Lymphatic System: MDCAT Biology notes

Why a lymphatic system is needed

As blood flows through capillaries, plasma filters out through the capillary walls to form interstitial (tissue) fluid, which bathes the cells. Most of it returns to the blood capillaries, but some is left behind along with proteins that have leaked out. The lymphatic system drains this excess interstitial fluid and returns it to the blood. Once the fluid enters lymph vessels it is called lymph.

Blood, tissue fluid and lymph compared

Component	Blood	Tissue fluid / lymph
Red blood cells	Present	Absent
Large plasma proteins	Present in plenty	Very few
Water, glucose, $CO_2$, salts, amino acids	Present	Present
White cells	Present	Lymphocytes present in lymph

So RBCs and large proteins distinguish blood from interstitial fluid, while water, glucose and $CO_2$ are common to both.

Structure

• Lymph capillaries begin blindly in the tissues among the cells. Their walls have larger intercellular gaps than blood capillaries, so proteins, debris and even bacteria can enter.
• They join into larger lymph vessels, which have valves (like veins) to prevent backflow. Lymph is moved along by contraction of surrounding skeletal muscles and breathing movements; there is no pump.
• Lacteals are lymph capillaries in the centre of each villus of the small intestine. They absorb digested fats (the fatty lymph is called chyle).
• Vessels finally join into two main ducts. The large thoracic duct empties lymph into the left subclavian vein; the right lymphatic duct into the right subclavian vein. So the final destination of lymph is the subclavian veins (i.e. the venous blood).

Lymphoid organs

Lymph nodes along the vessels filter lymph; their lymphocytes and macrophages remove microorganisms and debris. Other lymphoid organs and tissues are the spleen, thymus, tonsils and Peyer's patches of the intestine. The lungs are not part of the lymphatic system.

Functions

1. Returns tissue fluid and proteins to blood, maintaining blood volume, pressure and composition.
2. Absorbs fats from the intestine through lacteals.
3. Defence: lymphocytes and lymph nodes fight infection.

The lymphatic system does not carry oxygen; that is the work of red blood cells, so oxygen-carrying capacity is not affected by its absence.

Oedema

If lymph vessels are blocked (for example by parasites or surgery), tissue fluid cannot drain. The first result is excess fluid accumulating in the tissues, causing swelling called oedema.

Common MDCAT traps

• The thoracic duct is a lymph vessel, not the final destination; lymph ends in the subclavian vein.
• Lacteals are lymph capillaries, not blood capillaries or venules.
• Lymph contains no RBCs; "CO2, glucose and water" is the set common to blood and lymph.
• Backflow of lymph is prevented by valves, not by lymph nodes.
• Peyer's patches are lymphoid tissue; fat is absorbed by lacteals.

Quick revision

• Lymph = tissue fluid inside lymph vessels.
• Lymph capillaries end blindly in tissues and are very permeable.
• Lymph returns to blood via the subclavian veins.
• Lacteals absorb fats.
• Blocked lymph drainage causes oedema.