Your lungs are a pair of large sponge-like organs that almost fill your chest cavity. Your left lung is slightly smaller than your right lung, to make space for your heart.
When you breathe in, you suck air in through your nose and mouth and down a tube called the trachea. Your trachea divides into two tubes called the primary bronchi. One enters each lung. From there, the bronchi progressively branch into smaller airways, which eventually lead to tiny air sacs called alveoli. This intricate network of airways looks like an upside-down tree.