Thoracic Duct
Largest lymphatic vessel in body (great lymph channel). Drains lymph from most of body into venous bloodstream. Milky-white (chyle from fat digestion); beaded appearance (numerous valves).
Area of Drainage
- Drains all body except: right side of head/neck, right chest wall, right lung, right side of heart, right (upper) surface of liver
This right upper quadrant region is drained by right lymphatic duct instead. Thoracic duct drains remaining 3/4 of body.
Extent and Measurements
| Feature | Detail |
|---|---|
| Lower end | Upper end of cisterna chyli, posterior abdominal wall, lower border T12 |
| Upper end | Junction of left internal jugular + left subclavian veins, root of neck |
| Length | 45 cm (18 in) |
| Lumen width | 5 mm at ends; narrower in middle |
Formation, Course, Termination
| Segment | Description |
|---|---|
| Origin | Continuation of cisterna chyli (in front of L1–L2) at lower border T12 |
| Entry to thorax | Through aortic opening of diaphragm |
| Posterior mediastinum | Ascends right of midline, in front of vertebral bodies |
| Crossover | At T5, crosses midline right→left |
| Superior mediastinum | Ascends along left border of esophagus to root of neck |
| Neck arch | At C7, arches laterally; summit 3–4 cm above clavicle |
| Termination | Junction of left internal jugular + left subclavian veins |
Arch relations (neck):
| Relation | Structures |
|---|---|
| Anterior | Carotid system — left common carotid, left IJV, left vagus |
| Posterior | Vertebral system — vertebral artery/vein; left cervical sympathetic trunk |
Root of neck relations:
| Relation | Structures |
|---|---|
| Anterior | Carotid sheath — left common carotid, left IJV, left vagus |
| Posterior | Vertebral artery/vein; medial border scalenus anterior; phrenic nerve; thyrocervical trunk + branches (suprascapular, transverse cervical, inferior thyroid) |
Tributaries
Abdomen: efferents from lower 6 intercostal lymph nodes, both sides
Thorax:
- Ascending lymph trunks (pair) — upper lumbar/para-aortic nodes
- Descending lymph trunks (pair) — posterior intercostal nodes, upper 6 spaces
- Posterior mediastinal lymph node vessels
Neck:
- Left jugular lymph trunk (neck)
- Left subclavian lymph trunk (left upper limb)
- Left bronchomediastinal trunk
Development (3 stages)
| Stage | Description |
|---|---|
| I | Network of lymph channels in front of thoracic vertebral column |
| II | Two longitudinal channels (right + left) within network, cross-communications |
| III | Cross-communication persists opposite T5; right channel below T5 + left channel above T5 persist → thoracic duct; rest disappears |
Explains crossover at T5 — below = embryonic right channel, above = embryonic left channel
Applied Anatomy
Injury — thin-walled, may appear colourless → inadvertent surgical injury (esophageal/lung surgery)
- Chylothorax — laceration during lung surgery → chyle into pleural cavity
- Cervical injury — during neck block dissection; should be ligated immediately; if ligated, lymph returns via anastomotic channels; if undetected/unligated → chylous fistula (persistent leakage) — requires immediate ligation
Obstruction — filarial infection (Wuchereria bancrofti) → microfilarial obstruction → chylothorax, chyloperitoneum, chyluria, chylocele (chyle in tunica vaginalis)
The thoracic duct is the largest lymphatic vessel (also called the great lymph channel or thoracic lymphatic trunk) in the body. It drains lymph from most of the body into the venous bloodstream. The lymph within it is milky-white because it contains chyle (a product of fat digestion absorbed from the intestine). The duct appears beaded due to numerous valves in its lumen.
Area of Drainage
The thoracic duct drains lymph from all parts of the body except:
- Right side of the head and neck
- Right side of the chest wall
- Right lung
- Right side of the heart
- Right surface (upper surface) of the liver
N.B. This region — the right upper quadrant of the body — is drained instead by the right lymphatic duct. The thoracic duct drains the remaining three-quarters of the body.
