The atrium

In Embryology of the atrium, The primordial atrium is represented in the adult by the right auricle. The definitive atrium is enlarged by the incorporation of most of the embryonic sinus Denosus (Fig. B1.19A-C). The coronary sinus is also a derivative of this venous sinus. The part of the venous sinus incur-separated in the primordial atrium becomes the smooth-walled sinus venarum of the adult right atrium into which all the veins drain, including the Coro-nary sinus. The line of fusion of the primordial atrium (the adult auricle) and the sinus venarum (the derivative of the venous sinus) is indicated internally by the crista terminals and externally by the sulcus terminalis. The sinoatrial (SA)node (discussed earlier in this chapter) is located just in front of the opening of the SVC at the superior end of the crista terminals--that is, in the border between the primordial atrium and the sinus venosus; hence its name. Before birth, the valve of the IVC directs most of the oxygenated blood returning from the placenta in the umbilical-cal vein and IVC toward the oval foramen in the interatrial the septum, through which it passes into the left atrium The oval foramen has a flap-like valve that per-it's a right to the left shunt of blood but prevents a left too. At birth, when the baby takes its first breath, the lungs expand with air and pressure in the right atrium falls below that in the left atrium (Fig. B1.19E). Consequently, the oval foramen closes for its first and last time, and its valve usually fuses with the interatrial septum. The closed oval fora-men are represented in the postnatal interatrial septum by the depressed oval fossa. The border of the oval fossa (L.limbus fossae ovalis) surrounds the fossa. The floor of the fossa is formed by the valve of the oval foramen. The rudimentary IVC valve, a semilunar crescent of tissue, has no function after birth; it varies considerably in size and is occasionally absent.