Chapter 2: Anatomy and Physiology of the Heart
To fully understand what an echocardiogram reveals, it is essential to first understand the structure and function of the heart. An echocardiogram is much more than a simple picture—it is a moving image that allows healthcare providers to observe how each part of the heart works together to pump blood efficiently throughout the body.
The human heart is a powerful muscular organ located in the center of the chest, slightly to the left of the breastbone. Although it weighs only about 250 to 350 grams (9 to 12 ounces) in adults, it works continuously from before birth until the end of life without resting. During an average lifetime, the heart beats more than 2.5 billion times and pumps millions of liters of blood.
Every heartbeat supplies oxygen and nutrients to every cell while removing carbon dioxide and waste products. Even a brief interruption in this process can threaten life, making the heart one of the body’s most important organs.
The Location of the Heart
The heart is located in the mediastinum, the central compartment of the chest between the lungs. About two-thirds of the heart lies to the left of the body’s midline, while one-third lies to the right.
The heart rests behind the breastbone (sternum) and above the diaphragm. It is protected by:
- The rib cage
- The sternum
- The lungs
- The pericardium (a protective sac)
This protected position helps shield the heart from injury while allowing it enough room to expand and contract during each heartbeat.
Layers of the Heart
The heart consists of three main layers.
Endocardium
The endocardium is the smooth inner lining of the heart chambers and valves.
Its functions include:
- Providing a smooth surface for blood flow
- Reducing friction
- Helping prevent clot formation under normal conditions
Damage to the endocardium can lead to infections such as infective endocarditis.
Myocardium
The myocardium is the thick muscular middle layer responsible for pumping blood.
It is composed of specialized cardiac muscle cells that contract rhythmically throughout life.
The myocardium of the left ventricle is much thicker than that of the right ventricle because it must pump blood throughout the entire body.
Heart attacks primarily damage this layer.
Epicardium
The epicardium forms the thin outer layer of the heart.
It contains:
- Blood vessels
- Fat
- Nerves
- Connective tissue
It also contributes to protecting the heart.
The Pericardium
The heart is enclosed within a double-layered sac called the pericardium.
This sac serves several important functions:
- Protects the heart
- Anchors the heart in position
- Prevents excessive stretching
- Reduces friction during heartbeats
Between the two layers is a small amount of lubricating fluid called pericardial fluid.
An echocardiogram can detect excess fluid around the heart, known as a pericardial effusion, which may interfere with normal heart function.
The Four Chambers of the Heart
The heart has four chambers.
Right Atrium
The right atrium receives oxygen-poor blood returning from the body through:
- Superior vena cava
- Inferior vena cava
- Coronary sinus
After filling, the right atrium contracts and pushes blood into the right ventricle.
Right Ventricle
The right ventricle pumps oxygen-poor blood to the lungs through the pulmonary artery.
Its muscular wall is thinner than that of the left ventricle because pumping blood to the lungs requires much less pressure.
Left Atrium
The left atrium receives oxygen-rich blood returning from the lungs through four pulmonary veins.
It then delivers blood into the left ventricle.
Left Ventricle
The left ventricle is the heart’s strongest chamber.
It pumps oxygen-rich blood through the aorta to every organ and tissue in the body.
Because this chamber generates high pressure, its muscular wall is approximately three times thicker than that of the right ventricle.
An echocardiogram carefully evaluates left ventricular size and function because abnormalities are associated with many forms of heart disease.
The Heart Valves
The heart contains four valves that ensure blood flows in only one direction.
Tricuspid Valve
Located between:
- Right atrium
- Right ventricle
This valve prevents blood from flowing backward during ventricular contraction.
Pulmonary Valve
Located between:
- Right ventricle
- Pulmonary artery
It opens during ventricular contraction and closes afterward to prevent blood from returning to the heart.
Mitral Valve
Also called the bicuspid valve, it separates:
- Left atrium
- Left ventricle
The mitral valve is one of the valves most commonly affected by disease.
Aortic Valve
Located between:
- Left ventricle
- Aorta
It opens during each heartbeat to deliver oxygen-rich blood throughout the body.
Valve diseases such as stenosis and regurgitation are among the most common conditions diagnosed by echocardiography.
Major Blood Vessels Connected to the Heart
Several major blood vessels connect directly to the heart.
These include:
- Aorta
- Superior vena cava
- Inferior vena cava
- Pulmonary artery
- Pulmonary veins
An echocardiogram can evaluate portions of these vessels, particularly the aortic root and ascending aorta.
The Cardiac Cycle
Each heartbeat consists of two major phases.
Diastole
During diastole:
- The heart muscle relaxes.
- The chambers fill with blood.
- The ventricles prepare for contraction.
Adequate filling during diastole is essential for effective pumping.
Systole
During systole:
- The ventricles contract.
- Blood is pumped into the pulmonary artery and aorta.
