NAUI Nitrox Diver
36 The Physiology of Diving and Nitrox
Oxygen and Metabolism
Oxygen is our life-support gas, the gas we extract
from air in respiration and use in metabolism to generate
heat and energy. It is absorbed through our lungs,
combined with the hemoglobin in our red blood cells,
and delivered to the tissues via the arterial system. There
it is metabolized to produce our life-maintaining energy.
The primary waste product is carbon dioxide, which–
dissolved in the serum of our blood–is delivered back to
the lungs via the venous side of the circulatory system
and exhaled.
Physiological Effects of Low Oxygen Levels
(Hypoxia)
Our bodies are optimized to operate at greatest efficiency
when the oxygen partial pressure approximates
that of standard sea-level air, 0.21 atmospheres absolute.
People who live at altitude do become acclimated to
lower oxygen levels by increasing the number of red
blood cells in their bodies and other changes. But, if
the partial pressure of oxygen falls too low, the oxygen
supply is inadequate to the task of fully supporting us,
the symptoms of hypoxia, which means “low oxygen,”
begin to appear. The brain, which requires large
amounts of energy, is among the first to suffer.
Manifestations of hypoxia include impaired mental
performance and defective memory, blueness of the lips
(cyanosis), fatigue, visual disturbances, and dizziness.
For most persons, symptoms begin to appear if the partial
pressure of inspired oxygen falls below about 0.16 atmosphere.
If the partial pressure of inspired oxygen falls
below about 0.10 atmosphere more severe symptoms leading
to unconsciousness will occur. Because the breathing
stimulus is largely controlled by carbon dioxide levels
rather than oxygen levels, a hypoxic diver rarely feels any
warning of impending loss of consciousness.
Hypoxia is not a problem for divers breathing
either air or oxygen-enriched air, but it must be
monitored in some rebreather situations or deep diving
applications. Extremely deep dives may require
low percentages of oxygen in the “bottom mix” (the
breathing gas used at depth), and mistakenly breathing
the bottom mix at shallow depth can lead to hypoxia.
Although extremely rare (only one reported death since
accident records have been kept) hypoxia has also
occurred from internal rusting of a steel cylinder stored
over many months with some water inside, which supported
additional rusting that consumed the oxygen in
the stored air.
Physiological Effects of High Oxygen Levels
Oxygen is essential to us, and it also plays an
important role in the treatment of diving maladies such
as decompression sickness. Divers planning stage-decompression
dives will breathe high concentrations of
oxygen during decompression stops in order to offgas
nitrogen more quickly, but safely. As nitrox divers, we
use oxygen-enriched air to safely prolong our dive time
or to increase our nitrogen safety margins. However,
because we are utilizing an oxygen-enriched mixture,
we must control and monitor the inspired partial pressure
of oxygen we are breathing as well as pay attention
to the other parts of dive planning. If we did not do so,
we could easily get into trouble by diving to depths that
allow the oxygen partial pressure in our breathing gas to
become dangerously high.
With oxygen and nitrox, it is entirely possible to
get “too much of a good thing,” There are two types of
oxygen toxicity.
Central nervous system toxicity has a wide range
of signs and symptoms, the most dramatic being epilepsy
like convulsions. CNS toxicity can result from relatively
short exposures to high partial pressures of oxygen.
The other type is called pulmonary toxicity or whole
body toxicity. Pulmonary toxicity results from prolonged
exposure to elevated partial pressures of oxygen (above
about 0.5 atmosphere). As the name implies, its most
pronounced effects are on the lungs, producing signs
and symptoms such as chest tightness, breathing discomfort
and pain, shortness of breath, and coughing.
Development of pulmonary toxicity requires long-term
exposures such as encountered in saturation diving, cer