Atrio-ventricular Orifices
The Interventricular Septum
It corresponds to the anterior and inferior interventricular grooves. Its right side is convex and bulges to the right ventricle. The septum has a posterior border which separates the right and left AV orifices and is continuous with the interatrial septum. The greater part of the septum is thick and muscular but a small area near the posterior margin is membranous. The septal cusp of the tricuspid valve is attached vertically on this part of the septum and divides it into an anterior part that separates the right and left ventricles and a posterior part which separates the left ventricle from the right atrium. The latter part is therefore, referred to as the atrioventricular septum.
The right and left atrio-ventricular orifices are oval apertures. The openings lie in a plane that is almost vertical with a slight downward inclination. Each opening is directed forwards then left and slightly downwards. Each orifice is strengthened by a variable amount of fibrous tissue around it. It is guarded by a valve which allows flow of blood,only from atrium to ventricle but not in the reverse direction. The valves are made up of cusps which are thin leaflets of tissue. Each cusp consists of a double fold of endocardium within which is some fibrous tissue. It has two surfaces, ventricular and atrial. It has a base which is attached to the ring of fibrous tissue around the Atrio-ventricular orifice. It has an apex and free margins. The margins of adjoining cusps are fused to each other for some distance so that the cups form a continuous membrane. The apex and the margins give attachment to tendinous strands called the chordae tendinae. The chordae tendinae are also attached to the ventricular surfaces of the cusps which are therefore, rough in contrast to the atrial surface which are smooth. At their other end the chordae tendinae are attached to the apices of the papillary muscles. Each papillary muscle through the chordae tendinae is attached to the adjoining part of the two cusps. As a result the adjoining margins of the two cusps are drawn together when the papillary muscle contracts.
The features peculiar to each orifice are: the right orifice is larger than the left; it admits the tips of three fingers. In contrast the left orifice is smaller and admits the tips of two fingers. The arrangements of cusps and papillary muscles also differ.
The Tricuspid Valve
It is made up of three cusps, anterior, posterior and septal. The septal cusp is attached to the medial margin. The anterior cusp is attached to the superior-lateral part of the margin.The posterior cusp is attached to the inferio-lateral part of the margin. The anterior cusp separates the inflow part of the right ventricle from the infundibulum.The chordae tendinae attached to these cusps arise from a large anterior papillary muscle,a large posterior papillary muscle and directly from the interventricular septum or from small papillary muscles attached to the septum. The chordae tendinae arising from the anterior papillary muscle are attached to the anterior and posterior cusps, those from the posterior muscle to the posterior and septal cusps and those from the septal muscles to the anterior and septal cusps.The base of the anterior muscle is attached to the sternocostal wall of the right ventricle.The base of the posterior muscle is attached near the angle between the diaphragmatic wall and the interventricular septum. The base of the anterior papillary muscle is connected to the interventricular septum by a special band called the septo-marginal trabecula or the moderator band.
The features peculiar to each orifice are: the right orifice is larger than the left; it admits the tips of three fingers. In contrast the left orifice is smaller and admits the tips of two fingers. The arrangements of cusps and papillary muscles also differ.
The Tricuspid Valve
It is made up of three cusps, anterior, posterior and septal. The septal cusp is attached to the medial margin. The anterior cusp is attached to the superior-lateral part of the margin.The posterior cusp is attached to the inferio-lateral part of the margin. The anterior cusp separates the inflow part of the right ventricle from the infundibulum.The chordae tendinae attached to these cusps arise from a large anterior papillary muscle,a large posterior papillary muscle and directly from the interventricular septum or from small papillary muscles attached to the septum. The chordae tendinae arising from the anterior papillary muscle are attached to the anterior and posterior cusps, those from the posterior muscle to the posterior and septal cusps and those from the septal muscles to the anterior and septal cusps.The base of the anterior muscle is attached to the sternocostal wall of the right ventricle.The base of the posterior muscle is attached near the angle between the diaphragmatic wall and the interventricular septum. The base of the anterior papillary muscle is connected to the interventricular septum by a special band called the septo-marginal trabecula or the moderator band.
