Question

Why does the front side of the trachea have cartilage rings and the back side does not?

Answer 1

There's cartilage on the anterior side primarily for protection and it's absent in the posterior side in order to provide room for the oesophagus to distend.

Explanation:

The front of the entire trachea is covered with 16-20 C shaped hyaline cartilage with the posterior side being covered only by trachealis smooth muscle. This is due to the fact that some portions of the anterior side of the trachea is subcutaneous (means you can feel it from the outside, particularly at your neck). So, to protect the airway, there's cartilage at the front. Also, another important function of this cartilage is to prevent the airway from 'collapsing' due to negative pressure during inspiration.

So, that's good and all, but why's there no cartilage behind the trachea as with the bronchi? The reason is that, you don't really need protection on the posterior side, cause there are lots of structures behind the trachea and so, the chances of the trachea being injured from the outside on the posterior side is very low.

But, that's not the only reason. In fact, the primary reason is that the oesophagus is located closely behind the trachea and runs down with it. When food passes down the oesophagus, it gets distended substantially as opposed to when there's no food. So, in order to provide room for that distension, the trachea has a layer of smooth muscle instead of a cartilaginous layer on the posterior aspect. If there were cartilages in the back as like that at the front, it would've been very difficult for the oesophagus to dilate during the passage of food.

So, that is the reason why there's cartilage on the front side while the backside lacks that layer.