Like they do eyes and ears, human beings have two noses. Each nostril is its own pathway to the respiratory system and while both obviously work together, they can be considered separate in many ways. Notice that if you close each nostril and breathe in sequence, one of them will always seem slightly less clear than the other. If you repeat the process approximately every 3-6 hours, you will find that in a healthy nose each nostril alternates in clearness. This is called the nasal cycle, caused by increased blood flow to the structures inside the nose which continuously switches back and forth. Many people assume that congestion is caused by excess mucus, when in fact most cases of congestion are caused by a swelling of the nasal cavity that can result from a number of different stimuli, including the perfectly healthy alternation of blood flow outlined above.
The nose serves a few important purposes for the respiratory system. Because a large percentage of the air we breathe comes in through the nose, it acts as a first line of defense for your respiratory system. Air often enters the nose at a temperature that is too cold to be ideal, so the nostrils act as a temperature conditioning device that warms the air before it reaches the lungs. Likewise, breathing dry air directly into the lungs inhibits respiratory efficiency and so the nose acts as a humidifier. And because there are contaminants and irritants in the air that surrounds us, the nose acts as a filter. The lining of the nose, called mucosa, produces mucus which keeps the nose moist and traps tiny particles. Epithelial cells have cilia (hair-like projections) that line the inside of the nostrils which move the mucus, and tiny particles trapped in mucus, out of the nose. This is called mucocilliary clearance.