There is medical evidence that blood and water would come out from the pericardial sac around the heart of a crucified person:VI – Coming Out of Blood and Water
“Then came the soldiers and brake the legs of the first and of the other which was crucified with him. But when they came to Jesus and saw that he was dead already, they brake not his legs. But one of the soldiers with a spear pierced his side, and forthwith came there out blood and water”. (John 19: 3-234)
The soldiers did not break the legs of Jesus for they took him for a dead man while actually, he was in a state of swoon, or just to silence the angry Jews he was declared be a ‘dead’ man. But one of the soldiers incidentally “pierced his side and forthwith there came out blood and water”, which is a surer sign of life for blood and water do not come out of a dead man’s body. There is much food for thought here for those who care to ponder over it.
When they came to Jesus, He was already dead so they did not break His legs (John 19:33). Instead, the soldiers pierced His side (John 19:34) to assure that He was dead. In doing this, it is reported that “blood and water came out” (John 19:34), referring to the watery fluid surrounding the heart and lungs.
http://ronaldvhuggins.blogspot.com/2015/10/blood-and-water-pouring-from-jesuss.html
... the Ahmadiyyah representative, who was himself a doctor, insisted that the reference to blood and water coming out of Jesus's side could not indicate death, since, as every reader of good mystery stories knows, corpses don't bleed. So I put the question to Dr. Rob Cheeley, an old freind who runs a large medical work in the far east: "Question. The coming forth of blood and water in John 19:34. Corpses don't bleed. What do you see going on there?"
Here is his answer:
Yes, corpses don't bleed. However, fluids (basically blood OR water) gather in various spaces in the body if the death being suffered moves them there. So the idea that this is possible is not a problem at all. But then one must explain, in relation to HOW Christ died and WHERE He was stabbed with the spear, whether or not there should have been blood and water there, given the sort of death He suffered.
One thing to make clear in your mind as you seek to understand it, is that the diaphragm separates the chest cavity from the abdominal cavity. They are absolutely separated (unless the spear traversed from the abdominal cavity up through and into the chest cavity--a third possibility). So, then there are 3 different possibilities for the wound of the spear from which the blood and water came:
1) chest cavity wound (the blade went into the space outside the lung, might have gone into the lung, might have gone into the sack around the heart or the heart itself);
2) abdominal cavity wound (the blade went into the area which holds one's bowels, liver, spleen, bladder, kidneys);
3) both cavities pierced.
Most people who tackle this explanation say that the spear went into his thoracic (chest) cavity. I don't know why they say that. The bible says 'His side'. So it is hard to say. However, it does make more sense that the soldier would have done it as a diagnostic. If so, then the chest cavity makes more sense. Piercing the abdominal cavity really means nothing and diagnoses nothing, as one could be dead or alive and that action wouldn't really tell you anything. But what happens when one pierces the chest would be rather diagnostic ... etc
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Jesus couldn't have been dead for long. The piercing with the lance was deliberate; it was done to ensure that if Jesus was not dead, he definitely would be after a spear through the lung(s) and heart