Hematidrosis (also called hematohidrosis) is a very rare condition in which a human being sweats blood, though it has not been confirmed scientifically. It may occur when a person is suffering extreme levels of stress, for example, facing his or her own death. Several historical references have been described; notably by Leonardo da Vinci: describing a soldier who sweated blood before battle, men unexpectedly given a death sentence, as well as descriptions in the Bible, that Jesus experienced hematidrosis when he was praying in the garden of Gethsemane (Luke 22:44).
According to Dr. Frederick Zugibe (former Chief Medical Examiner of Rockland County, New York) it is well-known, and there have been many cases of it. The clinical term is hematohidrosis. "Around the sweat glands, there are multiple blood vessels in a net-like form. Under the pressure of great stress the vessels constrict. Then as the anxiety passes the blood vessels dilate to the point of rupture. The blood goes into the sweat glands. As the sweat glands are producing a lot of sweat, it pushes the blood to the surface - coming out as droplets of blood mixed with sweat."
In a lecture, Dr. Zugibe stated: "The severe mental anxiety...activated the sympathetic nervous system to invoke the stress-fight or flight reaction to such a degree causing hemorrhage of the vessels supplying the sweat glands into the ducts of the sweat glands and extruding out onto the skin. While hematidrosis has been reported to occur from other rare medical entities, the presence of profound fear accounted for a significant number of reported cases including six cases in men condemned to execution, a case occurring during the London blitz, a case involving a fear of being raped, a fear of a storm while sailing, etc. The effects on the body is that of weakness and mild to moderate dehydration from the severe anxiety and both the blood and sweat loss."
Another effect is that the skin become extremely tender and fragile, so that any pressure or damage to the skin is more than ordinarily painful.