LYMPH VESSELS

Lymph vessels are about the same thickness as a small vein with the same layers of tissue i.e. fibrous covering, middle smooth muscle and elastic tissue, and an inner lining of endothelium. Cup shaped valves in these vessels ensures that the lymph flows one way and not backwards. There is no pump involved like the heart in the movement of lymph, but the vessels do have an intrinsic ability to contract rhythmically. Any structure that is involved in this system can assist in the movement of lymph through contraction of adjacent muscles and the pulsation of large arteries. Lymph vessels increase in size forming together to form lymph ducts.

LYMPH CAPILLARIES

Originated from blind-end tubes in the interstitial spaces, the lymph capillaries are very similar to blood capillaries. Their walls however are more permeable to all interstitial fluid constituents, Inc proteins and cell debris. These capillaries join up to form lymph vessels.

LYMPH NODES

These are less than 1cm in thickness, and are bean shaped organs. They lie often grouped together along the length of lymph vessels. Lymph is drained through the nodes, before returning to the venous circulation. Their main function is to remove foreign materials which are extracted from the lymph and deposited there. Nodes contain macrophages (as discussed earlier in blood) which can engulf and destroy invaders. They also store lymphocytes (white blood cells), and release them when necessary.

LYMPH

This is a clear watery fluid which transport plasma proteins that seep out of the capillary beds back into the bloodstream. It is also responsible for transporting larger particles such as bacteria and cell debris from the damaged tissues back to the lymph nodes to be destroyed. Lymph contains lymphocytes (as discussed earlier in blood) which circulate in the lymphatic system.

TISSUE FLUID

All of our cells in our body are bathed in lymph, where they receive their vital nutrients and oxygen to survive. If cells are fixed to a tissue they are incapable of surviving independently, and protect themselves from toxic chemicals pH changes or extreme temperatures. They can also not seek food, ingest or remove their own toxin products. This is where tissue fluid is vital. This service which is carried out by both blood and lymph is responsible for:

·        Transportation of oxygen, nutrients and hormones to the tissues where they move out of the capillaries (atrial end) into the tissue fluid. Mechanisms of movement used include osmosis, diffusion and filtration. Movement is via the cells, and cells passing waste into the tissue fluid.

·        Large proportion of waste materials pass from tissue fluid back into blood in the capillaries (venous end)

·        Remaining waste passed into lymph in the lymph vessels. Lymph returned to the blood stream where the vessels join the subclavian veins.

The difference in protein of tissue fluid and blood plasma is all because of protein molecules that are too large to pass through the capillary membranes and so are retained in the blood. This helps to maintain the osmotic pressure of the blood.

The main differences between blood plasma, tissue fluid & lymph are shown in the table below.

SPLEEN

The spleen is the largest lymphatic organ and is formed by reticular and lymphatic tissue. It is situated in the abdominal cavity between the fundus of the diaphragm of the stomach. The spleen is a large oval reddish coloured organ. Its main function is to search for old, degenerated red blood cells and break them down into their component chemicals (Bilirubin and iron) for use else where in the body. They are transported via the splenic and portal veins to the liver. Unlike lymph nodes the spleen is not exposed to diseases spread by lymph because it has no afferent lymphatics entering it.

THYMUS GLAND

This is situated in the upper part of the medistinum behind the sternum and extends upwards into the root of the neck. This gland grows until puberty and then begins to atrophy, gradually returning to its weight that it was at birth. The structure of this gland consists of two lobes joined by areolar tissue. They are enclosed in a fibrous capsule which divides into lobules. These consist of an irregular branching framework of epithelial cells and lymphocytes. The main function is to help and develop active T-lymphocytes (as discussed previously)