Fluid that circulates throughout the lymphatic system
Structure of the immune system
Lymph
Diagram showing the formation of lymph frominterstitial fluid (labeled here as "Tissue fluid"). Note how the tissue fluid is entering the blind ends oflymph capillaries (shown as deep green arrows).
Lymph (from Latinlympha'water')[1] is the fluid that flows through thelymphatic system, a system composed oflymph vessels (channels) and interveninglymph nodes whose function, like thevenous system, is to return fluid from the tissues to be recirculated. At the origin of the fluid-return process,interstitial fluid—the fluid between the cells in allbody tissues[2]—enters thelymph capillaries. This lymphatic fluid is then transported via progressively larger lymphatic vessels through lymph nodes, where substances are removed by tissuelymphocytes and circulating lymphocytes are added to the fluid, before emptying ultimately into the right or the leftsubclavian vein, where it mixes with centralvenous blood.
Because it is derived from interstitial fluid, with which blood and surrounding cells continually exchange substances, lymph undergoes continual change in composition. It is generally similar toblood plasma, which is the fluid component of blood. Lymph returnsproteins and excess interstitial fluid to thebloodstream. Lymph also transports fats from thedigestive system (beginning in thelacteals) to the blood viachylomicrons.
Bacteria may enter the lymph channels and be transported tolymph nodes, where the bacteria are destroyed.Metastatic cancer cells can also be transported via lymph.
Lymph has a composition similar but not identical to that ofblood plasma. Lymph that leaves a lymph node is richer inlymphocytes than blood plasma is. The lymph formed in thehuman digestive system calledchyle is rich intriglycerides (fat), and looks milky white because of its lipid content.
Formation of interstitial fluid from blood.Starling forces are labelled: the hydrostatic pressure is higher proximally, driving fluid out; oncotic forces are higher distally, pulling fluid in.
Blood suppliesnutrients and importantmetabolites to the cells of atissue and collects back the waste products they produce, which requires exchange of respective constituents between the blood and tissue cells. This exchange is not direct, but instead occurs through an intermediary calledinterstitial fluid, which occupies the spaces between cells. As the blood and the surrounding cells continually add and remove substances from the interstitial fluid, its composition continually changes. Water andsolutes can pass between the interstitial fluid and blood viadiffusion across gaps incapillary walls calledintercellular clefts; thus, the blood and interstitial fluid are indynamic equilibrium with each other.[3]
Interstitial fluid forms at thearterial (coming from the heart) end of capillaries because of the higher pressure of blood compared toveins, and most of it returns to itsvenous ends andvenules; the rest (up to 10%) enters thelymph capillaries as lymph.[4] (Prior to entry, this fluid is referred to as thelymph obligatory load, or LOL, as the lymphatic system is effectively "obliged" to return it to the cardiovascular network.[5]) The lymph when formed is a watery clear liquid with the same composition as the interstitial fluid. However, as it flows through the lymph nodes it comes in contact with blood, and tends to accumulate more cells (particularly, lymphocytes) and proteins.[6]
Lymph returnsproteins and excess interstitial fluid to thebloodstream. Lymph may pick up bacteria and transport them to lymph nodes, where the bacteria are destroyed.Metastatic cancer cells can also be transported via lymph. Lymph also transports fats from thedigestive system (beginning in thelacteals) to the blood viachylomicrons.
Tubular vessels transport lymph back to the blood, ultimately replacing the volume lost during the formation of the interstitial fluid. These channels are the lymphatic channels, or simplylymphatics.[7]
Unlike the cardiovascular system, the lymphatic system is not closed. In some amphibian andreptilian species, the lymphatic system has central pumps, calledlymph hearts, which typically exist in pairs,[8][9] but humans and other mammals do not have a central lymph pump. Lymph transport is slow and sporadic.[8] Despite low pressure, lymph movement occurs due toperistalsis (propulsion of the lymph due to alternate contraction and relaxation ofsmooth muscle tissue), valves, and compression during contraction of adjacent skeletal muscle andarterialpulsation.[10]
Lymph that enters the lymph vessels from the interstitial spaces usually does not flow backwards along the vessels because of the presence of valves. If excessivehydrostatic pressure develops within the lymph vessels, though, some fluid can leak back into the interstitial spaces and contribute to formation ofedema.
The flow of lymph in thethoracic duct in an average resting person usually approximates 100ml per hour. Accompanied by another ~25ml per hour in other lymph vessels, the total lymph flow in the body is about 4 to 5 litres per day. This can be elevated several fold while exercising. It is estimated that without lymphatic flow, the average resting person would die within 24 hours.[11]
This sectionneeds expansion. You can help byadding to it.(March 2018)
Histopathological examination of the lymph system is used as a screening tool forimmune system analysis in conjunction with pathological changes in otherorgan systems andclinical pathology to assess disease status.[12] Although histological assessment of the lymph system does not directly measure immune function, it can be combined with identification of chemicalbiomarkers to determine underlying changes in the diseased immune system.[13]
In 1907 the zoologistRoss Granville Harrison demonstrated the growth of frog nerve cell processes in a medium of clotted lymph. It is made up of lymph nodes and vessels.
In 1913, E. Steinhardt, C. Israeli, and R. A. Lambert grewvaccinia virus in fragments oftissue culture from guinea pigcornea grown in lymph.[14]
^Warwick, Roger; Peter L. Williams (1973) [1858]. "Angiology (Chapter 6)".Gray's anatomy. illustrated by Richard E. M. Moore (Thirty-fifth ed.). London: Longman. pp. 588–785.
![[icon]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aWiki_letter_w_cropped.svg)
