The middle ear and the nasal sinuses are essentially closed cavities with a narrow drainage, pressure-equalizing tube.  As the aircraft climbs, air in the body cavities expands as the barometric pressure decreases.  Normally air will escape from the middle ear and the sinuses and the pilot will only notice the ears "popping".   The outlet of these tubes, however, is narrow and, if the aviator has a head cold or a throat infection, local swelling may reduce it.  On ascent air may still be able to escape but on descent, particularly at high rates, the outlet may close like a flap preventing air from re-entering the middle ear cavity.  The increasing ambient air pressure will then force the eardrum inward.  This can lead to severe pain and to an injury known as a "barotrauma" (pressure injury).  Pressure in the ears can be equalized by swallowing, yawning or by a Valsalva manoeuvre.