question: if pressure decreases as altitudes increase, how come pressure builds on the ears/head as airplanes ascend and descend?
comment: The pressure differential is in different directions. When the plane ascends, the pressure in is higher in your inner ear than in the cabin as the cabin air thins out. Once you equalize, the pressure is the same. On descent, the pressure in the cabin rises back to sea level, but the pressure in your inner ear is still at altitude, so it's lower that the ambient cabin pressure.
passages: The range of intraoral pressure involved in various classes of ethnic wind instruments, such as Native American flutes, has been shown to be generally lower than Western classical wind instruments.
  
  Section::::Influencing factors.:Drugs.
  
  Intraocular pressure also varies with a number of other factors such as heart rate, respiration, fluid intake, systemic medication and topical drugs. Alcohol and marijuana consumption leads to a transient decrease in intraocular pressure and caffeine may increase intraocular pressure.
   Pressure decreases as we rise above sea level, but less dramatically. At 3000 feet altitude (almost 1000 meters), the ambient pressure is almost 90% of sea level pressure. Ambient pressure does not drop to 50% of sea level pressure until 20,000 feet or 6,000 meters altitude.
  
  Section::::Effects of pressure on the body.
  
  Section::::Effects of pressure on the body.:Direct effects on tissues.
   The phenomenon is based on the differential in atmospheric pressure, directly correlated with the observer's frequency of and level of experience in high-altitude metabolism. As the external pressure decreases, the difference in pressure between the gas within the body and the atmosphere outside is higher, and the urge to expel gas to relieve the pressure is greater. Consistent with Boyle's law, controlling for dietary variance, the amount of gas produced is constant in mass, but the volume increases as the external pressure decreases.
   The effects of changes in atmospheric pressure upon water levels observed in wells has been known for many years. The effect is a direct one, an increase in atmospheric pressure is an increase in load on the water in the aquifer, which increases the depth to water (lowers the water level elevation). Pascal first qualitatively observed these effects in the 17th century, and they were more rigorously described by the soil physicist Edgar Buckingham (working for the United States Department of Agriculture (USDA)) using air flow models in 1907.
  
  Section::::Head loss.
   The baroreceptors are stretch-sensitive mechanoreceptors. At low pressures, baroreceptors become inactive. When blood pressure rises, the carotid and aortic sinuses are distended further, resulting in increased stretch and, therefore, a greater degree of activation of the baroreceptors. At normal resting blood pressures, many baroreceptors are actively reporting blood pressure information and the baroreflex is actively modulating autonomic activity. Active baroreceptors fire action potentials ("spikes") more frequently. The greater the stretch the more rapidly baroreceptors fire action potentials. Many individual baroreceptors are inactive at normal resting pressures and only become activated when their stretch or pressure threshold is exceeded.
   At low altitudes above sea level, the pressure decreases by about for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure "p" to altitude "h":
  
  formula_1
  
  where the constant parameters are as described below:
  
  Section::::Local variation.
  
  Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.
   Ambient pressure
  
  The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.
  
  Section::::Atmosphere.
  
  Within the atmosphere, the ambient pressure decreases as height increases. By measuring ambient atmospheric pressure, a pilot may determine altitude (see pitot-static system). Near sea level, a change of ambient pressure of 1 millibar is taken to represent a change of height of .
  
  Section::::Underwater.
   which will not change unless the temperature does. Integrating the above and assuming "P" is the pressure at height "z" = 0 (pressure at sea level) the pressure at height "z" can be written as:
  
