The heart is the central organ of the cardiovascular system. The walls that enclose the hollow, four-chambered interior of the heart consist of three definable layers.

• Myocardium: The middle muscular layer.
• Epicardium: Exterior covering of the heart.
• Endocardium: Interior covering of the heart; This layer consists of endothelium supported by connective tissue and is continuous with the tunica intima of blood vessels entering and leaving the heart.
  
  EXTERNAL ANATOMY
  
  

  The heart is described as having a base and an apex. The base of the heart faces posteriorly and is made up mainly of the left atrium and a small portion of the right atrium. The left atrium lies behind (not to the left) of the right atrium. This is attributed mainly to the obliquity of the interatrial septum.

  
  

  From the base, the heart projects forward to terminate in the apex. The apex of the heart points downward and toward the left as well as forward. The apex is formed by the left ventricle.

  
  
  

  The heart is described as having four surfaces. These include:

  • Diaphragmatic (inferior)
  • Sternocostal (anterior
  • Left surface
  • Right surface
    
    

    The diaphragmatic surface extends from the base to the apex, facing inferiorly, and rests on the diaphragm. The coronary sulcus separates this surface from the base. This surface is formed mainly by the left ventricle and a small portion of the right ventricle. The ventricles are separated, externally, by the posterior interventricular sulcus, which is occupied by an artery and vein. This sulcus runs from the coronary sulcus toward the apex.
    

    TheSternocostal surfacefaces anteriorly and is mainly composed by the right ventricle. The right atrium is visible to the right and the left ventricle is visible to the left.

    
    

    The left and the right surfaces of the heart both face the lungs. The left is usually the only one described, sometimes, as the pulmonary surface. The right surface consists of the right atrium, and the left, the left ventricle.

    
    
    
    
    INTERNAL ANATOMY
    
    

    The RIGHT ATRIUM Receives blood from three main sources. In the upper posterior corner enters the superior vena cava, whereas the inferior vena cava and the coronary sinus open into the lower posterior corner. From the atrium, the blood is emptied into the right ventricle through the right atrioventricular ostium. The ostium is guarded by the right atrioventricular or tricuspid valve.

    The crista terminalis divides the interior of the atrium into two main parts. This is a smooth muscular ridge that resides on the roof of the atrium just in front of the opening of the superior vena cava. It also extends on the lateral wall of the chamber to the anterior lip of the inferior vena cava. The cavity posterior to the crista terminalis is called the sinus venarum cavarum. The two vena cava open into this section. The anterior half of the chamber is called the atrium proper. This contains the right auricle. The walls of the sinus venarum cavarum are smooth, as is the surface of the interatrial septum. The walls of the anterior half of the chamber, including the auricle, are ridged by the musculi pectinati. These structures fan out from the crista terminalis.

    
    

    The interatrial septum lies behind the right atrium as well as to its left. The fossa ovalis is the depression in the septum just above the orifice of the inferior vena cava. The prominent margin of the fossa ovalis is called the limbus fossae ovalis. This fossa marks the location of the foramen ovale. The foramen ovale is an aperture through which blood flows from the right to the left atrium before birth. Since the lungs are not functioning before birth the fetus has no need to pump blood through the right ventricle and to the lungs. Therefore, this serves as a bypass so blood can go directly to the left atrium ,the left ventricle and then to the rest of the body.

    
    
    

    The RIGHT VENTRICLE receives blood from the right atrium in a horizontal and forward direction. The reason for this is the right ventricle is situated in front and to the left of the right atrioventricular opening. The cavity of the right ventricle extends to the apex of the heart. It projects onto most of the sternocostal surface and along the anterior edge of the diaphragmatic surface. If you were to take a crosssection of this chamber it would be "C" shaped. The surface of most of the myocardium in the right ventricle is composed mainly of trabeculae carneae. These are prominent ridges which also give rise to papillary muscles. The smooth portion of the myocardium in the right ventricle is the conus arteriosus or infundibulum. this portion of the right ventricle houses the outflow tract of the ventricle. Blood enters the pulmonary trunk from this tract.

    
    

    At least one of the trabeculae carneae is elevated into a band that forms a ridge between the interventricular septum and the anterior wall close to the apex of the ventricle. This ridge is known as the moderator band. Along this band runs a branch of the atrioventricular bundle.

    
    

    There are three sets of papillary muscles located in the right ventricle. They are named the anterior, posterior, and septal according to the location of their base. From the end of each papillary muscle several tendonlike fibrous cords, called chordae tendinae extend to the cusps of the atriventricular valve. The papillary muscles and chordae tendinae function to prevent the cusps of the valve from being everted into the atrium by the pressure developed in the contracting ventricle. This increased pressure closes the valve, and as the ventricle becomes smaller during contraction, the papillary muscles also contract so that they maintain tension on the cusps.

    
    

    The tricuspid valve (also known as the right atrioventricular valve) is responsible for preventing the backflow of blood into the right atrium when the right ventricle contracts. As previously mentioned, the chordae tendinae attach to the ventricular surface of the cusps. When the valve opens the cusps project into the ventricle. The three cusps are named according to their positioning: anterior, posterior, and septal. These cusps consist of dense fibrous connective tissue covered by endocardium.

    
    

    The right venticle pumps its blood through the pulmonary valve. This valve consists of three semilunar valves or cusps. Each cusp is formed by the reduplication of endothelium with a small amount of fibrous connective tissue between the layers. The function of the pulmonary valve is to prevent the regurgitation of blood into the right ventricle after it contracts and relaxes. The valves are named according to their positioning: right, left, and anterior.

    The LEFT ATRIUM, similar to the right atrium, is divided into a posterior and an anterior half. The posterior half receives blood from the four pulmonary veins. The anterior portion is continuous with the left auricle and contains musculi pectinati on its wall. The interatrial septum forms part of the anterior wall of the left atrium. Similar to the right atrium, there is a thin area located on the septum called the valvule of the foramen ovale. This functions to prevent the backflow of blood from the right atrium to the left atrium in the fetus.

    The LEFT VENTRICLE lie in front of the left atrium. This means blood entering the left ventricle from this atrium will be directed toward the apex in a forward direction. This chamber contains the thickest myocardium. The left ventricle also contains trabeculae carne, however, they are more fine than the trabeculae carnea located in the right ventricle, which is much more rigid. The interventricular septum occupies the anterior and right-hand wall of the left ventricle. The thickest portion of this chamber is called the pars muscularis, and the thinnest portion the pars membranecea.

    
    

    The mitral or bicuspid valve prevents the backflow of blood from the left ventricle to the left atrium. It is composed of two cusps also named according to their positioning: anterior and posterior. Since there are two cusps there are also two papillary muscles located in the left ventricle. The chordae tendinae of each go to both of the cusps of the mitral valve.

    
    

    Blood leaves the left ventricle via the aortic valve. This valve is very similar to the pulmonary valve. It contains three cusps named: right, left, and posterior according to their positioning. Each aortic cusp will house a sinus. The sinuses are matched with their respective cusp. From the right and left sinus originates the right and left coronary arteries. The posterior sinus and valve are designated as the noncornary sinus and cusp.

    
    
    
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