The Lymphatic System

It is made up of a fluid called lymph, the vessels that transport this liquid, called lymphatic vessels and a group of structures and organs related to the lymph.

The lymphatic system has three essential functions:

• Draining the interstitial liquid.
• Transporting fats from the digestive system to the blood.
• Defending our body. Lymph transport many leucocytes and some lymphatic organs and structures are related to the immune system.

There are two types of lymphatic vessels.

• Lymphatic capillaries: they are small, thin vessels, made of endothelial cells with bottle-like shape. This shape is related to the junction between successive cells, forming a valve that makes the lymph move only in one direction. These junctions, besides, can open and close to allow the interstitial liquids, the substances and the defensive cells to enter the lymphatic capillaries.
• Lymphatic vessels: they are vessels with thin walls and many valves that prevent lymph from flowing in the opposite direction. The lymphatic system doesn't have any organ to pump the lymph, so preventing the reflux is essential. The lymphatic vessels are formed when the capillaries join and, just like the blood vessels, they have anastomoses. The biggest lymph vessels are called lymphatic trunks. Through these vessels our body can transport between two and four litres of lymph. The lymph moves due to the peristaltic contractions of the muscular wall that can be found in the lymphatic vessels. Indeed, they can contract several times per minute. 

The big vessels, besides, run between skeletal muscles that press the vessels when they contract, draining the lymph and forcing it to move. This process is known as milking. All the blood vessels end up in to big trunks that are connected to the circulatory system in two points: the angle that form the left internal jugular vein and the left subclavian vein, and the angle that form the right internal jugular vein and the right subclavian vein.

Regarding the lymphatic organs, the most relevant ones are these:

• Lymphatic ganglions: they are oval in structure, with variable size, from one to twenty five centimetres in diameter, that are located along the lymphatic vessels. The human body has between six hundred and seven hundred ganglions, isolated or forming groups. Sometimes these groups are divided into superficial and deep groups. They are made up of an external capsule and an internal trabecular system. The capsule is made of dense connective tissue. The interior of the ganglion builds up lymphocytes and macrophages. The ganglions have two main functions. They can be used as reservoirs for lymph. The most relevant function, however, is the filtration of lymph, detecting invaders, such as viruses and bacteria that are trapped in the interior of the ganglion and attacked by the defensive cells. 
• Tonsils: they are an aggregation of lymphatic ganglions forming a ring that surrounds the pharynx. They protect the body against inhaled or ingested invaders.
• Spleen: this is an oval organ, around twelve centimetres long, located in the upper left part of the abdomen, between the diaphragm and the stomach. It is the place where lymphocytes B grow. It is also related to the destruction of damaged erythrocytes and lymphocytes. Blood can also be built up in this organ, being released it to the circulatory system when it is required.
• Thymus: bilobulated organ located in the upper part of the mediastinum. It is the place where the lymphocytes T grow. It is really active in children, becoming inactive in many adults.