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:
Once started ventricular fibrillation will often not stop without external action - usually very high magnitude electric shock - whose purpose is NOT to supply a heart start signal but to FULLY STOP the fibrillation signals so that the normal heart waveforms can re-establish.
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.
