BPK 142 Lecture Notes - Lecture 17: Snorkeling, Depth Gauge, Ambient Pressure

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Pressure Effects
Pressure of air at sea level = one atmosphere or 760 mm Hg
The weight of a column of water directly above a diver's body (hydrostatic
pressure) increases directly with increasing depth. The pressure increases by
one atmosphere for each additional 33 ft. of depth.
33 ft. = 2.0 atmosphere
66 ft. = 3.0
Because the tissues of the body are largely water, they are non-compressible.
However, the body contains air cavities - lungs, respiratory passages,
sinus and middle ear spaces - where volume and pressure will change
with increases or decreases in diving depth.
Boyle's Law: The volume of any gas varies inversely with the pressure on it
Example: if the pressure is doubled, volume is halved. II.
Snorkeling and Breathing-Hold Diving
● Snorkeling
There are limits to snorkel size because
1. Pressure effects: When breathing through a snorkel, the diver
must inspire air at atmospheric pressure. At a depth of only 3 ft., the
compressive force of water against the chest cavity is so large that the
inspiratory muscles are usually unable to overcome external pressure
and expand the thoracic cavity.
2. Increase in pulmonary dead space:
Normal anatomical dead space = 150 ml.
Dead space of regular snorkel = 150 ml.
VA = VT - VD:
At rest: 350 ml = 500 ml - 150 ml
With snorkel: 350 ml = 650 ml - 300 ml
As snorkel size increases, VD increases and it becomes more
difficult to maintain VA.
Breath Hold Diving
As the skin diver descends, the air in the lungs is compressed -> lung
When lung volume is compressed below residual volume -> lung
damage occurs as blood is sucked from the pulmonary capillaries into
the alveoli
"Normal" maximal breath holding time after a maximal inspiration of ambient
air is approximately 50 - 60 seconds. The arterial PO2 then drops to about 60
mm Hg and the arterial PCO2 rises to 50 mm Hg.
Hyperventilation prior to breath-hold diving can significantly extend the
breath-hold time. This is very dangerous due to the risk of blackout – loss of
consciousness in the water.
Hyperventilate forcefully for one minute - can decrease PaCO2 from
normal value of 40 mm Hg to 15-20 mmHg.
Normal PaO2 = 100 mmHg. Unconsciousness will occur when PaO2
decreases to approximately 30 mm Hg.
Paradoxical drowning - Diver hyperventilates, holds breath, and dives down
to a certain depth Æ gases in lung are compressed and partial pressures are
increased Æ diver holds breath as long as possible and then starts to ascend
Æ partial pressure of gases in lung decrease on ascent Æ PaO2 decreases
below critical point Æ diver loses consciousness and drowns.
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