Lymph is formed from blood. The blood pressure at the arterial end of the capillaries is high, so blood plasma is forced out through the capillary walls into the spaces between the cells. The white blood corpuscles can move. They can change their shape to squeeze through the capillary walls. Tissue fluid is diluted plasma containing white corpuscles. It contains no red blood corpuscles, because these are too large to be able to pass through the capillary walls.

If the blood plasma continuously moved out of the blood capillaries the blood would lose far too much liquid. To prevent this, a small amount of tissue fluid is absorbed by the blood capillaries at the vein end where the blood pressure is lower than the osmotic pressure in the blood capillaries. Most of the water is absorbed back into the blood capillary by osmosis.

Most of the tissue fluid is collected as lymph into another system of tubes that return it to the blood. These tubes are called lymphatic vessels or lymphatics.

Lymphatic Capillaries among the tissue cells differ from blood vessels in that they may be blind-end tubes or they may form a network into which excess lymph drains. Lymph capillaries unite to form bigger lymphatics. These eventually come together to form two large lymphatics, namely, the thoracic duct and the smaller right lymphatic duct. The thoracic duct passes from the abdomen right through the thorax or chest to open into the left subclavian vein under the collarbone. Opening into the thoracic duct are the lymphatics from the posterior part of the body, the left side of the thorax and the left side of the neck and head. Therefore, lymph from all these regions drains into the thoracic duct. The lacteals of the villi of the small intestine are a special set of lymphatics. They absorb the digested fats, carry them to the thoracic duct and then to the blood for distribution throughout the body.

The right lymphatic duct lies on the right side of the thorax. It collects lymph from the right side of the body and returns it to the blood streams as it opens into the right subclavian vein.

Lymph flows very slowly, being pushed along by the contraction and relaxation of muscles. When muscles contract they press onto the lymph vessel and this pushed the lymph along. Lymph vessels possess valves similar to those in the veins to prevent backflow so that the flow of lymph is only in one direction.

The Lymph Glands