Lymph

Lymph (also known as lymphatic fluid) is the name given to the interstitial fluid once it has passed into the lymphatic vessels.

Formation

As blood passes through capillary beds a significant proportion of the plasma is filtered into the extracellular space. Most of this filtered tissue fluid (a.k.a. interstitial fluid) passes into lymphatic capillaries and is returned by the lymphatics to the systemic circulation.

Constituents

An accurate composition of the lymph has only been elucidated since the early 2000s. This was primarily due to the difficulty in accessing the tiny lymphatic vessels and obtaining enough lymphatic fluid to examine, but these practical obstacles have now been largely resolved .

The water, glucose and electrolyte concentrations of the lymph in the initial lymphatics are broadly similar to the plasma; however the concentrations of sodium, potassium, calcium and magnesium ions are mildly decreased, whilst chloride and bicarbonate ions are mildly elevated .

Clotting factors are present, hence lymph will coagulate when in vitro. In general, it also has a significant protein content, which represents those plasma proteins that cross the capillary walls and are returned to the systemic circulation by the lymphatics. The quantity of lymphatic protein is less than that in the plasma and is dependent on location :

• liver: 6.2 g/dL
• heart: 4.4 g/dL
• GI tract: 4.1 g/dL
• lungs: 4 g/dL
• skin: 2 g/dL
• skeletal muscle: 2 g/dL
• ciliary body: 0 g/dL

In the postprandial state, aqueous-insoluble fats pass from the gut into their lymphatics, thus this lymph takes on a milky coloration due to its high fat concentration, and is known as chyle.

The predominant cellular component of the lymph are lymphocytes, which reach the systemic circulation via the thoracic or right lymphatic ducts.