Structure of the<unk> s Blood flow through the<unk> s The<unk> s and the chambers are lined with endocardium.<unk> s separate the atria from the ventricles, or the ventricles from a blood vessel.<unk> s are situated around the fibrous rings of the cardiac skeleton. The<unk> s incorporate flaps called leaflets or cusps, similar to a duckbill<unk>  or flutter<unk> , which are pushed open to allow blood flow and which then close together to seal and prevent backflow. The mitral<unk>  has two cusps, whereas the others have three. There are nodules at the tips of the cusps that make the seal tighter. The pulmonary<unk>  has left, right, and anterior cusps. The aortic<unk>  has left, right, and posterior cusps. The tricuspid<unk>  has anterior, posterior, and septal cusps; and the mitral<unk>  has just anterior and posterior cusps. The<unk> s of the human<unk>  can be grouped in two sets: * Two atrioventricular<unk> s to prevent backflow of blood from the ventricles into the atria: * Tricuspid<unk>  or right atrioventricular<unk> , between the right atrium and right ventricle * Mitral<unk>  or bicuspid<unk> , between the left atrium and left ventricle * Two semilunar<unk> s to prevent the backflow of blood into the ventricle: * Pulmonary<unk> , located at the opening between the right ventricle and the pulmonary trunk * Aortic<unk> , located at the opening between the left ventricle and the aorta.<unk>  Number of flaps/cusps location prevent backflow of blood Atrioventricular<unk> s 3 or 2 From the ventricles into the atria Tricuspid<unk>  3 between the right atrium and right ventricle. Bicuspid or mitral<unk>  2 between the left atrium and left ventricle. Semilunar<unk> s 3 (half-moon shaped) flaps into the ventricle Pulmonary semilunar<unk>  3 (half-moon shaped) flaps at the opening between the right ventricle and the pulmonary trunk Aortic semilunar<unk>  3 (half-moon shaped) flaps at the opening between the left ventricle and the aorta Atrioventricular<unk> s 3D - loop of a<unk>  viewed from the apex, with the apical part of the ventricles removed and the mitral<unk>  clearly visible. Due to missing data, the leaflets of the tricuspid and aortic<unk> s are not clearly visible, but the openings are; the pulmonary<unk>  is not visible. On the left are two standard 2D views (taken from the 3D dataset) showing tricuspid and mitral<unk> s (above) and aortal<unk>  (below). These are the mitral and tricuspid<unk> s, which are situated between the atria and the ventricles and prevent backflow from the ventricles into the atria during systole. They are anchored to the walls of the ventricles by chordae tendineae, which prevent the<unk> s from inverting. The chordae tendineae are attached to papillary muscles that cause tension to better hold the<unk> . Together, the papillary muscles and the chordae tendineae are known as the subvalvular apparatus. The function of the subvalvular apparatus is to keep the<unk> s from prolapsing into the atria when they close. The subvalvular apparatus has no effect on the opening and closure of the<unk> s, however, which is caused entirely by the pressure gradient across the<unk> . The peculiar insertion of chords on the leaflet free margin, however, provides systolic stress sharing between chords according to their different thickness. The closure of the AV<unk> s is<unk>  as lub, the first<unk>  sound (S1). The closure of the SL<unk> s is<unk>  as dub, the second<unk>  sound (S2). The mitral<unk>  is also called the bicuspid<unk>  because it contains two leaflets or cusps. The mitral<unk>  gets its name from the resemblance to a bishop's mitre (a type of hat). It is on the left side of the<unk>  and allows the blood to flow from the left atrium into the left ventricle. During diastole, a normally-functioning mitral<unk>  opens as a result of increased pressure from the left atrium as it fills with blood (preloading). As atrial pressure increases above that of the left ventricle, the mitral<unk>  opens. Opening facilitates the passive flow of blood into the left ventricle. Diastole ends with atrial contraction, which ejects the final 30% of blood that is transferred from the left atrium to the left ventricle. This amount of blood is known as the end diastolic volume (EDV), and the mitral<unk>  closes at the end of atrial contraction to prevent a reversal of blood flow. The tricuspid<unk>  has three leaflets or cusps and is on the right side of the<unk> . It is between the right atrium and the right ventricle, and stops the backflow of blood between the two. Semilunar<unk> s The aortic and pulmonary<unk> s are located at the base of the aorta and the pulmonary trunk respectively. These are also called the "semilunar<unk> s". These two arteries receive blood from the ventricles and their semilunar<unk> s permit blood to be forced into the arteries, and prevent backflow from the arteries into the ventricles. These<unk> s do not have chordae tendineae, and are more similar to the<unk> s in veins than they are to the atrioventricular<unk> s. The closure of the semilunar<unk> s causes the second<unk>  sound. The aortic<unk> , which has three cusps, lies between the left ventricle and the aorta. During ventricular systole, pressure rises in the left ventricle and when it is greater than the pressure in the aorta, the aortic<unk>  opens, allowing blood to exit the left ventricle into the aorta. When ventricular systole ends, pressure in the left ventricle rapidly drops and the pressure in the aorta forces the aortic<unk>  to close. The closure of the aortic<unk>  contributes the A2 component of the second<unk>  sound. The pulmonary<unk>  (sometimes referred to as the pulmonic<unk>  lies between the right ventricle and the pulmonary artery, and has three cusps. Similar to the aortic<unk> , the pulmonary<unk>  opens in ventricular systole, when the pressure in the right ventricle rises above the pressure in the pulmonary artery. At the end of ventricular systole, when the pressure in the right ventricle falls rapidly, the pressure in the pulmonary artery will close the pulmonary<unk> . The closure of the pulmonary<unk>  contributes the P2 component of the second<unk>  sound. The right<unk>  is a low-pressure system, so the P2 component of the second<unk>  sound is usually softer than the A2 component of the second<unk>  sound. However, it is physiologically normal in some young people to<unk> both components separated during inhalation. Development In the developing<unk> , the<unk> s between the atria and ventricles, the bicuspid and the tricuspid<unk> s, develop on either side of the atrioventricular canals. The upward extension of the bases of the ventricles causes the canal to become invaginated into the ventricle cavities. The invaginated margins form the rudiments of the lateral cusps of the AV<unk> s. The middle and septal cusps develop from the downward extension of the septum intermedium. The semilunar<unk> s (the pulmonary and aortic<unk> s) are formed from four thickenings at the cardiac end of the truncus arteriosus. These thickenings are called endocardial cushions. The truncus arteriosus is originally a single outflow tract from the embryonic<unk>  that will later split to become the ascending aorta and pulmonary trunk. Before it has split, four thickenings occur. There are anterior, posterior, and two lateral thickenings. A septum begins to form between what will later become the ascending aorta and pulmonary tract. As the septum forms, the two lateral thickenings are split, so that the ascending aorta and pulmonary trunk have three thickenings each (an anterior or posterior, and half of each of the lateral thickenings). The thickenings are the origins of the three cusps of the semilunar<unk> s. The<unk> s are visible as unique structures by the ninth week. As they mature, they rotate slightly as the outward vessels spiral, and move slightly closer to the<unk> .
Heart valve
Coronary circulation
Endobronchial valve
Pericardial heart valves
Electrical conduction system of the heart
Cranial nerves
Soft tissue
Cardiac cycle