There are lots of different ways of dying, but most of them end up with the heart stopping, which means that blood is no longer pumped around the body to supply it with oxygen. We can be declared dead by a doctor when our hearts stop, breathing stops and the doctor doesn’t think that the heart can be restarted again. However, while we might be legally dead, our cells aren’t.
Without oxygen being supplied by the circulation, the cells can’t last long. The brain cells would die first, after 5-10 minutes or so. They need oxygen to make ATP, a chemical that is used for longs of chemical reactions – it provides the energy. All cells have a membrane potential (a small voltage). Without any ATP they can’t recharge this voltage (like topping up a battery). Small holes open up (voltage gated calcium channels), letting an important ion called calcium into the cells. This activates lots of chemical process inside the cell, but most of these processes can’t activate properly because there is no ATP. The cell membrane becomes more leaky, and so the inside of the cells becomes more like the outside. The neurons in the brain stop being able to communicate with one another, and all chance of thinking again is gone.
Most muscles goes into a spasm, called rigor mortis, because the calcium causes a contraction that can’t be relaxed for a long time.
Then the micro-organisms move in to try to take whatever nutrition they can from us … nice to know that we’ll be recycled!