Effects of Barometric Pressure
At sea level, 760mmHg or 1 atm, air is 20.98% O 2 , 0.04% CO 2 , 78.06% N, and 0.92% other gases. Water vapor in the air reduces these percentages. Gases expand to fill the container they are in and will diffuse or spread from areas of greater concentration or pressure to areas of lower concentration or pressure. The partial pressure of a gas is a measure of its concentration in air or a liquid. For instance, in air at 1 atm of barometric pressure, oxygen has a partial pressure of 160 mmHg (760 x 0.21). Partial pressures are indicated by preceding the chemical symbol of the gas with a capital P . Humidity or water vapor in the air exerts a partial pressure that reduces the pressure of the other components. The water vapor in the alveolar air exerts a partial pressure of about 47 mmHg. This reduces the PO 2 to 149 mmHg.
Decreased barometric pressure, as in increased altitude, decreases the partial pressure of oxygen (PO 2 ) in the air. When the PO 2 is reduced to a significant degree, chemoreceptors sense the hypoxia and cause an increase in respiration rate. Such an increase in ventilation rate can excrete excessive CO 2 and cause a respiratory alkalosis.
A mountain sickness, characterized by irritability, headache, sleeplessness, breathlessness, nausea and vomiting may occur within the first 24 hours that a person arrives at a high altitude. This sickness can last 4-8 days. Generally, those ascending to high altitudes do so slowly with time for acclimatization to increase altitude tolerance. If an unacclimitized person ascends to above 6000 m they usually loose consciousness due to lack of oxygen when breathing ambient air. Persons breathing pure oxygen do not loose consciousness until they reach about 14000 m . Above this level, life is impossible with out pressurization.
Physiological mechanisms of acclimatization include a decreased affinity of hemoglobin for oxygen which makes more oxygen available to the tissues. The hormone erythropoetin increases and stimulates red blood cell production. This increases the oxygen carrying capacity of the blood. In the tissues, cellular adaptations include increased mitochondria and cytochrome oxidase.
Descending to depths within bodies of water increases ambient pressure by one atm for approximately each 10 m of depth ( 10 m for seawater, 10.4 m for freshwater). Divers, breathing pressurized air via scuba gear, are subject to the effects of changes in pressure in descent and ascent from the depths. Nitrogen narcosis can result from breathing pressurized air at depths of 4-5 atm. The diver experiences euphoria and a sort of intoxication where intellectual functions are diminished. Other gases are often substituted for nitrogen in scuba tanks. However, many gases that are inert at normal atmospheric pressure are anesthetics at increased pressures.
Divers may also experience decompression sickness when they rapidly ascend due to nitrogen leaving the blood and tissues and accumulating as bubbles. Bubbles can cause pain, neurologic symptoms, paralysis, and respiratory failure due to accumulation in the joints or blood stream where they might occlude blood flow in vital organs. Treatment is by recompression followed by slow decompression in a pressure chamber.