  This translates as the pressure decreasing exponentially with height.
   Rapid descents relate to dramatic changes in cabin air pressure—even pressurized aircraft—and can result in discomfort in the middle ear. Relief is achieved by decreasing relative pressure by equalizing the middle ear with ambient pressure ("popping ears") through swallowing, yawning, chewing, or the valsalva maneuver.
   All of the above influences of low atmospheric pressures on breathing are accommodated primarily by breathing deeper and faster (hyperpnea). The exact degree of hyperpnea is determined by the blood gas homeostat, which regulates the partial pressures of oxygen and carbon dioxide in the arterial blood. This homeostat prioritizes the regulation of the arterial partial pressure of carbon dioxide over that of oxygen at sea level. That is to say, at sea level the arterial partial pressure of CO is maintained at very close to 5.3 kPa (or 40 mmHg) under a wide range of circumstances, at the expense of the arterial partial pressure of O, which is allowed to vary within a very wide range of values, before eliciting a corrective ventilatory response. However, when the atmospheric pressure (and therefore the partial pressure of O in the ambient air) falls to below 50-75% of its value at sea level, oxygen homeostasis is given priority over carbon dioxide homeostasis. This switch-over occurs at an elevation of about 2500 m (or about 8000 ft). If this switch occurs relatively abruptly, the hyperpnea at high altitude will cause a severe fall in the arterial partial pressure of carbon dioxide, with a consequent rise in the pH of the arterial plasma. This is one contributor to high altitude sickness. On the other hand, if the switch to oxygen homeostasis is incomplete, then hypoxia may complicate the clinical picture with potentially fatal results.
   Arterial chemical receptors are stimulated by exposure to a low partial pressure and hence increase alveolar ventilation, up to a maximum of 1.65 times. Almost immediately, compensation for the higher altitude begins with an increase of pulmonary ventilation eliminating a large amount CO2. Carbon dioxide partial pressure reduces and corporal fluids pH increase. These actions inhibit the respiratory center of the encephalic trunk, but later this inhibition disappears and the respiratory center responds to the stimulation of the peripheral chemical receptors because of the hypoxia increasing ventilation up to six times.
   At about 150 thousand feet of altitude (28 miles (45 km)) on Earth, the atmospheric pressure starts to be equivalent to the surface of Mars.
  
  See also Effects of high altitude on humans
  
  BULLET::::- Temperature
   Altitude DCS became a problem in the 1930s with the development of high-altitude balloon and aircraft flights but not as great a problem as AMS, which drove the development of pressurized cabins, which coincidentally controlled DCS. Commercial aircraft are now required to maintain the cabin at or below a pressure altitude of even when flying above . Symptoms of DCS in healthy individuals are subsequently very rare unless there is a loss of pressurization or the individual has been diving recently. Divers who drive up a mountain or fly shortly after diving are at particular risk even in a pressurized aircraft because the regulatory cabin altitude of represents only 73% of sea level pressure.
   The alveoli are open (via the airways) to the atmosphere, with the result that alveolar air pressure is exactly the same as the ambient air pressure at sea level, at altitude, or in any artificial atmosphere (e.g. a diving chamber, or decompression chamber) in which the individual is breathing freely. With expansion of the lungs (through lowering of the diaphragm and expansion of the thoracic cage) the alveolar air now occupies a larger volume, and its pressure falls proportionally, causing air to flow in from the surroundings, through the airways, till the pressure in the alveoli is once again at the ambient air pressure. The reverse obviously happens during exhalation. This "process" (of inhalation and exhalation) is exactly the same at sea level, as on top of Mt. Everest, or in a diving chamber or decompression chamber.
   Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the (gauge) tire pressure goes up because atmospheric pressure goes down. The absolute pressure in the tire is essentially unchanged.
   Barotrauma located in the maxillary, ethmoidal, or sphenoid sinuses is observed less frequently and appears when the ostia are blocked; the majority of cases are probably caused by an acute upper respiratory tract infection. The magnitude of the pressure difference needed to produce a barotrauma probably shows great individual variation and is related to the size of the sinus ostium and the rate of ambient pressure change. Due to this, even commercial flying may produce severe cases of barotraumas, although most of the cases are observed in high performance aircraft with lower pressurized cabins.
  
