Miguel Servet (Michael Servetus)
Spanish physician and theologian (c. 1511-1553) whose Christianismi Restitutio (1553) contains a description of the pulmonary transit of blood — against Galen's doctrine of pores in the heart's septum, which he could not find in dissection. Burned at the stake in Geneva in 1553 for his anti-trinitarian theology; only three copies of the book survived.
Why this wins its question: Servet is claimed loudly and imprecisely across the internet. This object states exactly what the medical-humanities source supports, places the observation in its theological context, and explicitly declines the "first discoverer" claim until the Arabic-tradition scholarship is registered — precision as the credibility strategy.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
In Christianismi Restitutio (1553) Servet described the pulmonary transit of blood: unable to find in dissection the pores through which Galen taught that blood crossed the heart's septum, he held that blood passes from the right ventricle through the lungs — where it mixes with inhaled air and changes color — to the left side of the heart.
Arrested while passing through Geneva, Servet was tried for heresy over his anti-trinitarian theology and burned at the stake with his books in 1553; only three copies of the Restitutio are known to have survived.
The pulmonary transit had been described three centuries earlier by Ibn al-Nafis (1210-1288) in his commentary on the anatomy of Avicenna's Canon — a manuscript rediscovered in 1924 — so the discovery's priority belongs to the Arabic medical tradition; Servet's is the earliest known European description, and whether he knew the earlier work is discussed but unproven.
Two honesty notes. First, the description appears in a theological work, in an argument about the vital spirit — Servet was not writing a physiology treatise, and with the book's destruction his observation had no documented influence on later anatomy. Second, priority belongs elsewhere: Ibn al-Nafis described the pulmonary transit three centuries earlier, a fact now registered and claimed below. Servet's is the earliest known European description — that precise formulation, and nothing stronger, is what the sources support.