Composition of Lymph and its Function

The lymphatic system is a vital part of the circulatory and immune systems in the human body. It helps maintain fluid balance, defends the body against infections, and assists in the absorption of dietary fats.

1.0General Functions of the Lymphatic System

Returns Fluid from Tissues to Blood

• About 85% of the fluid that leaks from blood vessels is reabsorbed directly into blood capillaries.
• The remaining 15% is collected by lymphatic capillaries and returned to the bloodstream.
• In 24 hours, lymphatic vessels return a volume of fluid equivalent to the entire blood volume.
• If lymphatic flow is blocked, it can lead to oedema (fluid accumulation and swelling).

Returns Large Molecules to Blood

• Each day, 25–50% of plasma proteins escape from blood capillaries into surrounding tissues.
• These proteins cannot re-enter blood vessels directly and are instead absorbed by lymphatic capillaries and returned to the blood.
• If this process is disrupted, blood protein levels decrease, leading to a fluid imbalance in the body.

Absorption and Transport of Fats

• Specialised lymphatic capillaries called lacteals, located in the villi of the small intestine, absorb lipids and fat-soluble vitamins from digested food.
• These nutrients bypass the liver and are often delivered directly to adipose (fat) tissue.

Hemopoiesis (Formation of Blood Cells)

• Some white blood cells, particularly lymphocytes and monocytes, are produced in lymphatic tissues (rather than bone marrow).
• The lymphatic system is a primary source of lymphocytes.

Body Defence and Immunity

• Lymphatic (or lymphoid) tissue forms a vital part of the immune system, acting as a diffuse surveillance and defence system throughout body tissues and organs.
• It continuously monitors body fluids, removing pathogens, damaged cells, and other harmful substances.
• White blood cells (WBCs) play a central role in this defence.

2.0Lymph and Lymphatic Vessels (Lymphatics)

Lymph

Lymph is a clear, watery fluid similar to blood plasma but contains fewer proteins.

• Its composition varies depending on the specific organs or tissues it drains.
• The lymphatic system processes approximately 125 mL of lymph per hour, totalling around 2,500-2,800 mL per day.
• Nearly half of this lymph originates from the liver and small intestine.

Lymphatic Vessels (Lymphatics)

• Lymphatic capillaries begin in body tissues as tiny, blind-ended sacs.
• • They run parallel to blood capillaries.
  • It is composed of a single layer of endothelial cells, similar to those found in blood vessels, specifically blood capillaries.
  • However, they are much more permeable, allowing the entry of solvents, large solutes, and even whole cells.
• These capillaries merge to form larger lymphatic vessels, which gradually join together to become even larger channels.
• Lymphatic vessels resemble veins in structure but have some key differences:
• • They have three layers (tunica intima, media, and adventitia), but the walls are much thinner than those of veins.
  • They contain numerous one-way valves, more frequent than in veins (typically every few millimetres), ensuring unidirectional lymph flow.
  • Lymph nodes are located at intervals along vessels, where lymph is filtered and monitored by immune cells.
  • As the vessels progress, they enlarge and eventually drain into major lymphatic ducts.

Lymphatic Ducts

Lymphatic trunks merge to form two main lymphatic ducts, which are comparable to major blood vessels and more similar to veins than to arteries in structure.

Right Lymphatic Duct

• A short vessel that drains lymph from the upper right quadrant of the body:

• • Right side of the head and neck

  • Right arm

  • The right side of the thorax

• Empties into the right subclavian vein at its junction with the right internal jugular vein.

Thoracic Duct

• The larger and longer of the two ducts.
• Drains lymph from the remaining 3/4ths of the body, including:
• • Both lower limbs and abdomen (i.e., below the diaphragm)
  • Left arm
  • The left side of the head, neck, and thorax
• It begins just below the diaphragm, in front of the vertebral column.
• It is formed by the union of the lumbar trunks and the intestinal trunk, which together create a sac-like structure called the cisterna chyli.
• Ultimately, it drains into the left subclavian vein.

3.0Flow of Lymph

• Lymphatic circulation operates under very low pressure, similar to that in veins.
• Lymphatic vessels are thin-walled and equipped with one-way valves that prevent backflow, thereby maintaining the unidirectional flow of lymph.

