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Development of the Sinus Venosus
![Picture](/uploads/9/5/1/4/9514123/2201860.png?347)
Figure 1
The heart starts to beat on day 22, and by day 24, blood begins to circulate throughout the embryo. Venous return initially enters the right and left sinus horns via the common cardinal veins, where the horns receive blood from the vitelline and umbilical veins as well.
With obliteration of the right umbilical vein and the left vitelline vein during the fifth week, the left sinus horn rapidly loses its importance. When the left common cardinalvein is obliterated at 10 weeks, all that remains of the left sinus horn is the oblique vein of the left atrium
and the coronary sinus.
Within the next few weeks, the venous system is remodeled so that all systemic venous blood enters the right sinus horn via the superior and inferior venae cava as shown in figure 1. This shift is caused primarily by left-to-right shunts of blood, which occur in the venous system during the fourth and fifth weeks of development.
With obliteration of the right umbilical vein and the left vitelline vein during the fifth week, the left sinus horn rapidly loses its importance. When the left common cardinalvein is obliterated at 10 weeks, all that remains of the left sinus horn is the oblique vein of the left atrium
and the coronary sinus.
Within the next few weeks, the venous system is remodeled so that all systemic venous blood enters the right sinus horn via the superior and inferior venae cava as shown in figure 1. This shift is caused primarily by left-to-right shunts of blood, which occur in the venous system during the fourth and fifth weeks of development.
![Picture](/uploads/9/5/1/4/9514123/6587816.png?389)
Figure 2
As a result of left-to-right shunts of blood, the right sinus horn and veins enlarge greatly. The right horn, which now forms the only communi-cation between the original sinus venosus and the atrium, is incorporated into the right atrium to form the smooth-walled part of the right atrium as shown in figure 2.
Its entrance, the sinuatrial orifice, is flanked on each side by a valvular fold, the right and left venous valves.
Dorso-cranially, the valves fuse, forming a ridge known as the septum spurium (Fig. 2 A). Initially the valves are large, but when the right sinus hornis incorporated into the wall of the atrium, the left venous valve and the septum spurium fuse with the developing atrial septum.
The superior portion of the right venous valve disappears entirely. The inferior portion develops intotwo parts: (a) the valve of the inferior vena cava and (b) the valve of the coronary sinus.The crista terminalis forms thedividing line between the original trabeculated part of the right atrium and the smooth-walled sinus venarum.
We recommend taking this quiz that will help you evaluate your knowledge after taking the tutorial.
Prepared by Joy Y. Balta