Kia

Superior mesenteric vein

Portal vein

Left umbilical

Splenic vein

Left umbilical

Splenic vein

Figure 11.43 Development of vitelline and umbilical veins in the (A) second and (B) third months. Note formation of the ductus venosus, portal vein, and hepatic portion of the inferior vena cava. The splenic and superior mesenteric veins enter the portal vein.

vein (right hepatocardiac channel). Ultimately the right hepatocardiac channel forms the hepatocardiac portion of the inferior vena cava. The proximal part of the left vitelline vein disappears (Fig. 11.43, A and B). The anastomotic network around the duodenum develops into a single vessel, the portal vein (Fig. 11.43B). The superior mesenteric vein, which drains the primary intestinal loop, derives from the right vitelline vein. The distal portion of the left vitelline vein also disappears (Fig. 11.43, A and B).

Umbilical Veins

Initially the umbilical veins pass on each side of the liver, but some connect to the hepatic sinusoids (Fig. 11.42, A and B). The proximal part of both umbilical veins and the remainder of the right umbilical vein then disappear, so that the left vein is the only one to carry blood from the placenta to the liver (Fig. 11.43). With the increase of the placental circulation, a direct communication forms between the left umbilical vein and the right hepatocardiac channel, the duc-tus venosus (Fig. 11.43, A and B). This vessel bypasses the sinusoidal plexus of the liver. After birth the left umbilical vein and ductus venosus are obliterated and form the ligamentum teres hepatis and ligamentum venosum, respectively.

Cardinal Veins

Initially the cardinal veins form the main venous drainage system of the embryo. This system consists of the anterior cardinal veins, which drain the cephalic part of the embryo, and the posterior cardinal veins, which drain the rest of the embryo. The anterior and posterior veins join before entering the sinus horn and form the short common cardinal veins. During the fourth week, the cardinal veins form a symmetrical system (Fig. 11.44A).

During the fifth to the seventh week a number of additional veins are formed: (a) the subcardinal veins, which mainly drain the kidneys; (b) the sacrocardinal veins, which drain the lower extremities; and (c) the supracar-dinal veins, which drain the body wall by way of the intercostal veins, taking over the functions of the posterior cardinal veins (Fig. 11.44).

Formation of the vena cava system is characterized by the appearance of anastomoses between left and right in such a manner that the blood from the left is channeled to the right side.

The anastomosis between the anterior cardinal veins develops into the left brachiocephalic vein (Fig. 11.44, A and B). Most of the blood from the left side of the head and the left upper extremity is then channeled to the right. The terminal portion of the left posterior cardinal vein entering into the left brachiocephalic vein is retained as a small vessel, the left superior intercostal vein (Fig. 11.44B). This vessel receives blood from the second and third intercostal spaces. The superior vena cava is formed by the right common cardinal vein and the proximal portion of the right anterior cardinal vein.

The anastomosis between the subcardinal veins forms the left renal vein. When this communication has been established, the left subcardinal vein disappears, and only its distal portion remains as the left gonadal vein. Hence the right subcardinal vein becomes the main drainage channel and develops into the renal segment of the inferior vena cava (Fig. 11.44B).

The anastomosis between the sacrocardinal veins forms the left common iliac vein (Fig. 11.44B). The right sacrocardinal vein becomes the

Anterior cardinal vein

Common cardinal vein

Posterior cardinal vein

Subcardinal vein

Anastomosis anterior cardinal veins

Anastomosis anterior cardinal veins

Anterior cardinal vein

Common cardinal vein

Posterior cardinal vein

Renal segment inferior vena cava

Azygos Vein

. Left renal vein renal vein

Left gonadal vein

Superior vena cava Supracardinal vein

Azygos vein -

Hepatic segment inferior vena cava

Hepatic segment

Renal segment inferior vena cava

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