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Where do these humours come from? Food. All that we eat and digest travels to the liver, where it's converted into blood. From the liver, the blood is transported through veins to other organs – firstly, to supply them with nutrients, and secondly, to convert the blood into other humours. The brain is where the blood is made into phlegm; the two kinds of bile are produced closer to the liver: the regular yellow in the gallbladder and the black in the spleen. Other bodily fluids are produced in other organs still; for example, in the testicles, blood is converted into male seed (that is, semen), while in the ovaries, it is turned into female seed (which looks just like blood and is secreted out of a woman's body roughly once a month). As for the role that the heart and lungs play, the Galenic theory doesn't really hold much water, so we're just going to ignore it. It suffices to say that arteries, coming out of the heart, distribute blood enriched with '''pneuma''', a special airy substance which is reponsible for all those vital processes that can't be explained in any other way.