10.7 - Lymphatic System of Humans

The Formation of Tissue Fluid

When the blood reaches the arterial end of the capillaries, the high blood pressure will cause the fluid to leak out into the spaces between the cells. The fluid is known as interstitial fluid or tissue fluid.

Figure 17 - Higher blood pressure at the arterial end of the capillary will force fluid out into the intercellular spaces. The tissue fluid does not contain any red blood cells, platelets and plasma proteins as they are too big to diffuse through. Tissue fluid allows the exchange of nutrients and waste products between the blood and body cells.

Lymph Formation and Components of the Lymphatic System

Around 85% of the fluid that leaves the arterial end of the capillary re-enters at the venous end due to lower pressure. The remaining 15% of the fluid still remains in the interstitial space. However, they must return to the circulatory system to maintain normal blood volume. The fluid will return through the lymphatic system. The lymphatic system consists of a network of lymphatic capillaries, larger lymphatic vessels and lymph nodes. Once the fluid enters the lymphatic capillaries, they are known as lymph. From the lymphatic vessels, the lymph eventually drains into the thoracic duct or the right lymphatic duct.

• The thoracic duct receives the lymph from the left side of the head, neck, chest, left upper limb, and the entire body below the ribs.
• The right lymphatic duct receives the lymph from the right side of the head, neck, arm and shoulder area.

Figure 18 - The lymphatic system consists of lymph capillaries, lymphatic vessels, lymph nodes, thymus and spleen.

The lymphatic system does not have a pump to move the lymph through the lymphatic vessels. The lymph moves to the subclavian veins with the help of:

• One-way valves
• Contractions of the skeletal muscles
• Intestinal movements
• Changes in pressure during inhalation and exhalation

The thoracic duct empties the lymph into the left subclavian vein while the right lymphatic duct empties the lymph into the right subclavian vein.

Comparison between lymph and tissue fluid.

• Similarities

  • Plasma does not have plasma proteins, erythrocytes and platelets

• Differences

  • Lymph

    • Fats content: Higher
    • Lymphocytes content: High

  • Tissue fluid

    • Fats content: Lower
    • Lymphocytes content: Low

Comparison between lymph and blood.

• Similarities

  • Plasma contains nutrients, hormones, enzymes, cellular wastes, respiratory gases and leucocytes.

• Differences

  • Lymph

    • Plasma proteins: Absent
    • Erythrocytes: Absent
    • Platelets: Absent

  • Blood

    • Plasma proteins: Present
    • Erythrocytes: Present
    • Platelets: Present

The Necessity of the Lymphatic System

• Complements the blood circulatory system

  • Returns the excess fluid back into the blood system.
  • Maintain normal blood pressure and blood volume in the body.

• Body defense

  • Lymph nodes produce and store lymphocytes.
  • Protect body against infections.

• Transport fat-soluble substances

  • Lacteals are lymphatic capillaries that carry fat-soluble substances.
  • Lipids substances are transported to the thoracic duct and blood circulatory system via the left subclavian vein.