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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 muscle is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium). The heart's ability to contract and relax rhythmically is driven by complex molecular and cellular events, including the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum.
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What You'll Learn
The heart is a muscular organ
Cardiac muscle cells contain sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres consist of long proteins that organize into thick and thin filaments called myofilaments. When the heart beats, these myofilaments slide past each other, causing the muscle to contract and generating the force needed to pump blood. The contraction of cardiac muscle is triggered by electrical stimulation in the form of a cardiac action potential, which stimulates the release of calcium from the cell's internal store, the sarcoplasmic reticulum. This release of calcium causes the myofilaments to slide past each other in a process called excitation-contraction coupling.
The heart's pumping action is essential for the body's overall health and function. The blood pumped by the heart delivers oxygen and nutrients to the cells and tissues of the body, while also carrying away waste products like carbon dioxide to be expelled. The ventricles, the bottom two chambers of the heart, have thicker walls containing more cardiac muscle to pump blood to other parts of the body. The interior of the heart also contains valves that help keep blood flowing in the correct direction.
The heart's ability to contract and relax rhythmically is driven by complex molecular and cellular events, including the intricate interplay of proteins and ion movements. Studies have shown that the basic structure of cardiac muscle is conserved across different vertebrates, indicating a common evolutionary origin. Cardiac muscle in mammals has limited regenerative potential due to the absence of resident cardiac stem cells and the inability of adult cardiomyocytes to divide. However, individual cardiomyocytes can increase in size during childhood development and in response to extensive exercise, heart disease, or heart muscle injury.
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Cardiac muscle is one of three types of muscle tissues
The heart is a muscular organ, made up predominantly of a type of muscle tissue called cardiac muscle. This is one of three types of muscle tissues found in the human body, the other two being skeletal muscle and smooth muscle.
Cardiac muscle is also known as myocardium and forms the thick middle layer of the heart. The myocardium is surrounded by a thin outer layer called the epicardium (or visceral pericardium) and an inner endocardium. The heart wall is a three-layered structure, with the myocardium sandwiched between the inner and outer layers.
Cardiac muscle is made up of individual cardiac muscle cells, known as cardiomyocytes, which are joined by intercalated discs. These discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium. The cardiomyocytes are surrounded by an extracellular matrix produced by supporting fibroblast cells.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. This is achieved through the contraction of the heart, which is caused by the contraction of the cardiomyocytes. The contraction of these individual cells is triggered by electrical stimulation in the form of a cardiac action potential, which prompts the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling, resulting in the contraction of the heart.
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The heart has three layers
The heart is a muscular organ that pumps blood throughout the body. It is housed in a pericardial sac, which is a protective fluid-filled sac. The heart itself is made up of three layers of tissue, each with its own function.
The outer layer of the heart wall is called the epicardium or visceral pericardium. It forms the inner layer of the pericardium and is composed primarily of loose connective tissue, including elastic fibres and adipose tissue. The epicardium functions to protect the inner heart layers and assists in the production of pericardial fluid, which helps reduce friction between the pericardial membranes. This layer is also home to the coronary blood vessels, which supply the heart wall with blood.
The middle layer of the heart is the myocardium, which is the thickest layer of the three. It is composed of cardiac muscle fibres, which enable the heart to contract and pump blood. The myocardium is functionally the main constituent of the heart and is made up of cardiomyocytes, which are individual cardiac muscle cells. These cells are constantly contracting and therefore require a lot of energy. They are rich in mitochondria and glycogen deposits to meet these energy needs.
The inner layer of the heart is the endocardium, which lines the inner surfaces of the heart chambers and covers the cardiac valves. It has two layers: the inner layer, which is made of endothelial cells, and an outer layer that is in direct contact with the myocardium.
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The heart's ability to contract and relax rhythmically
The heart is a muscular organ, made up predominantly of a type of muscle tissue called cardiac muscle or myocardium. The myocardium forms a thick middle layer of the heart, sandwiched between the outer layer of the heart wall (the pericardium or visceral pericardium/epicardium) and the inner layer (the endocardium).
Cardiac muscle cells (cardiomyocytes) are the contractile cells of the cardiac muscle. They are self-stimulating and do not require individual innervation, relying instead on the heart's intrinsic conducting system to coordinate contractions. The primary function of cardiomyocytes is to contract, which creates the pressure required to pump blood through the circulatory system. Cardiomyocytes are composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments.
The sliding of myofilaments past each other during contraction and relaxation causes the formation of "cross-bridges", which leads to the contraction of the heart and the generation of force. The concentration of calcium in the cardiomyocyte is the critical factor that determines how much force is generated with each contraction. An increase in calcium causes the myofilaments to slide past each other in a process called excitation-contraction coupling. Electrical stimulation triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum.
Cardiac muscle fibres have their own auto-rhythmicity, with their own pacemaker cells that spontaneously depolarize. These depolarizations occur at a consistent pace, but the pacemaker cells can also receive input from the autonomic nervous system to decrease or increase the heart rate depending on the body's requirements.
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Cardiomyocytes are the individual cells that make up the heart
The heart is a muscle, specifically a cardiac muscle, and it is made up of individual cells called cardiomyocytes. These cells are responsible for the rhythmic beating of the heart. Humans are born with a set number of cardiomyocytes, which increase in size as the heart grows during childhood development.
Cardiomyocytes are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. These myofilaments slide past each other as the muscle contracts and relaxes, causing the heart to contract and generate force.
The contraction and relaxation of cardiomyocytes are orchestrated by cyclic increases and decreases in intracellular calcium ions. This process is initiated by the depolarization of the cell membrane, or sarcolemma, and sustained by the release and re-uptake of calcium ions by the sarcoplasmic reticulum. The concentration of calcium ions in the cell determines the force generated with each contraction.
Cardiomyocytes are connected to each other by intercalated discs, forming long fibers. These discs enable the rapid transmission of electrical impulses through the network of cardiomyocytes, allowing them to contract in a coordinated manner. The contraction of cardiomyocytes generates the pressure needed to pump blood through the circulatory system.
Cardiomyocytes also have T-tubules, which are pouches of cell membrane that improve the efficiency of contraction. Additionally, they contain a high number of mitochondria, which provide the energy needed for the cell in the form of adenosine triphosphate (ATP).
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Frequently asked questions
Yes, the heart is a muscular organ. It is made up of a type of muscle tissue called cardiac muscle or myocardium.
Cardiac muscle is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
The heart's ability to contract and relax rhythmically is driven by complex molecular and cellular events. Electrical stimulation triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. This causes the cell's myofilaments to slide past each other in a process called excitation-contraction coupling, which results in the contraction of the heart and the generation of force.
Unlike skeletal muscles, cardiac muscle cells are self-stimulating and do not require individual innervation. They rely on the heart's intrinsic conducting system to coordinate contractions. Cardiac muscle cells are also unable to regenerate, unlike skeletal muscle.