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| Base of ventricles exposed by removal of the atria |
The Mitral Valve
This valve has the same basic features as of the Tricuspid Valve. It has an anterior and a posterior cusp. The anterior cusp is larger and is attached on the upper right part of the margin of the left AV orifice. The posterior cusp is attached to the lower left part. The anterior cusp intervenes between the Mitral and Aortic orifices. There is, therefore, a forceful blood flow on both surfaces of this cusp. The papillary muscles connected to the cusp of the mitral valve are also called the anterior and posterior. These terms are misleading. The anterior papillary muscle arises from the sterno-costal wall of the ventricle near the lower end. The posterior papillary muscle arises from the diaphragmatic wall near its anterior end. The two muscles run backwards almost parallel to each other and their origins are close together. The chordae tendinae arising from these papillary muscles pass to the adjoining part of the two cusps of the mitral valve.
The Aortic and Pulmonary Orifices
These circular orifices are located at the upper ends of the outflow parts of the left and right ventricles respectively. The pulmonary orifice which is 3 cms, is 0.5 cm larger than the aortic orifice. The aortic orifice is placed in front and to the right of the mitral orifice.The pulmonary orifice is placed above and to the left of the tricuspid orifice and the aortic orifice intervening between them. The pulmonary orifice is guarded by the pulmonary valve and the aortic orifice by the aortic valve. Each valve consists of three semilunar cusps. Each cusp is triangular. It has a convex edge which is attached to the part of the margin of the orifice. It has two free margins that meet at the apex of the cusp. Each cusp consists of a double fold of endocardium with some fibrous tissue in it. The region of the apex is thickened to form a nodule while crescentric parts near the free edges are called lunules.
This valve has the same basic features as of the Tricuspid Valve. It has an anterior and a posterior cusp. The anterior cusp is larger and is attached on the upper right part of the margin of the left AV orifice. The posterior cusp is attached to the lower left part. The anterior cusp intervenes between the Mitral and Aortic orifices. There is, therefore, a forceful blood flow on both surfaces of this cusp. The papillary muscles connected to the cusp of the mitral valve are also called the anterior and posterior. These terms are misleading. The anterior papillary muscle arises from the sterno-costal wall of the ventricle near the lower end. The posterior papillary muscle arises from the diaphragmatic wall near its anterior end. The two muscles run backwards almost parallel to each other and their origins are close together. The chordae tendinae arising from these papillary muscles pass to the adjoining part of the two cusps of the mitral valve.
The Aortic and Pulmonary Orifices
These circular orifices are located at the upper ends of the outflow parts of the left and right ventricles respectively. The pulmonary orifice which is 3 cms, is 0.5 cm larger than the aortic orifice. The aortic orifice is placed in front and to the right of the mitral orifice.The pulmonary orifice is placed above and to the left of the tricuspid orifice and the aortic orifice intervening between them. The pulmonary orifice is guarded by the pulmonary valve and the aortic orifice by the aortic valve. Each valve consists of three semilunar cusps. Each cusp is triangular. It has a convex edge which is attached to the part of the margin of the orifice. It has two free margins that meet at the apex of the cusp. Each cusp consists of a double fold of endocardium with some fibrous tissue in it. The region of the apex is thickened to form a nodule while crescentric parts near the free edges are called lunules.
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| Aorta laid open to show the semilunar valves |
The Interventricular Septum
It corresponds to the anterior and inferior interventricular grooves. Its right side is convex and bulges to the right ventricle. The septum has a posterior border which separates the right and left AV orifices and is continuous with the interatrial septum. The greater part of the septum is thick and muscular but a small area near the posterior margin is membranous. The septal cusp of the tricuspid valve is attached vertically on this part of the septum and divides it into an anterior part that separates the right and left ventricles and a posterior part which separates the left ventricle from the right atrium. The latter part is therefore, referred to as the atrioventricular septum.
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| Section of the heart showing the ventricular septum |
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