9.4 The Effect of Electric Currents on Biological Media

The human body is a complex heterogenous system, exhibiting both passive(consumes, but does not produce energy) and active(can produce energy, or amplify it) electric properties.

  • In a biological environment the electric current is conducted by ions in a solution. The function of ions depends on their charge ( + / – ).
  • The cytoplasm of cells and the intra-cellular environment behave as a second order conductor defined by a frequency-independent resistance R (which obeys Ohm’s law). The impendence Z, is related to R and the capacitance Xc by the formula:


The biophysical effects depend on the electric current type:

  • DC exhibits polar effects (stimulation for instance)
  • AC, at high frequencies, exhibits stimulation and heating (1 – 3000 Hz). Currents which have (f)> 3000 Hz only heat up the medium.

Q(heat) = U(Voltage V) * I(Current) * t(time)

In a capacitative method, heating is due to the dielectric loss 1 .

In an inductive method, heating is the result of induced eddy currents 2.

The electric shock hazard depends on the voltage of the source, and on the internal resistance of the body (medium). Sources with high internal resistance (r) are not hazardous – but those with a low resistance are. This is because the excessive amount of energy is free to travel through the medium. As a result surrounding tissues are burned.

  • Electric shock hazard is directly proportional to air humidity. This is because humidity lowers resistance.

The most sensitive organs to electric shock are:

  • brain
  • respiratory system organs
  • heart

The biological effect of the electric shock depend on the intensity of the current. Generally AC is more hazardous than DC. The boundary within which it is safe to operate are:

  • 10 mA – 1kHz  – AC
  • 25 mA – 1kHz – DC

Currents up to 25 mA can induce respiratory arrests, while currents >25 mA may induce cardio-respiratory arrests and even cause death.

When a muscle fibre is stimulated, the direction of flow of the current is the direction of the muscle fibreThe muscle fibres of the heart run in all directions.

An AC/DC > 80 mA has irreversible effects.

Notes compiled by Andrei Cociug.

  1. The dissipation of electromagnetic energy from an insulator that can be polarised by an electric field. 
  2. Are loops of electric current induced within conductors by a changing magnetic field in the conductor, due to Faraday’s law of induction