Scanning electron micrograph of lung prepared by instillation of the fixative to prevent collapse of the alveoli and terminal segments of the bronchial tree. The lungs are the primary organs of respiration in humans and many other animals including a few fish and some snails. An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the terminal ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well. A bronchus, also known as a main or primary bronchus, is an airway in the respiratory tract that conducts air into the lungs. Magnification unknown.