The arrival of more blood turns the mucosa redder (hyperemia) and causes it to swell. The greater volume of the mucosa creates a blockage in the nasal cavities, causing the sensation of stuffy nose.

To this is added the fact that an abundant quantity of liquids exits the dilated capillaries; it unites with the excess mucus produced and aggravates the difficulty of breathing correctly.

It is the reaction of the physiological defense mechanisms of the mucosa during an infectious illness or an allergy attack which causes the unpleasant sensation of stuffy nose. We must keep this in mind. If we act forcefully against these defensive systems, we leave the body defenseless against aggression.

The nasal mucosa and congestion

The inflammation underlying nasal congestion provokes hyperemia and swelling of the mucosa as well as increased production of liquid and mucus. All this creates obstruction of the nasal cavities and the sensation of stuffy nose. 
Aboca Grintuss