Functions of lymph

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Functions of Lymph

Lymph is the tissue fluid that enters lymphatic capillaries. It is essentially a filtrate of plasma - slightly different in composition depending on where it drains from - and it performs several distinct physiological functions.

1. Return of Interstitial Fluid to the Circulation (Fluid Homeostasis)

The lymphatic system is the primary mechanism for returning excess interstitial fluid back to the blood vascular system. Under normal Starling forces, there is a slight net filtration of fluid out of capillaries into the interstitial space throughout the body. The lymphatics collect approximately 10% of this extravasated fluid (the remaining 90% is reabsorbed at the venous end of capillaries) and return it to the bloodstream via the thoracic duct and the right lymphatic trunk, which drain into the subclavian veins.

In healthy adults, the lymphatic system returns up to 8 litres of interstitial fluid per day to the circulation.

If lymphatic drainage is impaired (e.g., after lymph node removal, irradiation, or parasitic infection such as filariasis), this fluid accumulates in the tissues, causing lymphedema - swelling, tissue fibrosis, and susceptibility to infection.

Histology: A Text and Atlas | Costanzo Physiology, 7th Ed.

2. Return of Plasma Proteins

Proteins are continuously leaking from blood capillaries into the interstitial fluid. These molecules are too large to be reabsorbed directly back into blood capillaries, but lymphatic capillaries - which are more permeable than blood capillaries - can take them up easily. Lymph therefore recirculates approximately 20-30 g of protein per litre of fluid back to the bloodstream daily.

Without this mechanism, plasma oncotic pressure would fall progressively, leading to worsening oedema.

Histology: A Text and Atlas | Guyton and Hall Textbook of Medical Physiology

3. Absorption and Transport of Dietary Fats (Lipids)

In the gastrointestinal tract, the lymphatics play an indispensable role in fat absorption. Dietary fats (long-chain triglycerides, cholesterol, cholesteryl esters, and phospholipids) are packaged into chylomicrons within intestinal enterocytes. These particles are too large to cross into blood capillaries directly, so they are absorbed by specialised lymphatics in the intestinal villi called lacteals.

The chylomicrons travel via the intestinal lymphatics into the thoracic duct and enter the venous circulation at the left subclavian vein. This is also the route by which fat-soluble vitamins (A, D, E, K) are transported from the gut.

After a fatty meal, the lymph in the thoracic duct appears milky-white due to its high fat content - this is called chyle.

About 80-90% of all fat absorbed from the gut is transported to the blood via thoracic lymph in the form of chylomicrons.

Ganong's Review of Medical Physiology, 26th Ed. | Sabiston Textbook of Surgery, 11th Ed.

4. Immune Surveillance and Defence

Lymph is central to the body's immune system in several ways:

Antigen transport: Lymphatic vessels carry antigens and antigen-presenting cells (e.g., dendritic cells) from peripheral tissues to the draining lymph nodes, where immune responses are initiated.
Lymphocyte recirculation: Lymphocytes continuously exit the blood into tissues and re-enter the circulation via lymph, allowing them to patrol peripheral tissues.
Filtration by lymph nodes: Before lymph is returned to the blood, it passes through lymph nodes where it is exposed to macrophages and other immune cells. Particulate matter, bacteria, and cellular debris are trapped and destroyed here.

"Before lymph is returned to the blood, it passes through lymph nodes, where it is exposed to the cells of the immune system. Thus, the lymphatic vessels serve not only as an adjunct to the blood vascular system but also as an integral component of the immune system."

Histology: A Text and Atlas

5. Clotting Capacity

Lymph contains clotting factors (derived from plasma proteins that have crossed capillary walls) and will clot upon standing in vitro. This property gives it the ability to limit spread of infection or injury in peripheral tissues.

Ganong's Review of Medical Physiology, 26th Ed.

Summary Table

Function	Mechanism	Clinical Relevance
Fluid return	Collects ~10% of capillary filtrate	Lymphedema if impaired
Protein return	Uptake of large molecules from interstitium	Hypoproteinaemia if lymph blocked
Fat absorption	Chylomicron transport via lacteals and thoracic duct	Chyle leak, fat malabsorption
Immune surveillance	Antigen/APC transport to lymph nodes; lymphocyte recirculation	Reduced immunity after lymphadenectomy
Clotting	Contains plasma clotting factors	Local haemostasis

Sources: Costanzo Physiology 7th Ed. | Ganong's Review of Medical Physiology 26th Ed. | Guyton and Hall Textbook of Medical Physiology | Histology: A Text and Atlas (Pawlina & Ross)

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