Nature of osmotic pressure
Cellular vital activity directly depends on the stability of their surrounding environment. The most important characteristic of blood is osmotic pressure — the physical force with which various substances dissolved in plasma can retain water molecules within their hydration shell. In addition, this same force causes water to be drawn and move through semipermeable biological membranes toward a higher concentration of dissolved particles.
In medical practice and physiology, modern clinical indicators 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 — shows the concentration of the same active substances, but calculated per mass of pure solvent. Measured in milliosmoles per kilogram of water (mOsm/kg H₂O).
Traditionally, osmotic pressure may also be expressed in atmospheres or millimeters of mercury (mmHg).
Ionic composition and fluid balance
The normal blood plasma osmolality indicator is maintained at about 300 mOsm/kg H₂O. Normally, intracellular and extracellular fluids are always in a state of strict osmotic equilibrium. Only short-term physiological deviations are possible, which the body quickly compensates for.
Osmotic pressure is primarily created by electrolytes — low-molecular-weight substances such as inorganic salts and their ions. Osmotically active substances are recognized as those that poorly penetrate 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 ($Na^+$)
It is sodium ions that play the leading role: they account for over 90% of the total contribution to the total plasma osmolality.
Oncotic (colloid-osmotic) pressure
Oncotic pressure represents a small but critically important part of the total osmotic pressure. Unlike crystalloid pressure created by salts, oncotic pressure is formed exclusively by high-molecular-weight colloidal compounds, predominantly plasma proteins.
The mechanism of its action is that large protein molecules are physically unable to pass through the intact membrane of the capillary wall. Remaining in the vascular bed, they securely retain water within their massive hydration shells.
Despite the fact that the contribution of proteins to the total blood pressure is extremely small compared to ions, the value of oncotic pressure is a critical factor. It is this pressure that 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 intercellular space.
Quantitative parameters of oncotic pressure at a normal blood protein concentration (70 g/L) are:
- About 1.65 mOsm/kg H₂O (out of the total 300 mOsm/kg H₂O).
- In traditional units, this equals approximately 25 mmHg.
Briefly: answers to topical questions
Which specific plasma protein fractions make the main contribution to oncotic pressure?
The main contribution to the blood oncotic (colloid-osmotic) pressure is made by the albumin fraction.
They are proteins with a relatively low molecular weight that are synthesized predominantly in the liver. The pressure difference created by albumins counteracts hydrostatic pressure and retains fluid in the vascular bed, providing the absorbing force of plasma.
What are the consequences of a decrease in plasma oncotic pressure?
A decrease in blood plasma oncotic pressure leads to the transition of fluid from the vascular bed into tissues and the development of edema.
Main pathogenetic consequences:
- Increased filtration of fluid from arterioles and precapillaries into the interstitium.
- Decreased reabsorption of water from the interstitium into postcapillaries and venules.
In nephrotic edema, excessive urinary excretion of albumins decreases plasma oncotic pressure, enhances fluid filtration from capillaries into tissues, and leads to a decrease in circulating blood volume.
OSMOtic = Main pressure (salts, low molecular weight, 300 mOsm/kg). ONCOtic = Volume-Impermeable (proteins, high molecular weight, only 25 mmHg, but holds water in vessels).
Class and exam questions
What is the difference between osmolarity and osmolality?
Osmolarity is calculated per unit volume of solution (liter), while osmolality is calculated per mass of solvent (kilogram of water).
What creates the main part of plasma osmotic pressure?
The main part is created by low-molecular-weight electrolytes. The main contribution (over 90%) is made by sodium ions ($Na^+$).
Why is oncotic pressure so important if its value is only 1.65 mOsm/kg?
Because proteins do not penetrate the capillary wall. This small pressure difference is a critical factor that retains water in the bloodstream and prevents its excessive exit into tissues.
What else to study on this topic
- Mechanisms of water transport through the capillary wall
- Blood cells and their role
- Blood buffer systems
- Short-term disorders of osmotic equilibrium
- Formation of hydration shells around molecules
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