Lymphatic Vessels

Additional Specialisations of Vessels

Small arteries and veins often form anastomosing networks, which provides routes for alternative blood supply and drainage if one of the vessels should become occluded because of pathological or normal physiological circumstances. Some arteries are however the only supply of blood to their target tissues. These arteries are call end arteries. Tissues which are supplied by end arteries die if the arteries become occluded.

The segments of the kidney and the heads of the gastrocnemius muscle are examples of tissues supplied by end arteries.

Arteries and veins may also form arteriovenous shunts, which can shunt the blood flow that otherwise would enter the capillary network between the vessels. These shunts usually contain specialisations of the smooth muscle in the region of the shunt. Arteriovenous shunts are frequently seen in the blood supply of distal parts of the limbs and the nose (thermoregulation) and in the blood supply of endocrine organs.

Lymphatic Vessels

Parts of the blood plasma will exude from the blood vessels into the surrounding tissues because of transport across the endothelium or because of blood pressure and the fenestration of some capillaries (this process is partly counteracted by the higher osmotic pressure of the blood). The fluid entering tissues from capillaries adds to the interstitial fluid normally found in the tissue. The surplus of liquid needs to be returned to the circulation. Lymph vessels are dedicated to this unidirectional flow of liquid, the lymph. Three types of lymph vessels can be distinguished based on their size and morphology.

Lymph capillaries

are somewhat larger than blood capillaries and very irregularly shaped. They begin as blind-ending tubes in connective tissue. The basal lamina is almost completely absent and the endothelial cells do not form tight junctions, which facilitates the entry of liquids into the lymph capillary. Temporary openings in the endothelial lining of the lymph capillaries also allow the entry of larger particles into the lymph capillaries (lipid droplets, which are absorbed from the lumen of the gut do not enter blood capillaries, but enter the circulation via lymph vessels which are found in the villi of the ileum and jejunum). Lymph capillaries merge to form

Lymph collecting vessels

which are larger and form valvesbut otherwise appear similar to lymph capillaries. The lymph is moved by the compression of the lymph vessels by surrounding tissues. The direction of lymph flow is determined by the valves. Lymph vessels empty intermittently into lymph nodes from which the lymph continues in efferent lymph vessels.

Only very little lymph is returned from the limbs if they are immobilized, which illustrates the importance of muscular action in lymph transport. This is also the reason for immobilizing limbs that are either infected or that have been bitten by venomous Australians. The effect can also be observed after long intercontinental flight when you may feel that your shoes and socks are just about one number too small. Finally, impeded lymph drainage is one of the problems associated with surgery which requires the removal of lymph nodes and which thereby interrupts the lymph collecting vessels.

Eventually the lymph collecting vessels merge to form

Lymph ducts

which contain one or two layers of smooth muscle cells in their wall (some textbooks call this layer the tunica media of lymph vessels). They also form valves. The walls of the lymph ducts are less flexible in the region of the attachment of the valves to the wall of the duct, which may give a beaded appearance to the lymph ducts. Peristaltic contractions of the smooth muscle contribute to the movement of lymph towards the heart in addition to the compression of the ducts by surrounding tissues.

The largest lymph duct of the body, the thoracic duct, drains lymph from the lower half and upper left quadrant of the body and empties the lymph into the circulation by merging with the vascular system close to the junction of the left internal jugular and subclavian veins. That it is the largest lymph duct does not mean that it is a large vessel when compared to the large arteries and veins. It actually is not much larger (about 5mm in diameter) than one of the superficial forearm veins.

 
Suitable Slides

Sections of small intestine - H&E

Jejunum, baboon - H&E At low magnification you will see villi extending into the lumen of the jejunum. Find villi which are cut longitudinally and change to medium magnification. In some of the villi you will see fairly large open spaces, which are surrounded by a layer of flattened endothelial cells. They should not contain any red blood cells - if they do you are looking at a capillary, which also should be somewhat smaller. These openings represent the blind end of lymph capillaries which originate in the villi. Their name, lacteals, is derived from the milky appearance of the lymph. This appearance is caused by suspended lipid droplets which enter these lymph capillaries.  Draw a villus containing a lymph capillary. Focus on the features of the lymph capillary and the surrounding connective tissue. Include, if possible, a blood capillary in your drawing. The jejunum slide is also good for revision. You should be able to find columnar epithelium, goblet cells, smooth muscle, small nerves, a few ganglion cells and, of course, lots of loose connective tissue and blood vessels.