5.3. Veins That Drain Superiorly
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The inferior vena cava drains the blood from the abdomen, pelvis and lower limbs. It drains into the right atrium of the heart. The IVC is absent in the diagram, but if sketched in it would overlie the azygous vein, obscuring it from view. Although the IVC is a major thoracic feature, it does not directly drain much blood from the thorax. The aortic arch has been added for reference only.
REMEMBER that the 1st – 11th posterior intercostal veins drain the posterior aspects of the 1st – 11th intercostal spaces. However, these posterior intercostal veins then drain into a variety of other veins, some of which have already been mentioned. On each side, the supreme/highest intercostal vein drains the 1st posterior intercostal vein. Each drains into the brachiocephalic vein of that side. On each side, the superior intercostal vein on each side drains the 2nd, 3rd and 4th posterior intercostal veins into...the left brachiocephalic vein on the left and...the azygous vein on the right.
ON THE RIGHT, the azygous vein drains the 4th – 11th right posterior intercostal veins and the subcostal vein. The azygous vein is only present on the right, and runs up the right anterolateral aspect of the vertebral column.
ON THE LEFT, the accessory hemiazygous vein (also called the superior hemiazygous vein) drains the 5th – 8th posterior intercostal veins.
ALSO ON THE LEFT, the hemiazygous vein (also called the inferior hemiazygous vein) drains the 9th – 11th left posterior intercostal veins and the subcostal vein.
Note that there is no 12th posterior intercostal vein. The vein below the 12th rib is called the subcostal vein.
The azygous vein forms an anastomosis: it actually drains into both the IVC and the superior vena cava (SVC), although this is not clearly shown on the diagram. If one of these vessels is blocked, blood can still be returned to the heart.