Anatomy and Physiology HAP101 Chapter Notes - Chapter 22: Perforin, Haematopoiesis, Phagosome

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HAP201 Week Five/Chapter 22 The Lymphatic System and Immunity
22.1: Lymphatic System Structure and Function
This system consists of a fluid called lymph, vessels called lymphatic vessels that transport the lymph, a number
of structures and organs containing lymphatic tissue (lymphocytes within a filtering tissue), and red bone marrow
This system circulates the body to help defend against disease-causing agents
Most components of blood plasma filter through blood capillary walls to form interstitial fluid. Once this fluid
passes into lymphatic vessels, it is called lymph (clear fluid).
o The major difference between interstitial fluid and lymph is location: IF is found between cells; lymph is
located with lymphatic vessels and lymphatic tissue
Lymphatic tissue: specialized form of connective tissue that contains large numbers of lymphocytes. There are
two types of lymphocytes that participate in adaptive immune responses: B cells and T cells
Functions of the Lymphatic System
This system has three functions
o Drain excess interstitial fluid: lymphatic vessels drain excess interstitial fluid from tissue spaces and
return it to the blood. This function is related with the CVS; without it, the maintenance of circulating
blood volume is impossible
o Transports dietary lipids: lymphatic vessels transport lipids and lipid-soluble vitamins (A, D, E, and K)
absorbed by the GI tract
o Carries out immune responses: lymphatic tissue initiates highly specific response directed against
particular microbes or abnormal cells
Lymphatic Vessels and Lymph Circulation
Lymphatic vessels begin as lymphatic capillaries
These capillaries are located in spaces between cells and are closed at one end. They unite to form larger
lymphatic vessels, which resemble small veins in structure but have thinner walls and more valves.
Lymph flows through lymph nodes, which are masses of B and T cells
In the skin, these vessels lie in the subQ tissue; they follow the same route as veins
Tissues that lack lymphatic capillaries include avascular tissue (e.g. cartilage, epidermis, the cornea of the eye),
the CNS, portions of the spleen and red bone marrow
Lymphatic Capillaries
o Capable of absorbing large molecules, such as proteins and lipids, due to a greater permeability than
blood capillaries
o These capillaries are larger in diameter than blood capillaries. They have a one-way structure than permits
interstitial fluid to only flow into them, not out.
o Ends of endothelial cells make up the wall of a lymphatic capillary overlap.
o When pressure is greater in the interstitial fluid than in lymph, the cells separate slightly, causing this
fluid to enter the lymphatic capillary
o When pressure is greater inside the lymphatic capillary, the cells adhere more closely, and lymph cannot
escape back into the interstitial fluid
o This pressure is relieved as lymph moves further down its capillary
o Anchoring filaments are attached to the lymphatic capillaries and contain elastic fibers. They extend
from the capillaries, attaching lymphatic endothelial cells to surrounding tissues. When excess interstitial
fluid accumulates and causes tissue swelling, these filaments are pulled, making the openings between
cells larger for more fluid to flow into the lymphatic capillary
o Lacteals are specialized lymphatic capillaries in the small intestine. They carry dietary lipids into
lymphatic vessels and ultimately into the blood. These lipids cause chyle, a creamy white lymph, to drain
from the small intestines. Elsewhere, lymph is a clear, pale-yellow fluid
Lymph Trunks and Ducts
o As lymphatic vessels exist lymph nodes, they unite to form lymph trunks.
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o There are five principle trunks
Lumbar trunks: drain lymph from the lower limbs, the all and viscera of the pelvis, the kidneys,
the adrenal glands and the abdominal wall
Intestinal trunk: drains lymph from the stomach, intestines, pancreas, spleen and part of the
liver.
Bronchomediastinal trunks: drain lymph from the thoracic wall, lung, and heart
Subclavian trunks: drain the upper limbs
Jugular trunks: drain the head and neck
o From these trunks, lymph travels into two main channels, the thoracic duct and the right lymphatic duct,
and then drains into venous blood.
Thoracic duct: begins as a dilation called the cisterna chyli, anterior to the second lumbar
