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20-404: The AV node lies at the lower back section of the interatrial septum near the opening of the coronary sinus , which conducts the normal electrical impulse from the atria to the ventricles. The AV node is quite compact (~1 x 3 x 5 mm). It is located at the center of Koch's triangle —a triangle enclosed by the septal leaflet of the tricuspid valve, the coronary sinus, and the membranous part of

40-472: Is commonly seen in Down syndrome . Typically, this defect will cause a shunt to occur from the left atrium to the right atrium. Children born with this defect may be asymptomatic, however, over time pulmonary hypertension and the resulting hypertrophy of the right side of the heart will lead to a reversal of this shunt. This reversal is called Eisenmenger syndrome . Lipomatous atrial septal hypertrophy (LASH)

60-451: Is important because loss of the conduction system before the AV node should still result in pacing of the ventricles by the slower pacemaking ability of the AV node. Interatrial septum The interatrial septum is the wall of tissue that separates the right and left atria of the heart . The interatrial septum is a septum that lies between the left atrium and right atrium of

80-419: Is the fat deposition in the infoldings of the interatrial septum adjacent to the true atrial septum. It is shaped like a “dumbbell” because the deposition is located at the above and the below the fossa ovalis, sparing the fossa itself. The incidence of LASH increases with older age and obesity. On CT scan, it shows homogeneous, dumbbell mass which is non-enhancing at the interatrial septum. MRI shows extension of

100-415: Is the gap between the septum primum and the septum intermedium , which gets progressively smaller until it closes. Failure of the septum primum to fuse with the endocardial cushion can lead to an ostium primum atrial septal defect . This is the second most common type of atrial septal defect and is commonly seen in Down syndrome . Typically, this defect will cause a shunt to occur from the left atrium to

120-480: Is unique to the AV node is decremental conduction , in which the more frequently the node is stimulated the slower it conducts. This is the property of the AV node that prevents rapid conduction to the ventricle in cases of rapid atrial rhythms, such as atrial fibrillation or atrial flutter . The AV node's normal intrinsic firing rate without stimulation (such as that from the SA node) is 40–60 times/minute. This property

140-407: The human heart . The interatrial septum lies at angle of 65 degrees from right posterior to left anterior because right atrium is located at the right side of the body while left atrium is located at the left side of the body. The interatrial septum represents the posterior wall of the right atrium. The interatrial septum forms during the first and second months of fetal development . Formation of

160-486: The pulmonary circulation is greater than that of the systemic circulation . Consequently, the right atrium is generally under higher pressures than the left atrium, and the valve of the foramen ovale is normally open. At birth, there is a reversal in the pressure gradient between the atria, resulting in functional closure of the valve of the foramen ovale. Permanent anatomical closure of the foramen ovale occurs with time in normal infants. Inappropriate failure of closure of

180-559: The sinoatrial node through the atria along specialized conduction channels. This activates the AV node. The atrioventricular node delays impulses by approximately 0.09s. This delay in the cardiac pulse is extremely important: It ensures that the atria have ejected their blood into the ventricles first before the ventricles contract. This also protects the ventricles from excessively fast rate response to atrial arrhythmias (see below). AV conduction during normal cardiac rhythm occurs through two different pathways: An important property that

200-426: The foramen ovale results in patent foramen ovale . An Atrial septal defect is a relatively common heart malformation that occurs when the interatrial septum fails to develop properly. Persistence of the ostium secundum is the most common atrial septal defect . Additionally, in a subset of the population, the foramen ovale is not overtly patent but the two septa have not fused. In normal physiologic circumstances,

220-465: The formation of the atrial septum . The foramen lies beneath the edge of septum primum and the endocardial cushions . It progressively decreases in size as the septum grows downwards, and disappears with the formation of the atrial septum . The septum primum, a septum which grows down to separate the primitive atrium into the left atrium and right atrium , grows in size over the course of heart development . The primary interatrial foramen

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240-404: The interatrial septum. The blood supply of the AV node is from the atrioventricular nodal branch . The origin of this artery is most commonly (80–90% of hearts) a branch of the right coronary artery , with the remainder originating from the left circumflex artery . This is associated with the dominance of the coronary artery circulation. In right-dominant individuals the blood supply is from

260-443: The mass into interventricular septum and ventricular wall. Ostium primum In the developing heart , the atria are initially open to each other, with the opening known as the primary interatrial foramen or ostium primum (or interatrial foramen primum ). The foramen lies beneath the edge of septum primum and the endocardial cushions . It progressively decreases in size as the septum grows downwards, and disappears with

280-451: The right atrium to the left. As the septum primum grows, the ostium primum progressively narrows. Before the ostium primum is completely occluded, a second opening called the ostium secundum begins to form in the septum primum . The ostium secundum allows continued shunting of blood from the right atrium to the left. To the right of the septum primum, the septum secundum begins to form. This thick, muscular structure initially takes on

300-409: The right atrium: posteriorly, via the crista terminalis , and anteriorly, via the interatrial septum . Contraction of heart muscle cells requires depolarization and repolarization of their cell membranes. Movement of ions across cell membranes causes these events. The cardiac conduction system (and AV node part of it) coordinates myocyte mechanical activity. A wave of excitation spreads out from

320-644: The right coronary artery while in left dominant individuals it originates from the left circumflex artery. Bone morphogenetic protein (BMP) cell signaling plays a key role in diverse aspects of cardiac differentiation and morphogenesis. (BMPs) are multifunctional signaling molecules critical for the development of AV node. BMP influences AV node development through Alk3 receptor (Activin receptor-like kinase 3). Abnormalities seen in BMP and Alk3 are associated with some cardiovascular diseases like Ebstein's anomaly and AV conduction disease. The AV node receives two inputs from

340-427: The same crescent shape as the septum primum, except that it originates anteriorly, whereas the septum primum originates posteriorly. As the septum secundum grows, it leaves a small opening called the foramen ovale . The foramen ovale is continuous with the ostium secundum, again providing for continued shunting of blood. The ostium secundum progressively enlarges and the size of the septum primum diminishes. Eventually,

360-428: The septum occurs in several stages. The first is the development of the septum primum , a crescent-shaped piece of tissue forming the initial divider between the right and left atria. Because of its crescent shape, the septum primum does not fully occlude the space between the left and right atria; the opening that remains is called the ostium primum . During fetal development, this opening allows blood to be shunted from

380-421: The septum primum acts as a one-way valve preventing blood flow as described above; but, if pathologic conditions cause right atrial pressure to exceed left atrial pressure, blood may flow through the foramen ovale from right to left. Failure of the septum primum to fuse with the endocardial cushion can lead to an ostium primum atrial septal defect . This is the second most common type of atrial septal defect and

400-450: The septum primum is nothing more than a small flap that covers the foramen ovale on its left side. This flap of tissue is called the valve of the foramen ovale. It opens and closes in response to pressure gradients between the left and right atria. When the pressure is greater in the right atrium, the valve opens; when the pressure is greater in the left atrium, the valve closes. Because the lungs are nonfunctional in fetal life, pressure in

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