Heart, river, United States
Heart, river, 180 mi (290 km) long, rising in the low prairie country near the Little Missouri River, SW N.Dak., and flowing E to the Missouri at Mandan, N.Dak. The Heart Butte and Dickinson dams, irrigation and flood control units built by the U.S. Bureau of Reclamation as part of the Missouri River basin project Missouri River basin project, comprehensive plan authorized in 1944 for the coordinated development of water resources of the Missouri River and its tributaries, draining an area of c...... Click the link for more information. , have created the region's largest lakes, which are major recreation areas.
heart, in anatomy
heart, muscular organ that pumps blood to all parts of the body. The rhythmic beating of the heart is a ceaseless activity, lasting from before birth to the end of life.Anatomy and Function
The human heart is a pear-shaped structure about the size of a fist. It lies obliquely within the chest cavity just left of center, with the apex pointing downward. The heart is constructed of a special kind of muscle muscle, the contractile tissue that effects the movement of and within the body. Muscle tissue in the higher animals is classified as striated, smooth, or cardiac, according to its structure and function.
..... Click the link for more information. called myocardium or cardiac muscle, and is enclosed in a double-layered, membranous sac known as the pericardium. A wall of muscle divides the heart into two cavities: the left cavity pumps blood throughout the body, while the right cavity pumps blood only through the lungs. Each cavity is in turn divided into two chambers, the upper ones called atria, the lower ones ventricles. Venous blood from the body, containing large amounts of carbon dioxide, returns to the right atrium. It enters the right ventricle, which contracts, pumping blood through the pulmonary artery to the lungs. Oxygenated blood returns from the lungs to the left atrium and enters the left ventricle, which contracts, forcing the blood into the aorta aorta (āôr`tə), primary artery of the circulatory system in mammals, delivering oxygenated blood to all other arteries except
..... Click the link for more information. , from which it is distributed throughout the body. In addition, the heart employs a separate vascular system to obtain blood for its own nourishment. Two major coronary arteries regulate this blood supply.
Cardiac Cycle
Blood flows through the heart in one direction only. It is prevented from backing up by a series of valves at various openings: the tricuspid valve between the right atrium and right ventricle; the bicuspid, or mitral, valve between the left atrium and left ventricle; and the semilunar valves in the aorta and the pulmonary artery. Each heartbeat, or cardiac cycle, is divided into two phases. In the first phase, a short period of ventricular contraction known as the systole, the tricuspid and mitral valves snap shut, producing the familiar "lub" sound heard in the physician's stethoscope. In the second phase, a slightly longer period of ventricular relaxation known as the diastole, the pulmonary and aortic valves close up, producing the characteristic "dub" sound. Both sides of the heart contract, empty, relax, and fill simultaneously; therefore, only one systole and one diastole are felt. The normal heart has a rate of 72 beats per minute, but in infants the rate may be as high as 120 beats, and in children about 90 beats, per minute. Each heartbeat is stimulated by an electrical impulse that originates in a small strip of heart tissue known as the sinoatrial (S-A) node, or pacemaker.
Advances in Cardiology
One of the important advances in cardiology is the artificial pacemaker pacemaker, artificial, device used to stimulate a rhythmic heartbeat by means of electrical impulses. Implanted in the body when the heart's own electrical conduction system (natural pacemaker) does not function normally, the battery-powered device emits impulses
..... Click the link for more information. used to electrically initiate a normal heartbeat when the patient's own pacemaker is defective (see arrhythmia arrhythmia (ārĭth`mēə), disturbance in the rate or rhythm of the heartbeat.
..... Click the link for more information. ); it may be surgically implanted in the patient's body. Similarly, an internal defibrillator defibrillator, device that delivers an electrical shock to the heart in order to stop certain forms of rapid heart rhythm disturbances ( arrhythmias ). The shock changes a fibrillation to an organized rhythm or changes a very rapid and ineffective cardiac rhythm to a
..... Click the link for more information. may be implanted to deliver an electrical shock to the heart in order to stop certain forms of rapid heart rhythm disturbances. Another familiar tool of the cardiologist is the electrocardiograph (EKG), which is used to detect abnormalities that are not evident from a physical examination (see electrocardiography electrocardiography (ĭlĕk'trōkärdēŏg`rəfē)
..... Click the link for more information. ).
One of the most important advances in heart surgery during the 1960s was the transplantation of the healthy heart immediately after the death of an individual (the donor) to a recipient suffering from incurable heart disease (see transplantation, medical transplantation, medical, surgical procedure by which a tissue or organ is removed and replaced by a corresponding part, either from another part of the body or from another individual.
..... Click the link for more information. ). In the 1980s new advances in the design and construction of an artificial heart—both the entire organ and such parts as the valves and large blood vessels—showed some promise in treating cardiovascular disease (see heart, artificial ventricular assist device (VAD), or "artificial ventricle," is an internally implanted pump designed to aid a person with a failing left ventricle; unlike an artificial heart, it does not require removal of the patient's heart.