vertebra. It is the main duct for the return of lymph to blood. It receives lymph from the right and
left lumbar trunks and from the intestinal trunk. In the neck, this duct also receives lymph from
the left jugular, left subclavian, and left bronchomediastinal trunks. Thus, the thoracic duct
receives lymph from the left side of the head, neck, and chest, the upper limb, and the entire body
inferior to the ribs. The duct drains lymph into venous blood at the junction of the left internal
jugular and left subclavian veins
Right lymphatic duct: receives lymph from the right jugular, right subclavian, and right
bronchomediastinal trunks. Thus, the right lymphatic duct receives lymph from the upper right
side of the body. From here, lymph drains into venous blood at the junction of the right internal
jugular and right subclavian veins
Formation and Flow of Lymph
o Most components of blood plasma filter freely through capillary walls to form interstitial fluid, but more
fluid filters out of the blood capillaries than returns to them by reabsorption
o The excess fluid drains into lymphatic vessels and becomes lymph
o Interstitial fluid contains small amounts of protein. Proteins that do leave blood plasma cannot return to
the blood by diffusion, as the concentration gradient opposes such movement. The proteins can move
through the more permeable lymphatic capillaries into lymph. Thus, an important function of lymphatic
vessels is to return the lost plasma proteins and plasma to the bloodstream.
o The sequence of fluid flow is blood capillaries (blood) interstitial spaces (interstitial fluid) lymphatic
capillaries (lymph) lymphatic vessels (lymph) lymphatic ducts (lymph) junction of the internal
jugular and subclavian veins (blood).
o The same structures that help return venous blood to the heart also help maintain the flow of lymph
Skeletal muscle pump: the “milking action” of skeletal muscle contractions compresses
lymphatic vessels (and veins) and forces lymph towards the junction of the internal jugular and
subclavian veins
Respiratory pump: lymph is maintained by pressure changes that occur during inhalation. It
flows from the abdominal region (higher pressure) towards the thoracic region (lower pressure).
During exhalation, pressure reverses and the valves in the lymphatic vessels prevent backflow of
lymph. Also when the vessel distends, the smooth muscle in its walls contracts, helping lymph
move from one segment of the vessel to the next
Lymphatic Organs and Tissue
Lymphatic organs and tissues are classified into two groups based on their function
o Primary lymphatic organs: sites where stem cells divide and become immunocompetent, capable of
mounting an immune response. These organs include the red bone marrow and the thymus. Pluripotent
stem cells in red bone marrow produce mature, immunocompetent B cells and pre-T cells. The pre-T cells
migrate to the thymus to become immunocompetent
o Secondary lymphatic organs and tissues: sites where most immune responses occur. They include
lymph nodes, the spleen, and lymphatic nodules. The thymus, lymph nodes and spleen are considered
organs, as each is surrounded by a connective tissue capsule; lymphatic nodules are not organs, as they
lack a capsule.
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Document Summary

Hap201 week five/chapter 22 the lymphatic system and immunity. This system consists of a fluid called lymph, vessels called lymphatic vessels that transport the lymph, a number of structures and organs containing lymphatic tissue (lymphocytes within a filtering tissue), and red bone marrow. This system circulates the body to help defend against disease-causing agents. Most components of blood plasma filter through blood capillary walls to form interstitial fluid. Once this fluid passes into lymphatic vessels, it is called lymph (clear fluid): the major difference between interstitial fluid and lymph is location: if is found between cells; lymph is located with lymphatic vessels and lymphatic tissue. Lymphatic tissue: specialized form of connective tissue that contains large numbers of lymphocytes. There are two types of lymphocytes that participate in adaptive immune responses: b cells and t cells. This system has three functions: drain excess interstitial fluid: lymphatic vessels drain excess interstitial fluid from tissue spaces and return it to the blood.

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