lymphatic system

lymphatic system This comprises a body-wide network of branching lymphatic vessels (carrying the fluid lymph); the lymph nodes and other lymphoid tissue; the lymphocytes (in the circulating blood); the spleen, the bone marrow, and, in early life, the thymus.

That blood circulates is common knowledge. The less dramatic circulation of lymph is less well known, but although its interruption has less immediately hazardous consequences, it is physiologically essential. Also, the lymphoid tissue is a vital component of the body's immune system.

All blood capillaries are to some extent leaky, though the leakiness varies between different organs and tissues. The hydrostatic pressure inside the capillaries is greater than that in the tissues, pushing fluid out. The fluid that leaks out into the interstices among the cells is a little of the watery part of the blood plasma, with all the substances it contains except for the larger proteins. Conversely, some water moves into the capillaries because of the osmotic pull of those proteins. Normally, there is a small net loss from the blood. This fluid movement has the effect of refreshing the tissue fluid in the immediate environment of the cells. Fluid does not accumulate in the tissues, but moves away by entering the blind ends of microscopic lymphatic channels, which are present in all organs and tissues except the central nervous system. These vessels drain into progressively larger ones, and they have valves which maintain flow towards the chest. En route, lymph vessels encounter lymph nodes or other lymphoid tissue such as the patches which lie behind the lining of the large intestine, and the tonsils and adenoids at the back of the throat and nose. The lymph passing through these is exposed to phagocytes which pick up any foreign material, notably bacteria, and take part in the processes of the immune system mediated by the lymphocytes which populate the lymphoid tissue. Thus any invader which gets further than the point of entry, and travels in the lymph, will be ‘challenged’ at the first lymph node it reaches; for this reason an infection for example in a finger may give rise to an inflamed lymph node at the elbow, or if it gets past there, in the armpit; or a sore throat or mouth ulcer can cause tender swollen lymph nodes in the neck. The lymphoid tissue in the wall of the large intestine performs a similar function for any bacterial or other invasion from the faeces.

The lymph nodes are outposts of the immune system, well placed to challenge bacterial invaders. Cancer cells gain access to lymphatic vessels in tumour tissue, and spread by this route; further spread may be initially forestalled at the lymph nodes. For this reason, surgical removal of a malignant tumour may also involve a clearance of the lymph nodes to which its vessels drain. If lymph vessels draining a part of the body (such as an arm) are blocked or removed, there will inevitably be a collection of excess fluid in the catchment area of those vessels: this is one cause of oedema.

The lymph from the whole body (except the central nervous system) finally drains into vessels at the base of the neck (the major one is the thoracic duct) and flow through these into the venous blood stream, on its way to the heart. Thus the fluid lost from the blood capillaries is recycled. Overall, the rate of flow is about 4 litres in 24 hours.

The lymph drained from the small intestine has an additional function: it carries the fats absorbed from the food. Because of this, the lymph in the thoracic duct (known as chyle) has a high fat content; it also has a high protein content because although a very small fraction of the plasma proteins escape from blood capillaries in the tissues, the amount collected from the whole body is significant.

Illustration

Sheila Jennett

See also blood; immune system; oedema; thymus.