Born in Prairie du Chien, Wisconsin, he was associated with Harvard University for most of his career. He investigated many physiological problems, including digestion (using X-rays to study the movement of barium in the alimentary tract), and argued that the sympathetic nerves prepared an animal for "fight or flight", through increasing heart rate, blood pressure etc, with the two branches of the autonomic system acting together to maintain a large number of physiological functions, which he named homeostasis. During World War I, he studied the mechanism of traumatic shock on the battlefield, and in 1939 discovered sympathin, a stimulant for certain organs. After the Spanish civil war he helped to find posts for scientists and physicians who opposed Franco, and also assisted many victims of Nazi Germany. His death at the age of 73 was the result of a neoplasm caused by exposure to X-rays during his research.