Hemoglobin is a protein in red blood cells that transports oxygen from the lungs to tissues throughout the body. It binds oxygen when levels are high, such as in the lungs, and releases it where levels are low, such as in muscles. This binding and releasing of oxygen is an example of chemical bonding. Hemoglobin allows oxygen transport even during strenuous activities like climbing Mount Everest where oxygen levels are low. The pH level also affects hemoglobin's ability to bind and release oxygen. Blood doping artificially increases red blood cell counts to enhance oxygen delivery to muscles during exercise.