biomedicalephemera:

Longitudinal Thoracic Cross-Sections, Right and Left side

As you can see here, the right lung has three lobes, and the left lung only has two. These lobes are anatomically distinct, but perform the same respiratory functions.

I know this isn’t historical, but it’s bloody cool anyhow - these days, it’s actually possible to donate one lobe of your lung while you’re still alive! I always thought of it as something that was donated only when you died, like the heart. Due to a massive deficit of cadaver lungs, the living lung transplant was developed. One lobe of your lungs is removed (leaving you with four lobes), and one lobe of another matching donor is removed, and both of those lobes are transplanted into the recipient. Though your lung doesn’t regenerate the missing lobe, the capacity of the remaining lobes will increase to the point where it doesn’t need that last one, anyhow. The survival and complication rates of transplanted live-donor lobes are comparable to receiving complete cadaver lungs. 

Surgery of the Lung.  C. Garre and H. Quincke, 1913.