Chapter 1: Understanding the Human Heart and Blood Circulation
The human heart is one of the most extraordinary organs in the body. Roughly the size of a clenched fist, it works tirelessly from before birth until the end of life without taking a single break. Every day, it beats approximately 100,000 times and pumps between 7,000 and 8,000 liters of blood through an extensive network of blood vessels. This continuous circulation delivers oxygen and nutrients to every cell while removing carbon dioxide and other waste products. Every organ in the body—including the brain, kidneys, liver, lungs, and muscles—depends on this uninterrupted blood supply to function properly. When the heart cannot pump efficiently or when its own blood supply is interrupted, serious conditions such as a heart attack can occur.
The heart is located in the chest cavity between the lungs, slightly to the left of the body’s midline. It sits behind the breastbone, known as the sternum, and rests on the diaphragm. Contrary to popular belief, the heart is not positioned entirely on the left side of the chest. About two-thirds of the heart lies to the left of the center, while the remaining one-third extends to the right. It is enclosed within a double-layered protective sac called the pericardium. Between these two layers is a thin film of lubricating fluid that minimizes friction as the heart beats thousands of times every day.
An adult heart measures approximately 12 centimeters in length, 8 to 9 centimeters in width, and about 6 centimeters in thickness. It weighs between 250 and 350 grams, depending on a person’s sex and body size. Although relatively small, the heart performs an enormous amount of work throughout a lifetime. A person who lives to the age of 80 will have a heart that has beaten more than three billion times, pumping hundreds of millions of liters of blood throughout the body.
The heart is composed of three distinct layers. The outermost layer, known as the epicardium, protects the heart and forms the inner layer of the pericardial sac. Beneath it lies the myocardium, the thick muscular layer responsible for pumping blood. This is the most important layer because every heartbeat results from the contraction of these muscle fibers. The left ventricle has the thickest myocardium because it must generate enough force to pump blood throughout the entire body. The innermost layer is the endocardium, a smooth lining that covers the heart chambers and valves, allowing blood to flow without unnecessary friction and reducing the risk of clot formation.
The heart contains four chambers that work together in a coordinated manner. The upper chambers are called atria, while the lower chambers are known as ventricles. The right atrium receives oxygen-poor blood returning from the body through two large veins called the superior vena cava and the inferior vena cava. This blood then flows through the tricuspid valve into the right ventricle. The right ventricle pumps the oxygen-poor blood to the lungs through the pulmonary artery. Inside the lungs, carbon dioxide is released, and fresh oxygen is absorbed into the blood.
The oxygen-rich blood returns from the lungs through four pulmonary veins and enters the left atrium. It then passes through the mitral valve into the left ventricle, the strongest and most muscular chamber of the heart. Finally, the left ventricle pumps the oxygen-rich blood into the aorta, the largest artery in the body. From there, blood is distributed through countless arteries, arterioles, and capillaries, reaching every tissue and organ. This remarkable process repeats continuously, ensuring that every cell receives the oxygen and nutrients necessary for survival.
To ensure blood flows in only one direction, the heart contains four valves. The tricuspid valve lies between the right atrium and the right ventricle. The pulmonary valve separates the right ventricle from the pulmonary artery. On the left side of the heart, the mitral valve lies between the left atrium and left ventricle, while the aortic valve separates the left ventricle from the aorta. These valves open and close with every heartbeat, preventing blood from flowing backward. Healthy valves are essential for efficient circulation, and diseases affecting these valves can significantly impair heart function.
Unlike most muscles in the body, the heart possesses its own electrical conduction system that enables it to beat automatically. The heartbeat begins in a small group of specialized cells called the sinoatrial (SA) node, located in the upper portion of the right atrium. The SA node is often referred to as the heart’s natural pacemaker because it generates electrical impulses at a regular rate of approximately 60 to 100 beats per minute in healthy adults. These electrical signals spread through both atria, causing them to contract and push blood into the ventricles. The impulse then reaches the atrioventricular (AV) node, where it is briefly delayed to allow the ventricles to fill completely. From there, the electrical signal travels through the Bundle of His, the right and left bundle branches, and finally the Purkinje fibers, triggering a powerful contraction of the ventricles. This precisely coordinated sequence ensures efficient pumping of blood with every heartbeat.
Blood circulation occurs through two interconnected systems: pulmonary circulation and systemic circulation. In pulmonary circulation, oxygen-poor blood travels from the right side of the heart to the lungs, where it exchanges carbon dioxide for oxygen. The oxygen-rich blood then returns to the left side of the heart. Systemic circulation begins when the left ventricle pumps oxygenated blood through the aorta into arteries that branch throughout the body. Oxygen and nutrients are delivered to tissues, while carbon dioxide and waste products are collected. The oxygen-poor blood eventually returns to the right atrium, completing the cycle. This entire process takes less than one minute at rest and becomes even faster during physical activity.
Although the heart pumps blood to every part of the body, it cannot absorb oxygen directly from the blood flowing through its chambers. Instead, it depends on its own dedicated network of blood vessels known as the coronary arteries. These arteries originate from the base of the aorta immediately after blood leaves the left ventricle. The coronary arteries wrap around the surface of the heart like a crown, supplying oxygen and nutrients to the heart muscle itself.
