How easy the nose is to breathe through and how wet it is depends on the combination of (1) fixed architecture and (2) structures which adjust their size and degree of wetness.

The anatomy of the nose - the architecture - is the arena in which nasal breathing takes place. The nasal passage looks like the attic under a pitched roof, only with a wall down the center dividing it into two parts. Shelves called turbinates run along the side walls of the space. Turbinates can expand or contract in order to open or close the space. The wall between the two sides of the nose, the nasal septum, continues to grow throughout life. If one inherits a septum which is destined to be too big for one's nose, the septum will have to buckle to fit and contributes to obstruction.  If the pitch on the "roof" is too steep, making the nose too narrow, there is less room for the septum to twist.

Shelves on the side walls of the nose, in addition to the ability to expand and contract, have the job of secreting mucus into the nose. 

This architecture forms an arena in which a tug of war takes place. How swollen and wet the turbinates are depends on two competing streams of instructions coming through the housekeeper nervous system. This system, the autonomic, is the same system which
controls basic functions such as heart rate, blood pressure, and the progress of food through the gut.

One set of these forces, mediated by the sympathetic side of the autonomic nervous system, keeps the nose open  and dry, by shrinking the shelves or turbinates and reducing the amount of mucus being secreted. The other side of the autonomic nervous system, the parasympathetic side, sends a constant stream of instructions to keep the nose swollen and wet. Where the nose is at any one time actually depends on the balance between these two sets of forces. This balance changes throughout the day. Every two hours one side of the nose will  close and the other side will open. This balance is also affected by gravity so that when one lays down there is more of a tendency of the nose to be blocked up.

The sinus cavities are off to the sides and  have no direct effect on breathing. Sinuses are auxiliary mucus generators only. The mucus linings of the nose and sinuses have a conveyor belt quality (as a result of ciliary activity) which causes the constant  normal lubricating mucus to stream backward in the nose down into the throat where it meets another stream always being generated by the lungs. The combined flow from the nose and the lungs is designed to be  swallowed, not expectorated, so the mucus can be decontaminated in the stomach and the components recycled by the digestive system.

The onset of a blocked, stuffy nose is a sign that the balance of forces has been shifted in the direction of over-secretion and closure. Surgical procedures which change the  shape of the nose for appearance reasons usually diminish the amount of airway available for the function of the turbinates and the growth of the septum. Occasionally, nasal fractures can produce the same results. Over-the-counter nose sprays if used longer than two days will reverse their effects and block the nose by swelling turbinates. The most common example of a shift in the balance is the common cold which moves the  balance temporarily but dramatically in favor of the wet, blocked nose.

Hayfever accomplishes the same thing. Less obvious causes include the fluctuating levels of estrogen that accompany the menstrual cycle, since the hormones which are primarily designed to control the surfaces in the genitaltract have echoes in the airway. All of these effects are magnified in the dry air. Medications which change the balance in the housekeeper system such as those used for controlling blood pressure or stomach cramps have nasal side effects. Infection in the sinuses can affect the balance although this is relatively uncommon. Nasal polyps are fleshy exaggerations of the linings of unknown cause.

How much distress one experiences from a blocked nose probably has to do with how dependent one is on nasal breathing. Just as most people are right handed and a few are lefties, so most people are nasal breathers, but some are mouth breathers. The amount of
blockage which causes great distress in one person may not cause any problems for somebody who finds it easy to switch over to mouth breathing. Young children with their characteristically runny noses often are less bothered by the blockage than their parents. This probably reflects the "wiring" in the nervous system just as handedness does.

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