Is interstitial fluid (essentially) plasma or serum?
Basically, the interstitial fluid is found in spaces between the tissues and consists of a water solvent containing amino acids, sugars, fatty acids, coenzymes, hormones, neurotransmitters, salts, 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 passes 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.
