Body Fluid Compartments • and • Fluid Balance
Objectives • 1. Differentiate between the terms osmole, osmolarity, osmolality and tonicity. • 2. List the typical value and normal range for plasma osmolality. • 3. Explain osmosis in terms of a differentially permeable membrane. • 4. Understand the concepts of osmosis and osmotic pressure. • 5. Define tonicity and be able to use the terms isotonic, hypertonic, and hypotonic. • 6. State the water content of the body, and its physiological variations with age, fat content and gender • 7. Understand the distribution of body water in the various body fluid compartments. • 8. Explain the principles of methods of measurement of body fluids. • 9. List the concentration of some important solutes in intracellular and extra cellular compartments. • 10. Understand the role of osmotic pressure in the distribution of body water into compartments.
Some useful information -1 Mol OF Glucose solution? - 1 Mol NaCl solution? In the ECF, max contribution to Osmolalirty is by NaCl • One osmole is 1 gram molecular weight of undissociated solute. • Eg. 1 osmole of glucose = 180 grams of glucose , is equal to 1 osmole of glucose because glucose does not dissociate. • 1 gram molecular weight of NaCl (58.5 gm) is equal to 2 osmole because NaCl dissociates into Na+ and Cl- (both are osmotically active). • Osmolality A solution that has 1 osmole of solute dissolved in 1 Kilogram of water • Osmolarity A solution that has 1 osmole of solute dissolved in in 1 liter (1000 ml) of water . • Conventionally: because of low concentration, solute dissolved in mg/litre of water or Kg of H2O milliOsmole (mOsm) • Normal Osmolarity of plasma is about 300 mOsm/L
Osmolarity/Osmolality • Is a measure of the total number of dissolved particles in a solution • Ionic composition of the ICF fluidisdifferent from that of ECF • But Osmolarity of ICF is equal to that of ECF.
Concept of osmotic pressure Osmosis: Diffusion of H2O Across semi permeable membrane From high H2O concentration to low H2O concentration. In tissues - via lipid bilayer -Through water channels (acquaporins) H2O H2O Solute Movement of H2O Osmotic pressure is the pressure which is applied at the membrane to oppose osmosis Semi-permeable membrane Osmotic pressure is directly proportional To the osmolarity (solute concentration)
Concept of hydrostatic pressure Pressure = 2 mmHg Pressure = 30 mmHg
Principle of Osmosis Osmosis is the net movement of water molecules through a selectively permeable membrane During osmosis water molecules pass through a selectively permeable membrane in two ways (1) through the lipid bilayer and (2) through aquaporins (aqua = water) channels.
The osmosis, diffusion, filtration and reabsoption provide for continual exchange of water and solutes (electrolytes) among different body compartments Yet, the volume of the fluid in different body compartments remains remarkably stable
Body fluid compartment ECF also includes lymph, CSF, synovial fluid, aqueous & vitreous humor, endolymph & perilymph and fuild present in pleural, pericardial and peritoneal cavities