Superior vena cava, Inferior vena cava, Portal vein: Their branches and anastomoses

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Superior Vena Cava, Inferior Vena Cava & Portal Vein — Branches and Anastomoses

Source: Gray's Anatomy for Students

I. Superior Vena Cava (SVC)

Formation & Course

The SVC begins posterior to the lower edge of the right first costal cartilage, where the right and left brachiocephalic veins unite. It descends vertically and terminates at the lower edge of the right third costal cartilage, emptying into the right atrium. Its lower half lies within the pericardial sac (middle mediastinum).

Tributaries

The SVC itself receives only two direct tributaries:

Azygos vein — arches over the root of the right lung at ~T4 to join the SVC just before it enters the pericardial sac
Pericardial and mediastinal veins (minor)

Tributaries of the Brachiocephalic Veins (which form the SVC)

The right and left brachiocephalic veins each form at the junction of the internal jugular and subclavian veins on their respective sides.

Tributaries	Right Brachiocephalic	Left Brachiocephalic
Vertebral veins	✓	✓
1st posterior intercostal vein	✓	✓
Internal thoracic veins	✓	✓
Inferior thyroid veins	✓ (may)	✓
Left superior intercostal vein	—	✓
Thymic & pericardial veins	may	may

The Azygos System (key SVC tributary)

The azygos vein arises at L1/L2 from the junction of the right ascending lumbar vein and the right subcostal vein (or directly from the IVC). It enters the thorax through the aortic hiatus, ascends in the posterior mediastinum, and arches anteriorly at T4 to join the SVC.

Tributaries of the azygos vein:

Right superior intercostal vein (2nd–4th intercostal veins; the 1st drains into the right brachiocephalic vein)
5th–11th right posterior intercostal veins
Hemiazygos vein (inferior hemiazygos) — arises from junction of left ascending lumbar + left subcostal veins; ascends on the left to ~T9, then crosses the vertebral column posteriorly to join the azygos. Receives the lowest 4–5 left posterior intercostal veins + esophageal and mediastinal veins
Accessory hemiazygos vein (superior hemiazygos) — descends on the left from upper posterior mediastinum to ~T8, crosses to join azygos or hemiazygos; receives the 4th–8th left posterior intercostal veins; connects superiorly to the left superior intercostal vein
Esophageal, mediastinal, pericardial, and right bronchial veins

II. Inferior Vena Cava (IVC)

Formation & Course

The IVC is formed at vertebral level LV by the union of the two common iliac veins, just to the right of the midline. It ascends anterior to the vertebral column, immediately to the right of the abdominal aorta, and exits the abdomen by piercing the central tendon of the diaphragm at T8 to drain into the right atrium.

Tributaries (inferior → superior)

Tributary	Level / Note
Common iliac veins	L5 — formation
Lumbar veins (3rd & 4th)	Drain directly into IVC; 1st & 2nd may go to ascending lumbar veins; 5th goes to iliolumbar vein → common iliac
Right testicular/ovarian vein	Right side drains directly; left side drains into left renal vein
Renal veins	Both sides
Right suprarenal vein	Right drains directly; left suprarenal drains into left renal vein
Inferior phrenic veins	Near the diaphragm
Hepatic veins	Just before piercing the diaphragm

Note: Veins from the GI tract, spleen, pancreas, and gallbladder do not drain into the IVC directly — they form the portal system and pass through the liver first.

The Lumbar Veins and Their Anastomotic Role

The ascending lumbar veins connect the common iliac, iliolumbar, and lumbar veins below with the azygos and hemiazygos veins in the thorax above. This creates an important IVC–SVC collateral pathway: if the IVC becomes obstructed, the ascending lumbar veins serve as the major collateral channel between the lower and upper body.

