There aren't many common respiratory system words that start with the letter "K". Here are a few possibilities:

* Kink: This word can be used to describe a sharp bend in a bronchus, which could potentially obstruct airflow.

* Karyotype: This is a more general biology term, but it relates to the chromosomes found in lung cells, which can be affected by respiratory diseases.

* Klebsiella: This is a genus of bacteria that can cause pneumonia and other respiratory infections.

Let me know if you'd like more options or information on any of these terms!