Page 89 - Electrical Safety of Low Voltage Systems

72 Chapter Five

The physiologic effects of currents must be comprehended as they
dictate the minimum safety requirements in electrical installations.

5.2 The Human Body as an Electrical System
On a cellular level, the human body is an electrical system, as the flux
of information necessary to its proper functioning is propagated by
the means of electric charges, positive and negative, constituted by
ions. A clear example is the cardiac muscle, whose contractions are
the result of the biological electrical system.

5.2.1 On the Electrical Nature of the Cells
Biological tissues are formed by cells, in contact with each other, im-
mersed in the extracellular (or interstitial) fluid. The cell is enclosed in
a membrane, which contains the intracellular fluid. Both intracellular
and interstitial fluids contain electrically charged ions, whose relative
concentration is indicated by the size of the circles in Fig. 5.1.
Measurements indicate that the charges of the undisturbed cell,
uniformly distributed on each side of the membrane, create a perma-
nent potential difference across the cell, called resting (or membrane)
potential. This voltage can assume a value as high as −70 mV, where
the negative sign indicates that there is an excess of negative charge
inside of the cell, with respect to the interstitial liquid.
The resting potential is the result of the equilibrium between two
different forces. Ions on both sides of the membrane are subject to
the electric field reciprocally exerted by their charges. Ions are also
exposed to the forces of diffusion due to their chemical gradient. Ions,

FIGURE 5.1 Relative concentrations and forces of ions in equilibrium across
the cell’s membrane.