Osmosis and its physiological role
Osmotic pressure does not exist on its own — it arises exclusively at the interface of two liquid media separated by a semipermeable membrane. The main property of such a barrier is that it freely allows water molecules to pass through while retaining substances dissolved in it.
The process of directed water movement itself is called osmosis. Fluid always moves along the osmotic gradient: from the area with a lower solute concentration to the area with a higher concentration. Water tends to "dilute" the more concentrated solution.
In the human body, this pressure difference acts as the main driver of water exchange. The osmotic gradient ensures water exchange between:
- the gastrointestinal tract and the blood;
- the vascular bed and the extracellular fluid;
- the extracellular and intracellular fluids;
- the blood and urine.
It is important to emphasize that osmotic and oncotic pressures have a completely different physical nature than hydrostatic pressure or the partial pressure of gases.
Units of measurement
In modern normal physiology, the unit of measurement used to assess biological fluids is the milliosmole (mOsm).
The base unit, 1 osmole (osmol), equals the pressure generated by 1 gram-molecule of a substance dissolved in 1 liter of water. To understand the scale of this physical force: the osmotic pressure of 1 osmol/L is equivalent to a colossal pressure of 22.4 atm. The unit ratio is standard: 1 osmole equals 1000 milliosmoles.
Arterial blood parameters
In clinical and laboratory practice, osmotic pressure is studied primarily in arterial and venous blood. This is because these parameters are critical for assessing tissue pressure, and the sample itself is easy to collect.
For arterial blood, the total pressure is normally 300 mOsm. It is divided into two fractions:
- Major part (electrolytes): Formed by the hydration shells of ions. Cations (primarily sodium) account for 140 mOsm, and anions (chlorides, bicarbonates) account for 130 mOsm. Key ions include sodium, chlorine, and bicarbonates.
- Minor part (non-electrolytes): Accounts for only 30 mOsm. This part is generated by substances such as glucose, urea, cholesterol, and proteins.
Dynamics in the venous bed
Venous blood parameters differ from arterial ones. It has a higher osmotic pressure. This occurs due to the continuous influx of metabolic products from tissues into the vascular bed.
- At rest: the value is at 310–315 mOsm/L.
- During physical exertion: active release of metabolites leads to an increase in pressure up to 330 mOsm/L.
Role of oncotic pressure
A special fraction of osmotic pressure is designated as oncotic pressure. This is the force with which blood plasma proteins attract water.
Its value is traditionally expressed in millimeters of mercury. Normally, it is 25–30 mm Hg (equivalent to 1.3–1.8 mOsm/L). As can be seen from the figures, the value of oncotic pressure is much lower than the total osmotic pressure.
Despite its modest absolute values, this force plays a huge role in the continuous exchange of water between blood and tissues. In addition, protein pressure acts as a crucial factor in urine formation, ensuring adequate plasma filtration in the renal glomeruli.
Briefly: answers to topical questions
Which plasma protein fractions contribute the most to the generation of oncotic pressure?
The greatest contribution to the oncotic pressure of the blood is made by albumins. These are proteins with a relatively low molecular weight, synthesized primarily in the liver. Their normal concentration is 40–45 g/L. Albumins determine the oncotic (colloid-osmotic) blood pressure of about 25 mm Hg; this pressure difference opposes hydrostatic pressure and retains fluid within the vascular bed. With massive loss of albumins through the kidneys, "renal" edema develops, and in starvation, "hunger" edema.
How does oncotic pressure affect the filtration of primary urine in the kidneys?
Blood oncotic pressure impedes the filtration of primary urine in Bowman's capsule. Plasma proteins normally do not pass through the glomerular filtration barrier, remain in the vascular bed, and use the force of osmosis to attract water, retaining it within the vessel. The magnitude of this force in the glomerular capillaries is about 30 mm Hg. This value is subtracted from the hydrostatic blood pressure when calculating the effective filtration pressure (EFP). A decrease in blood oncotic pressure leads to a pathological increase in the effective filtration pressure.
What happens to red blood cells when placed in a hypertonic or hypotonic solution?
In a solution hypotonic relative to blood (<0.9%), hemolysis occurs — the rupture of red blood cell membranes. The initial stage of hemolysis occurs when osmotic pressure drops to 3.5–3.9 atm, and complete hemolysis occurs at 2.5–3.0 atm. In a solution hypertonic relative to blood (>0.9%), plasmolysis develops — dehydration of red blood cells. When administering solutions into the bloodstream, muscle tissue, or spinal canal, "osmotic conflict" must be avoided.
Osmosis is the "thirst" of a solution. Water always rushes to where there are more dissolved substances ("salts") to nourish the more concentrated medium.
Class and exam questions
What is the main difference between osmotic pressure and hydrostatic pressure?
Hydrostatic pressure is related to the mechanical pressure exerted by fluid on vessel walls. Osmotic pressure has a different physical nature and occurs only in the presence of a semipermeable membrane and a difference in solute concentrations.
Why is osmotic pressure higher in venous blood?
Passing through tissues, venous blood collects cellular metabolic products. The appearance of additional dissolved particles naturally increases pressure from 300 to 310–330 mOsm/L.
Which ions create the main part of plasma osmotic pressure?
Electrolytes play the main role due to their hydration shells. Key ions are sodium (cation), as well as chlorides and bicarbonates (anions).
Why is oncotic pressure needed if it is so small?
Plasma proteins (generating 25–30 mm Hg) cannot leave the vascular bed, so they act like a sponge, retaining water inside the vessels and regulating the filtration of primary urine in the kidneys.
What else to study on this topic
- Mechanisms of formation of hydration shells of sodium and chloride ions in blood plasma.
- The effect of glucose and urea concentration on the non-electrolyte fraction of pressure.
- Comparison of osmotic pressure in blood plasma, lymph, and capillary blood.
- Physiology of renal filtration under the action of the oncotic gradient.
- Pathophysiology of edema in decreased plasma protein levels.
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