- Oxygen-rich blood reaches the body.
An echocardiogram measures how effectively the heart performs both systole and diastole.
Blood Flow Through the Heart
The pathway of blood follows a precise sequence.
- Oxygen-poor blood returns from the body to the right atrium.
- Blood flows through the tricuspid valve into the right ventricle.
- The right ventricle pumps blood through the pulmonary valve into the pulmonary artery.
- Blood travels to the lungs, where it receives oxygen.
- Oxygen-rich blood returns through the pulmonary veins to the left atrium.
- Blood passes through the mitral valve into the left ventricle.
- The left ventricle pumps blood through the aortic valve into the aorta.
- Blood is distributed throughout the body.
This cycle repeats continuously throughout life.
Coronary Circulation
Although the heart pumps blood to the body, it also requires its own blood supply.
This is provided by the coronary arteries.
The two main coronary arteries are:
- Right coronary artery (RCA)
- Left main coronary artery
The left main artery divides into:
- Left anterior descending artery (LAD)
- Left circumflex artery (LCX)
Blockage of these arteries can cause myocardial infarction (heart attack).
While coronary arteries themselves are better visualized with coronary angiography or CT angiography, echocardiography can identify damage caused by reduced blood flow.
Electrical Conduction System
The heart has its own electrical system that controls each heartbeat.
Major components include:
Sinoatrial (SA) Node
Known as the heart’s natural pacemaker, the SA node generates electrical impulses that initiate each heartbeat.
Atrioventricular (AV) Node
The AV node briefly delays electrical impulses, allowing the ventricles to fill before contracting.
Bundle of His
This pathway carries electrical signals from the AV node toward the ventricles.
Right and Left Bundle Branches
These branches distribute electrical impulses to both ventricles.
Purkinje Fibers
These fibers rapidly spread electrical signals throughout the ventricular muscle, producing coordinated contraction.
Electrical abnormalities may affect heart function, although they are primarily diagnosed using electrocardiography (ECG).
Cardiac Output
Cardiac output is the amount of blood pumped by the heart each minute.
It depends on:
- Heart rate
- Stroke volume
The relationship is expressed as:
Cardiac Output = Heart Rate × Stroke Volume
A healthy adult typically pumps about 5 liters of blood per minute at rest.
During exercise, cardiac output may increase four to six times.
An echocardiogram helps estimate cardiac output and evaluate whether the heart pumps efficiently.
Ejection Fraction
One of the most important measurements obtained during an echocardiogram is the ejection fraction (EF).
Ejection fraction represents the percentage of blood pumped out of the left ventricle during each heartbeat.
General interpretation includes:
- 55–70%: Normal
- 41–54%: Mildly reduced
- 30–40%: Moderately reduced
- Below 30%: Severely reduced
A reduced ejection fraction often indicates heart failure or damage from a previous heart attack.
Heart Sounds
The familiar “lub-dub” sounds heard with a stethoscope result from valve closure.
- First heart sound (S1): Closure of the mitral and tricuspid valves.
- Second heart sound (S2): Closure of the aortic and pulmonary valves.
Additional heart sounds or murmurs may indicate valve disease or abnormal blood flow.
An echocardiogram helps determine the exact cause of these abnormal sounds.
How Echocardiography Evaluates Heart Anatomy
During an echocardiogram, healthcare providers assess:
- Chamber size
- Wall thickness
- Valve movement
- Pumping strength
- Blood flow direction
- Valve leakage
- Valve narrowing
- Congenital abnormalities
- Pericardial fluid
- Blood clots
- Heart muscle motion
Because the examination is performed in real time, doctors can observe how all parts of the heart function together during every heartbeat.
Why Understanding Heart Anatomy Matters
Learning the normal anatomy of the heart helps patients better understand their echocardiogram results. Terms such as “left ventricular function,” “mitral regurgitation,” “aortic stenosis,” or “ejection fraction” become much easier to understand when the basic structure and function of the heart are familiar.
Knowledge also enables patients to participate more actively in discussions with their healthcare providers and make informed decisions about their care.
Chapter Summary
The heart is a complex but highly organized organ composed of four chambers, four valves, specialized muscle tissue, and an intricate electrical conduction system. Its primary function is to pump oxygen-rich blood throughout the body while returning oxygen-poor blood to the lungs for reoxygenation. Echocardiography provides a detailed, real-time view of this remarkable organ, allowing healthcare providers to evaluate heart structure, valve function, blood flow, pumping efficiency, and overall cardiac performance. A solid understanding of heart anatomy forms the foundation for interpreting echocardiographic images and recognizing the changes associated with heart disease.
In the next chapter, we will explore how echocardiography works, including the science of ultrasound, image formation, Doppler technology, and the advanced techniques that make modern cardiac ultrasound one of the most powerful diagnostic tools in medicine.
