Physiology Topics
The total amount of water in a human varies from 45-75%. Females have less water because they have more fat than males (muscle is a water-rich tissue). Body water can be classified as intracellular (cytoplasm) or extracellular fluids. The composition of the intracellular fluid is very different from that of the extracellular fluid.
An important difference is in Na+ levels, which are very low in the intracellular fluid and very high in the extracellular fluid. The reverse is true for K+ levels. This differrences are set up and maitained by Na+/K+ ATPases. Other solutes are also found in different in intracellular versus extracelluar fluids: levels of Mg2+ and proteins (usually negativelly charged) are relativelly higher inside cells, while Cl- levels are higher in the extracellular fluid. Other charged molecules like ATP and GTP are found in higer levels inside cells.
An electrolyte is a charged ion, and its concentrtation is measured in mEq. An osmolyte is a particle, regardless of its charge, and is meaured in mOs. For example:
1 mM glucose = 0 mEq = 1 mOs
1 mM NaCl = 2 mEg = 2 mOs
1 mM CaCl2 = 4 mEq = 3 mOs
Usually the electrolyte concentration is 210 mEq inside the cells and 54 mEq outside, while the osmolyte concentration is the same inside and outside the cell. Since osmolytes and electrolytes are basically the same solutes, the osmotic balance depends on the electrolyte balance (WHAT!!!???).
The extracellular fluid can be further subclassified into interstitial fluid and plasma. These two fluids are very similar regarding osmolarity and electrolyte content, the major difference being the blood cells and proteins present in plasma. Plasma and interstitial fluid are separated by a very permeable membrane: the endothelial cells of blood vessels. The cell membrane is mostly permeable to water. All other substances will move through active or passive transporters.
Body water will move acording to osmolarity. When a volume of water enters the extracellular fluid, it readily equilibrates between the extracellular and intracellular compartments according to their osmolarity, soon increasing the volume and decreasing the osmolaruity of both compartments. In contrast, when a volume isotonic fluid enters the extracellular compartment, since the osmolarity of neither compartment has changed, the added fluid will remain in the blood or interstitial space. If a hypertonic fluid is added to the extraccellular fluid,water will move out of the cells to balance the change in osmolarity, leading to an increase in osmolarity of both compartments, decrase intracellular volume, and increased extracellular volume.
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