Extent and Measurements
| Feature | Detail |
|---|---|
| Lower end | Upper end of the cisterna chyli, on the posterior abdominal wall, at the lower border of T12 vertebra |
| Upper end | Junction of the left internal jugular vein and left subclavian vein, at the root of the neck |
| Length | 45 cm (18 inches) |
| Width of lumen | 5 mm at the ends; narrower in the middle |
Formation, Course, and Termination
| Segment | Description |
|---|---|
| Origin | Begins as a continuation of the cisterna chyli (lying in front of L1–L2 vertebral bodies) at the lower border of T12 |
| Entry into thorax | Enters through the aortic opening of the diaphragm |
| Posterior mediastinum | Ascends to the right of the midline, on the front of the vertebral bodies |
| Crossover at T5 | At the level of T5 vertebra, crosses the midline from right to left |
| Superior mediastinum | Ascends along the left border of the esophagus to reach the root of the neck |
| Arch in the neck | At the level of C7 vertebra, arches laterally — summit of arch lies 3–4 cm above the clavicle |
| Termination | Opens into the junction of the left internal jugular vein and left subclavian vein |
Relations of the Arch in the Neck
| Relation | Structures |
|---|---|
| Anterior to the arch (in front) | Carotid system — left common carotid artery, left internal jugular vein, left vagus nerve |
| Posterior to the arch (behind) | Vertebral system — vertebral artery and vertebral vein; left cervical sympathetic trunk |
Relations in the Root of the Neck
| Relation | Structures |
|---|---|
| Anterior | Carotid sheath — left common carotid artery, left internal jugular vein, left vagus nerve |
| Posterior | (1) Vertebral artery and vein; (2) medial border of scalenus anterior; (3) phrenic nerve; (4) thyrocervical trunk and its branches (suprascapular, transverse cervical, inferior thyroid arteries) |
Tributaries
A. In the Abdomen
- Efferents from the lower six intercostal lymph nodes of both sides
B. In the Thorax
- A pair of ascending lymph trunks — drain the upper lumbar (para-aortic) lymph nodes
- A pair of descending lymph trunks — drain the posterior intercostal lymph nodes of the upper six spaces
- Lymph vessels from the posterior mediastinal lymph nodes
C. In the Neck
- Left jugular lymph trunk — drains the neck
- Left subclavian lymph trunk — drains the left upper limb
- Left bronchomediastinal trunk
Development
The thoracic duct develops in three stages:
| Stage | Description |
|---|---|
| Stage I | A network of lymph channels appears in front of the thoracic vertebral column |
| Stage II | Two longitudinal lymph channels appear (right and left) within the network, with cross-communications |
| Stage III | A cross-communication persists opposite T5; the right channel below this level and the left channel above this level persist and form the thoracic duct; all other parts disappear |
This developmental pattern explains the crossover of the thoracic duct at T5 — below T5 it follows the embryonic right channel, above T5 it follows the embryonic left channel.
Applied Anatomy
Injury of the Thoracic Duct
The duct is thin-walled and may appear colourless, so it can be injured inadvertently during surgery in the posterior mediastinum (e.g., oesophageal or lung surgery).
- Chylothorax — laceration during lung surgery allows chyle to enter the pleural cavity
- Cervical injury — may occur during block dissection of the neck; the duct should be ligated immediately. If ligated, lymph returns via anastomotic channels. If not detected and not ligated, it causes a chylous fistula with persistent lymph leakage — immediate ligation is required.
Obstruction of the Thoracic Duct
In filarial infection (Wuchereria bancrofti), the thoracic duct may be obstructed by microfilarial parasites, producing widespread effects:
- Chylothorax
- Chyloperitoneum
- Chyluria
- Chylocele — accumulation of chyle in the tunica vaginalis
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