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How did the Hippocratic humoral theory explain disease?

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Diagram explaining ATP's role in powering cellular activities, with arrows showing ATP produced by mitochondria, chloroplasts, and glycolysis, and used for making biomolecules, transporting substances, and driving energy-requiring processes; includes a structural diagram of ATP with labeled adenine, ribose, and phosphate groups. — Physiology Adenosine triphosphate (ATP) is the power source of many biochemical reactions. It is produced in the cell structures and system listed at the left to energize the important life processes listed on the right. An abbreviated chemical formula of the structure of ATP is also shown. The two high-energy P―O―P bonds are responsible for its power.

The Hippocratic humoral theory did not originate solely with Hippocrates but was rooted in the earlier philosophical work of Empedocles, who proposed that the universe was composed of four elements: fire, air, earth, and water.1

The Hippocratic school expanded this concept to human physiology, positing that the body was composed of four humours: blood, black bile, phlegm, and yellow bile. Health was defined as a state of harmony or balance among these fluids. Conversely, disease was understood to be the result of an imbalance or disharmony of these humours.21

This framework also linked the humours to specific temperaments—sanguine, melancholy, phlegmatic, or choleric—based on which fluid was in excess. Because this theory attributed illness to an internal imbalance, it led to medical practices such as bloodletting, which was intended to remove excessive humours and restore the body's natural equilibrium.32