Page 164 - 84 human physiolofy part-1

Chapter 9

lymph nodes due to a radical mastectomy). Edema is common in the lower extremities when people
spend a lot of time sitting, because the fluid return is based largely on the massaging action of skeletal
muscles.

Lymphatic Vessels and Ducts

The lymphatic vessels are similar in structure to the cardiovascular veins, meaning they also have
valves. They are dependent upon the contraction of skeletal muscle, respiratory movements and valves
that do not allow backward flow. The vessels merge before entering one of two ducts.

• Thoracic Duct: This duct is much larger than the lymphatic duct. It serves the abdomen,
lower extremities and the left side of the upper body (head, neck, and arm)
• Right Lymphatic Duct: This duct serves all of the right side of the upper body and thoracic
area (head, neck).

Organs, Tissues and Cells of the Immune System

The immune system consists of a network of lymphatic organs, tissues, and cells. These structures
are supported by the reticuloendothelial system: loose connective tissue with a network of reticular
fibers. Phagocytic cells, including monocytes and macrophages, are located in the reticular connective
tissue. When micro-organisms invade the body, or the body encounters antigens (such as pollen),
antigens are transported to the lymph. Lymph is carried through the lymph vessels to regional lymph
nodes. In the lymph nodes, the macrophages and dendritic cells phagocytose the antigens, process
them, and present the antigens to lymphocytes, which can then start producing antibodies or serve as
memory cells. The function of memory cells is to recognize specific antigens in the future.

Primary Lymphatic Organs The primary lymphatic organs are the red bone marrow and the
thymus. They and are the site of production and maturation of lymphocytes, the type of white blood
cell that carries out the most important work of the immune system.

• Red Bone Marrow Red bone marrow, the soft, spongy, nutrient rich tissue in the cavities
of certain long bones, is the organ that is the site of blood cell production.

Some of the white blood cells produced in the marrow are: neutrophils, basophils, eosinophils,
monocytes, and lymphocytes. Lymphocytes differentiate into B lymphocytes and T lymphocytes. Red
bone marrow is also the site of maturation of B lymphocytes. T lymphocytes mature in the thymus.

• Thymus Gland The thymus gland is located in the upper thoracic cavity posterior to the
sternum and anterior to the ascending aorta. The thymus is an organ that is more active in
children, and shrinks as we get older. Connective tissue separates the thymus into lobules,
which contain lymphocytes. Thymic hormones such as thymosin are produced in the thymus
gland. Thymosin is thought to aid in the maturation of T lymphocytes. The Thymus is critical
to the immune system. Without a thymus, a person has no ability to reject foreign substances,
blood lymphocyte level is very poor, and the body’s response to most antigens is either absent
or very weak

Immature T lymphocytes travel from the bone marrow through the bloodstream to reach the

164 | Human Physiology