What is "sinus venosus"

Sinus \Si"nus\, n.; pl. L. Sinus, E. Sinuses. [L., a bent surface, a curve, the folds or bosom of a garment, etc., a bay. Cf. Sine, n.]

1. An opening; a hollow; a bending.

2. A bay of the sea; a recess in the shore.

3. (Anat. & Zo["o]l.) A cavity; a depression. Specifically:

   1. A cavity in a bone or other part, either closed or with a narrow opening.

   2. A dilated vessel or canal.

4. (Med.) A narrow, elongated cavity, in which pus is collected; an elongated abscess with only a small orifice.

5. (Bot.) A depression between adjoining lobes. Note: A sinus may be rounded, as in the leaf of the white oak, or acute, as in that of the red maple. Pallial sinus. (Zo["o]l.) See under Pallial. Sinus venosus. [L., venous dilatation.] (Anat.)

   1. The main part of the cavity of the right auricle of the heart in the higher vertebrates.

   2. In the lower vertebrates, a distinct chamber of the heart formed by the union of the large systematic veins and opening into the auricle.

The sinus venosus is a large quadrangular cavity which precedes the atrium on the venous side of the chordate heart. In mammals, it exists distinctly only in the embryonic heart (where it is found between the two venae cavae); however, the sinus venosus persists in the adult. In the adult, it is incorporated into the wall of the right atrium to form a smooth part called the sinus venarum, also known as the venarum sinus, which is separated from the rest of the atrium by a ridge of fibres called the crista terminalis. The sinus venosus also forms the SA node and the coronary sinus.

In the embryo, the thin walls of the sinus venosus are connected below with the right ventricle, and medially with the left atrium, but are free in the rest of their extent. It receives blood from the vitelline vein, umbilical vein and common cardinal vein.

It originally starts as a paired structure but shifts towards associating only with the right atrium as the embryonic heart develops. The left portion shrinks in size and eventually forms the coronary sinus (right atrium) and oblique vein of the left atrium, whereas the right part becomes incorporated into the right atrium to form the sinus venarum.