The humoral scheme sets four fluids in the body and explains health as their balance and illness as an excess or a deficiency of one of them. It appears in its familiar four-part form in a Hippocratic treatise on the nature of man written in the late fifth century BCE, and it was argued for, systematised and made dominant by Galen in the second century CE. Blood is the humour of this temperament, and in the physiology that went with the scheme it was made in the liver out of digested food and carried outward through the veins.
Each humour was assigned a pair of the primary qualities, hot or cold together with moist or dry, and blood took hot and moist. That pairing fixes the rest of the correspondences: air among the elements, spring among the seasons, and childhood or early youth among the ages of life, each of which is likewise hot and moist. The same four-part structure repeats across all of them, and the neatness of that repetition is a large part of why the scheme was found satisfying for so long.
Bloodletting, the most persistent therapeutic practice in the Western tradition, follows directly from it. If illness is an excess and blood is the humour most easily removed, then drawing blood is the obvious correction, and physicians drew it for conditions that have nothing to do with any modern account of the blood for the better part of two thousand years.