Obstructive lung disease also changes the appearance of the flow volume curve. As with a normal curve, there is a rapid peak expiratory flow, but the curve descends more quickly than normal and takes on a concave shape, reflected by a marked decrease in the FEF25-75. With more severe disease, the peak becomes sharper and the expiratory flow rate drops precipitously. This results from dynamic airway collapse which occurs as diseased conducting airways are more readily compressed during forced expiratory efforts. On the volume time curve, this is seen as a slower ascent to maximum volume, with a gradual upsloping versus the rapid rate seen in normal individuals. This equates with a prolonged forced expiratory time demonstrable on physical exam.