Scanning electron micrograph of lung showing thin alveolar septa and the interior of numerous alveoli. 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. Magnification @ 68x, 3x5 print.