The blood is basically the transport system of the entire body and when the heart stops pumping it, sudden death is inevitable.
To put it simply, the brain needs oxygen to function and what gives the brain oxygen is blood, if the brain doesn't have oxygen for a few seconds, you will die.
The livers and kidneys depend on the blood to carry all the toxens and waste products of the body to them, without blood, those water products would remain in the body and poison us more.
Heat is also transferred to the skin to regulate the body's temperature and it's the bloods job to get it there.
If the heart should stop pumping blood then all these processes stop dead in their tracks, and incidentally, so will you.