Lymphatic system

# Overview
The lymphatic system is a complex network of lymphoid organs, lymph nodes, lymph ducts, lymphatic tissues, lymph capillaries and lymph vessels that produce and transport lymph fluid from tissues to the circulatory system. The lymphatic system is a major part of the immune system.
The lymphatic system has three interrelated functions: (1) removal of excess fluids from body tissues, (2) absorption of fatty acids and subsequent transport of fat, as chyle, to the circulatory system and, (3) production of immune cells such as lymphocytes (e.g. antibody producing plasma cells) and monocytes.

# Lymphatic circulation
Unlike the blood system, the lymphatic system is not closed and has no central pump.  Lymph movement occurs slowly with low pressure due to peristalsis, valves, and the milking action of skeletal muscles. Like veins, lymph travels through vessels in one way only, due to semilunar valves. This depends mainly on the movement of skeletal muscles to squeeze fluid through them, especially near the joints.  Rhythmic contraction of the vessel walls through movements may also help draw fluid into the smallest lymphatic vessels, capillaries. Tight clothing can restrict this, thus reducing the removal of wastes and allowing them to accumulate. If tissue fluid builds up the tissue will swell; this is called edema. As the circular path through the body's system continues, the fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); both ducts drain into the circulatory system at the right and left subclavian veins. The system collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria. This is known as a secondary circulatory system.

# Function of the fatty acid transport system
Lymph vessels called lacteals are present in the lining of the gastrointestinal tract, predominately in the small intestine. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain, via the portal vein, into the liver for processing, fats (lipids) are passed on to the lymphatic system, to be transported to the blood circulation via the thoracic duct.  The enriched lymph originating in the lymphatics of the small intestine is called chyle. As the blood circulates, fluid leaks out into the body tissues.  This fluid is important because it carries food to the cells and waste back to the bloodstream.  The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation.  The lymph system is a one-way system, transporting interstitial fluid back to blood.

# Pathology
In elephantiasis, infection of the lymphatic vessels cause a thickening of the skin and enlargement of underlying tissues, especially in the legs and genitals. It is most commonly caused by a parasitic disease known as lymphatic filariasis.
Lymphedema also causes abnormal swelling, especially in the appendages (though the face, neck, and abdomen can also be affected).  It occurs if the lymphatic system is damaged, or underdeveloped in some way.  An estimated 170 million suffer with the disorder.
There are three stages:
Stage 1: Pressing the swollen limb leaves a pit that takes a while to fill back in.  Because there is little fibrosis (hardening) it is often reversible.  Elevation reduces swelling.
Stage 2: Pressure does not leave a pit.  Elevation does not help.  If left untreated, the limb becomes fibrotic.
Stage 3: This stage of lymphedema is often called elephantiasis.  It is generally only in the legs after lymphedema that has gone long untreated.  While treatment can help a little, it is not reversible.
Some common causes of swollen lymph nodes include staph infections, infectious mononucleosis and neoplasm, e.g. Hodgkin's and non-Hodgkin's lymphoma, and metastasis of cancerous cells via the lymphatic system.

# Development of lymphatic tissues
Lymphatic tissues begin to develop by the end of the fifth week of embryonic life. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm.
The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck and head. Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct.
The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. Capillary plexuses and lymphatic vessels spread form the retroperitoneal lymph sac to the abdominal viscera and diaphragm. The sac establishes connections with the cisterna chyli but loses its connections with neighboring veins.
The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.
With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes.
The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch.