According to Ancient Greek physicians, vital heat was produced by the heart, maintained by the pneuma (spirit or soul), and circulated throughout the body by blood vessels, which were thought to be intact tubes using blood to transmit heat. Aristotle supported this argument by showing that when the heart is made cold compared to other organs, the individual dies. He believed that the heat produced in the heart caused blood to react in a similar way to boiling, expanding out through the blood vessels with every beat. This extreme heat, according to him, can lead to a self-consuming flame if it is not cooled by air from the lungs.
Galen wrote in On the Usefulness of the Parts of the Body (170): "The heart is, as it were, the hearthstone and source of the innate heat by which the animal is governed." In the 11th century, Avicenna agreed with this notion, stating that the heart produced breath, the "vital power or innate heat" within the body, in his Canon of Medicine.
Later, the term innate heat was attributed to friction caused by the motion of blood through arteries, as evidenced by the Cyclopaedia (1728):