..... Click the link for more information. ), but the limited success that has characterized artificial heart implantation thus far has led many experts to question the efficacy of such measures. Although the artificial heart has often been used as a temporary measure until a permanent human donor heart can be located, a number of recipients have not fared well, even for a limited duration. In addition, it is often unclear how long the recipient will have to wait for a donor. Proponents of the artificial heart hope that technological advances will allow the permanent replacement of human hearts with artificial ones.
See circulatory system circulatory system, group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the
..... Click the link for more information. ; heart disease heart disease, any of several abnormalities of the heart and its function in maintaining blood circulation. Heart disease is the cause of approximately half the deaths in the United States each year.
..... Click the link for more information. .
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www.americanheart.org
heart [härt]
Heart (invertebrate)
Hearts of invertebrates can be categorized according to the source of the electrical rhythmicity that underlies their beat. Rhythmic electrical activity can arise in the muscle itself (myogenic hearts) or in neurons that drive the heart muscle (neurogenic hearts). Most mollusks and some insects appear to have purely myogenic hearts; these hearts beat normally when isolated from neural inputs. Conversely, the hearts of the higher crustaceans and the xiphosuran Limulus are usually considered to be purely neurogenic: motor neurons impose their rhythmic electrical activity on heart muscle fibers by means of direct excitatory synapses. Without neural input, the heart ceases to beat. Other invertebrates, including gnathobdellid leeches and some insects, have hearts that can produce a myogenic beat but require rhythmic neural input to coordinate that beat and maintain the proper rate.
In the marine snail Aplysia, an organism with a myogenic heart, a muscular heart consisting of an auricle and a ventricle is located in a dorsal pericardial cavity. The rhythmic contractions of the auricle fill the ventricle with hemolymph, which is then pumped through the open circulatory system by the rhythmic contractions of the ventricle. The normal heartbeat period lasts about 3 s. A pair of semilunar valves prevents backflow of hemolymph into the auricle during ventricular contraction. Three arteries issue from the ventricle toward the anterior, and a single semilunar valve prevents backflow from them during ventricular expansion. The arteries carry the hemolymph to the various body organs, where they end in tissue spaces. The hemolymph then collects in the hemocoel and returns to the heart by two parallel veins, one through the kidney and one through the gill. Although the Aplysia heart is innervated, its normal beat persists after denervation.
The lobster is an example of an organism with a neurogenic heart. A muscular heart pumps hemolymph through the open circulatory system. This heart is located dorsally along the thoracic midline and is suspended within a pericardial cavity by ligaments. The heartbeat period lasts about 2 s. Large anterior- and posterior-going arteries, which branch extensively to supply various body organs, issue from the heart. Semilunar valves, located at the juncture of each artery with the heart, prevent backflow of blood into the heart when it relaxes. Hemolymph enters the heart from the pericardial sinus through six ostia, which have valves to ensure unidirectional flow. The rhythmic discharge of motor neurons innervating the heart by way of excitatory chemical synapses produces the heartbeat. These motor neurons are located in the cardiac ganglion on the inner dorsal surface of the heart. The cardiac ganglion contains only nine neurons, which generate a simple two-phased rhythm of electrical activity. The four posterior small cells (cells 6–9) are interneurons, and the five anterior large cells (cells 1–5) are the motor neurons. See Nervous system (invertebrate)
Heart (vertebrate)
The muscular pumping organ of the cardiovascular system. The heart typically lies ventrally, near the anterior end of the trunk; it is ventral and medial to the gills in fish and at the base of the neck or in the chest region of tetrapods. In humans it is located behind the breastbone and ribs between the third and fifth costal cartilages. Its anterior portion or base is directed to the right and dorsally and is the area where the great vessels enter and leave the heart. The lower muscular portion ends in a blunt apex which lies behind the fifth costal cartilage on the left.
The muscular wall of the heart, the myocardium, is lined by an inner endocardium and is covered externally by membranous visceral pericardium. There are coronary arteries and veins to and from the heart, which has a specialized neuromuscular conducting system and autonomic nerve supply.
In fishes the heart is basically a simple tube which becomes subdivided into four successive chambers, the sinus venosus, atrium, ventricle, and conus arteriosus. Blood from the body enters the sinus and leaves the conus to go to the gills to be oxygenated. The ventricle supplies the main pumping force.
When lungs are introduced into the system in lungfish and tetrapods, the mixing of oxygenated and nonoxygenated blood becomes a problem. In brief, the sinus venosus and conus arteriosus disappear, becoming incorporated into the other chambers or the bases of the great vessels. At the same time the atrium and later the ventricle become divided into right and left chambers by a median septum.
In birds and mammals including humans (see illustration) the medial fibromuscular septum divides the heart into two lateral halves, each consisting of a thin-walled receiving chamber or atrium and a thicker, muscular pumping chamber or ventricle. Blood enters the right atrium from the superior and inferior venae cavae which drain most of the body. It passes through the tricuspid valve to the right ventricle and is pumped to the lungs during systole, or contraction of the heart. Blood returns from the lungs by way of the pulmonary veins to the left atrium, passes into the left ventricle through the mitral valve, and during contraction is pumped out into the aorta. See Cardiovascular system
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