John 19:34 "But one of the soldiers with a spear pierced his side, and forthwith came there out blood and water." (KJV) - a sign of life!
The side of Jesus being pierced, blood rushed out and this was a certain sign of life. This is corroborated later when Jesus invited Thomas "reach hither thy hand, and thrust it into my side." (John 20:27) Dean Farar in his Life of Christ (p. 421) had to concede that when the Roman soldiers thrust the broad head of the spear into Jesus' side, "he might have only been in a syncope;" he only appeared to be dead but had in fact fallen into a comatose state. In the Encyclopedia Biblica, under the article "cross," column 960, that "Jesus was alive when the spear was thrust."
John 19:34: "Instead, one of the soldiers pierced his side with a spear, and at once blood and water came out."
If he was dead and his heart had stopped beating, such acute bleeding as would cause the blood to rush our gush out would be impossible. At most, coagulated blood and plasma could have passively seeped out. But that is not the picture which the NT presents. It says that blood and water rushed out.
As far as water being mentioned, it should not be surprising for Jesus to have developed pleurisy during the extremely exacting and punishing hours of trial that he spent on the cross. Also, the stress of the crucifixion could have resulted in exudates from the pleura to collect like bags of water, which is medically termed as wet pleurisy. This condition, which is otherwise dangerous and painful, seems to have turned out into an advantage for Jesus because when his side was pierced the swollen pleura could easily have played the role of cushion protecting the chest organs from being directly penetrated by the spear. Water mixed with blood rushed out because of an active heart beat.
Let us examine the issue of the lance that was 'thrust' into Jesus' side. Holger Kersten points out (p. 249, His Unknown Life Before And After The Crucifixion) that the original Greek word for the thrust by the soldier is nyssein. That word does not suggest a forceful thrust. The word means a light scratch, or light puncture of the skin. This word was considered a test designed to determine if the victim was actually dead. If the victim showed no reaction to this light stabbing, then it was assumed that the victim had died. [Add sources]