16 The lymphatic system
As blood passes through the capillaries in the tissues, fluid oozes out through the porous walls and circulates through the tissues, bathing every cell. This fluid is called tissue or interstitial fluid; it fills the interstices or the spaces between the cells that form the different tissues (Fig. 16.1). It is a clear, watery, straw-coloured fluid similar to the plasma of the blood from which it is derived. While blood circulates only through the blood vessels, tissue fluid circulates through the actual tissue and carries nutrients, oxygen and water from the blood stream to each individual cell and carries away its waste products such as carbon dioxide, urea and water, transmitting them to the blood. It is, in other words, the carrying medium between the tissue cells and the blood.
Fig. 16.1 The circulation of tissue fluid. Fluid passes out into the tissues and is collected partly by the blood capillary and partly by the lymph capillary.
Of the fluid that escapes from the capillaries into the tissues, a certain amount passes back through the capillary wall but its return is more difficult than its escape owing to the constant stream of oncoming blood that fills the capillaries. The excess fluid that cannot return directly into the blood stream is collected and returned to the blood by a second set of vessels, which form the lymphatic system; the fluid that these vessels contain is called lymph. If fluid accumulates in the interstitial tissues and is not removed by the lymphatic system, oedema results.
The components of the lymphatic system
The lymphatic system comprises four types of structure:
Lymphatic capillaries
The lymphatic capillaries arise in the spaces in the tissues as fine hair-like vessels with porous walls. These gather up excess fluid from the tissues and unite to form the lymphatic vessels. The walls of lymph capillaries are permeable to substances of greater molecular size than those that can pass through the walls of blood capillaries.
Lymphatic vessels
The lymphatic vessels are thin-walled, collapsible tubes similar in structure to the veins but carrying lymph instead of blood. They are finer and more numerous than the veins and, like them, are provided with valves to prevent the lymph moving in the wrong direction (Fig. 16.2). Lymphatic vessels are found in most tissues except the central nervous system but run particularly in the subcutaneous tissues and pass through one or more lymphatic nodes (Figs 16.3–16.5).
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