This chapter argues that physicians in the sixteenth century already, long before the rise of “mechanical philosophy”, resorted to a range of mechanical and, in particular, hydraulic notions and images to explain specific physiological and pathological phenomena in the body. Obstructions of vessels and organs, described in analogy with the accidental or deliberate blocking of water pipes, brooks, and channels, played a prominent role in explaining many diseases. Renaissance anatomists, in turn, devoted considerable attention to the precise anatomy of the arteries and veins and of other vessels and ducts which channeled and guided the flow of blood and spirits and of other fluids and vapors in the body. And they developed a particular interest in valves and their function. Those in the heart were known since Antiquity but the anatomists now discovered other valvular structures between the small and the large intestines and at the insertion of the ureters into the bladder, which both prevented a reflux of matter, and, around 1550 already, they described the venous valves which eventually became a major building stone for William Harvey’s[aut]Harvey, William theory of blood circulation.