Impact of isotonic changes
The distribution of fluids in the body strictly depends on the concentration of dissolved substances. If the body experiences an excess or, conversely, a deficiency of water and salts while maintaining the isotonicity of the media, the physiological consequences have a strictly defined localization.
- Exclusively the volume of the extracellular space changes.
- First and foremost, these shifts affect the volume of blood plasma and intercellular fluid.
- Crucial fact: the volume of intracellular water remains completely unchanged.
Since the intracellular volume is stable, normal cellular vitality and functions are not disrupted during isotonic shifts. Fluid does not rush into the cell nor leave it, as there is no gradient to trigger this movement.
Role of osmotic pressure
A completely different picture is observed when a shift in the osmotic pressure of blood plasma occurs. In this case, the barriers between compartments become sites of active water movement.
When osmolarity changes, the volume of not only the extracellular but also the intracellular fluid is transformed. Water begins to move along the concentration gradient, striving to equalize the osmotic pressure on both sides of the cell membrane.
Consequences for the organism: Such fluctuations in intracellular volume are extremely critical. Cell swelling or, conversely, shrinking naturally causes severe disruptions in their viability and functional activity. It is precisely the difference in osmotic pressure that is the decisive factor in water exchange between the extracellular and intracellular spaces.
Fluid exchange in capillaries
If osmosis plays the leading role at the cellular level, other physical forces operate at the level of tissue microvessels. Fluid exchange between the blood flowing within the vessel lumen and the surrounding intercellular space occurs directly in the capillaries.
The leading role in this process belongs to the balance of two forces:
- Hydrostatic pressure of blood and intercellular fluid (the force pushing water out).
- Oncotic pressure of blood and intercellular fluid (the force holding water in).
It is the difference between these pressures that determines whether fluid will be filtered from the capillary into the tissue or return back into the bloodstream. Any shifts in these parameters instantly affect the volume of plasma and tissue fluid.
Briefly: answers to topical questions
Which blood plasma proteins create the oncotic pressure that keeps water in the vessels?
The oncotic pressure that retains water in the vascular bed is generated predominantly by albumins. These are high-molecular-weight colloidal compounds of blood plasma that attract water and prevent its excessive filtration into the interstitial space or nephron capsule. The magnitude of oncotic pressure is 25–30 mm Hg, and it plays a key role in fluid exchange between blood and tissues.
What is the normal resting value of blood plasma osmolarity?
The normal value of blood plasma osmolarity is 280–290 mOsm.
Where are the central osmoreceptors located that trigger thirst when osmolarity changes?
Central osmoreceptors are localized in the supraoptic and paraventricular nuclei of the anterior hypothalamus. These are neurons sensitive to the osmotic pressure of blood in brain capillaries; their cytoplasm contains a vacuole whose size depends on osmolarity. In a hyperosmotic state, water leaves the vacuoles, reducing their size; when osmotic pressure rises, the anterior nuclei of the hypothalamus are excited, a signal is sent to the thirst center, and the sensation of thirst and water intake occur.
Which hormones are the main regulators of water reabsorption in the kidneys?
Hormones play a leading role in the regulation of reabsorption. Vasopressin (ADH) acts on the collecting ducts, dramatically increasing the permeability of their walls to H2O and enhancing water reabsorption, thus decreasing diuresis. Aldosterone acts on the distal convoluted tubules and collecting ducts, increasing the reabsorption of Na+, Cl-, and H2O, retaining water and salts. Natriuretic hormone enhances diuresis and the excretion of sodium and water.
What is the essence of the Starling equation for capillary filtration?
The essence of the Starling force balance lies in determining the effective filtration pressure (EFP) through the difference between hydrostatic and oncotic pressures. EFP is calculated as the difference between blood pressure, which promotes fluid extrusion from the capillary, and the sum of forces opposing filtration. Opposing forces include the oncotic pressure of plasma proteins and the hydrostatic pressure of the fluid outside the capillary itself (e.g., primary urine or interstitial fluid).
Localization rule: Osmosis controls cells, while hydrostatic and oncotic pressures control capillaries.
Class and exam questions
What happens to cells during an isotonic excess of water?
In isotonic changes, cell function and volume are not disturbed. Only the volume of the extracellular space, such as blood plasma, increases.
Why is a change in plasma osmotic pressure dangerous?
It forces water to move between the extracellular and intracellular sectors. This changes the volume of the cells themselves, leading to the disruption of their normal vital activity.
What factors determine water exchange in the capillaries?
The main factors of transcapillary exchange are the gradients of hydrostatic and oncotic pressures between blood and intercellular fluid.
What else to study on this topic
- Mechanisms of cell damage during sharp fluctuations in osmotic pressure
- Dynamics of plasma volume changes during isotonic dehydration
- Interaction of hydrostatic and oncotic pressures at the arterial and venous ends of the capillary
- Compensatory reactions of the extracellular space to an excess of salts
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