Riolan is remembered for his traditional views towards medicine, and was a major proponent concerning the teachings of Galen. He held a different viewpoint regarding William Harvey's (1578-1657) theory involving the blood's circulatory system. Riolan calculated that the blood traveled through the arteries and veins to the body's extremities and returned to the heart only two or three times a day. He postulated that blood ebbed and flowed in the veins and that it was taken in as nutrient by different parts of the body. Riolan also did not believe that the heart propelled the blood, instead he proposed that the blood kept the heart in motion, analogous to a stream moving the wheel of a water mill.
Riolan had other disagreements with Harvey, including the role of the liver as a blood-manufacturing organ. Riolan was an opponent to the practice of vivisection, believing that violent and painful deaths suffered by research animals, placed them in an unnatural condition that led to incorrect assumptions about the functionality of healthy animals.
Riolan's best known written works are l'Anthropographie (1618), regarding human anatomy and Opuscula anatomica (1649), where he criticizes Harvey's views of the circulatory system. The eponymous anastomosis of Riolan is named after him, which is the mesenteric arterial connection between the superior and inferior mesenteric arteries.