Vena Cava - Function, Structure, Anatomy, Diagram

The venae cavae are the two largest systemic veins in the human body, responsible for returning deoxygenated blood from the body to the right atrium of the heart. They are composed of the superior vena cava (SVC) and the inferior vena cava (IVC), each serving a distinct region. The SVC drains the head, neck, upper limbs, and upper thorax, while the IVC drains the abdomen, pelvis, and lower limbs.

Superior Vena Cava

Origin and Course

The superior vena cava is formed by the union of the right and left brachiocephalic veins behind the lower border of the right first costal cartilage. From there, it descends vertically along the right side of the superior mediastinum and enters the pericardial sac at the level of the second costal cartilage, draining into the upper posterior wall of the right atrium.

Length and Location

Length: Approximately 7 cm
Position: Entirely within the thorax

Relations

Anterior: Thymus, right lung and pleura, sternum
Posterior: Right pulmonary artery, right main bronchus
Right: Right phrenic nerve, right lung and pleura
Left: Ascending aorta

Tributaries

Right and left brachiocephalic veins
Azygos vein (joins SVC just before it enters the pericardium)

Inferior Vena Cava

Origin and Course

The inferior vena cava is formed by the union of the right and left common iliac veins at the level of L5. It ascends retroperitoneally on the right side of the vertebral column, passes through the caval opening of the diaphragm at T8, and enters the thoracic cavity for a short distance before emptying into the lower posterior wall of the right atrium.

Length and Location

Length: Approximately 22–24 cm
Abdominal portion: Majority of its length
Thoracic portion: Very short, just before entering the right atrium

Relations (Abdominal Portion)

Anterior: Duodenum (3rd part), head of pancreas, liver
Posterior: Right psoas major, right renal artery, lumbar vertebrae
Right: Right kidney and adrenal gland
Left: Aorta

Relations (Thoracic Portion)

Passes through the central tendon of the diaphragm at the T8 level (caval opening)
Lies in the fibrous pericardium before entering the right atrium

Tributaries

Function

Both the superior and inferior vena cava serve as major conduits for the return of deoxygenated blood to the heart:

Superior vena cava: Returns blood from the head, neck, upper limbs, and thorax
Inferior vena cava: Returns blood from the lower limbs, pelvis, and abdomen

Histological Structure

The walls of both venae cavae are thin compared to arteries, with large lumens and fewer elastic fibers. They are composed of three layers:

Tunica intima: Endothelium and subendothelial layer
Tunica media: Sparse smooth muscle
Tunica adventitia: Thickest layer with collagen and vasa vasorum

Clinical Significance (Minimal)

SVC Syndrome: Obstruction of the superior vena cava, often due to a tumor (e.g., lung cancer), causes venous congestion in the upper body
IVC Obstruction: May lead to collateral vein dilation and lower extremity edema; can result from tumors, thrombosis, or pregnancy
Caval filters: Inferior vena cava filters are sometimes used to prevent pulmonary embolism from deep vein thrombosis

Topographic Summary

SVC: Formed by brachiocephalic veins, drains head, neck, upper limbs, upper thorax
IVC: Formed by common iliac veins, drains lower limbs, abdomen, pelvis
Entry into heart: Both drain into the right atrium at different levels
Position: SVC – entirely thoracic; IVC – mainly abdominal with short thoracic portion