Physiology and anatomy stood still during the medieval period. Galen’s work was studied and followed slavishly.
The Church forbade dissection on religious grounds, and also much other knowledge which contradicted Galen was lost when the Roman Empire slowly collapsed like a flan in cupboard. In the Eastern Islamic work, Galen’s work also survived, and a similar ban on dissection prevented his ideas from been disproved.
Ibn al Nafis disproved Galen’s blood flow theory, however no one would accept this as it was against the teachings of the Church and Galen whose ideas had lasted so long.