Charles Ellicott Commentary John 19:34

"howbeit one of the soldiers with a spear pierced his side, and straightway there came out blood and water." — John 19:34 (ASV)

But one of the soldiers with a spear pierced his side.—They had seen that He was dead, and therefore did not break the legs. To cause death was not, then, the object in piercing the side; and yet it may have seemed to make death doubly sure. The word rendered “pierced” occurs nowhere else in the New Testament, but it is certain, from John 20:27, that the act caused a deep wound, and that the point of the lance therefore penetrated to the interior organs of the body. If the soldier stood before the cross, this wound would naturally be in the left side.

And forthwith came there out blood and water.—“Various physiological explanations have been given of this fact, such as:

1. that the lance pierced the pericardium, which contained a small quantity of watery lymph, which immediately flowed out, and also the heart, from which the blood flowed, the actual death taking place at this moment;
2. that the physical death of Christ resulted from rupture of the heart, and that the cavities of the heart and the surrounding vessels contained a watery fluid;
3. that decomposition of the blood in the corpse had taken place, the solid matter being separated from the fluid, so that it would appear to be blood mixed with water. (Compare Notes on 1 John 5:5–6.)”

Whatever solution we adopt, it is clear that death had taken place some time previously (John 19:30). It is also clear that, while we cannot say which physical explanation is the true one, there is quite sufficient within the region of natural occurrences to account for the impression on the mind of St. John which he records here. We have to think of the disciple whom Jesus loved looking at the crucified and pierced body of his Lord, remembering the picture in later years, and telling that there flowed from that pierced side both blood and water.