Formation of the Ventricles

Fate of the Sinus Venosus (Formation of the Right Atrium)

Figure 1

Unlike the atria, the sinus vinosus remains a paired structure with right and left horns. Each horn receives venous blood from three vessels:

• 1. Vitelline vein
• 2. Umbilical vein
• 3. Commom cardinal vein

Communication between the sinus venosus and the primitive atrium, the sinoatrial oriface, is centrally located.

• VV = Vitelline vein
• UV = Umbilical vein
• CC = Common cardinal vein
• SA = Sinoatrial oriface
• RSH, LSH = right, left sinus horn
• RA, LA = right, left atrium

Figure 2

Gradually the sinoatrial oriface shifts to the right, due to the shunting of blood to the right, until the sinus venosus communicates with only the right atrium. The fate of each structure is as follows:

• the right sinus horn becomes enlarged
• the right anterior cardinal vein becomes the superior vena cava.
• the right vitelline vein becomes the inferior vena cava
• the right umbilical vein is obliterated

Conversely, the left vein counterparts are obliterated and the left sinus horn diminishes in size and forms the coronary sinus and the oblique vein of the left ventricle.

• SVC = Superior vena cava
• IVC = Inferior vena cava
• SA = Sinoatrial junction
• CS/OV = Coronary sinus/ oblique vein of left ventricle
• LA, RA = Left, right atrium
• LV, RV = Left, right ventricle

Figure 3

Internally, the sinoatrial oriface is flanked by two valves, the right and left venous valves. Superiorly these two valves meet to form the septum spurium. Note that the left horn opens up underneath the oriface of the right horn (sinoatrial oriface). This is the orifice of the coronary sinus.

• LVV, RVV = left, right venous valve
• SS = Septum spurium
• SA = Sinoatrial oriface
• OCS = oriface of the coronary sinus

Figure 4

Further into development the right sinus horn is incorperated into the expanding right atium. As the atrium expands the smooth tissue of the sinus venosus displaces the trabeculated tissue of the primitive right atium anteriorly and laterally where it becomes the adult right auricle. The smooth tissue forms part of the atium called the sinus venarum. Crista Terminalis, a ridge of tissue located to the right of the sinoatrial oriface, forms the boundry between the auricle and the sinus venarum.

• RAu = Right auricle
• SV = Sinus venarum
• CT = Crista terminalis
• OCS = Oriface of the coronary sinus