I usually try to do some research before asking questions on this site, but everything I've found has been contradictory.
If I were to direct the positive and negative ends of a 9 volt battery through open wounds on either of my hands, allowing the electricity to travel through my bloodstream and presumably past my heart, would it kill me? What if I were able to let go as soon as I felt the shock?
IF you get cause currents in the 10's of microamps range onto the actual surface of the heart death can occur.
This paper makes that statement Electrical Safety in the Operating Room: Dry Versus Wet and my recollection is that the writer has come to specialise in establishing best safety practices in that area.
He says:
Microshock refers to very small currents (as little as 10–50 μA) and applies only to the electrically susceptible patient, such as an individual who has an internal conduit that is in direct contact with the heart. This conduit can be a pacing wire or a saline-filled central venous or pulmonary artery catheter. In the electrically susceptible patient, even minute amounts of current (10 μA) may cause ventricular fibrillation.
Once started, the chaotic rhythm known as ventricular fibrillation (VF) will often not stop by itself without external intervention - usually a very high magnitude electric shock from a defibrillator*- whose purpose is NOT to supply a heart start signal but to FULLY STOP the fibrillation rhythm so that the normal heart waveforms (normal sinus rhythm) can be restored.
*A defibrillator is usually a purpose designed item of medical equipment. In an emergency any source of significant but not too-significant electrical energy can be used and *may work. Risk of causing death or injury by this means is high and the risks of acting or not acting need to be weighed. (In a long ago movie the 'hero' used a mains cord from a torn apart table lamp as a defibrillator to successfully save a life. You'd need to be brave, desperate and lucky to perform such a feat successfully. In some cases it may be the best choice :-(.)
This question stems from the rumor that some navy tech wanted to test the conductivity of their body so they pushed the meter beneath their skin and got electrocuted. (at least the version I heard, I'm sure there are many variants by now.)
Yes, it can, it only takes 10-20mA to stop a human heart. A 9V battery can provide much more than that. Your skin has sufficient resistance that it can stop current. If the skin is broken the resistance drops significantly. The current must be across the heart.
Offhand it would seem that a shock of 10,000 volts would be more deadly than 100 volts. But this is not so! Individuals have been electrocuted by appliances using ordinary house currents of 110 volts and by electrical apparatus in industry using as little as 42 volts direct current. The real measure of shock's intensity lies in the amount of current (amperes) forced though the body, and not the voltage. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal current.
While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) are lethal. Currents above 200 milliamps (0.2 amp), while producing severe burns and unconsciousness, do not usually cause death if the victim is given immediate attention. Resuscitation, consisting of artificial respiration, will usually revive the victim.
From a practical viewpoint, after a person is knocked out by an electrical shock it is impossible to tell how much current has passed through the vital organs of his body. Artificial respiration must be applied immediately if breathing has stopped.
Source: https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.html
I wrote a new answer that will be more precise after more resarch.
DC current is about 2-4 times less dangerous than AC current because the AC current will cause faster ventricular fibrillation which is often the cause of death from electric shock.
So what rates are dangerous?
If I were to direct the positive and negative ends of a 9 volt battery through open wounds on either of my hands, allowing the electricity to travel through my bloodstream and presumably past my heart, would it kill me? What if I were able to let go as soon as I felt the shock?
Applying 9V from your hand to hand directly in your bloodstream would then give 30mA DC which is highly unlikely to kill you.
While not pleasant it seems that connecting a 9V battery directly to your heart is in the danger zone but not necessarily going to kill you. Connecting a 9V battery however from hand to hand in the bloodstream is highly unlikely to kill you.
"It's the volts that jolts, but the mills that kills"
Roughly speaking, humans can feel voltage (via muscular contraction) but death comes from milliamps of current passing through the sinoatrial node near the heart.
If you could construct a scenario where you could present a conductive path that includes the sinoatrial node, of resistance less than about 90Ω, to the terminals of a 9V battery, then you might have an electrocution risk. The human blood vessel system with wounds at either end is probably not sufficient. But since the 1930's, experiments of this nature have fallen out of favour so the state of the art is really just a lot of extrapolation.
The state of the art is that <120Vdc is low risk in normal circumstances, but you could still come up with a scenario with electrocution risk. Thus <60Vdc is often used as a safer limit in various worldwide standards. Finally, in particular hazardous scenarios like swimming pools (think of underwater lighting), <25Vdc is required to be considered safe.
9V is considerably lower than this already very low limit, so would take some very special circumstances to present an electrocution risk. But I reckon if you worked hard enough at it, you could kill some very unlucky, highly susceptible, high sodium content individual.
To answer your second question: yes, it is possible to let go and save yourself - electrocution only occurs if the shock coincides with a vulnerable phase (about 10%) of the sinus cycle. At 9V the muscular convulsion would not be enough to render you paralysed, so if you're lucky, you might be able to sense the shock and disconnect before the vulnerable period arrives.
If I were to direct the positive and negative ends of a 9 volt battery through open wounds on either of my hands, allowing the electricity to travel through my bloodstream and presumably past my heart, would it kill me?
Other answers have addressed the electrical interaction with the heart's rhythm center and conduction systems - the aspects typically associated with electric shock.
A 9V battery in the scenario you propose is a bit more insidious since if your heart won't be instantaneously affected, you'll be bound to think "oh, it's not doing anything".
Well, it is actually doing something and that is facilitating galvanic chemical reactions that will be destroying the various components of the blood (a liquid tissue), and the tissues adjacent to the site of the contact with the battery terminals. A 9V battery discharging through the bloodstream will definitely not make you feel any good the longer you keep it going. The charge in a 2032 coin cell is enough to cause pretty bad lesions in the moist environment of esophagus - bad enough to cause life-long complications and death if untreated. And that is a completely intact 2032 cell with nothing leaching out - just the stored charge in it is able to do a lot of localized damage.
A 9V battery's worth of charge going through the bloodstream is not going to be a fun experience in any shape or form. Leave it connected until it fully discharges and the experiment may be interrupted by systemic effects bad enough to warrant an emergency room visit. Do not try this at home, folks.
Electricity shouldn't be much different if it hits you in a wound or not. It's not traveling on the outside of your skin, outside a wound and then going in only at the wound. A 9v battery shouldn't kill a human though.
