The lymphatic system includes lymph vessels, lymph nodes, spleen, thymus, tonsils, and bone marrow. Lymph vessels carry lymph fluid and white blood cells through the body and to lymph nodes, which filter the lymph and harbor lymphocytes. The spleen, thymus, tonsils, and bone marrow are lymphoid tissues that help produce and store lymphocytes to fight infection and disease. Together, these components of the lymphatic system help maintain fluid balance in tissues, absorb fats and transport white blood cells throughout the body as part of the immune system.

1. LYMPHATIC SYSTEM
Definition
Lymphatic system is a closed system of vessels which draws the extra-tissue
fluid into the blood vascular system.
Components of Lymphatic System-
 Lymph and lymph vessels
 Lymph
 Lymph capillaries Lymph vessels proper
 Terminal lymph ducts (trunks)
 Lymphoid tissue
 Lymph nodes
 Spleen
 Tonsils
 Thymus
 Bone marrow
Functions of Lymph and Lymphatic System-
1. It helps to maintain interstitial tissue pressure.
2. Lymph carries protein molecules, electrolytes and other macromolecules
back from interstitial fluid to circulation.
3. It helps in transport of lymphocytes, red blood cells (RBCs), antigens and
antigen presenting cells to the secondary lymphoid organs.
4. Lymph nodes and spleen help to destroy any foreign particles and
microorganisms in circulation, thus guarding against them.
5. The digested fats in small intestines are absorbed into the lymph vessels
and carried to the liver and the circulation.
6. It supplies oxygen and nutrients to selected parts of the body.
LYMPH
 Lymph is a clear, colourless liquid with a composition similar to blood
plasma.
 Lymph carries particulate material, colloids, and macromolecules from
tissue fluid.
 This helps maintain the low protein concentration of tissue fluid.

2.  Lymph also clots on standing due to presence of clotting factors.
 Lymphocytes are the most abundant cellular component of the lymph.
Rate of flow of Lymph
About 120 mL of lymph flows into blood per hour. Out of this, about 100
mL/hour flows through thoracic duct and 20 mL/hour flows through the right
lymphatic duct.
Composition of Lymph
It is formed by 96% water and 4% solids.
Origin of the lymph

3. Function of lymph-
 Lymph act as “middle man” which transport oxygen, food material and
hormones
 Lymph components perform phagocytosis
 It brings CO2 and other metabolic wastes from the body cell to blood
 Cell are kept moist by lymph
LYMPH VESSELS
Lymph Capillaries
o These begin blindly in the extracellular spaces and communicate freely
with adjacent lymph capillaries.
o The structure of lymph capillaries is basically similar to that of blood
capillaries, but is adapted for much greater permeability. There is an inner
lining of endothelium.
o As compared to blood capillaries, much larger molecules can pass
through the walls of lymph capillaries. These include colloidal material,
fat droplets, and particulate matter, such as bacteria.
Places Where Lymph Capillaries are Absent
 Avascular structures like epidermis, cornea, and cartilage
 Brain and spinal cord
 Splenic pulp
 Bone marrow
 Liver lobule
 Lung units Superficial fascia

4. Lymph Vessels-
They are formed by the convergence of lymph capillaries,
Characteristics of Lymph Vessels
They consist of single layer of endothelium surrounded by smooth muscle fibers
and elastic tissue in their walls.
Large trunks and thoracic duct have structure similar to that of vein, i.e., they
have three distinct layers
(i) tunica intima
(ii) tunica wall
(iii) tunica adventitia
Numerous valves, similar to those in veins are present in small as well as large
lymphatic vessels. They are more in number than in veins. The valves often
give lymph vessels a beaded appearance.
TERMINAL LYMPH DUCTS
Lymph vessels become larger as they join together, eventually forming the two
terminal ducts-
(i)right lymphatic duct
(ii) thoracic duct which empty lymph into the subclavian vein.
Thoracic Duct
It is a common lymphatic trunk which begins at upper end of the confluence of
lymphatics a dilated sac like structure present at the level of first two lumbar
vertebrae. The duct is 40-45 cm long with a beaded appearance due to presence
of numerous valves in lumen.
It enters thorax along with aorta through aortic opening of diaphragm and ends
by opening into the junction of subclavian vein and internal jugular vein.

