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Cardiac muscle, also known as the myocardium, is one of the three major types of muscles in the human body, alongside smooth and skeletal muscle. The myocardium is responsible for the contractility of the heart and, therefore, its pumping action. This pumping action requires a constant supply of energy, which is produced through the metabolism of glucose and lipids by oxidative reactions. In the absence of oxygen, the heart can use anaerobic processes to generate energy, which involves the fermentation of pyruvic acid. This process results in the production of lactic acid, which must be removed by the blood circulation and brought to the liver for further metabolism. Therefore, cardiac muscle can undergo fermentation under anaerobic conditions to maintain energy production and support its vital function of pumping blood throughout the body.
| Characteristics | Values |
|---|---|
| Cardiac muscle cells | Striated, branched, contain many mitochondria, and are under involuntary control |
| Cardiac muscle composition | Sarcomeres, sarcolemma, voltage-gated calcium channels, intercalated discs, gap junctions, desmosomes |
| Primary function | To pump blood into circulation by generating sufficient force |
| Contractile functions | Require ATP, obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones |
| Anaerobic metabolism | Plays a role in myocardial preservation during ischemia or hypoxia |
| Major energy-yielding process | Metabolism of glucose and lipids by oxidative reactions |
| Fatty acids | Considered the major fuel consumed by cardiac muscle |
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What You'll Learn
- Cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria
- The heart is made up of three layers—the pericardium, myocardium, and endocardium
- The cardiac muscle must contract with enough force to pump blood into circulation and supply blood to the body's metabolic demands
- The major energy-yielding process in the heart is through the metabolism of glucose and lipids by oxidative reactions
- The heart may use anaerobic processes in a limited capacity, such as during a heart attack or open-heart surgery
Cardiac muscle cells, or cardiomyocytes, are striated, branched, and contain many mitochondria
Cardiac muscle cells, also called cardiomyocytes, are striated, branched, and contain many mitochondria. They are the contractile myocytes of the cardiac muscle and are under involuntary control. Each cardiomyocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which are specialised ion channels that skeletal muscle does not possess.
The sarcolemma of cardiac muscle cells contains folds known as T-tubules, which are highly branched invaginations that function in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction. The T-tubules penetrate deep into the interior of the cell, allowing electrical impulses to reach the inner parts of the cell. The cardiac syncytium is a network of cardiomyocytes connected by intercalated discs that enable the rapid transmission of electrical impulses through the network, enabling the syncytium to act in a coordinated contraction of the myocardium.
Each cell contains myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other. These are organised into sarcomeres, the fundamental contractile units of muscle cells. The regular organisation of myofibrils into sarcomeres gives cardiac muscle cells a striped or striated appearance when viewed through a microscope, similar to skeletal muscle. The sarcomere, which consists of thick (myosin) and thin (actin) filaments, forms the basis of the sliding filament theory.
Mitochondria are present in large numbers in cardiac muscle fibres to provide energy for muscle contraction. The major energy-yielding process in the heart is through the metabolism of glucose and lipids by oxidative reactions.
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The heart is made up of three layers—the pericardium, myocardium, and endocardium
The heart is a vital organ that supplies blood and oxygen to the body. It is located in the chest cavity, slightly to the left of the breastbone, between the lungs, and superior to the diaphragm. This small but mighty organ is protected by three layers of tissue: the pericardium, the myocardium, and the endocardium.
The pericardium is a fluid-filled sac that surrounds the heart and the roots of the major blood vessels extending from the heart. It acts as a protective cushion, holding the heart in place and preventing it from expanding too much. The pericardium is composed of two main layers: the fibrous pericardium, which is the tough, outermost layer made of connective tissue, and the serous pericardium, which is the inner layer. The serous pericardium is further divided into two layers: the parietal pericardium, which is firmly attached to the fibrous pericardium, and the visceral pericardium (also known as the epicardium), which is the innermost layer that directly covers the heart. The space between the two layers of the serous pericardium is filled with pericardial fluid, which lubricates the heart and reduces friction during its contractions.
The myocardium is the muscular middle layer of the heart wall. It is composed of cardiac muscle fibres that enable the heart to contract and pump blood. The thickness of the myocardium varies, with the left ventricle being the thickest part, as it generates the power needed to pump oxygenated blood from the heart to the rest of the body.
The endocardium is the thin, delicate inner layer of the heart. It lines the inner surfaces of the heart chambers and covers the heart valves. The endocardium consists of two layers: an inner layer of endothelial cells and a subendocardial connective tissue layer that is continuous with the myocardium. The endocardium is susceptible to infections, which can lead to a serious condition called endocarditis.
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The cardiac muscle must contract with enough force to pump blood into circulation and supply blood to the body's metabolic demands
The human heart is a powerful muscle that pumps oxygenated blood to the body. The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. The cardiac muscle must contract with enough force to pump blood into circulation and supply blood to meet the body's metabolic demands. This is known as cardiac output and is defined as heart rate multiplied by stroke volume, which is determined by the contractile forces of the cardiac muscle and the frequency of their activation.
