78 Steps Health Journal

Blood and lymph tissues play important roles in defense systems

The components of the mammalian defense system are dispersed throughout the body and interact with almost all of its other tissues and organs. The lymphoid tissues, which include the thymus, bone marrow, spleen, and lymph nodes, are essential parts of the defense system (Figure 18.1), but central to their functioning are the blood and lymph.

Blood and lymph are both fluid tissues that consist of water, dissolved solutes, and cells. Blood plasma is a yellowish solution containing ions, small molecular solutes, and solu

In the lymph nodes, fluids are filtered and white blood cells mature.

Lymph ducts conduct lymph.

T cells mature in the thymus.

Heart

Thoracic duct

Lymphocytes accumulate and mature in the spleen.

B cells mature in the bone marrow.

In the lymph nodes, fluids are filtered and white blood cells mature.

Lymph ducts conduct lymph.

T cells mature in the thymus.

Heart

Thoracic duct

Lymphocytes accumulate and mature in the spleen.

B cells mature in the bone marrow.

18.1 The Human Lymphatic System A network of ducts and vessels collects lymph from the body's tissues and carries it toward the heart, where it mixes with blood to be pumped back to the tissues. The thymus, spleen, and bone marrow are also essential to the body's defense system.

ble proteins. Suspended in the plasma are red blood cells, white blood cells, and platelets (cell fragments essential to clotting) (Figure 18.2). While red blood cells are normally confined to the closed circulatory system (the heart, arteries, capillaries, and veins), white blood cells and platelets are also found in the lymph.

Lymph is a fluid derived from blood and other tissues that accumulates in intercellular spaces throughout the body. From these spaces, the lymph moves slowly into the vessels of the lymphatic system. Tiny lymph capillaries conduct this fluid to larger vessels that eventually join together, forming one large vessel, the thoracic duct, which joins a major vein (the left subclavian vein) near the heart. By this system of vessels, the lymph is eventually returned to the blood and the circulatory system.

At many sites along the lymph vessels are small, roundish structures called lymph nodes, which contain a variety of white blood cells. As fluid passes through a node, it is filtered and "inspected" for nonself materials by these defensive cells.

Myeloid progenitor cell

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