In medicine, a bleb is a blister (often hemispherical) filled with serous fluid. Blebs can form in a number of tissues by different pathologies, including frostbite. In pathology pulmonary blebs are small subpleural thin-walled air-containing spaces, not larger than 1-2 cm in diameter. Their walls are less than 1 mm thick. If they rupture, they allow air to escape into pleural space, resulting in a spontaneous pneumothorax. In the lungs, a bleb is a collection of air within the layers of the visceral pleura.