Formation and entry of lymph

Why fluid leaves the blood capillaries

At the arterial end of a capillary, blood pressure pushes water and small dissolved substances out through the vessel wall into the tissue space. This movement is called filtration, and it delivers oxygen, nutrients and water to the cells. At the venous end, the pressure inside the capillary is lower and the osmotic pull of plasma proteins draws most of the fluid back in. In a healthy person, the two movements nearly balance: most of the filtered fluid is reabsorbed. A small fraction, perhaps two to four litres a day, is not, and it remains in the tissue spaces. If it were allowed to accumulate, the tissues would swell. The lymphatic system exists to collect this residual fluid and return it to the blood.

For example: When you stand for a long time, capillary pressure in the legs is higher, more fluid filters out and the ankles may swell slightly until lymph drainage catches up.

Interstitial fluid becomes lymph

The fluid in the tissue spaces is interstitial fluid. It contains water, dissolved ions and nutrients, small proteins that have leaked from the capillaries, waste products from cell activity and sometimes particles such as bacteria. When this fluid enters a lymphatic capillary, it is called lymph. The change of name marks the change of compartment, not a change in the fluid itself: interstitial fluid is outside the vessels, and lymph is inside a lymphatic vessel. Lymph is normally clear or pale yellow and similar in composition to plasma but with much less protein. Because it comes from the tissue spaces, its composition reflects what is happening locally; lymph from the intestine is rich in absorbed fats, and lymph from an infected area contains microbes and immune cells.

For example: Lymph collected from the small intestine after a fatty meal looks milky, because it carries absorbed fat; lymph elsewhere is clear.

How lymph enters a lymphatic capillary

Lymphatic capillaries are blind-ended: one end is closed, and the open end continues into larger vessels. They begin in the tissue spaces, so their closed tips lie close to the cells. Their walls are made of endothelial cells that overlap one another rather than forming a sealed tube, like tiles on a roof. When pressure in the tissue fluid rises above the pressure inside the capillary, the overlapping edges are pushed apart and fluid enters through the gaps. Fine anchoring filaments connect the endothelial cells to the surrounding tissue; when the tissue swells, the filaments pull the cells apart and widen the openings, so more fluid can enter exactly when it is most needed. This is a passive, pressure-driven mechanism that requires no pump.

For example: If excess fluid makes a tissue swell, the anchoring filaments are stretched and pull the capillary cells apart, so the entry openings widen automatically.

Enlarge illustration ↗

Why lymph does not flow straight back out

The same overlapping-cell design that lets fluid in also prevents it from leaving. Once fluid has entered, pressure inside the capillary rises. That internal pressure pushes the overlapping edges of the endothelial cells together, closing the gaps from the inside, rather than apart. The result is a one-way valve made of the wall itself: fluid can enter when outside pressure is higher, and backflow is blocked when inside pressure is higher. Larger lymphatic vessels add conventional valves along their length, which keep lymph moving in one direction as it is squeezed by body movements. This combination means that lymph moves from the tissue spaces towards the centre of the body and does not return to the spaces it came from.

For example: If you press on a swollen area, you do not push lymph back into the tissue; the overlapping cell edges close against the pressure.

Common misconception: Lymphatic capillaries are not open pipes. Their walls are sealed by overlapping cells that open only when outside pressure exceeds inside pressure.

Where lymphatic capillaries are found

Lymphatic capillaries are present in most tissues, but not all. They are absent from the epidermis, from cartilage and the cornea, and from the central nervous system, which has its own fluid drainage arrangements. Where they are present, they lie close to blood capillaries so that they can collect the fluid that filters out. In the small intestine, specialised lymphatic capillaries called lacteals extend into each villus and absorb fats and fat-soluble vitamins that cannot enter the blood capillaries directly. Because lymphatic capillaries depend on tissue pressure and movement rather than on the heart, drainage is slow and low-pressure, and it depends on the skeletal muscle pump, breathing movements and the pulsation of nearby arteries to keep lymph moving.

For example: A scratch on the cornea heals without lymphatic drainage because the cornea has no lymphatic capillaries; the aqueous humour and other routes handle fluid.