What is Lymphatic System And Function with Structure

What is Lymphatic system?

Lymphatic system Structure and Function

The lymphatic system (lim-FAT-ik) 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. The lymphatic system assists in circulating body fluids and helps defend the body against disease-causing agents.

As you will see shortly, most components of blood plasma filter

through blood capillary walls to form interstitial fluid. After interstitial fluid passes into lymphatic vessels, it is called lymph (LIMF - clear fluid). The major difference between interstitial fluid and lymph is location: Interstitial fluid is found between cells, and lymph is located within lymphatic vessels and lymphatic tissue.

Lymphatic tissue is a specialized form of reticular connective tissue  that contains large numbers of lymphocytes.

Recall from Chapter 19 that lymphocytes are agranular white

blood cells . Two types of lymphocytes participate in adaptive immune responses: B cells and T cells.

Functions of the Lymphatic System

The lymphatic system has three primary functions:

1. Drains excess interstitial fluid. Lymphatic vessels drain excess interstitial fluid from tissue spaces and return it to the blood. This function closely links it with the cardiovascular

system. In fact, without this function, the maintenance of circulating blood volume would not be possible.

2. Transports dietary lipids. Lymphatic vessels transport lipids and lipid-soluble vitamins (A, D, E, and K) absorbed by the gastrointestinal tract.

3. Carries out immune responses. Lymphatic tissue initiates highly specific responses directed against particular microbes or abnormal cells.

Lymphatic Vessels and Lymph Circulation

Lymphatic vessels begin as lymphatic capillaries. These capillaries, which are located in the spaces between cells, are closed at one end . Just as blood capillaries converge to form

venules and then veins, lymphatic capillaries unite to form larger lymphatic vessels , which resemble small veins in structure but have thinner walls and more valves. At intervals along the lymphatic vessels, lymph flows through lymph nodes,

encapsulated bean-shaped organs consisting of masses of B cells and T cells. In the skin, lymphatic vessels lie in the subcutaneous Tissue and generally follow the same route as veins; lymphatic vessels of the viscera generally follow arteries, forming plexuses (networks) around them. Tissues that lack lymphatic capillaries

include avascular tissues (such as cartilage, the epidermis, and the

cornea of the eye), the central nervous system, portions of the spleen, and red bone marrow.

Lymphatic Capillaries

Lymphatic capillaries have greater permeability than blood capillaries and thus can absorb large molecules such as proteins and lipids. Lymphatic capillaries are also slightly larger in diameter than blood capillaries and have a unique one-way structure that

permits interstitial fluid to flow into them but not out. The ends of

endothelial cells that make up the wall of a lymphatic capillary overlap. When pressure is greater in the interstitial

fluid than in lymph, the cells separate slightly, like the opening of a one-way swinging door, and interstitial fluid enters the lymphatic capillary. When pressure is greater inside the lymphatic capillary,the cells adhere more closely, and lymph cannot escape back into

interstitial fluid. The pressure is relieved as lymph moves further

down the lymphatic capillary. Attached to the lymphatic capillaries are anchoring filaments, which contain elastic fibers. They extend out from the lymphatic capillary, attaching lymphatic endothelial cells to surrounding tissues. When excess interstitial fluid

accumulates and causes tissue swelling, the anchoring filaments are pulled, making the openings between cells even larger so that more fluid can flow into the lymphatic capillary.

 In the small intestine, specialized lymphatic capillaries called lacteals carry dietary lipids into lymphatic vessels and ultimately into the blood .

The presence of these lipids causes the lymph draining from the small intestine to appear creamy white; such lymph is referred to as chyle  Elsewhere, lymph is a clear, pale-yellow Fluid.

Lymph Trunks and Ducts

As you have already learned, lymph passes from lymphatic capillaries into lymphatic vessels and then through lymph nodes. As lymphatic vessels exit lymph nodes in a particular region of the body, they unite to form lymph trunks. The principal trunks are

the lumbar, intestinal,bronchomediastinal, subclavian, and jugular trunks . The lumbar trunks drain lymph

from the lower limbs, the wall and viscera of the pelvis, the kidneys, the adrenal glands, and the abdominal wall. The intestinal trunk drains lymph from the stomach, intestines, pancreas, spleen,and part of the liver. The bronchomediastinal trunks drain lymph from the thoracic wall, lung, and

heart. The subclavian trunks drain the upper limbs. The jugular trunks drain the head and neck. 

 Lymph passes from lymph trunks into two main channels, the thoracic duct and the right lymphatic duct, and then drains into venous blood. The thoracic (left lymphatic) duct is about 38–45 cm (15–18 in.) long and begins as a dilation called the cisterna chyli anterior to the second lumbar vertebra. The thoracic duct is the main duct for the return of lymph to blood. The cisterna chyli receives lymph from the right and left lumbar trunks and from the intestinal trunk. 

In the neck, the thoracic duct also receives lymph from the left

jugular, left subclavian, and left bronchomediastinal trunks. Therefore, the thoracic duct receives lymph from the left side of the head, neck, and chest, the left upper limb, and the entire body

inferior to the ribs . The thoracic duct in turn drains lymph into venous blood at the junction of the left internal jugular and left subclavian veins.

 The right lymphatic duct is about 1.2 cm(0.5 in.) long and 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 the right lymphatic duct, lymph drains into venous blood at the junction of the right internal jugular and right subclavian veins.

Formation and Flow of Lymph

Most components of blood plasma, such as nutrients, gases, and hormones, filter freely through the capillary walls to form interstitial fluid, but more fluid filters out of blood capillaries than re-

turns to them by reabsorption . The excess filtered fluid—about 3 liters per day—drains into lymphatic vessels

and becomes lymph. Because most plasma proteins are too large to leave blood vessels, interstitial fluid contains only a small amount of protein. Proteins that do leave blood plasma cannot

return to the blood by diffusion because the concentration gradient (high level of proteins inside blood capillaries, low level outside) opposes such movement. The proteins can, however, move readily 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.

 Like veins, lymphatic vessels contain valves, which ensure the

one-way movement of lymph. As noted previously, lymph drains into venous blood through the right lymphatic duct and the thoracic duct at the junction of the internal jugular and subclavian

veins . Thus, the sequence of fluid flow is blood capillaries (blood) n interstitial spaces (interstitial fluid) n lymphatic

capillaries (lymph) n lymphatic vessels (lymph) n lymphatic

ducts (lymph) n junction of the internal jugular and subclavian veins (blood).  illustrates this sequence, along with the

relationship of the lymphatic and cardiovascular systems. Both systems form a very efficient circulatory system.

 The same two “pumps” that aid the return of venous blood to the heart maintain the flow of lymph.