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The heart is a muscular organ that pumps blood throughout the body. It is made up of a type of muscle tissue called cardiac muscle or myocardium. This is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. The heart is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium is the thickest layer and is made up of cardiac muscle, which contracts when the heart beats, pumping blood through the body.
| Characteristics | Values |
|---|---|
| Type of muscle | Cardiac muscle, also known as myocardium |
| Other types of muscles in the body | Skeletal muscle and smooth muscle |
| Number of chambers | 4 |
| Number of layers | 3 |
| Outer layer | Pericardium or epicardium |
| Middle layer | Myocardium |
| Inner layer | Endocardium |
| Function | To pump blood throughout the body |
| Average number of beats per day | 100,000 |
| Average number of beats in a lifetime | 2.5 billion |
| Average amount of blood pumped per minute | 5.6 litres |
| Cells | Cardiomyocytes |
| Cardiomyocyte shape | Cylindrical |
| Cardiomyocyte length | 100μm |
| Cardiomyocyte diameter | 10–25μm |
| Contractibility | Controlled by electrical impulses and calcium release |
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What You'll Learn
The heart is a muscular organ
The myocardium forms the thick middle layer of the heart wall, between the outer layer (the pericardium or epicardium) and the inner layer (the endocardium). The heart wall is made up of three layers in total, with the myocardium being the thickest. The heart consists mostly of cardiac muscle cells, which are striated, branched, and contain many mitochondria. They are under involuntary control, contracting and releasing without conscious direction from the brain.
The heart's primary function is to pump blood around the body through the circulatory system. The rhythmic contraction of the heart is regulated by the sinoatrial node, which serves as the heart's pacemaker. This is made up of pacemaker cells, which are a specialised type of cardiac tissue. These cells contract and expand in response to electrical impulses from the nervous system, which tell the cardiac muscle cells to contract and relax. The rate at which the heart contracts depends on the electrical properties of the cardiac muscle cells and the conduction of electrical information from one region of the heart to another.
Cardiomyopathy is a disease that affects the heart's ability to pump blood. There are several types of cardiomyopathy, including hypertrophic, dilated, restrictive, and arrhythmogenic right ventricular dysplasia. These conditions can cause the cardiac muscle cells to enlarge and thicken, the ventricles to become larger and weaker, the ventricles to become stiff, and the cardiac muscle tissue of the right ventricle to be replaced with fatty or fiber-rich tissue, respectively.
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Cardiac muscle, or myocardium
The heart is a muscular organ that pumps blood throughout the body. It is made up of a type of muscle tissue called cardiac muscle or myocardium. This tissue forms the bulk of the heart and is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle.
Cardiac muscle is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. It forms a thick middle layer between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium). The myocardium is surrounded and protected by these two layers. The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels that connect to the heart. The outer epicardium forms part of the pericardial sac that surrounds and lubricates the heart.
The cardiac muscle is composed of individual cardiac muscle cells or cardiomyocytes that are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. The intercalated discs enable the rapid transmission of electrical impulses through the network, allowing the syncytium to act in a coordinated contraction of the myocardium. The individual cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. Cardiomyocytes are the contractile myocytes of the cardiac muscle and are the cells responsible for the contraction of the myocardium.
Cardiomyocytes are linked to the basement membrane, which is mainly composed of type IV collagen and laminin, via specialised glycoproteins called integrins. Humans are born with a set number of cardiomyocytes, which increase in size as the heart grows during childhood development. Evidence suggests that less than 50% of the cardiomyocytes present at birth are replaced during a normal lifespan.
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Heart muscle cells, or cardiomyocytes
The heart is a muscular organ that pumps blood throughout the body. It is made up of a type of muscle tissue called cardiac muscle, or myocardium. This muscle tissue forms the bulk of the heart and is responsible for the heart's pumping action.
Cardiac muscle is composed of individual cardiac muscle cells, or cardiomyocytes, which are joined by intercalated discs and encased by collagen fibers and other substances that form the extracellular matrix. These cardiomyocytes are the contractile myocytes of the cardiac muscle and they work together to pump blood from the heart. Each cardiomyocyte needs to contract in coordination with its neighboring cells, and this coordination is facilitated by gap junctions that permit intercellular communication.
