By our own modern reckoning, poorly. But the body fluids were viewed as having utmost importance. Medieval medical thinking was dominated heavily by the Greek medical tradition, which had been passed on through the Romans. The Greeks held that there were four essential humors (fluids) that when out of balance caused illness and poor health. Each humor also had a personality trait that went along with it. The humors correspond with the four elements that were thought to form all matter. The four humors were blood, phlegm, yellow bile, and black bile. Hippocrates wrote that:

"The Human body contains blood, phlegm, yellow bile and black bile. These are the things that make up its constitution and cause its pains and health. Health is primarily that state in which these constituent substances are in the correct proportion to each other, both in strength and quantity, and are well mixed. Pain occurs when one of the substances presents either a deficiency or an excess, or is separated in the body and not mixed with others."

Blood was thought to arise from the liver and was associated with air. Those in whom blood predominated had the calm or sanguine temperament.

Yellow bile was believed to come from the spleen and was associated with fire and anger. It caused the choleric temperament. Even today we refer to someone venting their spleen if they are angry.

Black bile came from the gall bladder, was associated with earth, and disposed on to depression. The Greek words for black and bile give us the term melancholy.

Phlegm came from the brain or lungs, was associated with water, and was associated with apathy or a phlegmatic mood.

The humoral theory dominated Western medicine until well into the 1800s. Ancient and medieval treatments like bloodletting and the use of purgatives were designed to eliminate some humor that was felt to be out of balance. In 1838, a lecturer at the Royal College of Physicians admonished that "blood-letting is a remedy which, when judiciously employed, it is hardly possible to estimate too highly"

So the fluids of the body were considered absolutely crucial in medieval times. But their true structures and functions were to be unknown for hundreds of years until the advent of microbiology, physiology, and microscopic anatomy. But the next time you feel sanguine about the news, or perhaps melancholy instead, you're thinking like a medieval physician would have.

Sourced, in part, "The Western Medical Tradition: 800 BC to AD 1800" Conrad et al.