We have studied that blood carries food and oxygen around the body. But blood never comes in contact with body cells. So, how do food and oxygen get from the blood to the body cells where they are needed? This happens with the help of plasma which leaks from the blood capillaries around the body cells.
This plasma which leaks out from the blood capillaries is called tissue fluid. We can now say that: The liquid from the blood which is forced out through the capillary walls and moves between all the body cells (providing them with food and oxygen, and removing carbon dioxide) is called tissue fluid.
Actually, the walls of blood capillaries are very thin. So, when blood flows through the capillaries, a liquid called tissue fluid leaks from the blood capillaries and goes into tiny spaces between the various body cells in the tissues.
The tissue fluid carries food and oxygen from the blood to the cells, and picks up their waste products like carbon dioxide. After doing its job, most of the tissue fluid seeps back into blood capillaries.
The remaining tissue fluid carrying large protein molecules, digested fat, germs from the cells and fragments of dead cells, enters into another type of tiny tubes called lymph capillaries and it becomes lymph. This lymph (along with its contents) is returned to the blood by another type of transport system in the human body called lymphatic system. We will now describe the lymphatic system in brief.
Lymphatic System :
A system of tiny tubes called lymph vessels (or lymphatics) and lymph nodes (or lymph glands) in the human body which transports the liquid called lymph from the body tissues to the blood circulatory system is called lymphatic system. The lymphatic system consists of the following parts:
(i) Lymph capillaries,
(ii) Larger lymph vessels,
(iii) Lymph nodes (or Lymph glands), and
Lymph capillaries are tiny tubes which are present in the whole body (just like blood capillaries). Lymph capillaries, however, differ from blood capillaries in two ways: lymph capillaries are closed ended (the end of lymph capillaries in the tissues of the body is closed), and the pores in the walls of lymph capillaries are bigger in size (than that of blood capillaries).
Since the ends of the lymph capillaries in the body tissues are closed, so the tissue fluid can only seep into the walls of the lymph capillaries present in the body tissues. Moreover, since the pores in the walls of the lymph capillaries are somewhat: bigger, so even large protein molecules present in the tissue fluid can enter into lymph capillaries (which could not pass into blood capillaries).
The lymph capillaries join to form larger lymph vessels. The lymph vessels have lymph nodes (or lymph glands) at intervals. The lymph nodes contain special type of cells called lymphocytes. Lymph nodes containing lymphocytes are involved in the cleaning of lymph and protecting the body from disease. The lymph vessels are connected to large veins of the blood circulatory system.
Lymph is a light yellow liquid which is somewhat similar in composition to blood plasma. Lymph is not red like blood because it does not contain red blood cells. Lymph contains large protein molecules and digested food (which come into it from the tissue fluid between the cells).
It also contains germs from the cells and fragments of dead cells. Lymph is another medium of circulation in the human body. But lymph flows in only one direction - from body tissues to the heart.
Since lymph is derived from the tissue fluid which remains outside the cells of the body, so it is also called extracellular fluid. Lymph contains a special type of white blood cells called lymphocytes which help in fighting infection and disease.
Lymph containing large protein molecules, digested fat, germs and fragments of dead cells from the tissue fluid around the body cells seeps into the lymph capillaries present throughout the body. From lymph capillaries, lymph passes into larger lymph vessels containing lymph nodes.
In the lymph nodes, lymph is cleaned by white blood cells called lymphocytes. These white blood cells eat the germs and dead cells, and also make antibodies for protecting the body from disease.
The cleaned lymph containing large protein molecules, digested fat and other useful materials is transported by lymph vessels to the large veins (called subclavian veins) which run just beneath the collar bone. These veins carry the lymph to the heart. In this way, the circulation of lymph from the body tissues to the heart is completed.