Krogh, Schack August Steenberg (1874-1949) was a Danish physiologist and zoologist who won the Nobel Prize in physiology or medicine in 1920 for his discoveries regarding the regulating mechanisms of the small blood vessels, or capillaries. He was particularly interested in the mechanisms by which the circulatory system controls the supply of oxygen within bodily tissue, a focus that grew out of his early studies of respiration.

Krogh attended the University of Copenhagen with the intention of studying medicine but soon began to pursue a course in zoology. After graduating, and while working as an assistant to the eminent physiologist Christian Bohr, he became interested in the mechanisms of gas exchange in living organisms. His thesis on this subject earned him his Ph.D. degree in zoology in 1903. In 1906, he was awarded the Austrian Academy of Sciences Seegen prize for a treatise on the pulmonary exchange of nitrogen and, over the course of his lifetime, did much breakthrough research and published many more books and papers in the field.

In 1908, Krogh returned to the University of Copenhagen as a professor in zoophysiology, a position he retained until his retirement in 1945. Among his many important contributions to the science of physiology, perhaps his most significant was in describing the contraction and dilation of the body's capillaries according to its need for blood, particularly within muscle tissue. He found that when muscles are being actively used, many more capillaries open up to supply oxygen, and that when the body is at rest and muscle activity is minimal, those capillaries contract again, as less oxygen is required. These discoveries greatly expanded the understanding of circulation, respiratory metabolism, and the body's mechanisms of water balance and heat regulation, and led to Krogh's 1920 Nobel Prize.