10.7 - Lymphatic System of Humans

Formation of Tissue Fluid

At the Arteriole End of a Blood Capillary

• Blood pressure is high because capillary diameter is small and the heart exerts a pumping force.
• Blood plasma diffuses continuously from the capillary into intercellular spaces.
• Plasma occupying intercellular spaces and bathing cells is called tissue fluid.
• Tissue fluid lacks erythrocytes, platelets and plasma proteins because they are too large to diffuse through the capillary wall.

Exchange Between Blood and Body Cells

• Nutrients and oxygen diffuse from tissue fluid into body cells.
• Excretory products and carbon dioxide diffuse from body cells through tissue fluid into blood capillaries.

Formation of Lymph

At the Venule End of a Blood Capillary

• Blood plasma is hypertonic to the surrounding tissue fluid and blood pressure is lower.
• Water, mineral salts and wastes are reabsorbed into the venule capillary.
• About 85% of fluid that left at the arteriole end diffuses back at the venule end.

Entry into Lymphatic Capillaries

• The remaining 15%, about 4 litres daily, enters lymphatic capillaries and is returned to the blood.
• This pale-yellow fluid is called lymph.
• Lymphatic capillary walls are one cell thick; one end is closed and the other joins a lymphatic vessel.

Comparison of Lymph, Tissue Fluid and Blood

• Lymph and tissue fluid: Both contain plasma without plasma proteins, erythrocytes and platelets.
• Lymph has more fat, fat-soluble substances and lymphocytes than tissue fluid.
• Lymph and blood: Both contain nutrients, hormones, enzymes, cellular wastes, respiratory gases and leucocytes.
• Blood, unlike lymph, contains plasma proteins, erythrocytes and platelets.

Components of the Lymphatic System

Lymph, Lymphatic Capillaries and Lymphatic Vessels

• Lymphatic capillaries join to form larger lymphatic vessels; lymph nodes occur along the vessels.
• The lymphatic system has no pump.
• Heartbeat pulse, skeletal muscle contractions, digestive-tract peristalsis and pressure changes during breathing assist lymph flow.
• One-way valves prevent backflow and direct lymph towards the heart.

Lymph Nodes and Lymphatic Organs

• Components include lymph, lymphatic capillaries, lymphatic vessels, lymph nodes and lymphatic organs.
• Lymphatic organs include lymph nodes, spleen, thymus gland, bone marrow, tonsils and appendix.

Re-entry into the Blood Circulatory System

• Right lymphatic duct: Receives lymph from the right hand, chest and right side of the head and neck; drains into the right subclavian vein.
• Thoracic duct: Receives lymph from the left side of the head, neck and chest and all body parts below the ribs; drains into the left subclavian vein.
• Lymph from the whole body returns to the blood circulatory system.

Necessity of the Lymphatic System

1. Complements the blood circulatory system: Returns excess tissue fluid to blood; maintains normal blood composition, pressure and volume.
2. Transports fat-soluble substances: Fats and fat-soluble substances enter lacteals in small-intestinal villi, travel through the thoracic duct and enter blood through the left subclavian vein.
3. Body defence: Lymph nodes produce and store lymphocytes involved in antibody production.