The heart is made up of three layers: the pericardium, myocardium, and endocardium. The endocardium is not cardiac muscle; it is made up of simple squamous epithelial cells and forms the inner lining of the heart chambers and valves. The pericardium is a fibrous sac that surrounds the heart. The myocardium, or cardiac muscle, is responsible for the contractility of the heart and, therefore, the pumping action. The cardiac muscle is made up of sarcomeres that allow for contractility. Unlike skeletal muscle, cardiac muscle is under involuntary control.
The atria and ventricles work together, contracting and relaxing to make the heart beat and pump blood. The electrical system of the heart is the power source that makes this possible. Electrical impulses trigger each heartbeat and travel through the heart via a special pathway. The impulse starts in a small bundle of specialized cells called the SA node (sinoatrial node), in the right atrium. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Aerobic production is the core utilization process; however, the heart may use anaerobic processes in limited circumstances, such as during a heart attack or open-heart surgery, where there is impairment of blood flow to the myocardium.
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The major energy-yielding process in the heart is through the metabolism of glucose and lipids by oxidative reactions
The heart is one of the most metabolically active organs, with a prolific capacity for energy generation. The major energy-yielding process in the heart is through the metabolism of glucose and lipids by oxidative reactions. This process involves the breakdown of glucose and lipids to produce energy in the form of adenosine triphosphate (ATP).
Glucose, derived from carbohydrates, is an important fuel for the heart. It can generate ATP through cytoplasmic glycolysis and the mitochondrial oxidation of pyruvate, which is produced from glycolysis. During glycolysis, glucose is broken down to release energy, with two ATP molecules being consumed initially and a net gain of two ATP molecules produced for every molecule of glucose metabolized. Pyruvate, derived from glycolysis, is imported into the mitochondria and converted into acetyl-CoA, which then enters the tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle.
Lipids, including fatty acids and ketone bodies, are also important energy substrates for the heart. Fatty acids produce the greatest ATP yield per two carbon units compared to other energy substrates. However, they also have the highest oxygen requirement for ATP production, making them the least efficient in terms of ATP produced per oxygen consumed. The oxidation of fatty acids by the heart can increase or decrease depending on the type of heart failure. For example, in heart failure associated with diabetes and obesity, myocardial fatty acid oxidation increases. Ketone bodies, such as β-hydroxybutyrate (βOHB), are produced in the liver from acetyl-CoA, which is predominantly sourced from fatty acid oxidation. Ketones are readily metabolized by the heart and can become a major fuel source if circulating ketone levels are elevated.
Under normal physiological conditions, the heart generates more than 95% of its ATP through oxidative metabolism, with 60-70% from fatty acid oxidation and 30-40% from glucose oxidation and other substrates like lactate, amino acids, and ketone bodies. However, during ischemia or hypoxia, when there is an impairment of blood flow to the myocardium, the heart relies more on anaerobic metabolism, with a shift towards greater glucose utilization and a reduction in mitochondrial oxidative metabolism. This anaerobic glycolysis is important for the preservation of myocardial viability during ischemia, but the accumulation of its products, protons and lactate, can inhibit glycolysis and lead to a depression of anaerobic metabolism.
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The heart may use anaerobic processes in a limited capacity, such as during a heart attack or open-heart surgery
The cardiac muscle is responsible for the contractility of the heart and, therefore, its pumping action. The heart derives energy from aerobic metabolism via many different types of nutrients. Sixty percent of the energy to power the heart is derived from fat, 35% from carbohydrates, and 5% from amino acids and ketone bodies from proteins.
The heart muscle pumps continuously throughout life and is adapted to be highly resistant to fatigue. However, the heart may use anaerobic processes in a limited capacity, such as during a heart attack or open-heart surgery. In an ischemic state, such as during a heart attack or the induced ischemia of open-heart surgery, there is an impairment of blood flow to the myocardium. This results in hypoxia, where there is a lack of oxygen supply to the myocardium.
Anaerobic metabolism in the heart muscle helps maintain myocardial preservation during ischemia or hypoxia. Under anaerobic conditions, oxygen is not available to accept the electrons in the metabolic degradation of substrates, and anaerobic glycolysis becomes important in preserving myocardial viability. However, the accumulated products of glycolysis, namely protons and lactate, inhibit glycolysis, ultimately resulting in a depression of anaerobic metabolism.
Lactate, created from lactic acid fermentation, accounts for the anaerobic component of cardiac metabolism. At normal metabolic rates, about 1% of energy is derived from lactate, and this can increase to about 10% under moderately hypoxic conditions. During severe hypoxia, not enough energy can be generated by lactate production to sustain ventricular contraction, leading to heart failure.
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Frequently asked questions
Cardiac muscle, also called the myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The heart is made up of three layers—the pericardium, myocardium, and endocardium. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses.
Anaerobic metabolism in heart muscle is important in maintaining myocardial preservation during ischemia or hypoxia, such as during a heart attack or open-heart surgery. Under these conditions, anaerobic glycolysis becomes important for preserving myocardial viability.
Cardiac muscle cells can obtain ATP through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Fatty acids are considered the major fuel consumed by cardiac muscle, meeting 90% of their ATP demands by oxidizing fatty acids between meals.
Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. They are under involuntary control and contain voltage-gated calcium channels, which skeletal muscle does not possess. Gap junctions between cardiomyocytes allow for electrical coupling, enabling synchronous contractions of the heart.
