William Zacharias, M.D., Ph.D., Zacharias Lung Lab, Department of Pulmonary Biology, Cincinnati Children’s Hospital Medical Center
As we breathe in, the air flows down the trachea into the left and right bronchi (tubes) that feed the left and right lungs. Bronchi further branch into smaller and smaller tubes called bronchioles (imagine an inverted tree). At the ends of the smallest branches are the delicate air sacs (alveoli) where gas exchange actually occurs. It has been estimated that the lung contains ~600 million of these alveoli. In this image of a normal adult mouse lung, we can see a pair of branching bronchioles surrounded by alveoli. Extremely thin cells (type I pneumocytes) form the walls of the alveoli; within those walls are fine capillaries through which red blood cells can flow, absorbing oxygen and releasing carbon dioxide and other toxins.