<unk> uman<unk>  during an autopsy Computer-generated animation of a beating<unk> uman<unk>  Cardiology video Location and shape Real-time MRI of the<unk> uman<unk>  The<unk> uman<unk>  is in the middle of the thorax, with its apex pointing to the left. The<unk> uman<unk>  is situated in the mediastinum, at the level of thoracic vertebrae T5-T8. A double-membraned sac called the pericardium surrounds the<unk>  and attaches to the mediastinum. The back surface of the<unk>  lies near the vertebral column, and the front surface sits behind the sternum and rib cartilages. The upper part of the<unk>  is the attachment point for several large blood vessels—the venae cavae, aorta and pulmonary trunk. The upper part of the<unk>  is located at the level of the third costal cartilage. The lower tip of the<unk> , the apex, lies to the left of the sternum (8 to 9 cm from the midsternal line) between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The largest part of the<unk>  is usually slightly offset to the left side of the chest (though occasionally it may be offset to the right) and is felt to be on the left because the left<unk>  is stronger and larger, since it pumps to all body parts. Because the<unk>  is between the lungs, the left lung is smaller than the right lung and<unk> as a cardiac notch in its border to accommodate the<unk> . The<unk>  is cone-shaped, with its base positioned upwards and tapering down to the apex. An adult<unk> <unk> as a mass of 250–350 grams (9–12 oz). The<unk>  is often described as the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness, although this description is disputed, as the<unk>  is likely to be slightly larger. Well-trained athletes can<unk> ave much larger<unk> s due to the effects of exercise on the<unk>  muscle, similar to the response of skeletal muscle. Chambers<unk>  being dissected showing right and left ventricles, from above The<unk> <unk> as four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum. This distinction is visible also on the surface of the<unk>  as the<unk> onary sulcus. There is an ear-shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage. The right atrium and the right ventricle together are sometimes referred to as the right<unk> . Similarly, the left atrium and the left ventricle together are sometimes referred to as the left<unk> . The ventricles are separated from each other by the interventricular septum, visible on the surface of the<unk>  as the anterior longitudinal sulcus and the posterior interventricular sulcus. The fibrous cardiac skeleton gives structure to the<unk> . It forms the atrioventricular septum, which separates the atria from the ventricles, and the fibrous rings, which serve as bases for the four<unk>  valves. The cardiac skeleton also provides an important boundary in the<unk> 's electrical conduction system since collagen cannot conduct electricity. The interatrial septum separates the atria, and the interventricular septum separates the ventricles. The interventricular septum is much thicker than the interatrial septum since the ventricles need to generate greater pressure when they contract. Valves Frontal section showing papillary muscles attached to the tricuspid valve on the right and to the mitral valve on the left via chordae tendineae. The<unk> <unk> as four valves, which separate its chambers. One valve lies between each atrium and ventricle, and one valve rests at the exit of each ventricle. The valves between the atria and ventricles are called the atrioventricular valves. Between the right atrium and the right ventricle is the tricuspid valve. The tricuspid valve<unk> as three cusps, which connect to chordae tendinae and three papillary muscles named the anterior, posterior, and septal muscles, after their relative positions. The mitral valve lies between the left atrium and left ventricle. It is also known as the bicuspid valve due to its<unk> aving two cusps, an anterior and a posterior cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall. The papillary muscles extend from the walls of the<unk>  to valves by cartilaginous connections called chordae tendinae. These muscles prevent the valves from falling too far back when they close. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. As the<unk>  chambers contract, so do the papillary muscles. This creates tension on the chordae tendineae,<unk> elping to<unk> old the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria. Two additional semilunar valves sit at the exit of each of the ventricles. The pulmonary valve is located at the base of the pulmonary artery. This<unk> as three cusps which are not attached to any papillary muscles. When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket-like valve, pressing against the cusps which close to seal the valve. The semilunar aortic valve is at the base of the aorta and also is not attached to papillary muscles. This too<unk> as three cusps which close with the pressure of the blood flowing back from the aorta. Right<unk>  The right<unk>  consists of two chambers, the right atrium and the right ventricle, separated by a valve, the tricuspid valve. The right atrium receives blood almost continuously from the body's two major veins, the superior and inferior venae cavae. A small amount of blood from the<unk> onary circulation also drains into the right atrium via the<unk> onary sinus, which is immediately above and to the middle of the opening of the inferior vena cava. In the wall of the right atrium is an oval-shaped depression known as the fossa ovalis, which is a remnant of an opening in the fetal<unk>  known as the foramen ovale. Most of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface<unk> as prominent ridges of pectinate muscles, which are also present in the right atrial appendage. The right atrium is connected to the right ventricle by the tricuspid valve. The walls of the right ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the lower part of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle. The right ventricle tapers into the pulmonary trunk, into which it ejects blood when contracting. The pulmonary trunk branches into the left and right pulmonary arteries that carry the blood to each lung. The pulmonary valve lies between the right<unk>  and the pulmonary trunk. Left<unk>  The left<unk> <unk> as two chambers: the left atrium and the left ventricle, separated by the mitral valve. The left atrium receives oxygenated blood back from the lungs via one of the four pulmonary veins. The left atrium<unk> as an outpouching called the left atrial appendage. Like the right atrium, the left atrium is lined by pectinate muscles. The left atrium is connected to the left ventricle by the mitral valve. The left ventricle is much thicker as compared with the right, due to the greater force needed to pump blood to the entire body. Like the right ventricle, the left also<unk> as trabeculae carneae, but there is no moderator band. The left ventricle pumps blood to the body through the aortic valve and into the aorta. Two small openings above the aortic valve carry blood to the<unk>  muscle; the left<unk> onary artery is above the left cusp of the valve, and the right<unk> onary artery is above the right cusp. Wall Layers of the<unk>  wall, including visceral and parietal pericardium The<unk>  wall is made up of three layers: the inner endocardium, middle myocardium and outer epicardium. These are surrounded by a double-membraned sac called the pericardium. The innermost layer of the<unk>  is called the endocardium. It is made up of a lining of simple squamous epithelium and covers<unk>  chambers and valves. It is continuous with the endothelium of the veins and arteries of the<unk> , and is joined to the myocardium with a thin layer of connective tissue. The endocardium, by secreting endothelins, may also play a role in regulating the contraction of the myocardium. The swirling pattern of myocardium<unk> elps the<unk>  pump effectivelyThe middle layer of the<unk>  wall is the myocardium, which is the
Heart
Kidney
Lung
Brain
Liver
Central nervous system
Peripheral nervous system
Skin