Cranial vena cava

The cranial vena cava in domestic animals is a large, unpaired venous trunk responsible for returning deoxygenated blood from the head, neck, thoracic limbs, and cranial thoracic wall to the right atrium of the heart. It is formed by the confluence of the right and left brachiocephalic veins at the thoracic inlet. It courses caudally along the right side of the mediastinum, ventral to the trachea and esophagus, and enters the right atrium at the base of the heart.

Course:

The cranial vena cava is formed by the union of the right and left brachiocephalic veins, which drain blood from the head, neck, thoracic limbs, and part of the thoracic wall. This junction occurs near the thoracic inlet, typically around the level of the first rib or manubrium. It proceeds caudally and slightly to the right within the cranial mediastinum. It enters the pericardial sac and terminates at the right atrium of the heart by entering at the cranial aspect of the sinus venarum.

Tributaries

The cranial vena cava receives several tributaries along its course:

• Brachiocephalic veins: These are the principal tributaries formed by:

  • External jugular veins (main drainage of the head and neck)

  • Subclavian veins (draining the forelimbs)

  • Occasionally includes the internal jugular and vertebral veins (species-dependent)

• Vertebral vein (sometimes joins the brachiocephalic veins before forming the vena cava)

• Costocervical trunks: Drain blood from dorsal intercostal spaces and deep neck regions

• Bronchoesophageal veins

• Internal thoracic veins: Run along the ventral thoracic wall

• Azygos vein:

  • Dog and cat: Only the right azygos vein usually drains into the cranial vena cava (sometimes directly into the right atrium).

  • Horse and pig: The left azygos vein is often present and drains into the coronary sinus or right atrium.

  • Ruminants: May have both right and left azygos veins. One may drain into the vena cava, while the other terminates in the coronary sinus.

Species-Specific Notes

• Dog and Cat: Cranial vena cava is relatively short and consistently formed by the brachiocephalic veins. The right azygos vein is the main dorsal contributor.

• Horse: Cranial vena cava receives drainage from the left azygos vein, and sometimes it may enter the right atrium directly. The jugular veins are prominent, and the cranial vena cava is relatively robust.

• Ruminants: Often have both azygos veins (right and left); both may contribute to thoracic drainage. The jugular vein anastomosis forms a jugular groove prominent in cattle.

• Pig: The left azygos vein is usually the main dorsal tributary. The internal thoracic and costocervical veins are particularly large in swine.