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 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

-1 Mol OF Glucose solution?

- 1 Mol NaCl solution?

In the ECF, max contribution to Osmolalirty is by NaCl

Osmolarity/Osmolality

Is a measure of the total number of dissolved particles in a solution

Ionic composition of the ICF fluid is different from that of ECF

But Osmolarity of ICF is equal to that of ECF.

Concept of osmotic pressure

H2OH2O

Solute

Semi-permeable membrane

Osmosis: Diffusion of H2OAcross semi permeable membrane From high H2O concentration to low H2O concentration. In tissues- via lipid bilayer -Through water channels(acquaporins)

Osmotic pressure is thepressure which is appliedat the membrane to oppose osmosis

Movement of H2O

Osmotic pressure is directly proportionalTo the osmolarity (solute concentration)

Concept of hydrostatic pressure

Pressure = 2 mmHg

Pressure = 30 mmHg

Principle of OsmosisOsmosis 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

Constitution of Extra and Intra cellular fluids

Distribution of important ionic and organic constituents