonary veins.
Left Ventricle: Pumps oxygenated blood to the entire body through the aorta.

The chambers work in perfect coordination — first the atria contract to fill the ventricles, then the ventricles contract to pump blood forward.

Heart Valves

There are four main valves in the heart:

Tricuspid Valve: Between the right atrium and right ventricle.
Mitral Valve: Between the left atrium and left ventricle.
Pulmonary Valve: Between the right ventricle and pulmonary artery.
Aortic Valve: Between the left ventricle and aorta.

These valves ensure unidirectional flow of blood and prevent backflow. Dysfunction of valves leads to murmurs which can be detected by auscultation.

Blood Circulation – Double Pump System

The heart works as a double pump:

Pulmonary Circulation: Right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium.
Systemic Circulation: Left atrium → left ventricle → aorta → body tissues → systemic veins → right atrium.

This continuous circulation ensures proper oxygenation and nutrient supply to all tissues.

Coronary Circulation – The Heart’s Own Blood Supply

The heart requires its own blood supply to keep functioning. This is provided by the coronary arteries:

Right Coronary Artery (RCA): Supplies the right atrium, right ventricle, part of the conduction system.
Left Coronary Artery (LCA): Divides into LAD and LCX branches which mainly supply the left ventricle.

Blockage of coronary arteries results in myocardial infarction (heart attack), one of the leading causes of death worldwide.

Electrical Conduction System

The heart has an intrinsic conduction system that allows it to beat on its own:

SA Node: Natural pacemaker, generates 60–100 impulses per minute.
AV Node: Delays impulse slightly, allowing atrial emptying before ventricles contract.
Bundle of His → Right and Left Bundle Branches → Purkinje Fibers: Spread impulse through ventricles for synchronized contraction.

This property is called automaticity.

The Cardiac Cycle

A cardiac cycle is the complete sequence of events from one heartbeat to the next.

Systole: Ventricular contraction and blood ejection.
Diastole: Ventricular relaxation and filling with blood.

Heart sounds correspond to this cycle:

S1: Closure of AV valves (start of systole).
S2: Closure of semilunar valves (start of diastole).

Sometimes S3 and S4 may be heard in pathological states like heart failure or stiff ventricles.

Cardiac Output and Regulation

Cardiac output is the amount of blood pumped per minute:

Cardiac Output = Stroke Volume × Heart Rate

Stroke Volume: Amount of blood pumped per beat (about 70 ml).
Preload: Degree of ventricular filling.
Afterload: Resistance the ventricle must pump against.
Contractility: Strength of ventricular contraction.

According to the Frank–Starling law, the greater the filling (preload), the stronger the contraction (within limits). Regulation also occurs via the autonomic nervous system — sympathetic stimulation increases heart rate and contractility, parasympathetic stimulation decreases heart rate.

Clinical Importance

Understanding cardiac anatomy and physiology helps with:

Interpreting ECGs.
Identifying normal and abnormal heart sounds.
Recognizing pathologies like myocardial infarction, heart failure, and arrhythmias.
Understanding the action of drugs and surgical treatments.