The human heart is the central organ of the cardiovascular system. It is a muscular organ divided into four chambers that functions as a pressure and suction pump. In a healthy adult at rest, the heart pumps about 5 to 6 liters of blood per minute (cardiac output) into the circulatory system.
The heart is located in the mediastinum posterior to the sternum and between the lungs, enclosed in a double-walled sac known as the pericardium. The largest fraction of the cardiac mass consists of heart muscle — the myocardium. The inner surface of the myocardium faces the lumen of the heart's chambers and is covered by a thin layer of endocardium. The outer surface of the myocardium is covered by the thin epicardium layer.
The four chambers of the heart are comprised of two atria (located at the superior end of the heart) and two ventricles (forming the inferior end of the heart). Each atrium is separated from its corresponding ventricle by an atrioventricular valve (also known as an AV valve). Blood enters the heart from the circulatory system and is pumped out of the heart back into this system. Between the atrial and ventricular myocardium there is a separating layer of connective tissue. The coronary arteries provide the functional blood supply to the heart tissues.
The heart is approximately the size of its owner's fist. The adult heart is about 12 to 14 cm long while its widest portion, measuring about 9 cm, is at its base. The normal weight of the adult heart varies in men between 280 and 340 grams and in women between 230 and 280 gm. The weight of the heart increases continually for the majority of a person's life.
The heart is located in the thorax within its medial cavity, the mediastinum, and extends from the second rib to the fifth intercostal space. The heart lies obliquely with its apex directed anteroinferiorly, nearly reaching the left midclavicular line at the fifth intercostal space, while its base is found posterosuperiorly directed towards the right shoulder. The apex points toward the left hip and lies on the diaphragm. At this apical end of the heart, in the fifth intercostal space and just below the left nipple, is the point of maximum intensity, where the beating of the heart can be easily palpated.
Laterally the heart borders on the parietal pleura of lungs. Dorsally the left atrium (atrium sinistrum) contacts the esophagus. The ventral side of the heart lies immediately dorsal to the sternum. The caudal end of the heart lies on the diaphragm.
The septum cardiale divides the heart longitudinally into a right and left side. The upper portion of the septum cardiale, the interatrial septum or septum interatriale, separates the left and right atria, while its lower portion, the interventricular septum or septum interventriculare, separates the left and right ventricles. Horizontally a circular constriction and the AV valves separate the two atria from the two ventricles. Thereby, the heart is divided into its four chambers:
These divisions are also visible on the surface of the heart as two grooves in which blood vessels are found that supply the myocardium. Between the atria and the ventricles is the circular constriction known as the atrioventricular groove, coronary sulcus or sulcus coronarius. The anterior interventricular sulcus and the posterior interventricular sulcus indicate the position of the interventricular septum separating the left and right ventricles.
The exterior surface of the heart is anatomically divided into:
The interior surfaces of the heart comprise the walls of the heart chambers, which are covered by the endocardium. The inner surface is not a flat smooth surface, but is mostly rather structured.
The internal walls of the ventricles display a network of irregular ridges of muscle known as the trabeculae carneae as well as several conical muscle bundles projecting into the ventricular cavity, the papillary muscles. In contrast, the atrial walls are quite smooth, although the anterior wall of the right atrium also displays thin bundles of muscle, the pectinate muscles. The right atrium has a shallow depression, the fossa ovalis, visible on the surface of the interatrial septum. The fossa ovalis marks where an opening between the right and left atrium, the foramen ovale cordis, existed in the fetal heart.
The heart wall consists of three layers. From the inside (the walls of the heart chambers) towards the surface these are:
The heart contains four valves:
The valves cannot move themselves as they contain no muscular tissue, but consist of connective tissue that is covered by a layer of endocardium.
The AV valves are located at the junction of each atrium with its respective ventricle form the opening for blood to enter the ventricle. When the heart is relaxed the flaps of these valves hang downwards into their ventricular chamber and the valves are open, permitting blood to flow from the atria into their respective ventricles.
Each heartbeat can be divided into a phase during which the heart muscle contracts, referred to as cardiac systole (or simply as systole), followed by a phase during which the heart muscle relaxes, referred to as cardiac diastole (or simply as diastole). During systole and diastole, the four heart valves are found to be in opened or closed states, depending on the valve and the phase of the heartbeat.
