Tiny Hairs of the Respiratory System Faultlessly Determine Direction
Along with air, we actually breathe in a lot of dust. But this and many other substances harmful to the body are held at certain "security zones" where they are neutralized before they ever get a chance to reach the lungs.
From the nose to the bronchioles, the entire surface of the respiratory pathway is coated with a layer of mucus. This substance, which also acts as a moisturizer for the respiratory surface, also traps small breathed-in particles like dust, preventing them from reaching the lungs. After these foreign particles have been caught by the mucus, however, they must be ejected out of the body, lest they build up in the respiratory passages. To effect this, another security mechanism comes into play.
Lining the respiratory surfaces are sharp-pointed flagella known as cilia, approximately 200 present on each cell. By whipping back and forth in waves, from ten to twenty times a second, these cilia facilitate regular movement—always up towards the pharynx. This way, any mucus that has trapped foreign particles is directed towards the pharynx at a rate of one centimeter per minute.
In the nose, however, mucus needs to be directed downwards, and so the cilia move in the opposite direction. This way, any foreign matter in the nasal mucus is also moved towards the pharynx. Later on, any foreign matter is either swallowed along with the mucus and taken to the digestive system, or expelled from the body through coughing.
As these examples show, these tiny hairlike structures can determine the location of the pharynx which is, relatively speaking, quite a distance away, even though they have no eyes to see with, and no brain to think. In addition, they know that if this foreign matter travels down to the lungs, it may well harm the body. To prevent this from happening, they act in a most harmonious way, beating always in the same direction.
In various experiments and using many different methods, scientists have studied this mechanism for many years. However, they still don't completely understand this mechanism, which tiny hairs only two millionth of a meter in size have been carrying on faultlessly since the first human was alive. Acting with the inspiration given to them by God Who created them, cilia perform in an ingenious way that cannot have been "rehearsed" in a series of coincidences.
Cilia in the windpipe, photographed by an electron microscope