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The lymphatic system

Trace lymph from tissue fluid to the veins, and explain what lymph nodes, nodules, the spleen and the thymus each do.

In the transport section you saw tissue fluid draining into a lymph vessel. Follow that fluid the rest of the way, and meet the organs that clean and monitor it.

Read one idea at a time1 / 10

Lymph

Lymph is interstitial fluid that has entered a lymphatic capillary. About three litres drain this way each day; the fluid is clear and similar to plasma but contains less protein, because most plasma proteins are too large to leave blood vessels.

For example

A swollen ankle after a long flight has extra interstitial fluid. Lymph capillaries in the foot take up part of it and carry it back towards the blood.

Read all notes for this section

Lymph: Lymph is interstitial fluid that has entered a lymphatic capillary. About three litres drain this way each day; the fluid is clear and similar to plasma but contains less protein, because most plasma proteins are too large to leave blood vessels. A swollen ankle after a long flight has extra interstitial fluid. Lymph capillaries in the foot take up part of it and carry it back towards the blood.
Valves: Valves are folds inside lymphatic vessels that open in one direction only. They keep lymph moving towards the ducts instead of sliding back. Lymph has no pump of its own; skeletal muscle contractions squeeze the vessels, and pressure changes during breathing push lymph onwards. Walking contracts the leg muscles, which milks the leg lymph vessels upwards. Standing still for hours removes that help and fluid collects.
Lymph node: A lymph node is a small encapsulated organ along a lymphatic vessel. Lymph enters through afferent vessels, crosses a maze of sinuses, and leaves through efferent vessels at a hollow called the hilum. B cells gather in the outer cortex and T cells in the inner cortex. Macrophages trap and destroy some foreign material, and lymphocytes mount immune responses. During a throat infection the neck nodes may enlarge and feel tender because immune cells inside are multiplying. That is a response to filtered organisms, not the node itself fighting alone.
Lymphatic nodule: A lymphatic nodule is a cluster of lymphatic tissue without a capsule. Nodules lie in the lining of the digestive tract, urinary and reproductive tracts and airways, where together they are called mucosa-associated lymphatic tissue (MALT). Large groups include the tonsils of the throat and the Peyer’s patches in the ileum of the small intestine. Tonsils sit where swallowed and inhaled material first arrives, so their nodules meet microbes early. They are not enclosed organs like lymph nodes.
Spleen: The spleen is the largest single mass of lymphatic tissue. It lies in the left hypochondriac region, above and behind the stomach. White pulp contains lymphocytes and macrophages that respond to substances in blood; red pulp removes worn-out red blood cells and platelets, stores up to one third of the body’s platelets, and produced blood cells before birth. White pulp surrounds branches of the splenic artery, and the organ also has a hilum where its artery, vein and lymphatic vessels enter and leave. Because the spleen samples blood rather than lymph, it removes old red blood cells from circulation. You will not find lymph flowing through it.
Thymus: The thymus is a bilobed organ in the mediastinum, between the sternum and the aorta. Immature T cells arrive from red bone marrow, multiply in the outer cortex and are selected for usefulness; only a small share survive. The survivors mature in the medulla and then populate other lymphatic tissues. The thymus is largest at puberty and is gradually replaced by fat. A child without a working thymus cannot build a normal population of T cells, so defences against some infections are weak. The organ matters most early in life.

Formation and flow of lymph

More fluid filters out of blood capillaries than returns to them. The excess becomes interstitial fluid, the fluid between tissue cells. When that fluid enters a lymphatic capillary it is called lymph. Lymphatic capillaries have a closed end and walls of overlapping cells, so fluid can enter but not flow back. The smallest lymph vessels join into larger ones, and the whole network carries lymph towards the veins.

Vessels, trunks and the two ducts

Lymph travels from lymphatic capillaries, through lymphatic vessels with valves, into lymph trunks, and finally into two ducts. The thoracic duct is the main channel: it drains the lower body, the left arm and the left side of the head, neck and chest. The right lymphatic duct is shorter and drains the upper right side of the body. Both ducts empty into large veins near the neck, so lymph returns to the blood.

Lymph nodes and lymphatic nodules

A lymph node is a small, bean-shaped organ along a lymphatic vessel. Lymph enters through afferent vessels and leaves through efferent vessels, so it flows in one direction. Inside, macrophages trap and destroy some foreign material while lymphocytes start immune responses. Lymphatic nodules are different: they are clusters of lymphatic tissue without a capsule, found in the lining of the gut, airways and throat, including the tonsils and the Peyer’s patches of the small intestine.

