But one of the soldiers pierced his side with a spear, and at once there came out blood and water. He who saw it has borne witness—his testimony is true, and he knows that he is telling the truth—that you also may believe (John 19:34 – 35).
 
Blood and water? A.T. Robertson, Word Pictures in the New Testament (1934), notes:
 
Dr. W. Stroud (Physical Cause of the Death of Christ, 1871) argues that this fact proves that the spear pierced the left side of Jesus near the heart and that Jesus had died literally of a broken heart since blood was mixed with water.[1]
 
Stroud’s suggestion has been repeated in many sermons (including my own). It’s a very moving illustration that I was reluctant to let go of, so I asked my Christian doctor, Diane, about it. She smiled and shook her head. “No. I’ve never seen that and don’t believe Stroud is correct.”
 
Her answer set me back. I hate giving out sermon retractions, so I began to investigate further. The old English exegete, B.F. Westcott wrote long before Robertson:
 
The immediate cause of death was (it is said) a rupture of the heart, which was followed by a large effusion of blood into the pericardium. This blood, it is supposed, rapidly separated into its more solid and liquid parts (crassamentum and serum), which flowed forth in a mingled stream, when the pericardium was pierced by the spear from below. But it appears that both this and the other naturalistic explanations of the sign are not only inadequate but also inconsistent with the real facts. There is not sufficient evidence to shew that such a flow of blood and water as is described would occur under the circumstances supposed, and the separation of the blood into its constituent parts is a process of corruption [italics added], and we cannot but believe that even from the moment of death the Body of the Lord underwent the beginnings of that mange which issued in the Resurrection. The issuing of the blood and water from His side must therefore be regarded as a sign of life in death. It shewed both His true humanity and (in some mysterious sense) the permanence of His human life. Though dead, dead in regard to our mortal life, the Lord yet lived; and as He hung upon the cross He was shewn openly to be the source of a double cleansing and vivifying power, which followed from His death and life. [2]
 
In other words, if the blood and water were a natural phenomenon, the body of Jesus had rapidly begun to decay – something the Bible denies: “For you will not abandon my soul to Hades, or let your Holy One see corruption” (Acts 2:27 quoting Psalms 16). Therefore, the outpouring of actual water (see John 4:10 and elsewhere) was a miracle, but what did it represent? Jesus gave his blood (redemption) and living water (life) for us. The Apostle John witnessed the outpouring and was amazed: “He who saw it has borne witness—his testimony is true, and he knows that he is telling the truth—that you also may believe” (John 19:35).
 
Three years before, a very tired and very thirsty Jesus met the Samaritan woman at the well and told her, “Whoever drinks of the water that I will give him will never be thirsty again. The water that I will give him will become in him a spring of water welling up to eternal life.” On the cross, Jesus gave us the water of life!

[1] Robertson, A. T. (1933). Word Pictures in the New Testament (Jn 19:34). Broadman Press.

[2] Westcott, B. F., & Westcott, A., eds. (1908). The Gospel according to St. John Introduction and notes on the Authorized Version (p. 279). J. Murray.