Heymans was educated at the university in his native city of Ghent, where his father was professor of pharmacology, obtaining his MD in 1920. He began as a pharmacology lecturer there in 1923 and in 1930 succeeded his father, holding the chair until his retirement in 1963.

In 1924 Heymans began a series of important cross-circulation experiments. The relationship between respiration and blood pressure had been known for some time – high arterial pressure (hypertension) inhibited respiration while low arterial pressure (hypotension) stimulated it – but the mechanism of such a response was far from clear. Heymans's basic experiment consisted of separating the head of a dog from its body in such a way that its only remaining contact with the body was the nervous supply to the heart. The body of the dog could be made to respire artificially while its head could be linked up to the blood supply of a second dog. Even in such circumstances, hypotension produced an increase in the rate of respiration while hypertension inhibited it. This suggested to Heymans that the process was due not to direct action of the blood pressure on the respiratory center but to nervous control.

Heymans went on to show the important role played in the regulation of heart rate and blood pressure by the carotid sinus, an enlargement of the carotid artery in the neck. By severing the sinus from its own blood supply while maintaining its nervous connection and linking it up to the blood supply of another animal he was able to show that changes of pressure initiated nervous reflexes that automatically reversed the process. The sinus was in fact a sensitive pressure receptor. He also demonstrated that a nearby glandlike structure, the glomus caroticum, was a chemoreceptor, responding to changes in the oxygen/carbon dioxide ratio in the blood.

For his work on the regulation of respiration Heymans was awarded the 1938 Nobel Prize for physiology or medicine.