The spleen and the thymus

The spleen and the thymus are lymphatic organs. The spleen lies in the upper left abdomen and surveys blood: it removes worn-out red blood cells, stores platelets and helps immune defence. The thymus lies in the upper chest, behind the sternum, and it is where T cells mature. T cells are lymphocytes that recognise specific threats. The thymus is largest in childhood and shrinks after puberty; lymph nodes and nodules stay throughout life.

Explore the illustration

Look at the whole view first. Then select one label and connect it to the idea you just read.

Lymph drainage and the lymphatic organs

Anterior schematic view, not to scale. Select a number below to trace one connection at a time. The patient’s right is on your left. Green marks lymphatic routes; purple marks lymphoid tissue.

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01 / 18 Initial lymphatic capillaries

A closed lymphatic capillary

Guided anatomy tour: each step identifies one structure and explains its relationship to the next. The highlight moves; the anatomy stays still. Reduced motion is enabled. Use the arrows to study one step at a time, or choose Play.

Initial lymphatic capillaries

These blind-ended vessels begin in most tissues. Overlapping cells in their wall let interstitial fluid enter, and anchoring filaments open the gaps when the tissue swells. Fluid inside them is now called lymph.

Illustration credits

Goran tek-en · Male front 3d-shaded human illustration.svg · CC BY-SA 4.0.

Adapted for this lesson: labels removed, artwork cropped or recoloured, and interactive teaching overlays added. Adapted artwork retains its source licence.

Go deeper

In-depth reading

Follow this example step by step, then try explaining it yourself. The extra reading is optional: you can return to it after exploring the illustration.

Worked example: from fluid in the foot to blood at the neck

After a long flight, extra interstitial fluid collects around the cells of one foot. Blood capillaries have reabsorbed less fluid than they filtered out, so the surplus has to leave by another route.
The fluid enters a lymphatic capillary in the foot. That capillary begins as a closed tube with overlapping cells in its wall, so fluid can enter through the openings but cannot flow back into the tissue. Once inside, the fluid is called lymph.
Lymph passes into collecting vessels with valves. Walking squeezes these vessels, and each valve lets lymph move upward only. Lymph does not climb straight to the neck: it passes through the inguinal lymph nodes in the groin, where macrophages and lymphocytes act on anything foreign.
The cleaned lymph continues into the lumbar trunks, then into the cisterna chyli, and from there into the thoracic duct. Because the foot is below the ribs, its lymph belongs to the thoracic duct and not to the right lymphatic duct.
The thoracic duct empties into the junction of the left internal jugular and left subclavian veins. The recovered fluid is blood plasma again, and the loop that began in a capillary ends in a vein.

Explain it yourself

The right arm swells because its lymph cannot drain. Which duct would you expect to be blocked, and why?

Show the explanation

The right lymphatic duct. It drains the upper right side of the body, including the right arm, while the thoracic duct drains the lower body, the left arm and the left side of the head, neck and chest. Fluid that cannot leave the tissue accumulates, so the arm swells.

More detail — when you are ready

These notes expand the mechanisms and vocabulary. Read one topic at a time and find the structures in the illustration.

How lymph is formed and kept moving

More fluid filters out of blood capillaries than returns to them. The excess — roughly three litres a day — leaves the interstitial space through lymphatic capillaries and becomes lymph.

Lymphatic capillaries are slightly wider than blood capillaries and begin with a closed end. Their wall is built from endothelial cells whose edges overlap. When pressure in the interstitial fluid is higher than inside the capillary, the edges separate like a one-way door and fluid enters. When pressure inside is higher, the edges press together and lymph cannot escape. Elastic anchoring filaments fasten the capillary to surrounding tissue; when the tissue swells, they pull the openings wider.

Most plasma proteins are too large to leave a blood capillary. A small amount does escape, and it cannot return by diffusion because the protein concentration is higher inside the blood. Lymphatic vessels therefore also return lost plasma proteins to the blood — a function that fluid reabsorption alone cannot perform.

There is no lymphatic heart. Lymph moves because skeletal muscle contractions squeeze the vessels, and because breathing changes pressure between the abdomen and thorax. Valves along the vessels open in one direction only, so lymph keeps moving towards the ducts instead of sliding back.

