Electric current in the human body

[infomike]

You mostly hear about electrons as being the vehicle of an electric current because we think of wires and metal. However, if an electric current traveled through the human body, it would be in the form of ions, right? But what happens at the interface between body and wire? I am guessing that if the electrons in a current of a wire contacts flesh, there must be some kind of oxidation/reduction occurring at that point and also when the current leaves the body to go to Earth or another wire. Is that correct? So, if electrons pass into my body, the electrons may attach themselves to positively charged potassium or sodium ions, converting them to metallic potassium or sodium atoms? Would these metal atoms then give up their electrons at the exit point of the body as they transfer those electrons into the wire or into the earth? Or would some of the negative ions such as sulfate, phosphate, chloride, pass their electrons into the exit point, forming dissolved gases such as sulfur trioxide, chlorine, etc.? Since free electrons don't travel in fluids as they do in metal, this is the only explanation that I can envision.

 

[berkeman]

You mostly hear about electrons as being the vehicle of an electric current because we think of wires and metal. However, if an electric current traveled through the human body, it would be in the form of ions, right? But what happens at the interface between body and wire? I am guessing that if the electrons in a current of a wire contacts flesh, there must be some kind of oxidation/reduction occurring at that point and also when the current leaves the body to go to Earth or another wire. Is that correct? So, if electrons pass into my body, the electrons may attach themselves to positively charged potassium or sodium ions, converting them to metallic potassium or sodium atoms? Would these metal atoms then give up their electrons at the exit point of the body as they transfer those electrons into the wire or into the earth? Or would some of the negative ions such as sulfate, phosphate, chloride, pass their electrons into the exit point, forming dissolved gases such as sulfur trioxide, chlorine, etc.? Since free electrons don't travel in fluids as they do in metal, this is the only explanation that I can envision.

I think I see what is confusing you.

When there is no external voltage source connected to the body, then you are correct that electrical impulses are propagated by ions crossing boundaries. Na+ and K+ are two such ions.

http://www.zerobio.com/central/na_k.htm

But if you connect an external voltage source to the body, then the conduction is via electrons flowing through the conductivity of the body tissues and fluids. The skin has a relatively high resistance (thankfully!), but the internal body tissues and fluids have a relatively low resistance.

Does that help?

 

[infomike]

But if you connect an external voltage source to the body, then the conduction is via electrons flowing through the conductivity of the body tissues and fluids. The skin has a relatively high resistance (thankfully!), but the internal body tissues and fluids have a relatively low resistance.

Does that help?

You are saying that free electrons flow through the body fluids?? I don't see how that is possible from my chemistry background.

 

[davenn]

You are saying that free electrons flow through the body fluids?? I don't see how that is possible from my chemistry background.

look at it in a more simpler way ( I have to, I'm not a chemist ;) )

current won't flow too well, if at all through pure water, it makes a pretty good insulator
where as as the water get a higher and higher concentration of salts, it becomes a better conductor.
I can only see the fluids and inside tissues of the human body as a really good conductor with all the salts and
minerals present in that fluid mix :)

Dave

 

[Crazymechanic]

Suggestion: before electrocuting yourself keep a good diet... certainly don't use salt in food, it's not only bad for your liver but also good for current.

 

[berkeman]

You are saying that free electrons flow through the body fluids?? I don't see how that is possible from my chemistry background.

Not "free" electrons. Electrons in the conduction band of the conductive tissues and fluids in the body. Just the same as electrons flowing in a 1/4 Watt resistor or other resistive conductor..

 

[DrZoidberg]

Of course there are no free electrons flowing through the human body. In water just like in insulators the conduction band is higher than the valence band. Thats why pure water is an insulator. Current in the body always flows in the form of moving ions. At the contact point between a wire and the body you get electrolysis.

 

[Naty1]

Informike:

I think this is the best explanation but I'm not knowledgeable about the human body chemistry...however I am VERY familiar with electrolysis and this may be the most complete answer:

...Current in the body always flows in the form of moving ions. At the contact point between a wire and the body you get electrolysis.

On the other hand, apply 250,000 volts [to pick an example] that a transmission lineman might encounter and I'd guess some electrons start flowing...that's why they burn I would guess.

The above description DOES covers galvanic corrosion and electrolysis in salt water...a subject I have studied a lot. Ions conduct the 'current flow' in salt water.

Wikipedia suggests to me the possibility of further complexities in the human body:

See here for an example: http://en.wikipedia.org/wiki/Synapse

There are two fundamentally different types of synapses:
In a chemical synapse, the presynaptic neuron releases a chemical called a neurotransmitter that binds to receptors located in the postsynaptic cell, usually embedded in the plasma membrane. The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron. Because of the complexity of receptor signal transduction, chemical synapses can have complex effects on the postsynaptic cell.
In an electrical synapse, the presynaptic and postsynaptic cell membranes are connected by special channels called gap junctions that are capable of passing electric current, causing voltage changes in the presynaptic cell to induce voltage changes in the postsynaptic cell. The main advantage of an electrical synapse is the rapid transfer of signals from one cell to the next.[3]

 

[anonmily]

Imagine the (neutral) metal of an electrode in contact with skin. When current runs through the metal, the negative electrons give it a net negative charge on the surface. This negative charge then attracts the positive ions in the skin* towards it, resulting in a voltage difference across the skin membrane (positive ions migrate toward the surface closest to the electrode while negative ions go the other way, in the direction of another conductor with a source of positive ions/charge). Positive ions/charges in the tissues/membranes or electrolytes in touch with this membrane will then be attracted to the now "negative" (many negative ions present) internal side of the skin membrane, resulting in yet another net voltage difference. This transfer of voltage difference will then continue to be passed through all membranes/electrolytes in the circuit (e.g. if if were a human holding onto a transmission line while standing on the ground, the circuit through which current will flow is: wire--> entire human body --> ground. Not fun.).

If the voltage difference is very large, then the resulting voltage difference in all your tissues will also be very large, and many voltage-gated ion channels in your membranes will open. For example, the Na+ channels are a major contributor to action potentials in the nervous system, and they open when a sufficiently high voltage difference is applied across the membrane. (Hence the twitching you'd probably see if you ever witness anyone getting shocked from holding on to an electrical line...). Thus, electrical stimulation applied to nerves can be used to artificially cause action potentials.

Essentially:
electron flow through electrode --> negative charge at electrode surface --> attracts positive ions in the tissue --> voltage difference at electrode-tissue interface --> voltage difference across tissue membrane --> voltage difference across all adjacent conductors --> possibly may cause opening of voltage-gated ion channels

Here's a good article if you're interested in more aspects of electrical stimulation:

https://www.dropbox.com/s/ohcu7yfpdyzdan2/Motor Prostheses.pdf

**I think it is possible that the negative charge may favor reactions for the formation of ions, e.g H2O (l) --> H+ (aq) + OH- (aq), in addition to the ions already existing in the tissue such as Na+, K+, H+ etc

 

[infomike]

anonmily, I think that your answer is the best of all and is consistent with the chemistry and physics that I know.

As I predicted, there is some oxidation/reduction (electrolysis in the case of water )going on at the interfaces between the body and the wire.

 

[Naty1]

anonmily ...

that sounds just like salt water reactions...not surprising, but interesting!