Nature of Osmotic Pressure
Cellular viability directly depends on the stability of the surrounding environment. A critical characteristic of blood is osmotic pressure — the physical force with which various substances dissolved in plasma can retain water molecules within their hydration shells. Additionally, this same force causes water to be drawn and move across semipermeable biological membranes toward a higher concentration of dissolved particles.
In medical practice and physiology, modern clinical parameters are used to assess this force:
- Osmolarity — reflects the concentration of osmotically active substances per unit volume of the total solution. Measured in milliosmoles per liter of solution (mOsm/L).
- Osmolality — indicates the concentration of the same active substances, calculated per mass of the pure solvent. Measured in milliosmoles per kilogram of water (mOsm/kg H₂O).
Traditionally, osmotic pressure can also be expressed in atmospheres or millimeters of mercury (mm Hg).
Ionic Composition and Fluid Balance
The normal plasma osmolality level is maintained at approximately 300 mOsm/kg H₂O. Normally, intracellular and extracellular fluids are always in a state of strict osmotic equilibrium. Only brief physiological deviations occur, which the body rapidly compensates for.
Osmotic pressure is primarily generated by electrolytes — low-molecular-weight substances such as inorganic salts and their ions. Osmotically active substances are those that poorly cross the cell membrane, thereby forcing water to move to equalize concentrations.
The extracellular fluid contains various ions, the main ones being:
- Chloride ions ($Cl^-$)
- Bicarbonate ions ($HCO_3^-$)
- Sodium ions ($Na^+$)
Sodium ions play the leading role: they account for over 90% of the total contribution to the overall osmolality of blood plasma.
Oncotic (Colloid-Osmotic) Pressure
Oncotic pressure represents a small but critically important fraction of the total osmotic pressure. Unlike crystalloid pressure generated by salts, oncotic pressure is formed exclusively by high-molecular-weight colloidal compounds, predominantly plasma proteins.
The mechanism of its action lies in the fact that large protein molecules are physically unable to pass through an intact capillary wall membrane. Remaining within the vascular bed, they reliably hold water within their massive hydration shells.
Despite the fact that the contribution of proteins to total blood pressure is extremely small compared to ions, the magnitude of oncotic pressure is a critical factor. It determines the vector and volume of transport of water and substances dissolved in it across the capillary wall, regulating the constant fluid exchange between blood and the interstitial space.
Quantitative parameters of oncotic pressure at a normal blood protein concentration (70 g/L) are:
- Approximately 1.65 mOsm/kg H₂O (out of a total 300 mOsm/kg H₂O).
- In traditional units, this equals approximately 25 mm Hg.