The lymphatic system has four major functions that are equalization of interstitial fluid distribution, transport of plasma proteins and triglycerides, production of lymphocytes, and return of the material to the blood.

Interstitial fluid accumulates in the tissues, generally as a result of the pressure exerted from capillaries or protein leakage into the tissues. These conditions force fluid from the capillaries into the tissues. The lymphatic system drains the excess interstitial fluid that accumulates. When the interstitial fluid pressure increases, the flaps of the lymphatic vessels open and the fluid enters these vessels. The fluid moves to the veins that merge with the lymphatic vessels and thus help to reduce the interstitial pressure by returning the fluid to the circulation.

The digested fats in the small intestine are absorbed in the lacteals that are lymphatic capillaries. Via these lacteals, the fat molecules enter the circulation. The plasma proteins that leak into the interstitial spaces are also transported by lymphatic vessels along with the interstitial fluid.

The lymphatic system plays a key role in the immune system, as the proliferation and maturation of lymphocytes takes place in the lymphoid tissues present in the thymus, lymph nodes, etc. Lymphocytes are make in the bone marrow, however.

The plasma proteins, cells, interstitial fluid present in the interstitial space is brought back to the circulation via the lymphatic capillaries and vessels.

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Key Points

• The lymphatic system is responsible for the removal of interstitial fluid from tissues into lymph fluid, which is filtered and brought back into the bloodstream through the subclavian veins near the heart.

• It absorbs and transports fats from the intestine after digestion.

• It helps in the proliferation and maturation of the lymphocytes that are involved in the immune defence of the body.

• The lymphatic vessels help to bring back the molecules like proteins and the cells that leak into the interstitial system back to the blood.

Key Terms

lacteal: A lymphatic capillary that absorbs dietary fats in the villi of the small intestine.

interstitial fluid: Also called tissue fluid, a solution that bathes and surrounds the cells of multicellular animals.

lymphocytes: white blood cells that are involved in the immune defence of the body, ie T cells and B cells

thymus: a lymphoid organ in which lymphocytes mature

capillaries: tiny blood vessels that connect arteries and veins

veins: blood vessels carrying deoxygenated blood

plasma proteins: proteins present in blood plasma. They serve many different functions, including transport of lipids, hormones and vitamins