III. Portal Vein

Formation & Course

The portal vein is formed posterior to the neck of the pancreas at L2, by the union of the:

Superior mesenteric vein
Splenic vein

It ascends, passes posterior to the superior duodenum, enters the right free margin of the lesser omentum, and runs anterior to the omental (epiploic) foramen. In the lesser omentum it lies:

Posterior to the bile duct (right) and hepatic artery proper (left)
At the porta hepatis it divides into right and left branches entering the liver parenchyma

Tributaries of the Portal Vein

A. Splenic Vein

Formed from multiple vessels at the hilum of the spleen; passes through the splenorenal ligament with the splenic artery; runs along the posterior surface of the body of the pancreas.

Tributaries of the splenic vein:

Short gastric veins (fundus + left greater curvature)
Left gastro-omental vein (greater curvature)
Pancreatic veins (body and tail)
Inferior mesenteric vein (usually)

B. Superior Mesenteric Vein (SMV)

Drains the small intestine, cecum, ascending colon, and transverse colon. Begins in the right iliac fossa; ascends to the right of the superior mesenteric artery.

Tributaries of the SMV:

Jejunal and ileal veins
Ileocolic vein
Right colic vein
Middle colic vein
Right gastro-omental vein (right greater curvature)
Anterior and posterior inferior pancreaticoduodenal veins

C. Inferior Mesenteric Vein (IMV)

Drains rectum, sigmoid, descending colon, and splenic flexure. Begins as the superior rectal vein; ascends receiving sigmoid and left colic veins; usually joins the splenic vein (occasionally the junction of splenic + SMV, or SMV alone).

D. Direct Portal Vein Tributaries

Right and left gastric veins — drain the lesser curvature of the stomach and abdominal esophagus
Cystic veins — from the gallbladder
Para-umbilical veins — associated with the obliterated umbilical vein (ligamentum teres); connect to veins of the anterior abdominal wall

IV. Portosystemic (Portocaval) Anastomoses

These are sites where portal tributaries communicate with systemic (caval) veins. In health they are small; in portal hypertension (e.g., cirrhosis) they dilate into varices.

Site	Portal Side	Systemic Side	Varix Formed
Gastroesophageal junction	Left gastric vein + tributaries	Azygos system (SVC)	Esophageal varices — most dangerous; may bleed profusely
Anorectal junction	Superior rectal vein (IMV → portal)	Middle & inferior rectal veins → internal iliac → IVC	Anorectal varices
Umbilicus / anterior abdominal wall	Para-umbilical veins	Superficial veins of anterior abdominal wall → SVC (above) and femoral/external iliac (below)	Caput medusae
Retroperitoneum (minor)	Colic and mesenteric veins	Retroperitoneal veins → renal, suprarenal, gonadal (IVC)	Veins of Retzius (less clinically prominent)

IVC–SVC Anastomoses (Caval Collaterals)

When the IVC itself is obstructed, blood finds alternative routes to the SVC via:

Ascending lumbar veins → azygos/hemiazygos → SVC
Vertebral venous plexus (Batson's plexus)
Superficial epigastric / thoracoepigastric veins along the abdominal wall

Clinical Correlations at a Glance

Finding	Mechanism	Vein System
SVC syndrome	Obstruction (usually tumor) → facial/arm swelling, engorged neck veins	SVC
Pulmonary embolism	DVT → thrombus via IVC → right heart → pulmonary artery	IVC
IVC filter placement	Via SVC → right atrium → IVC to catch emboli	SVC/IVC
TIPS procedure	Transjugular route: SVC → IVC → hepatic vein → portal vein	SVC/IVC/Portal
Esophageal varices	Portal hypertension → left gastric → azygos collaterals	Portal–SVC
Caput medusae	Portal hypertension → para-umbilical → abdominal wall veins	Portal–SVC/IVC
Anorectal varices	Superior rectal (portal) ↔ middle/inferior rectal (IVC)	Portal–IVC

— Gray's Anatomy for Students, pp. 254–257, 268–269, 417–420, 453–454

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