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Area 1 - Basic Biomedical Sciences (16%)

1A Physiology

1A01 Lymphatic System

The lymphatic system consists of:

lymph

lymph vessels

lymph nodes

lymph organs, e.g. spleen and thymus

diffuse lymphoid tissue, e.g. tonsils

bone marrow.

Functions of the lymphatic system include the following.

Tissue drainage. 

Every day, 3-4 litres of fluid are drained away by the lymphatic vessels. Without this, the tissues would rapidly become waterlogged, and the cardiovascular system would begin to fail as the blood volume falls.

Absorption in the small intestine. 

Fat and fat-soluble materials, e.g. the fat-soluble vitamins, are absorbed into the central lacteals           (lymphatic vessels) of the villi.

Immunity. 
The lymphatic organs are concerned with the production and maturation of lymphocytes.

The Lympahtic System

lymph.

•  Lymph is a clear watery fluid. Lymph transports the plasma proteins that seep out of the capillary beds back to the bloodstream.
• It also carries away larger particles, e.g. bacteria and cell debris from damaged tissues
• Lymph contains lymphocytes, which circulate in the lymphatic system allowing them to patrol the different regions of the body

Lymph capillaries

• All tissues of the body have a network of lymphatic vessels, with the exception of the central nervous system, the bones and the most superficial layers of the skin.

Larger lymph vessels

• The walls of lymph vessels are about the same thickness as those of small veins and have the same layers of tissue, i.e. a fibrous covering, a middle layer of smooth muscle and elastic tissue and an inner lining of endothelium.
• Lymph vessels have numerous cup-shaped valves which ensure that lymph flows in one way only, i.e. towards the thorax.
• In addition, any structure that periodically compresses the lymphatic vessels can assist in the movement of lymph along the vessels, commonly including the contraction of adjacent muscles and the pulsation of large arteries.

Lymph nodes

•  Lymph nodes are oval or bean-shaped organs that lie, often in groups, along the length of lymph vessels.
•  The lymph drains through a number of nodes, usually 8 to 10, before returning to the venous circulation. These nodes vary considerably in size: some are as small as a pin head and the largest are about the size of an almond.

Spleen

• The Spleen is  formed by reticular and lymphatic tissue and is the largest lymph organ.
• The spleen lies in the left hypochondriac region of the abdominal cavity between the fundus of the stomach and the diaphragm.
•  It is purplish in colour and varies in size in different individuals, but is usually about 12 cm long, 7 cm wide and 2.5 cm thick. It weighs about 200 g.

Functions

Phagocytosis
Old and abnormal erythrocytes are destroyed in the spleen and the breakdown products, bilirubin and iron, are passed to the liver via the splenic and portal veins.

Storage of blood
The spleen contains up to 350 ml of blood, and in response to sympathetic stimulation can rapidly return a large part of this volume to the circulation, e.g. in haemorrhage.

Immune response
The spleen contains T- and B-lymphocytes, which are activated by the presence of antigens, e.g. in infection. Lymphocyte proliferation during serious infection can cause enlargement of the spleen (splenomegaly}.

Erythropoiesis

The spleen and liver are important sites of fetal blood cell production, and the spleen can also fulfil this function in adults in times of great need.

Thymus gland

The thymus gland lies in the upper part of the mediastinum behind the sternum and extends upwards into the root of the neck.

Function

Lymphocytes originate from pluripotent stem cells in red bone marrow. Those that enter the thymus develop into activated T-lymphocytes.