Mechanisms Assisting Lymph Flow

• Rhythmic contractions of smooth muscle in the vessel walls.
• Body movements and skeletal muscle contractions compress vessels, helping to propel lymph.
• Pulsation of nearby arteries (many lymph vessels are bundled with arteries in the connective tissue) aids lymph movement.
• The respiratory pump (changes in thoracic pressure during breathing) also promotes lymph flow.

Role of Lymphatic Circulation

• Collects excess interstitial fluid and proteins that leak from blood capillaries and returns them to the general circulation.
• It acts as a reservoir for plasma components and even some cells that escape from the vascular system.
• Provides a one-way route for the transport of interstitial fluid back into the cardiovascular system.

Lymph Nodes

• Also known as lymph glands.
• Small, oval-shaped structures ranging in size from a pinhead to a lima bean.
• Among the most numerous lymphatic organs, there are hundreds distributed throughout the body.

Functions of Lymph Nodes

Filtration of Lymph

• As lymph flows through the sinus channels within the node, it slows down, allowing microorganisms, foreign particles, and debris to be removed.
• Phagocytic white blood cells (WBCs) lining the sinuses play a crucial role in this filtration process.

Immune Surveillance

• Lymph nodes help alert the immune system to the presence of pathogens, triggering an immune response.

Hemopoiesis (Blood Cell Formation)

• Lymphocytes and monocytes are produced within lymph nodes, thereby contributing to the immune system's function.

Structure and Flow Through Lymph Nodes

• Lymph enters the node through multiple afferent lymphatic vessels.
• It flows slowly through sinusoidal channels lined with immune cells.
• 1 to 3 efferent lymphatic vessels carry the filtered lymph out of the node.
• As lymph passes from node to node, about 99% of impurities are removed, ensuring that lymph re-entering the bloodstream is clean.

Distribution of Lymph Nodes

Lymph nodes are spread throughout the body but are often found in clusters or groups, especially in key regions:

• Submental & Submaxillary Nodes:
• It is located on the floor of the mouth and drains the nose, lips, and teeth.
• Cervical Nodes:
• It is located in the neck and drains the head and neck regions.
• Axillary Nodes:
• Found in the armpits and upper chest, draining the arms, upper thorax, and breasts.

Lymphatics of the Breast

• The breast has two sets of lymphatic vessels (distinct from mammary glands):
• • Vessels drain the skin over the breast, excluding the areola and nipple.
  • Vessels drain the deeper tissues of the breast, including the areola and nipple.
• Lymph from the breast has multiple drainage connections, including:
• • The opposite breast
  • Axillary lymph nodes (receive about 85% of breast lymph)
  • Abdominal lymph nodes
• Inguinal lymph nodes, located in the groin, drain the legs and the external genitalia.

4.0Disorders of the Lymphatic System

1. Edema

• Edema refers to the excessive accumulation of interstitial fluid due to disrupted lymphatic drainage.
• It can result from injury, inflammation, surgical damage, or parasitic infections (e.g., elephantiasis).

2. Metastatic Cancers

• Metastasis occurs when cancer cells spread from the original tumor to other parts of the body.
• Lymph nodes are common sites for metastatic cancer because:
• • Lymphatic capillaries are highly permeable, allowing easy entry of cancer cells.
  • Cancer cells often lodge in the first lymph node they encounter, causing it to enlarge and become destroyed, forming a lymphoma.
  • From there, cancer cells may spread to other lymph nodes.

3. Hodgkin Disease

• A type of lymph node cancer (malignancy).
• Early symptoms include:
• • Enlarged, often painful lymph nodes, especially in the neck
  • Fever, weight loss, anorexia, night sweats, and intense itching
• Tends to progress to nearby lymph nodes over time.

4. Non-Hodgkin Lymphoma

• A broader group of lymphatic cancers is more common than Hodgkin disease.
• Typically shows a more widespread distribution in the body.
• Has a higher mortality rate and may be less predictable in progression.

5. Ruptured Spleen

• A common result of blunt trauma to the left side of the abdomen or thorax.
• The spleen is highly vascular so that rupture can lead to life-threatening internal bleeding.
• Often requires surgical removal (splenectomy).
• This is typically not critical, as other organs (like the liver and bone marrow) can compensate for its functions.

6. Elephantiasis

• A tropical parasitic disease caused by the blockage of lymphatic vessels.
• Transmitted by mosquito bites carrying the filarial worm (a roundworm).
• Leads to severe edema, followed by fibrosis and thickening of the skin, giving an elephant-like appearance.