Sinus Venosus

Notes

Key points

Between days 24-56 (approximately), asymmetrical heart and vascular remodeling ensures that all systemic blood flows into the right atrium.
Ultimately:

Features on Day 24:

Primitive left and right atria and ventricles
Outflow tract arises ventrally, and the singular orifice of the pulmonary vein drains into the area of the left atrium.

The sinus venosus bilaterally drains the following veins:

Features on Day 50:

Outcroppings of the atria, called auricles, are forming.
The outflow tract has separated and formed the pulmonary trunk and arteries, which delivers blood to the pulmonary circulation.
The aorta sends blood into the systemic circulation.
The orifice of the pulmonary vein now comprises four separate entrances; recall that two right and two left pulmonary veins return blood to the adult heart.

Sinus venosus and tributaries:

The left vitelline and proximal portions of both umbilical veins are obliterated (the distal portion of the left umbilical veins persists and anastomoses with the ductus venosus to bypass the liver).
Both posterior cardinal veins, and the left anterior cardinal vein are in the process of obliterating.
As the left anterior cardinal vein degenerates, its branches (the thymic and thyroid veins) form anastomoses with those of the right anterior cardinal vein.

Features on Day 56:

Left horn becomes the coronary sinus and oblique vein of the left atrium.
Inferior vena cava comprises remnants of the right vitelline vein; for reference, sub- and supra-cardinal veins also contribute to this section of the inferior vena cava.
The right anterior cardinal vein gives rise to the superior vena cava, and, via anastomoses with the left anterior cardinal vein, the left brachiocephalic vein.

Each brachiocephalic vein drains blood from the jugular veins, which are also derived from the anterior cardinal veins, and, the subclavian veins, which drain the upper extremities.

Summary of embryonic veins and their adult derivatives.

Sinus venosus is incorporated into the heart as the sinus venarum of the right heart; in the adult, it is identified by its smooth surface.
Right vitelline vein gives rise to a portion of the inferior vena cava, and the left vitelline vein has no adult derivatives.
Right umbilical vein degenerates.
After birth, the portion of the of the left umbilical vein that connected with the ductus venosus degenerates to form the ligamentum teres hepatis.
Via anastomoses, the anterior cardinal veins give rise to the superior vena cava, left brachiocephalic vein, and the jugular veins.
The posterior cardinal veins have no adult derivatives.

Bear in mind that the venous circulation is even more variable than the arteries:

A common variation in the sidedness or number of superior vena cavae results when the left anterior cardinal vein persists:

If the right anterior cardinal vein also persists, double superior vena cava result;
If the right anterior cardinal vein degenerates, the superior vena cava will form on the left from the left anterior cardinal vein.