Why blood and water came out

Why did blood and water come out of Jesus' side when He was pierced?

The Roman flogging or scourging that Jesus endured prior to being crucified normally consisted of 39 lashes, but could have been more (Mark 15:15; John 19:1). The whip that was used, called a flagrum, consisted of braided leather thongs with metal balls and pieces of sharp bone woven into or intertwined with the braids. The balls added weight to the whip, causing deep bruising and contusions as the victim was struck. The pieces of bone served to cut into the flesh. As the beating continued, the resulting cuts were so severe that the skeletal muscles, underlying veins, sinews, and bowels of victims were exposed. This beating was so severe that at times victims would not survive it in order to go on to be crucified.

Those who were flogged would often go into hypovolemic shock, a term that refers to low blood volume. In other words, the person would have lost so much blood he would go into shock. The results of this would be

1) The heart would race to pump blood that was not there.

2) The victim would collapse or faint due to low blood pressure.

3) The kidneys would shut down to preserve body fluids.

4) The person would experience extreme thirst as the body desired to replenish lost fluids.

There is evidence from Scripture that Jesus experienced hypovolemic shock as a result of being flogged. As Jesus carried His own cross to Golgotha (John 19:17), He collapsed, and a man named Simon was forced to either carry the cross or help Jesus carry the cross the rest of way to the hill (Matthew 27:32–33; Mark 15:21–22; Luke 23:26). This collapse indicates Jesus had low blood pressure. Another indicator that Jesus suffered from hypovolemic shock was that He declared He was thirsty as He hung on the cross (John 19:28), indicating His body’s desire to replenish fluids.

Prior to death, the sustained rapid heartbeat caused by hypovolemic shock also causes fluid to gather in the sack around the heart and around the lungs. This gathering of fluid in the membrane around the heart is called pericardial effusion, and the fluid gathering around the lungs is called pleural effusion. This explains why, after Jesus died and a Roman soldier thrust a spear through Jesus’ side (probably His right side, piercing both the lungs and the heart), blood and water came from His side just as John recorded in his Gospel (John 19:34).

Source:
http://www.gotquestions.org/blood-water-Jesus.html