5. Thoracic duct drains the lymphatics from the entire body except the right side of
head and neck, right upper limb, right lung, right thoracic wall, right half of
heart and the convex surface of liver.
Right Lymphatic Duct
The right lymphatic duct is about 1 cm long. It opens into the right subclavian
vein and drains lymph from right side of head and neck, right thorax, and right
arm.
LYMPHATIC ORGANS
Lymphoid Tissue-
The lymphoid tissues are part of tissue macrophage system that plays an
important role in immunological surveillance. They are formed by aggregation
of lymphocytes, macrophages, plasma cells. They help to destroy bacteria,
foreign bodies, old RBCs and white blood cells (WBCs). The lymphoid tissue
can be primary or secondary-

6. Lymph Nodes-
 It is also known as a “check post”
 They are small masses of lymphoid tissue usually present in groups along
the course of lymphatic vessels.
 As a rule, lymph from any part of the body passes through one or more
lymph nodes before entering the bloodstream. Lymph nodes act as filters
removing bacteria and other particulate matter from lymph. Lymphocytes
are added to lymph in these nodes.
Structure
Lymph node is of bean-shaped and 2.5cm in length. Its concave surface is
called the hilum, through which blood vessels enter and leave the lymph
node. Several lymph vessels enter the node on its convex surface.
The lymph node is covered by a connective tissue capsule A number of
septa extend into the node from the capsule dividing the node into a number
of lobules.
Lymph node has an outer cortex and inner medulla. The cortex does not
extend into the hilum
Cortex
The cortex is made up of densely packed lymphocytes and stains darkly.
There are several rounded masses of lymphocytes called lymphatic follicles
or lymphatic nodules mainly composed of B-lymphocytes.
Medulla
In this zone, the lymphocytes are arranged in the form of branching and
composed of both B- lymphocytes and T -lymphocytes.
Capsule
A lymph node is surrounded by a capsule.
Cells of Lymph Node
The cells are predominantly lymphocytes. Both B-lymphocytes and T-
lymphocytes are present in lymph nodes.
Afferent lymph vessels are numerous while efferent lymph vessel is single.

7. 3 Superficial regions where lymph nodes cluster -
 Inguinal nodes of groin
 Auxiliary nodes of armpit
 Cervical nodes of neck
Function of lymph node-
filter Lymph Preventing foreign particles from entering bloods
Preventing foreign particles from entering blood stream .
Produces lymphocytes
SPLEEN
Spleen is the oval shaped largest single mass of lymphoid tissue in the body.
It is red-brown (purplish) in color, soft and friable. Normally, it is a blood-
forming organ in fetal life and Blood-destroying organ in postnatal life
(graveyard of RBCs). Since it is in the bloodstream, it filters the blood from
blood borne antigens and microorganisms.
Location
The spleen is situated in the left hypochondrium opposite the Levels of 9th
,
10th
and 11th
ribs.
Dimensions
It is a wedge-shaped organ. It is about 2.5 cm thick, 7 cm wide, 12cm long,
200 gm in weight and is related to 9th
10th
11th
ribs.

8. Like other lymphoid tissues the spleen is a center where both B-lymphocytes
and T-lymphocytes multiply, and play an important role in immune
responses., the spleen is the only site where an immune response can be
started against antigens present in circulating blood.
The spleen contains the largest aggregations of macrophages of the
mononuclear phagocyte system. (In the spleen the main function of these
cells is the destruction of RBCs that have completed their useful life.
This is facilitated by the intimate contact of blood with the macrophages
because of the presence of an open circulation. Macrophages also destroy
worn out leukocytes and bacteria.
In fetal life the spleen is a center for production of all blood cells. In later life
only lymphocytes are produced here.
The spleen is often regarded as a store of blood that can be thrown into the
circulation when required. This function is much less important in man than
in some other species.
Blood supply
Arterial supply- Splenic artery branch of coelic trunk
Venous return- through splenic vein
TONSILS
Tonsils are the collection of lymphoid tissues facing into aerodigestive tract
The tonsils are the first line of defence of the body against bacterial invasion.
They differ from a lymph node in having an incomplete capsule and no
lymph sinus, so that, the tissue fluid is filtered directly.
Waldeyer’s Ring
At the entrance into the alimentary tract, there collections of lymphoid tissue
which are collectively called are the “Waldeyer’s ring”

9. THYMUS
The thymus is a lymphatic pinkish grey color, roughly pyramidal mass of
lymphoid tissue situated in the superior mediastinum (it may extend into the
anterior mediastinum and also into the root of the neck).
The weight of the thymus is 10-15gm at birth, the thymus is relatively large; its
size increases gradually till puberty. Then it atrophies gradually, getting
infiltrated by fatty and fibrous tissue.
The thymus is situated behind the manubrium sterni, and anterior to the aortic
arch and its branches. It is supplied by branches of internal thoracic and inferior
thyroid arteries. The veins drain into the corresponding veins
Functions of Thymus
 The stem cells coming to the thymus from the bone marrow, mature in
the thymus and become immunologically competent, i.e. they react only

10. against proteins foreign to the body. These lymphocytes are thrown into
the circulation. They lodge themselves in lymph nodes and spleen.
 Thymus produces a number of hormones, such as thymulin, thymopoietin
and thymosin.