“When we breathe in, oxygen breaks down (or oxidizes) chemical bonds in calories, for example, and releases the energy for use by our cells. The product of the process is CO2. While this basic exchange has been understood for centuries, what wasn’t known is how cells adapt and respond to changing oxygen levels. How, for example, do our cells know to ramp up breathing when we exercise and need more energy — and oxygen — to keep going? How do humans live at high altitudes?” Vox: William G. Kaelin Jr, Sir Peter J. Ratcliffe, and Gregg L. Semenza have jointly been awarded the 2019 Nobel Prize in physiology or medicine for discovering how cells sense and adapt to changing oxygen levels in the body.