The sweat glands are surrounded by the blood vessels. These vessels can constrict and then dilate to the point of rupture where the blood will then effuse into the sweat glands. It's cause--extreme anguish. In the other Gospels accounts, we see Jesus' level of anguish. "My soul is overwhelmed with sorrow to the point of death." (Matthew 26:38, cf. Mark 14:34) (gotquestions?.org)
We are not sure that this was the condition that produced the Savior's sweat to be like drops of blood. But the anguish in His heart, was certainly enough to bring it on.
He knew the extent of His suffering beforehand. If I had been in His shoes, I don't think I would have made it out of the Garden of Gethsemane, knowing all that would be happening in just a short time.
I am very thankful that an angel came and strengthened Him.
If there was ever a need for one to come, it was then.
But Jesus prayed that the Father's will be done, and He left that garden to journey up a lonely agonizing hill to die in our place. "Oh, how He loved you, and me!" And today because of His love, we have victory. Let us never forget the battle that was fought in the garden, that led to victory for us, for eternity. GP