Answer:
The best answer to the question: Explain the reason for this functional difference, would be: that both movements seek to propel mucus and debris, those who enter through the nasal cavity and the nasopharynx, and the mucus that is produced by the respiratory tract as a defense mechanism against debris that might have entered the tract, towards the esophagus, for expulsion from the body.
Explanation:
The air that we breathe does not simply carry gases that we use for survival; it also carries in it debris, dust particles and microorganisms that may enter the body through it and cause damage to the respiratory tract. In order to protect us from that, our bodies, and especially our upper and lower airways count with a series of protective mechanisms. One of those mechanisms is the production of mucus, both in the upper and lower airways, that traps these unwanted invaders and prevents them from reaching the sensitive and vulnerable lungs. However, this mucus needs to be moved, and for that, the airways count with cilia, brush-like extensions that propel the mucus and the debris, microorganisms and dust, that might have been caught in it, so that they can be expelled. While in the nasal cavity and nasopharynx the need is for this mucus and its passangers to reach the esophagus below, the movement will be downwards, while, those in the respiratory tract, need the mucus to reach the esophagus, whose opening lies superiorly; therefore, the cilia here will propel the mucus upwards, and towards the esophagus.
