A bulla is a fluid-filled sac or lesion that appears when fluid is trapped under a thin layer of your skin. It's a type of blister. Bullae (pronounced as “bully”) is the plural word for bulla. To be classified as a bulla, the blister must be larger than 0.5 centimeters (5 millimeters) in diameter.

How are bullae formed?

In bullous emphysema, bullae are created as areas of severe emphysema coalesce due to progressive loss of alveolar attachments. In contrast, bullae surrounded by normal lung likely form because of a focal anatomic defect that results in localized air trapping.

Why do bullae form?

When alveoli are damaged, they form larger spaces called bullae that simply take up space. Bullae can't absorb oxygen and transfer it into your blood. Bullae often result from chronic obstructive pulmonary disease (COPD).