Oxygen binds with high affinity to myoglobin. Therefore, the curve immediately rises sharply as the pO2 increases and then levels off. The hemoglobin, on the other hand, is a sigmoid curve, that demonstrates the cooperative properties of hemoglobin. At low oxygen levels, the curve of hemoglobin demonstrates an inability to become fully saturated. However, as the concentration increases, hemoglobin, by the curve, shows a stronger affinity for the oxygen, rising steadily, until eventually, it levels off at its maximum saturation. It gets half saturated at p26. When the pH decreases, such as in actively metabolizing tissues, the pO2 is lower; and so, oxygen is released and the saturation level drops.