The intracellular fluid of the cytosol or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells. It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments. The contents of a eukaryotic cell within the cell membrane, excluding the cell nucleus and other organelles (e.g. mitochondria, plastides, lumen of ER etc.), is referred to as the cytoplasm.
The cytosol is a complex mixture of substances dissolved in water. Although water forms the large majority of the cytosol, its structure and properties within cells is not well understood. The concentrations of ions such as sodium and potassium are different in the cytosol than in the extracellular fluid; these differences in ion levels are important in processes such as osmoregulation and cell signaling. The cytosol also contains large amounts of macromolecules, which can alter how molecules behave, through macromolecular crowding.
Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all body fluid outside of cells . In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma. The extracellular fluid also includes the transcellular fluid; making up only about 2.5% of the ECF. In humans, the normal glucose concentration of extracellular fluid that is regulated by homeostasis is approximately 5 mm. The pH of extracellular fluid is tightly regulated by buffers and maintained around 7.4. The volume of ECF is typically 15L (of which 12L is interstitial fluid and 3L is plasma).
Blood plasma is the straw-colored/pale-yellow liquid component of blood that normally holds the blood cells in whole blood in suspension. It makes up about 55% of total blood volume. It is the intravascular fluid part of extracellular fluid (all body fluid outside of cells). It is mostly water (93% by volume) and contains dissolved proteins (major proteins are fibrinogens, globulins and albumins), glucose, clotting factors, mineral ions (Na+, Ca++, Mg++, HCO3- Cl- etc.), hormones, and carbon dioxide (plasma being the main medium for excretory product transportation). Plasma also serves as the protein reserve of the human body. It plays a vital role in intravascular osmotic effects that keep electrolyte levels balanced and protects the body from infection and other blood disorders.
Interstitial fluid (or tissue fluid) is a solution that bathes and surrounds the cells of multicellular animals. The interstitial fluid is found in the interstitial spaces, also known as the tissue spaces. On average, a person has about 11 liters (2.4 imperial gallons or ~2.9 U.S. gal) of interstitial fluid, providing the cells of the body with nutrients and a means of waste removal.
Transcellular fluid is the portion of total body water contained within epithelial lined spaces. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. It is often not calculated as a fraction of the extracellular fluid, but it is about 2.5% of the total body water. Examples of this fluid are cerebrospinal fluid, and ocular fluid, joint fluid.