The human body is about 70 percent water, and also contains a great deal of salt. Salt water is an excellent conductor. But the body surprisingly resists electric charges. That's because our bodies are made of cells: tiny packets of salty water (containing proteins and DNA, for example) surrounded by membranes made of fats, oils and proteins. Oils are very poor conductors. So even though the presence of salt water allows for some electrical conductance in the body, the membranes serve as barriers.
Electric currents encounter less resistance when the surface of the skin is sweaty; the salt water of the sweat is an excellent conductor. Polygraph machines (more commonly known as lie detectors) use this property as one measurement to determine whether someone is being deceptive. And once an electric current passes into the blood stream, it experiences almost no resistance, because blood plasma is almost entirely made of salt water.
Electroshock therapy uses the body's natural conductance, causing small seizures. Scientists disagree on why the method seems to work. Some believe that shock works like antidepressants, changing the way brain receptors receive mood-related chemicals, such as serotonin or dopamine. Others believe the seizure causes the part of the brain that regulates water balance and body temperature (the hypothalamus) to release chemicals that regulate mood.
Opponents to electroshock therapy believe that the shocks damage the brain, causing memory loss and disorientation. Many patients do report some short-term or long-term memory loss. But the American Psychiatric Association reports that the success rate of electroshock therapy is about 80 percent, compared to success rates between 50 percent and 60 percent for most antidepressant medications.