There are two main coronary arteries: the left coronary artery and the right coronary artery. The left coronary artery quickly divides into two major branches. The first is the left anterior descending (LAD) artery, which supplies blood to the front wall of the heart, the interventricular septum, and a large portion of the left ventricle. Because this artery supplies such a significant area of heart muscle, blockage of the LAD can cause a particularly severe heart attack and is often referred to as the “widow-maker.” The second major branch is the left circumflex (LCX) artery, which supplies the side and back portions of the left ventricle as well as the left atrium. The right coronary artery supplies blood to the right atrium, right ventricle, the inferior portion of the left ventricle, and in many individuals, the sinoatrial and atrioventricular nodes. Blockage of the right coronary artery can therefore lead not only to damage of the heart muscle but also to disturbances in heart rhythm.
The coronary arteries are vulnerable to a condition called atherosclerosis, which is the underlying cause of most heart attacks. Atherosclerosis begins with injury to the inner lining of an artery. High levels of low-density lipoprotein (LDL) cholesterol, smoking, high blood pressure, diabetes, chronic inflammation, and other risk factors contribute to this injury. Cholesterol particles enter the damaged artery wall and accumulate over time. The immune system responds by sending white blood cells to the site, resulting in inflammation. As the process continues, fatty deposits called plaques gradually enlarge. Calcium may also accumulate within these plaques, making them harder and more rigid.
Initially, these plaques may not produce any symptoms because they develop slowly over many years. However, as they grow, they narrow the artery and reduce blood flow to the heart muscle, especially during physical exertion or emotional stress. This reduced blood flow may cause chest discomfort known as angina. The greatest danger occurs when a plaque suddenly ruptures. The body interprets the rupture as an injury and rapidly forms a blood clot at the site. If the clot completely blocks the artery, blood flow stops abruptly. Without oxygen, the heart muscle supplied by that artery begins to die. This process is known as myocardial infarction, or heart attack.
The heart has one of the highest oxygen demands of any organ in the human body. Even though it represents only a small percentage of total body weight, it consumes a significant proportion of the body’s oxygen supply. Unlike skeletal muscles, which can temporarily function with reduced oxygen, heart muscle depends almost entirely on a continuous supply of oxygen-rich blood. When blood flow stops, heart muscle cells begin to suffer within minutes. Permanent damage usually begins after approximately 20 to 30 minutes of complete blockage, and the amount of damage increases as time passes. This is why cardiologists often say, “Time is muscle.” Every minute of delay results in the loss of additional heart muscle, reducing the heart’s ability to pump effectively.
Each heartbeat consists of two major phases known as the cardiac cycle. During diastole, the heart relaxes and fills with blood returning from the body and lungs. This phase is especially important because most blood flow through the coronary arteries occurs during relaxation. During systole, the ventricles contract forcefully, pumping blood into the pulmonary artery and the aorta. These alternating phases occur continuously, adapting automatically to the body’s needs. During exercise, excitement, or stress, the heart beats faster and more forcefully to supply additional oxygen to active muscles. During sleep or rest, the heart slows down while still maintaining adequate circulation.
Blood pressure is another essential component of cardiovascular health. It measures the force exerted by blood against the walls of arteries as the heart pumps. A normal blood pressure for many healthy adults is around 120/80 mmHg, although ideal targets may differ depending on age, existing medical conditions, and current clinical guidelines. Persistently elevated blood pressure forces the heart to work harder, thickens the heart muscle, damages blood vessels, and accelerates the development of atherosclerosis. Over many years, uncontrolled hypertension significantly increases the risk of heart attack, stroke, kidney disease, and heart failure.
Fortunately, many of the factors that contribute to heart disease are preventable. Adopting a healthy lifestyle is one of the most effective ways to protect the heart. A balanced diet rich in fruits, vegetables, whole grains, legumes, lean proteins, fish, nuts, and healthy fats supports cardiovascular health. Limiting foods high in saturated fats, trans fats, added sugars, and excessive salt helps reduce cholesterol levels and blood pressure. Regular physical activity, such as brisk walking, cycling, swimming, or other moderate exercise for at least 150 minutes each week, strengthens the heart and improves circulation. Maintaining a healthy body weight reduces strain on the cardiovascular system. Avoiding tobacco products and secondhand smoke dramatically lowers the risk of heart attack. Limiting alcohol consumption, managing stress through relaxation techniques or mindfulness, obtaining sufficient sleep, and regularly monitoring blood pressure, blood sugar, and cholesterol further reduce cardiovascular risk.
Medical checkups play an equally important role. Many people with high blood pressure, diabetes, or elevated cholesterol have no symptoms until complications develop. Regular screening allows these conditions to be detected and treated early. When prescribed by a healthcare professional, medications such as statins, blood pressure-lowering drugs, or diabetes medications should be taken consistently to reduce the risk of heart disease and its complications.
Understanding the structure and function of the heart provides the essential foundation for understanding myocardial infarction. The heart is a remarkable muscular pump that depends on healthy coronary arteries to supply its own oxygen needs. When these arteries become narrowed or suddenly blocked, heart muscle begins to die, creating a medical emergency that requires immediate treatment. Recognizing how the heart works also highlights the importance of preventive measures, early diagnosis, and timely intervention.
In the next chapter, we will examine what a heart attack (myocardial infarction) is, how it develops, the different types of heart attacks—including STEMI and NSTEMI—how it differs from cardiac arrest and heart failure, and why rapid treatment can mean the difference between life and death.
