Besides guarding muscle and internal organs, the skin of a mouse can sense oxygen levels in the local environment and adjust the animal's physiological response accordingly, report University of California, San Diego, biology professor Randall S. Johnson and coworkers (Cell 2008, 133, 223). Amphibians such as frogs are known to monitor oxygen levels and even breathe through their skin, but this is the first time the oxygen-monitoring capability has been observed in a mammal, Johnson says. The research team found that mouse skin reacts to a drop in air oxygen levels by increasing blood flow in the skin. That enhanced flow signals the kidneys to ramp up production of the hormone erythropoietin (EPO). Extra EPO boosts formation of red blood cells, which carry oxygen around the body. Together, these alterations increase the oxygen-carrying capacity of the blood in low-oxygen conditions. If human skin responds in the same way, the results could have a positive impact on treatment of anemia and other diseases that involve red blood cell counts, Johnson says.