Re: Why are the left and right lungs two different sizes?

Great question! There are actually several laboratories around the world researching the issue of Left-Right Asymmetry, of which the lungs are a good example, at this very moment.

Externally, humans and all other vertebrates have bilateral symmetry, meaning that their left side is a mirror image of their right. However internally, vertebrates are not symmetric - there are differences in the internal organs on the left and right sides of the body. In humans, the heart, stomach, and spleen are on the left, while the appendix and gall bladder are on the right. Likewise, there is less liver on the left, to make room for the stomach, and there is less lung on the left, to make room for the heart.

The reason for the asymmetry of these organs has to do with the limited space available in the torso, e.g. the intestinal tract is too long to remain linear, so it must be looped and curled to fit in the abdominal cavity while leaving room for the other organs. The reason why the asymmetries are so tightly regulated also has to do with the limited space, e.g. if the heart ended up on the right without the lungs switching sides as well, the heart applies congestive pressure on the right lung while the left lung overinflates into the vacant cavity (this condition, called dextrocardia, occurs in a small percentage of the population, and often requires surgery to correct).

The recent interest in Left-Right Asymmetry arose from the discovery of a handful of genes which cause situs inversus (complete reversal of all of the internal organs) in humans and mice (and more recently, frogs and zebra fish), and other genes which are only active on one side of the developing embryo. The field is still fairly young, but some great strides have been made. It appears that very early in developement, a left-right axis is generated based on the orientations of the head-tail (anterior-posterior) and back-front (dorsal-ventral) axes. This initial left-right axis is translated into expression of different genes on each side of the embryo, which then act as markers to tell the tissues that develop later which side they are on. So, for example, when it comes time to form a spleen, the cells on the left side of the body have a "left" signal that promotes spleen growth, while the same cells on the right side don't see the "left" signal, so they don't form a spleen. The same is true in the thoracic cavity, where the branching of the lung buds to form the lobes of the lungs is very sensitive to which side of the body they occupy.

So the anatomical reason for the difference between the left and right lungs is to leave room in the chest for the heart, while the actual developmental reason is genetic cues set up in the embryo before the lungs form that tell the left lung to branch less and grow less than the right.