CHAPTER 3 Underwater Physiology
16 oxygen bottle containing 360 standard liters (3.96 scf) of usable gas will last
225 minutes at an oxygen consumption rate of 1.6 liters per minute at any depth,
provided no gas leaks from the rig.
Minute ventilation, or respiratory minute volume (RMV), is measured at BTPS
(body temperature 37°C/98.6°F, ambient barometric pressure, saturated with
water vapor at body temperature) and varies depending on a persons activity
level, as shown in Figure 3-6. Surface RMV can be approximated by multiplying
the oxygen consumption rate by 25. Although this 25:1 ratio decreases with
increasing gas density and high inhaled oxygen concentrations, it is a good rule-
of-thumb approximation for computing how long the breathing gas will last.
Unlike oxygen consumption, the amount of gas exhaled by the lungs is depth
dependent. At the surface, a diver swimming at 0.5 knot exhales 20 l/min of gas. A
scuba cylinder containing 71.2 standard cubic feet (scf) of air (approximately
2,000 standard liters) lasts approximately 100 minutes. At 33 fsw, the diver still
exhales 20 l/min at BTPS, but the gas is twice as dense; thus, the exhalation would
be approximately 40 standard l/min and the cylinder would last only half as long,
or 50 minutes. At three atmospheres, the same cylinder would last only one-third
as long as at the surface.
Carbon dioxide production depends only on the level of exertion and can be
assumed to be independent of depth. Carbon dioxide production and RQ are used
to compute ventilation rates for chambers and free-flow diving helmets. These
factors may also be used to determine whether the oxygen supply or the duration
of the CO2 absorbent will limit a divers time in a closed or semi-closed system.
RESPIRATORY PROBLEMS IN DIVING
Physiological problems often occur when divers are exposed to the pressures of
depth. However, some of the difficulties related to respiratory processes can occur
at any time because of an inadequate supply of oxygen or inadequate removal of
carbon dioxide from the tissue cells. Depth may modify these problems for the
diver, but the basic difficulties remain the same. Fortunately, the diver has normal
physiological reserves to adapt to environmental changes and is only marginally
aware of small changes. The extra work of breathing reduces the divers ability to
do heavy work at depth, but moderate work can be done with adequate equipment
at the maximum depths currently achieved in diving.
Oxygen Deficiency (Hypoxia). Oxygen deficiency, or hypoxia, is an abnormal
deficiency of oxygen in the arterial blood that causes the tissue cells to be unable
to receive sufficient oxygen to maintain normal function. Severe hypoxia will stop
the normal function of any tissue cell in the body and will eventually kill it, but the
cells of the brain tissue are by far the most susceptible to its effects.
The partial pressure of oxygen determines whether the amount of oxygen in a
breathing medium is adequate. For example, air contains about 21 percent oxygen
and thus provides an oxygen partial pressure of about 0.21 ata at the surface. This