Physiological Gradients: Normal Electrolyte Distribution
In a healthy cell (e.g., a cardiomyocyte), strict electrolyte asymmetry is maintained. This concentration difference is essential for normal cellular function.
- Inward-directed gradient: Sodium (Na+), calcium (Ca2+), and chloride (Cl-) ions are predominantly located in the extracellular environment. For instance, extracellular sodium concentration is 140 mM, while intracellular is only 30 mM. Extracellular calcium is 2.0 mM, whereas cytosolic calcium is merely 0.1 mM. Chloride mirrors sodium distribution: 140 mM extracellular and 30 mM intracellular.
- Outward-directed gradient: Potassium (K+) and magnesium (Mg2+) are typical intracellular residents. Intracellular potassium reaches 140 mM, whereas in the intercellular space it is only 4.5 mM. Intracellular magnesium is 15 mM, versus 0.9 mM extracellularly.
Any injury disrupts this fragile system, triggering a cascade of pathological reactions.
Pathogenesis of Ionic Shifts
When pathogenic factors act on a cell, the membrane loses its ability to maintain physiological gradients. Two reciprocal processes occur:
- Loss of intracellular ions: Potassium and magnesium rapidly leave the cytoplasm, passing into the extracellular environment.
- Accumulation of extracellular ions: Sodium, calcium, and hydrogen ions (H+) rush inward, leading to the development of intracellular acidosis.
The consequence of these shifts is a transmembrane ionic imbalance. The ratio of electrolytes inside the cell and in the extracellular matrix is disrupted, inevitably entailing a disorder of water metabolism.
Mechanisms of Water Metabolism Disorders
Water balance disturbances can develop via two scenarios: the cell either becomes overloaded with fluid or critically loses it.
Hyperhydration (swelling and edema) develops due to the accumulation of hydrophilic ions (primarily Na+ and Ca2+) and various organic substances in the cytoplasm. This sharply increases intracellular osmotic pressure. According to the laws of osmosis, water actively rushes inward, and the cell volume increases. Morphologically, this manifests as overstretching of the plasmalemma (cytolemma) and micro-ruptures of both the cell membrane itself and the membranes of internal organelles.
Hypohydration (shrinkage) occurs against the background of overall fluid loss by the body. Etiological factors include fever, hyperthermia, pronounced polyuria, or severe infectious diseases (cholera, dysentery, typhoid fever). Water exits the cells, dragging along dissolved proteins (including important enzymes), as well as water-soluble organic and inorganic compounds.
Disorders of Membrane Electrogenesis
Ionic chaos inevitably affects the electrical properties of the membrane. Under imbalance conditions, the formation of both the resting membrane potential (RMP) and the action potential (AP) is disrupted. Characteristic changes include a decrease in potential amplitudes, impaired development velocity, and altered duration.
The pathogenesis of these disorders includes three main shifts:
- Decrease in resting potential magnitude leads to an alteration in the cell's excitation threshold.
- Decrease in action potential amplitude causes impaired conduction of excitation.
- Changes in AP development velocity and duration disrupt the processes of impulse generation and conduction.
These electrophysiological changes are of immense clinical significance. They serve as reliable diagnostic signs of cellular injury, which clinicians record using instrumental methods:
- ECG (electrocardiogram): reflects cardiomyocyte injury in arrhythmias, angina pectoris, and cardiomyopathies.
- EEG (electroencephalogram): shows impaired structure and function of brain neurons.
- EMG (electromyogram): registers pathological changes in muscle cells.