Biology 12

10.6 The Lymphatic System

The lymphatic system consists of lymphatic vessels and the lymphoid organs. It is closely associated with the circulatory system.The lymphatic system has three main functions:• Lymphatic capillaries absorb excess tissue fluid and

return it to the bloodstream• Lymphatic capillaries absorb fats from the digestive

tract and transport them to the bloodstream• Lymphoid organs defend the body against disease

Lymphatic vessels form a one-way system that begins with the lymphatic capillaries.Lymphatic capillaries (tiny, closed-ended vessels) absorb excess tissue fluid called lymph.• Tissue fluid contains water, solutes (nutrients,

electrolytes, oxygen), and cellular products (hormones, enzymes, wastes)

Movement of Lymph in Lymphatic VesselsThe lymphatic capillaries join to form lymphatic vessels that merge before entering one of two ducts: • Thoracic duct: returns lymph collected from the left side of the body into the left

subclavian vein• Right lymphatic duct: returns lymph collected from the right side of the body into the

right subclavian vein

Movement of Lymph in Lymphatic Vessels• Lymph percolates through various lymph nodes, where foreign

material can be recognized by the immune system• Movement of lymph in the lymphatic capillaries is

dependent on skeletal muscle contraction• One-way valves in the vessels prevent lymph

from flowing backward

Biology 12

EdemaEdema is localized swelling caused by the accumulation of tissue fluid that has not been collected by the lymphatic system. • Occurs if too much tissue fluid is made and/or if not enough is drained away• Can lead to tissue damage and death

Lymphoid OrgansLymphoid organs contain large numbers of lymphocytes (white blood cells involved in adaptive immunity).There are two types of lymphoid organs:• Primary lymphoid organs: red bone marrow and thymus, where lymphocytes develop

and mature• Secondary lymphoid organs: lymph nodes and spleen, where lymphocytes become

activated

Primary Lymphoid Organs

Red bone marrow• Contains a network of connective tissue fibres, along

with stem cells that can divide and produce blood cells• Lymphocytes begin development in the bone marrow

o B lymphocytes (B cells) begin in bone marrow and then migrate to secondary lymphoid organs to mature

o T lymphocytes (T cells) begin in bone marrow and then migrate to the thymus, where they mature and differentiate

Thymus• Connective tissue divides the thymus into lobules, which are

filled with T cells and supporting cells• Lobules are lined with epithelial cells that secrete hormones

called thymosins, which are involved in the differentiation of T cells

• T cells that react to the body’s own cells undergo apoptosis (programmed cell death)

• T cells that leave the thymus can react to foreign molecules

Biology 12

Secondary Lymphoid Organs

Spleen• Consists of blood vessels and sinuses where

macrophages remove old and defective blood cells• Also contains small areas of lymphoid tissue, where

lymphocytes can react to foreign invaders in the blood• May be surgically removed due to trauma or disease,

however the body becomes more susceptible to certain types of infections

Lymph nodes• Occur along lymphatic vessels• Connective tissue divides nodes into nodules, each of which

contain B cells, T cells, and a sinus• As lymph passes through the sinuses, macrophages engulf

pathogens in the lymph• Cancer cells can enter the lymph nodes and lymphatic

vessels and move through to other regions of the body, where they produce secondary tumours (metastasis)