The lymphatic system - physiology

The body cells are bathed in  tissue fluid, which leaks constantly out of the bloodstream through the permeable walls of blood capillaries . 

it is therefore very similar in composition to blood plasma. Some tissue fluid returns to the capillaries at their venous end and the reminder diffuses through the more permeable walls of lymph capillaries, forming in the lymph.

Lymph passes through vessels of increasing size and a varying number of lymph nodes before returning to the blood.

The lymphatic system consists of:

  • Lymph
  • Lymph vessels
  • Lymph nodes 
  • Lymph organs 
  • Mucosa-associated lymphocytes tissue ( MALT)

1. Lymph

Lymph is a clear watery fluid, similar in. composition to plasma, with the important exception of plasma proteins, and identical in composition to interstitial (tissue) fluid.

 Lymph transport 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 tissue, which can then be filtered out and destroyed by the lymphoid nodes.

 Lymph contains lymphocytes, which circulate in the lymphatic system allowing them to patrol the different regions of the body.

2. Lymph capillaries

 These originate as blind-end tubes in the. interstitial spaces.

 They have the same structure as a blood capillary, e.g. a single layer of endothelial cells, but. their. walls are more permeable to all interstitial fluid constitutes, including. proteins and cell debris. 

The tiny capillaries join up to form larger lymph vessels.
3. Larger lymph vessels

 Lymph vessels are often found running. alongside the arteries and veins serving. the area. Their walls 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. linings of endothelium.

like - veins, lymph vessels have numerous cup-shaped valves. to ensure that lymph vessels have in a. one- way system towards the thorax. 

 There is no pump, like the heart, involved in the onward movement of. lymph, but the muscle layer in the walls of the large lymph vessels have an.  intrinsic ability to contract rhythmically.

  Lymph vessels become larger as they join together eventually forming two larger. ducts - 

a) Thoracic duct 

b) Right lymphatic duct

a). Thoracic duct  
 .   this duct begins at the cisterna chyli, which is a dilated lymph be situated and front of the bodies of the first two lumbar vertebrae. The duct is about 40. cm long and opens into the left subclavian vein in the root of the neck.

b) Right lymphatic duct

  this is a dilated lymph vessel about 1cm long. It lies in the root of the neck and. opens into the right subclavian vein. It. drains lymph from the right half of the. thorax.

4) Lymphatic organs and tissue 

   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 the number of nodes, usually 8-10 before returning to the. venous circulation. 

These nodes vary considerably in size: some are as small as a pinhead and the largest are about the size of an almond.


  1: Filtering and phagocytosis 

  2:Proliferation of lymphocytes 


 the spleen contains reticular and lymphatic tissue and is the largest lymph 

  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.


  1.  Phagocytosis 
  2. Storage of blood 
  3. Immune response
  4. Erythropoiesis

Thymus gland 

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

 it weighs about 10 to 15g at birth and. grows until puberty, when it begins to atrophy. Its maximum weight, at puberty, is between 30 and 40 g and by middle age it has returned to approximately its weight at birth.


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

 Mucosa - Associated Lymphoid Tissue (MALT)

. they contain B and T lymphocytes, which have migrated from bone marrow
  and the thymus, and are important in the early detection of invaders. 

however, as  they have no afferent lymphatic vessels, The main group of MALT are the tonsils and aggregated lymphoid follicles.


these are located in the mouth and throat, and will therefore destroy swallowed and inhaled antigens.

  Aggregated lymphoid follicles: these large collections of lymphoid tissue are found in the small intestine, and intercept swallowed antigens.