Naming the pathway from capillary to vein

The route is a hierarchy, and each step is wider than the last. Lymphatic capillaries join to form lymphatic vessels, which look like veins but have thinner walls and more valves. Vessels from a region gather into a lymph trunk. The lumbar and intestinal trunks drain into the cisterna chyli, a sac at the upper end of the abdomen.

The thoracic duct is the main channel. It begins at the cisterna chyli, collects the left jugular, left subclavian and left bronchomediastinal trunks in the neck, and carries lymph from the left side of the head, neck and chest, the left upper limb and the whole body below the ribs. It empties into the junction of the left internal jugular and left subclavian veins.

The right lymphatic duct is only about 1.2 cm long. It receives the right jugular, right subclavian and right bronchomediastinal trunks and drains the upper right side of the body — the right side of the head and chest and the right arm. It empties into the matching veins on the right.

Learn the two territories together. Everything below the ribs goes to the thoracic duct, whichever side of the body it is on.

Inside a lymph node

A lymph node is a filter placed in the path of lymph. Lymph enters the convex surface through several afferent lymphatic vessels whose valves open inward. Inside the node it crosses a series of irregular channels called sinuses: first the subcapsular sinus just under the capsule, then trabecular sinuses through the cortex, then medullary sinuses in the centre. It leaves through one or two efferent lymphatic vessels that are wider and fewer, with valves opening away from the centre.

The functional tissue is arranged in zones. The outer cortex holds rounded clusters of B cells. A cluster that has met its antigen develops a paler germinal centre and is then called a secondary nodule. The inner cortex holds mainly T cells and dendritic cells that present antigen to them. The medulla contains B cells, antibody-producing plasma cells and macrophages. Reticular fibres form the supporting mesh that slows the flow and gives cells something to cling to.

The outcome is two-layered. Macrophages destroy some foreign material by phagocytosis, and lymphocytes respond to the rest. The filtered lymph then continues its journey.

Lymphatic nodules are not small lymph nodes. A nodule is a cluster of lymphatic tissue with no capsule; nodules sit in the lamina propria of mucous membranes in the digestive, urinary and reproductive tracts and the airways, where the group is called mucosa-associated lymphatic tissue (MALT). The tonsils form a ring at the entrance to the throat, the aggregated lymphatic follicles or Peyer’s patches lie in the ileum, and further nodules occur in the appendix. Their position at an entrance lets them meet microbes early.

The spleen and the thymus: two different jobs

The spleen is the largest single mass of lymphatic tissue, about 12 cm long. It lies in the left hypochondriac region between the stomach and the diaphragm, and neighbouring organs leave impressions on its visceral surface — gastric, renal and colic. Blood reaches it through the splenic artery and leaves through the splenic vein; both, with efferent lymphatic vessels, pass through its hilum.

Its tissue is divided into white pulp and red pulp. White pulp is lymphatic tissue arranged around branches of the splenic artery, called central arteries. There, B cells and T cells carry out immune responses, and macrophages destroy blood-borne pathogens. Red pulp is built from blood-filled venous sinuses and splenic cords. There the spleen removes ruptured, worn-out or defective red blood cells and platelets, stores up to one third of the body’s platelets, and produced blood cells during fetal life.

Because it samples blood, the spleen is not a filter of lymph travelling in lymphatic vessels. That job belongs to lymph nodes.

The thymus is the second primary lymphatic organ. It sits in the mediastinum between the sternum and the aorta, and a capsule with inward extensions called trabeculae divides it into lobules. Each lobule has a dark outer cortex and a paler central medulla.

Immature T cells travel from red bone marrow to the cortex, multiply, and are tested for whether they recognise useful targets. Only about two per cent survive selection; the rest die by apoptosis, and macrophages clear the debris. Survivors mature in the medulla and then populate lymph nodes, the spleen and nodules.

The thymus is largest at puberty and afterwards adipose and areolar tissue replace much of it, so in old age it may weigh only about 3 g. The organ shapes the T-cell population early in life; afterwards, those cells continue to divide elsewhere.

A useful distinction: primary lymphatic organs — red bone marrow and the thymus — are where cells become immunocompetent. Secondary organs and tissues — lymph nodes, the spleen and nodules — are where most immune responses happen.