Ross Young; Week 1 MED1011; Physiology
Osmotic potential in mammalian cells is equivalent to 0.9% NaCl in water; 'normal saline' and has 1/4 the concentration of sea water. This environment forms ECF. Most of the body's water is found inside cells. The ICF has the same concentration of dissolved particles as the ECF but has a different ionic composition.
- ICF is about 28L, plasma about 3L, and ISF 11L. Cell membrane between ISF and ICF is permeable to lipids and water, and the capillary membrane between ISF and plama is permeable to substances up to about 4000 MW, though this varies between organs due to pore size changes.
Water can pass through protein pores in membranes, The positive charges of these pores keeps cations out.
Beverages supply 60% of water (1.5L), foods 30% (750mL) and metabolism 10% (250mL). 60% excreted through urine (1.5L), 700mL through skin and lungs (28%), 200mL (8%) through sweat and 100mL (4%) in faeces.
The solutes in ICF and ECF include crystalloids, which are small ions and molecules that dissolve and don't scatter light so liquids are clear, eg inorganic ions, sugar. Colloids are larger molecules that don't dissolve, but scatter light so a light beam shining through is visible. Proteins are the principal colloids. Some colloids change from sol to gel state, forming gelatin solutions.
With a few exceptions (CSF, synovial fluid, lymph, plasma) ECF is not free water but exists in the gel state. This is because it is bound to substances of a high molecular weight in the extracellular matrix, particularly proteoglycans.
Diffusion is solute or solvent moving from area of high concentration to that of low concentration. Water diffuses freely across cell membranes. Filtration is forced movement through a membrane permeable to a solvent, while particulates larger than the pores do not pass the membrane. Osmosis is net movement of water across a cell membrane caused by a concentration difference.
When a concentration difference exists, we say a concentration gradient exists. This includes the size of the concentration difference and the distance a diffusable substance has to move to obliterate the concentration difference.
Hydrostatic pressure due to difference in water level when membrane separating concentration gradient is only permeable to water is equivalent to the osmotic pressure
Molality is the number of moles of solute per kg of water. Molarity is the number of moles per L of solution; these are equivalent for physiological fluids (in clinical practive molarity used because measuring solution more practical than the mass of water it contains)
Osmotic pressure depends on number of particles, not on their mass or charge, molar concentration of a solution determines the osmotic activity of the solution, can be calculated as sum of the molarities of all the individual solutes.
- NaCl forms 2 ions in solution, so has 2x osmotic activity of non-dissociating solute
MW of glucose 180; a 90g/L solution has molarity of 0.5 as there are 0.5 osmole of particles in solution. MW of NaCl is 58.5, 29g/L is 0.5 molar but has 1 osmole of particles in solution as there is 0.5 osmoles of Na and 0.5 osmoles of Cl
- 1mOsm = 1/1000 mole
Body fluids typically 300mOsm; 1mOsm exerts pressure of 19.3mmHg, total osmotic pressure of body fluids expected to be 5790mmHg, in practice about 5500 as some solutes are attracted to each other and function as single units, not separate. 1mOsm exerts a lot of pressure and hence there can be large pressure differences generated across cell membranes. Can cause large changes in cell volume with quite small differences in the concentration of osmotically active particles across the membrane (only occurs when solute cannot permeate the membrane).
Tonicity has 2 definitions: the ability of a solution to alter the fluid volume within a cell; the osmotic pressure produced across a cell membrane separating it from plasma. Tonicity depends on the ability to permeate the membrane; lower concentration of impermeant solutes will cause water to move in, hypotonic. Higher impermeants is hypertonic, equal is isotonic. Isotonic is 0.9% NaCl, 5% glucose.