  Section::::Diagnosis.
   Gas is very compressible. Humans have many air spaces: sinuses, middle ears, gas in our bowels, cavities in our teeth, and largest of all, our lungs. On land in our daily lives, the pressure in our air spaces is usually exactly the same as the pressure outside, because our air spaces are connected to the outside world. If there was a pressure difference between the outside world and one of our air spaces, then we experience painful pressure on the walls of that air space, as air pushes from the higher-pressure side to the lower-pressure side. This is why we sometimes get painful ears on air trips.
   Pressure on Earth varies with the altitude of the surface; so air pressure on mountains is usually lower than air pressure at sea level. Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude. Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%.
   The physics that affect the body in the sky or in space are different from the ground. For example, barometric pressure is different at different heights. At sea level barometric pressure is 760 mmHg; at 3.048 m above sea level, barometric pressure is 523 mmHg, and at 15.240 m, the barometric pressure is 87 mmHg. As the barometric pressure decreases, atmospheric partial pressure decreases also. This pressure is always below 20% of the total barometric pressure. At sea level, alveolar partial pressure of oxygen is 104 mmHg, reaching 6000 meters above the sea level. This pressure will decrease up to 40 mmHg in a non-acclimated person, but in an acclimated person, it will decrease as much as 52 mmHg. This is because alveolar ventilation will increase more in the acclimated person. Aviation physiology can also include the effect in humans and animals exposed for long periods of time inside pressurized cabins
   The physiology of altitude sickness centres around the alveolar gas equation; the atmospheric pressure is low, but there is still 20.9% Oxygen. Water vapour still occupies the same pressure too—this means that there is less oxygen pressure available in the lungs and blood. Compare these two equations comparing the amount of oxygen in blood at altitude:
   There is some inconclusive research that indicates that exercise could possibly affect IOP (some positively and some negatively).
  
  Section::::Influencing factors.:Musical instruments.
  
  Playing some musical wind instruments has been linked to increases in intraocular pressure. One 2011 study focused on brass and woodwind instruments observed "temporary and sometimes dramatic elevations and fluctuations in IOP".
  
  Another study found that the magnitude of increase in intraocular pressure correlates with the intraoral resistance associated with the instrument, and linked intermittent elevation of intraocular pressure from playing high-resistance wind instruments to incidence of visual field loss.
   Negative pressure in air fill spaces results from increased barometric pressure during repressurization after an EVA or following a planned restoration of a reduced cabin pressure. Common symptoms include ear or sinus pain, decreased hearing, and tooth or jaw pain.
  
  Treatment may include active positive pressure equalization of ears and sinuses, oral and nasal decongestants, or oral and nasal steroids, and appropriate pain medication if needed.
  
  Section::::Effects of space-travel.:Decreased immune system functioning.
   Normally, the Eustachian tube is collapsed, but it gapes open both with swallowing and with positive pressure. When taking off in an airplane, the surrounding air pressure goes from higher (on the ground) to lower (in the sky). The air in the middle ear expands as the plane gains altitude, and pushes its way into the back of the nose and mouth. On the way down, the volume of air in the middle ear shrinks, and a slight vacuum is produced. "Active" opening of the Eustachian tube is required to equalize the pressure between the middle ear and the surrounding atmosphere as the plane descends. A diver also experiences this change in pressure, but with greater rates of pressure change; active opening of the Eustachian tube is required more frequently as the diver goes "deeper" into higher pressure.
   The other main issue with altitude is hypoxia, caused by both the lack of barometric pressure and the decrease in oxygen as the body rises. With exposure at higher altitudes, alveolar carbon dioxide partial pressure (PCO2) decreases from 40 mmHg (sea level) to lower levels. With a person acclimated to sea level, ventilation increases about five times and the carbon dioxide partial pressure decreases up to 6 mmHg. In an altitude of 3040 meters, arterial saturation of oxygen elevates to 90%, but over this altitude arterial saturation of oxygen decreases rapidly as much as 70% (6000 m), and decreases more at higher altitudes.
   Although the exact cause is not known at this time, it is suspected that microgravity-induced cephalad fluid shift and comparable physiological changes play a significant role in these changes. Other contributing factors may include pockets of increased CO and an increase in sodium intake. It seems unlikely that resistive or aerobic exercise are contributing factors, but they may be potential countermeasures to reduce intraocular pressure (IOP) or intracranial pressure (ICP) in-flight.
  
  Section::::Causes and current studies.
  
presuppositions: Pressure builds as planes ascend and descend. Pressure only decreases as altitudes increase.
corrections: You are feeling a difference in pressure not an absolute increase in pressure.  Pressure can increase or decrease differently in different directions of altitude.
raw_presuppositions: Pressure builds as planes ascend and descend. Pressure only decreases as altitudes increase.
raw_corrections: You are feeling a difference in pressure not an absolute increase in pressure.  Pressure can increase or decrease differently in different directions of altitude.