The individual cardiomyocyte is a tubular structure composed of chains of myofibrils, which are made up of repeating sections of sarcomeres. Sarcomeres are the fundamental contractile units of the muscle cells and they consist of long proteins that organize into thick and thin filaments, called myofilaments. These myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of "cross-bridges" which cause the contraction of the heart and the generation of force.
Cardiomyocytes typically contain only one centrally-located nucleus, unlike skeletal muscle cells which often contain many nuclei. They also contain many mitochondria to produce large amounts of adenosine triphosphate (ATP) and myoglobin to store oxygen to meet the demands of muscle contraction. A healthy adult cardiomyocyte has a cylindrical shape that is approximately 100 μm long and 10-25 μm in diameter. Humans are born with a set number of cardiomyocytes, which increase in size as the heart grows during childhood development and in response to extensive exercise, heart disease, or heart muscle injury.
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Heart muscle diseases, or cardiomyopathies
The heart is a muscle, specifically a highly specialised and efficient one. Its unique structural and functional characteristics enable it to perform the vital task of pumping blood throughout the body. The heart is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium, or cardiac muscle, forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
There are several types of cardiomyopathies, and they can affect people of any sex, age, or race. Some people may experience no symptoms and not require treatment, while others may develop symptoms as the disease progresses. Symptoms of cardiomyopathy can include shortness of breath, fatigue, dizziness, fainting, swelling in the ankles or legs, and chest pain. As cardiomyopathy worsens, it can lead to other heart problems, including arrhythmias, heart valve disease, and cardiac arrest.
Treatment options for cardiomyopathy include medications, procedures, healthy lifestyle changes, implanted devices, and therapy to lower stress. While these treatments may not cure the condition, they can help manage symptoms, slow down the disease's progression, and improve quality of life.
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Heart muscle function and physiology
The heart is a muscular organ, responsible for pumping blood throughout the body. It is made up of cardiac muscle tissue, also known as myocardium, which is one of three types of vertebrate muscle tissues, the others being skeletal and smooth muscle.
Cardiac muscle is involuntary, contracting and releasing without conscious control, and is striated. It forms the middle layer of the heart wall, between the outer layer, the pericardium, and the inner layer, the endocardium. The myocardium is thicker than the other two layers and is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs. These discs are responsible for force transmission during muscle contraction, allowing action potentials to spread between cardiac cells.
The heart contains four muscular chambers, two atria on top and two ventricles on the bottom. The right atrium receives oxygen-poor blood from the body's veins, which is then pumped to the right ventricle. The right ventricle pumps this blood to the lungs, where it is reloaded with oxygen. The pulmonary veins then carry the oxygen-rich blood to the left atrium, which pumps it to the left ventricle. The left ventricle is slightly larger than the right and pumps oxygen-rich blood to the rest of the body.
The heart's muscular walls contract and relax to send blood throughout the body. This process is controlled by the brain and nervous system, which sends signals to increase or decrease the heart rate depending on the body's needs. The heart also contains specialised "pacemaker" cells that generate electrical impulses, controlling the heart rate and determining how fast the heart pumps blood. These pacemaker cells are located in the sinoatrial node, positioned on the wall of the right atrium.
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Frequently asked questions
Yes, the heart is a muscle, specifically a highly specialized and efficient one. It is also an organ, made up of muscle tissue called cardiac muscle or myocardium.
The heart's primary function is to pump blood throughout the body.
The heart is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium is the thickest of the three layers and is made up of cardiac muscle.
The heart contracts and relaxes rhythmically, driven by complex molecular and cellular events. This movement is made possible by the release of calcium, which causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling.
Yes, "broken heart syndrome" is a real condition, also known as stress-induced cardiomyopathy or takotsubo cardiomyopathy. It is caused by a surge of stress hormones brought on by an emotional event and is usually treatable.
