Major Functions of the Lymphatic System

The lymphatic system is essential for our survival. This system has three main functions:

1. To collect and recycle the excess interstitial fluid and its dissolved substances,

2. To absorb fats and other substances from the digestive tract (this topic will be discussed in the Digestive System Unit), and

3. To initiate and coordinate an immune response to remove cellular debris, bacteria, toxins, fungi, parasites, and viruses that accumulate in our bodies.

Because this system has the two very different functions of maintaining the proper fluid balance in the body and protecting the body from harmful infections, we will begin its study by 1) investigating the lympathic vessels and lymph which function in fluid balance and then 2) investigate how these structures along with lymphatic cells, tissues and organs function in protecting the body from infections. Infection can be viewed as the invasion and multiplication of microorganisms that are not normally present within the body. An infection may remain at the location where it entered the body, or it may spread through the body via blood or lymphatic vessels. Immunity is the state of having sufficient defenses (resistance) against infections that might disrupt homeostasis. Immunity involves both non-specific, inherent components (innate immunity) and specific, acquired from previous exposure components (adaptive immunity).

It is important to realize that although immunity will be considered here in the context of human anatomy and physiology, it is not restricted to humans or animals. The ability to defend itself from “non-self” invaders appears, as early as in bacteria defending themselves from viral attacks, and it is an inherent homeostatic mechanism present in all types of cells, plants, and animals. As organisms evolved, so did the immune system. Thus, while the innate

system is present in all animals, only vertebrates present the adaptive response.

Lymph Tissue and Lymphatic Vessels

Like the circulatory system that carries blood throughout the body, the lymphatic system is made of a series of vessels, capillaries, and organs. These structures collect excess fluid and cellular debris from the tissues and return them back to the blood.

In the circulatory system, blood flows from arteries, through capillaries and into veins to be returned to the heart. On its way through the capillaries, some of the fluid passes out across the capillary wall and into the interstitial fluid in a process called capillary filtration. This filtration tends to occur across the arterial end of the capillary, with most of the filtered fluid being reabsorbed at the venous end of the capillary. This leaves a small amount of fluid that remains in the interstitial spaces between cells. This filtered fluid is mostly plasma plus any plasma proteins that might have leaked from the blood vessel as well. This excess interstitial fluid is collected by the lymphatic system. The fluid flows through the lymphatic vessels until it is returned to the circulatory system to again become a component of blood. Once interstitial fluid passes into lymphatic vessels, it is called lymph. Lymph is a clear, pale-yellow fluid connective tissue.

The lymphatic system consists of many different tissues and organs that are found throughout the body. Some organs provide the environment for the development and maturation of leukocytes. Other tissues and organs trap pathogen and are the sites where leukocytes can interact with the pathogen. The circulatory and lymphatic systems interact to connect these organs and tissues.

Fluid moves from blood capillaries into the interstitial spaces. Most of the fluid returns to the blood, but some of the fluid moves from the interstitial spaces into lymphatic capillaries to become lymph. To collect the lymph from the interstitial space, lymph capillaries originate in the blood capillary beds, and lymph vessels run parallel to the veins. At intervals along the lymphatic vessels, lymph flows through lymph nodes. Fluid collected in the lymph system is returned to the heart via veins in the chest. Unlike the circulatory system, the lymphatic system does not flow through a closed, circular system. There are no lymph arteries. Lymph fluid is not pumped around the body. Instead, the lymph system collects the lymph into vein-like structures called lymph vessels and returns it to the bloodstream.

Lymph Vessels and Capillaries

The lymphatic system contains both capillaries and vessels. Lymphatic vessels begin as capillaries. Both of these structures are thin walled, which allows lymph to be transported across the membrane and collected in the vessels. Lymphatic capillaries have greater permeability than blood capillaries and can absorb large molecules such as proteins and lipids. The endothelial cells that make up the wall of a lymphatic capillary lack a basement membrane loosely attach to each other and slightly overlap. Interstitial fluid enters the lymphatic vessel when the pressure is greater in the interstitial fluid than in lymph and nothing in the interstitial fluid is excluded from entering the lymphatic capillaries. When pressure is greater inside the lymphatic capillary, the endothelial cells prevent lymph from passing back into the interstitial spaces by acting like a one-way swinging door. Lymphatic capillaries are found wherever blood capillaries are located except in the central nervous system and bone marrow.

Anatomy of a Lymphatic Vessel

Lymphatic capillaries unite to form larger lymphatic vessels. Structurally, lymphatic vessels are similar to veins because they also have one-way valves that function like gates to ensure the lymph only flows in one direction. Like veins, skeletal muscle contraction exerts pressure on the lymph vessels and forces the lymph forward through them. Lymph vessels are like one-way roads, with the lymph being collected at the capillary beds and travels through the body into the thoracic cavity. Lymph is deposited in one of two large ducts in the chest region: the right lymphatic duct and the thoracic duct. The lymph then travels from these ducts into venous circulation via the subclavian and jugular veins.Unlike the cardiovascular circulation, the lymphatic circulation lacks a pump like the heart. Lymphatic vessels are low-pressure vessels similar to veins and the same muscle pump and respiratory pump that promote venous return also facilitate lymph flow. Therefore, even though there is some smooth muscle in lymphatic vessels, movement of the body is important to lymph circulation.

Common Dysfunctions of the Lymphatic Circulation Anything that would disrupt the flow of lymph could contribute to significant swelling of tissues (edema). Lymphedema is a condition of localized fluid retention and a tissue swelling caused by a compromised lymphatic system. Lymphedema can be primarily caused genetically or secondarily due to injury or obstruction of lymphatic vessels. It is most frequently seen after lymph node dissection, surgery and/or radiation, in which lymphatic system damage is caused during the treatment of cancer, usually breast cancer. Lymphedema may also be associated with parasitic infections in which parasites obstruct lymph vessels. Symptoms may include fatigue, a swollen limb or localized fluid accumulation in other body areas, including the head and neck, discoloration of the skin overlying the swollen tissue and eventually deformity (elephantiasis).