Interstitial fluids

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Interstitial fluid makes up approximately 25% of the extracellular body fluid compartments whereas plasma makes up the remaining 8% of the extracellular body fluid compartment. the leaky epithelium situated between the interstitial fluid and the plasma allows the exchange of major cations such as sodium and potassium and it also allows the exchange of major anions such as chloride and phosphates however due to despite proteins being one of the major anions, due to the size of the proteins they cannot pass through the leaky epithelium. The movement of water between the extracellular body fluid compartments is propagated by osmotic and hydrostatic pressures which is based on the Starling-Landis hypothesis which states that a decrease in interstitial fluid hydrostatic pressure releases water into the capillaries, and an increase in interstitial fluid colloid osmotic pressure draws the water back into the interstitial fluid. therefore to be able to deduce which component of the extracellular fluid we are analysing all we need to do is analyse the protein content whereby the plasma would have a higher concentration of protein due to the inability of the proteins passing through the leaky epithelium. Alongside this, if an increase is detected in the fluid amount or the protein content within the interstitial fluid, then the lymphatic system releases the contents in the form of lymphatic vessels. Due to the difference in composition of anions and cations within the body fluid compartments the system is said to be in a state of chemical disequilibrium this is mainly due to large differences in the sodium content between the extracellular and intracellular body fluid compartments and the presence of membranes which prevents diffusion from occuring [1][2][3].


  1. (2012) Interstitial Fluid and Lymph Formation and Transport: Physiological Regulation and Roles in Inflammation and Cancer. [online] Available at: [Accessed: 25 Nov 2012].
  2. Sloane, E. (1994) Untitled. london: jones and bartlett publishers international, p.336-339
  3. wiseGEEK (2003) What Is Interstitial Fluid?. [online] Available at: [Accessed: 25 Nov 2012].


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