Lymphatic vessels

Vasa lymphatica

Also known as: lymphatics, collecting lymphatic vessels, lymph vessels

The network of valved channels — capillaries, collecting vessels, trunks and ducts — that carries lymph one way from the tissues, through lymph nodes, to the great veins of the neck.

Overview

Collecting lymphatic vessels form from the union of lymphatic capillaries and precollectors. Their walls resemble thin veins, with an endothelium, a thin layer of smooth muscle and an adventitia, but they are thinner-walled and contain far more valves — paired semilunar (bicuspid) cusps every few millimetres, giving distended lymphatics a beaded appearance. The segment between two valves is a lymphangion, the functional pumping unit: its smooth muscle contracts rhythmically (several times per minute, faster when stretched), driving lymph past the downstream valve.

Superficial lymphatics lie in the subcutaneous tissue and generally follow superficial veins (e.g. along the great saphenous vein to the superficial inguinal nodes); deep lymphatics accompany deep arteries and veins. Every collecting vessel passes through at least one lymph node before lymph reaches the venous system: vessels entering a node are afferent, those leaving at the hilum are efferent. Efferent vessels of the most proximal nodes unite to form lymphatic trunks, which empty into the thoracic duct or right lymphatic duct.

As a body-wide system the lymphatics return interstitial fluid and protein, transport dietary fat and form highways for immune cells. Surgeons use their predictable drainage patterns for cancer staging (sentinel lymph node biopsy uses radiocolloid or blue dye injected near a tumour to identify the first draining node).

Location

Throughout the body: superficial vessels in subcutaneous tissue following superficial veins; deep vessels with the neurovascular bundles; visceral vessels following the arteries of each organ.

Parts & subdivisions

Lymphatic capillaries

Blind-ended, highly permeable initial lymphatics.

Collecting vessels

Valved vessels with smooth muscle that pump lymph through regional nodes; superficial and deep sets.

Lymphangion

Segment between two adjacent valves that acts as a small contractile pump.

Afferent and efferent lymphatics

Afferent vessels enter a node at its convex surface; efferent vessels leave at the hilum.

Lymphatic trunks

Jugular, subclavian, bronchomediastinal, lumbar and intestinal trunks.

Lymphatic ducts

Thoracic duct and right lymphatic duct, emptying into the venous angles.

Function

Conduct lymph one way towards the venous circulation.

Pump lymph actively via lymphangion contraction.

Deliver lymph to regional lymph nodes for filtration and immune surveillance.

Physiology

Flow is driven by the intrinsic lymphatic pump (lymphangion contraction triggered by stretch, modulated by sympathetic nerves and nitric oxide) and extrinsic compression (skeletal-muscle pump, arterial pulsation, respiratory movements, peristalsis), with valves ensuring one-way flow.

Clinical significance

Lymphangitis

Infection spreading along superficial lymphatics, usually from a skin wound infected with Streptococcus pyogenes — tender red streaks running proximally towards enlarged, tender regional nodes. Needs prompt antibiotics.

Lymphatic imaging and sentinel node biopsy

Lymphoscintigraphy (radiolabelled colloid), indocyanine-green lymphography and blue dye trace lymph flow. The sentinel node — the first node draining a tumour — is biopsied in breast cancer and melanoma; if negative, more extensive node clearance (with its higher lymphoedema risk) is usually avoided.

Test yourself

A testicular tumour metastasises via lymphatics first to which nodes?

Superficial inguinal nodes
Deep inguinal nodes
Para-aortic (lumbar) nodes at the level of the renal vessels
External iliac nodes

Answer: Para-aortic (lumbar) nodes at the level of the renal vessels

The testis develops on the posterior abdominal wall and its lymphatics follow the testicular vessels back to the para-aortic (lumbar) nodes near L1–L2. The scrotal skin, in contrast, drains to the superficial inguinal nodes.

More lymphatic & immune system questions