Both AV valves have thin white collagen cords, the chordae tendineae, attached to the edges of the valve flaps. These cords are also attached to the papillary muscles within the ventricular chamber. In this arrangement the cords are able to hold the valve flaps in their closed positions during ventricular systole and the ensuing ejection of blood from the heart. Without this reinforcement the blood pressure in the ventricles would force the AV valve flaps upwards into the atria and permit blood to flow back into the atria (reflux).
Each semilunar valve consists of three valve flaps and forms the opening from its ventricle that permits blood to exit the chamber during ventricular systole. The pulmonary semilunar valve connects the right ventricle to the pulmonary trunk that directs blood towards the lungs. The aortic valve connects the left ventricle to the aorta that directs blood into the systemic circulation. The increasing blood pressure within the ventricles during systole forces the semilunar valves to open, thereby permitting blood to flow out of the ventricles. During diastole the blood pressure in the pulmonary trunk and the aorta is greater than the pressure in their respective ventricles so that the blood within these vessels attempts to flow back into the ventricles, but this just pushes the flaps of the semilunar valves into the closed position so that no blood reflux occurs.
The functional blood supply to the heart muscle and tissues is provided by two coronary arteries, the right coronary artery and left coronary artery. Both arteries originate at the base of the aorta immediately outside of the aortic valve and encircle the heart in the atrioventricular groove. They form numerous anatomoses, but there are also numerous end arteries within the system of coronary vessels.
The two main coronary arteries and their major branches are located in the epicardium. They send branches into the myocardium to supply it with nutrients and oxygen.
The cardiac veins follow approximately the same routes as the coronary arteries. Most of the veins empty into the coronary sinus on the posterior side of the heart, from which the blood is emptied into the right atrium. Some of the veins on the anterior side of the heart empty directly into the right atrium.
For further information refer to: coronary circulation
Although the basic heart rate is set by the heart itself, the nerves of the autonomic nervous system modulate the heart's activity. The cardiac regulatory centers are located in the medulla oblongata of the brainstem. The sympathetic nervous system sends its modulating signals to the heart by means of the cardiac nerves. These nerves have their origins in the [[pars thoracica medullae spinalis|thoracic spinal cord] in the levels T1 to T5. Activation of these sympathetic nerves result in an increase in the heart rate (positive chronotropic effect), an increase the strength of the heart's muscle contraction (positive inotropic effect) and an increase in the speed of impulse conduction within the heart (positive dromotropic effect).
The parasympathetic nervous system modulates the heart through the vagus nerve (cranial nerve X) whose origin is in the brainstem's medulla oblongata. Activation of the vagus nerve will result in slowing of the heart rate (negative chronotropic effect), and usually also to weakening of the muscle contraction strength of the heart (negative inotropic effect) and to slowing of the conduction speed within the heart (negative dromotropic effect).
The primary function of the heart is to generate the propulsive force that pumps blood through the circulatory system of the body. The heart also assists in the regulation of the blood pressure. Increased blood volume causes greater stretching of cardiac muscle cells in the atria, and to some degree also in the ventricles. In response they release atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) that decrease blood volume and lower systemic blood pressure.
The heartbeat is made possible by the special anatomy and physiology of the cardiac muscle tissue. The heart's ability to rhythmically contract and relax in a coordinated manner derives from not only the cardiac muscle cells that can contract but also from specialized noncontractile cardiac cells that can produce and distribute action potentials throughout the heart. These noncontractile cells form the heart's intrinsic cardiac conduction system. Important components of this system are the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, the right and left bundle branches and the Purkinje fibers.
The amount of blood that the heart pumps out of each ventricle in one minute (ml/min) is known as the cardiac output. This quantity is calculated by multiplying the heart rate (beats/min) by the stroke volume (ml/beat):
Cardiac output (ml/min) = HR (beats/min) x SV (ml/beat)
where HR means heart rate, and SV means stroke volume. From this relationship it is clear that the cardiac output increases when heart rate increases or the stroke volume increases or both. Decreases in either or both factors result in a decrease in the cardiac output.
Using typical values for an adult at rest (HR = 72 beats/min and SV = 70 ml/beat) this equation shows that the typical cardiac output for an adult at rest is about 5040 ml/min or approximately 5 L/min. In trained athletes the cardiac output may increase to 35 L/min during competition.
The diseases and disorders of the heart belong to the domain of cardiology. Since the heart and the circulatory system are functionally interrelated, one often speaks of cardiovascular diseases or disorders. Some of the more common and important of these diseases are:
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