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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 Vessels
The 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
Edema
Edema 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 Organs
Lymphoid 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)