Lecture 9 : Fluid Compartments and Osmosis Flashcards
Fluid def
A FLUID is a substance that deforms under shear stress. Important fluids in physiology include those in which water, fats or lipids, are the solvent.
Body water content
Total body water is approx. 42 L for a 70 kg person, and it is proportionally greater than men than in women, and reduces with age.
Examples of transcellular compartments
- Peritoneal space (can greatly expand, used therapeutically during peritoneal dialysis)
- CSF (protected by blood brain barrier, which are endothelial cells joined by tight junctions)
- Pleural cavity
- Synovial fluid
Cell Volume Regulation
- Facilitated diffusion
- Requires aquaporins
- Requires driving force (osmosis, and NOT hydrostatic pressure. Hydrostatic pressure is force per unit area in a fluid)
TO regulate volume :
- Express aquaporins (affects rate but not equilibrium position)
- Change driving force (change concentration of solute)
- Osmotic pressure depends on concentration of solutes in cells
How can solute concentration be changed?
- Change concentration of small molecules through metabolic processes
- Eg. Amino acids from proteins, betaine from glycine, glucose from glycogen, sorbitol from glucose)
- Change influx of ions and small molecules
- Volume regulated anion channels (VRACs)
- If cells swell, VRACs open to let Cl- and other osmolytes out
- Stretch-activated cation channels in response to swelling, eg. Letting Na+ and Ca2+ in to trigger cellular responses, (different types of TRP channels)
- Volume regulated anion channels (VRACs)
Plasma
Plasma is the fluid component of the blood and represents roughly 55% of the blood by volume, rest is occupied by cells. Haematocrit is a measure of the proportion of blood occupied by cells (roughly 45%)
Osmolality vs Osmolarity
Osmolality - number of osmoles per unit MASS of the SOLVENT (Osm.kg^-1)
Osmolarity - Number of osmoles per unit VOLUME of the SOLUTION (Osm.l^-1)
Osmotic Pressure
- At the interface between 2 solutions, molecules exchange via diffusion
- If concentration of any species is different on either side of the interface, there is a net movement of molecules from one side of the membrane to the other
- Osmotic pressure refers to water only
- Semi-permeable membrane is the ‘interface’ is the body (the plasma membrane)
- The inverse of water potential??
Oncotic Pressure
Total osmotic pressure of plasma is VERY HIGH
Osmotic pressure estimated by Morse equation : Osmotic pressure = nCRT
nC = Osmolality (0.28 osm.kg^-1)
R = Ideal gas constant (0.082 L.atm.mol.K^-1)
T = Temp (310K)
KEY CONCEPT : Osmotic pressure is directly proportional to osmolality
Isosmotic vs Isotonic
Isosmotic - 2 solutions share the same osmolality
Isotonic - Applying the solution to RBCs will not cause net fluid movement
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