the normal level of carbon dioxide in the alveoli is 40 mmHg (0.055 bar)
when diving the arterial and alveolar carbon dioxide tensions should be maintained at approximately 40 mmHg
with increasing depth, the alveolar pressures of nitrogen and oxygen increase and therefore the alveolar percentage of carbon dioxide decreases
however, when energy expenditure is high, the level of carbon dioxide can increase dramatically
most commonly encountered in divers using rebreather sets
may also be seen in the recompression chamber if there is inadequate flushing of the chamber with fresh gas
if the level of carbon dioxide increases then clinical features of hypercapnia may occur:
breathlessness (carbon dioxide present at about 3%)
distress and dyspnoea (5-6%)
rise in blood pressure and heart rate, mental confusion and lack of coordination (10%)
followed by loss of consciousness and death (12-14%)
although carbon dioxide is a respiratory stimulant, most of its effects are related to the metabolic acidosis it produces and are neurologically depressant
Reference:
atmospheric pressure at sea level is 1 atm (equivalent to 101.3 kPa) and is the pressure that is experienced by all parts of the human body at sea level
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