How the lymphatic system works !

Concept
For completeness

Lymphatics collect the fluid and proteins that leak out of capillaries and return them to the blood.

Updated 2026-11-01

See it in 3D: Thoracic duct · Cisterna chyli · Subclavian vein · Mediastinal lymph nodes · Para-aortic lymph nodes

What it is

Every day capillaries filter more fluid than they reabsorb; that surplus, with the proteins that leak out, can only return to the blood through the lymphatics. Without them, fluid and protein would build up in the interstitium and edema would form. Lymphatics also carry fats absorbed in the gut and take antigens to the lymph nodes.

How it works, step by step

Filtered fluid enters the initial lymphatic capillaries through endothelial junctions that work as one-way valves.
Anchoring filaments pull those junctions open when the interstitium fills, and more fluid gets in.
The segments of lymph vessel between two valves (lymphangions) contract when stretched and push lymph forward.
Lymph nodes filter the lymph and present antigens to lymphocytes.
In the gut, lacteals collect chylomicrons, which reach the cisterna chyli.
Lymph enters venous blood at the base of the neck through the thoracic duct and the right lymphatic duct.

Starling forces along a capillary

Arteriolar NFP +13 mmHgVenular NFP −4 mmHgFiltration 5.0Reabsorption 0.5Net filtration 4.5 a.u. (lymph capacity 1.0)

Lymphatic obstruction (surgery, radiotherapy, filariasis): the forces are nearly normal, but filtered fluid and protein do not return (↑ πif): lymphedema.

Edema: net filtration (4.5 a.u.) is more than the lymph can return (1.0 a.u.), so fluid collects in the interstitium.

Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.

Filtration (fluid out)
Absorption (fluid in)
Net filtration pressure
Normal net pressure
Lymphatic (returns the excess)
Edema

Block lymphatic drainage and watch fluid build up while filtration is unchanged; compare with "normal".

Reference values (adult)

Parameter	Value	Note
Total lymph flow	≈2–3 L/day	≈120 mL/h
Interstitial pressure in subcutaneous tissue	≈−3 mmHg	approx.
Protein content of thoracic duct lymph	≈3–5 g/dL	mix of liver and gut lymph

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Clinical notes

Secondary lymphedema: after lymph node dissection or radiotherapy for cancer, or from filariasis in tropical areas; a protein-rich edema that hardens the skin.
In heart failure and cirrhosis lymph flow rises a lot, but once filtration outruns drainage, edema and ascites form.
Injury to the thoracic duct causes chylothorax: milky pleural fluid with high triglycerides.

Test yourself

Think of the answer, then tap the question to see it.

Why is lymphedema protein-rich, unlike the edema of heart failure?

Without lymphatic drainage filtered proteins cannot get back to the blood and pile up in the interstitium; in heart failure the excess is mostly water filtered by high pressure.

What moves lymph if the lymphatic system has no central pump?

Intrinsic contraction of the lymphangions when stretched, with valves that stop backflow, plus outside compression by muscles, arterial pulsation and breathing.

Why is negative interstitial pressure a safety factor against edema?

While the interstitium stays at negative pressure the tissue is stiff, and rising lymph flow drains the excess before free fluid accumulates.

What fluid appears in the pleura if the thoracic duct is injured, and why?

A milky fluid (chyle) high in triglycerides, because the thoracic duct carries the chylomicrons absorbed in the gut.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

In Biochemistry

Albumin

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 21.1 Anatomy of the Lymphatic and Immune Systems (OpenStax (Rice University), 2022); 20.3 Capillary Exchange (OpenStax (Rice University), 2022); Lymphedema (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 16). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.