Interstitial Fluid - Composition

Composition

Interstitial fluid consists of a water solvent containing sugars, salts, fatty acids, amino acids, coenzymes, hormones, neurotransmitters, as well as waste products from the cells.

The composition of tissue fluid depends upon the exchanges between the cells in the biological tissue and the blood. This means that tissue fluid has a different composition in different tissues and in different areas of the body.

Not all of the contents of the blood pass into the tissue, which means that tissue fluid and blood are not the same. Red blood cells, platelets, and plasma proteins cannot pass through the walls of the capillaries. The resulting mixture that does pass through is, in essence, blood plasma without the plasma proteins. Tissue fluid also contains some types of white blood cell, which help combat infection.

Lymph is considered to be extracellular fluid until it enters the lymphatic vessels where it is then considered to be lymph. The lymphatic system returns protein and excess interstitial fluid to the circulation.

The ionic composition of the interstitial fluid and blood plasma vary due to the Gibbs-Donnan effect. This causes a slight difference in the concentration of cations and anions between the two fluid compartments.