British physiologist and philosopher chiefly noted for his work on the physiology of respiration.

Haldane developed several procedures for studying the physiology of breathing and the physiology of the blood and for the analysis of gases consumed or produced by the body. Among his most widely used devices were the hemoglobinometer, an apparatus for the analysis of blood gas, and an apparatus for the analysis of mixtures of gases.

In 1905 Haldane reported his fundamental discovery that the regulation of breathing is usually determined by the effect of the tension of carbon dioxide in the blood on the respiratory centre in the brain. In 1911 he led an expedition to Pikes Peak, Colo., where he studied the physiological effects of low barometric pressure. He investigated the action of gases that caused suffocation in coal miners and the pathological effects of carbon monoxide present after a mine explosion. His report in 1896 on the causes of death resulting from mine explosions and fires was an important contribution to mine safety. In 1907 he developed a method of stage decompression that made it possible for a deep-sea diver to ascend to the surface safely. Haldane was also a notable thinker who throughout his life tried to clarify the philosophical basis of biology, its relation to physics and chemistry, and the problems of mechanism and personality.