Most doctors have only heard of Ernest Starling through his law of the heart, although this was not a particularly important part of his research output. Shortly after qualifying in medicine at Guy’s Hospital, London, in 1888 (where he won the university gold medal in medicine), he began investigating the formation of lymph. To explain his findings, he proposed an inward osmotic force at the capillary: the only possible source of this force was the plasma proteins. At the capillary there was a balance between an inward (osmotic) force and an outward (hydrostatic) force. This became Starling’s ‘Filtration Principle’, which, in retrospect, was a paradigm shift in our understanding of the circulation.