Cardiac tissue is the muscle tissue found in the heart.
The heart is a muscle at the center of the circulatory system. It lies in the chest, protected by the ribs and surrounded by a sac of connective tissue called the pericardium. Each minute, the heart contracts about 70 times and pumps about 5 quarts (5 L) of blood through the arteries, capillaries, and veins of the circulatory system.
Cardiac muscle is a type of muscle tissue found only in the heart. It has certain characteristics that allow it to sustain relatively slow, strong, regular, coordinated contractions throughout an individual's life. Cardiac muscle is different from both skeletal muscle that controls voluntary movements of bone and smooth muscle that controls involuntary internal functions such as digestion. However, cardiac muscle shares some qualities with each of these other types of muscle.
Cardiac muscle cells, like smooth muscle cells, contain a single central nucleus. By contrast, skeletal muscle cells contain many nuclei. Cardiac muscle cells are branched and lie parallel to each other. Their alignment, like that of skeletal muscle, makes them appear striated or lined when seen under the microscope. Contraction of cardiac muscle is involuntary. A person cannot consciously stop his heart from beating the way he can consciously contract muscles to move his arm or leg. Unlike skeletal muscle that requires stimulation by a nerve to begin a contraction, cardiac muscle cells are myogenic, meaning that they can self-initiate contraction without stimulation by a nerve. The ability of cardiac muscle cells to contract without external stimulation was demonstrated in 1912 by Alexis Carrell, a Nobel Prize-winning French scientist. Carrell removed a tiny piece of heart muscle from a chicken embryo and kept it alive in a nutrient solution. The heart cells continued to contract and relax in a regular pattern for 34 years, after which the experiment was intentionally ended.
The function of cardiac tissue is to contract in order to produce the force necessary to move blood through the circulatory system. Blood pressure is one measure of the force that cardiac tissue must generate to move blood through the arteries. Blood pressure measurements consist of two numbers such as 120/70 (a good reading for a healthy young adult). The top number is the systolic pressure, and it measures the pressure on the artery walls when the cardiac tissue in the left ventricle contracts. (See blood circulation entry for more details on heart anatomy.) The bottom number is the diastolic pressure, and it measures the pressure in major arteries between contractions when cardiac tissue is relaxed and at rest. The higher the blood pressure numbers, the harder the heart must work to circulate the blood. Another way of determining the force of cardiac tissue contraction is to measure the stroke volume, which is the amount of blood pumped out of the heart on each contraction. Cardiac output, or the total amount of blood pumped in one minute, can increase five-fold from resting values during heavy exercise.
To effectively deliver blood to all cells in the body, the contractions must be regular and coordinated. In a healthy heart, the signal to contract begins in the sinoatrial node (SA node), a patch of electrically active cardiac tissue located in the right ventricle. The SA node acts as a pacemaker for the heart. The rate and strength of heart contractions can be modified by hormonal and nerve stimulation (for example, the hormone adrenaline causes heart rate to increase), but the SA node alone initiates the electrical impulse needed to contract. The SA node creates an electrical impulse through changes in the chemistry of the cell membrane of cardiac muscle. Because one of the features of cardiac muscle cells is that they are connected to each other by special junctions called intercalcated discs, this electrical impulse can perpetuate itself by stimulating the cell next to it. As a result, the electrical impulse that stimulates cardiac tissue contraction moves directly through the heart from one cell to the next.
The electrical impulse initiated by the SA node causes the right atrium to contract. This, in turn, stimulates another patch of electrically active tissue, the atrio-ventricular node (AV node) located in the wall between the right atrium and the right ventricle. After a brief delay necessary to coordinate the contraction, an electrical impulse from the AV node travels down either side of the septum, the tissue separating the right and left sides of the heart. When the impulse reaches the tip of the ventricles, they begin to contract from the tip upward, squeezing blood out of the ventricles and into the atria. After contraction, there is a period of relaxation during which the heart again fills with blood.
Role in human health
Healthy cardiac tissue is essential for life. Blood must circulate constantly to bring oxygen and nutrients to all cells in the body and to remove metabolic wastes. Blood circulation is driven by the force of cardiac tissue contractions. Tissue damage and death result within minutes when cardiac tissue fails to contract or contracts in a disorganized way.
Common diseases and disorders
Cardiac tissue damage is diagnosed primarily through an electrocardiogram (EKG). During this procedure, electric leads are placed on the individual's chest, and the electrical activity of the heart is recorded by an oscilloscope. Experienced health care professionals can tell from the pattern of electrical activity whether the heart is healthy or whether there is damage to cardiac tissue and where the damage is likely to have occurred.
Damage to cardiac tissue can be congenital or occur later in life either suddenly or gradually. Congenital cardiac tissue disorders are usually the result of gene mutations. Over one hundred different mutations affecting the proteins in cardiac tissue have been identified as of 2005. Myocardial infarctions (MI), more commonly called heart attacks, cause rapid cardiac tissue death. MIs occur when blood circulation to cardiac tissue is interrupted. The heart receives blood through the coronary arteries. When one of these arteries is blocked by a clot, cardiac tissue dies from lack of oxygen. Individuals may die during a heart attack, or they may live but develop cardiac arrhythmias as a result of cardiac cell death in a localized patch of cardiac tissue.
Arrhythmias are irregular heart contractions. They cause fibrillation or disorganized contraction of cardiac tissue so that blood is not pumped out of the heart. Fibrillation may occur in any part of the heart. Ventricular fibrillation can cause sudden death. Ventricular fibrillation can sometimes be successfully treated with an immediate electric shock from a device called a defibrillator. The electric shock "resets" the pattern of contraction. Pacemaker devices may be surgically installed to treat certain other cardiac arrhythmias.
Gradual heart failure or pump failure occurs when cardiac output decreases below the needs of the body. Pump failure can be a complication of MI, or it can be a chronic condition associated with old age, high blood pressure, and other ongoing disorders. Pump failure causes the body to retain fluids. When the lungs fill up with fluid, breathing becomes difficult and eventually death results.
Cardiac tissue has a limited ability to repair itself. About 25,000 children are born each year needing corrective heart surgery and many thousands more adults experience heart attacks and cardiac tissue damage. Research is taking place that scientists hope will lead to the ability to artificially grow cardiac tissue to replace malformations and damaged tissue.
Congenital— Present at birth.
Fibrillation— Disorganized contraction of cardiac tissue that leaves the heart quivering but unable to contract in a way that moves blood through the circulatory system.
Myogenic— Muscle tissue that contracts in an orderly, regular fashion because of its inherent properties.
Pericardium— A double-walled sac that surrounds the heart.
Sinoatrial node— The section of cardiac tissue that initiates contraction of the heart and acts as a pacemaker.
Striation— A stripe of contrasting color. In muscles these are caused by the alignment of the muscle cells and the proteins causing contraction.
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