Page 147 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Effects of Diving on Respiration (! p. 132) trigger a sensation of shortness of
breath, signaling that it is time to resurface.
Diving creates a problem for respiration due to To delay the time to resurface, it is possible to lower
the lack of normal ambient air supply and to the P CO 2 in blood by hyperventilating before diving.
higher the outside pressures exerted on the Experienced divers use this trick to stay under water
body. The total pressure on the body under- longer. The course of alveolar partial pressures over
water is equal to the water pressure (98 kPa or time and the direction of alveolar gas exchange while
735 mmHg for each 10 m of water) plus the at- diving (depth: 10 m; duration 40 s) is shown in C: Hy-
perventilating before a dive reduces the P CO 2 (solid
mospheric pressure at the water surface. green line) and slightly increases the P O 2 (red line) in
A snorkel can be used when diving just the alveoli (and in blood). Diving at a depth of 10 m
below the water surface (! A), but it increases doubles the pressure on the chest and abdominal
dead space (! pp. 114 and 120), making it wall. As a result, the partial pressures of gases in the
harder to breathe. The additional pressure load alveoli (P CO 2 , P O 2 , P N 2 ) increase sharply. Increased
from the water on chest and abdomen must quantities of O 2 and CO 2 therefore diffuse from the
also be overcome with each breath. alveoli into the blood. Once the P CO 2 in blood rises to a
certain level, the body signals that it is time to resur-
The depth at which a snorkel can be used is limited 1) face. If the diver resurfaces at this time, the P O 2 in the
because an intolerable increase in dead space or air- alveoli and blood drops rapidly (O 2 consumption +
Respiration long or narrow snorkel, and 2) because the water stops. Back at the water surface, the P O 2 reaches a
way resistance will occur when using an extremely
pressure decrease) and the alveolar O 2 exchange
pressure at lower depths will prevent inhalation. The
level that is just tolerable. If the diver excessively hy-
maximum suction produced on inspiration is about
perventilates before the dive, the signal to resurface
pressure, ! p. 116). Inspiration therefore is no
(anoxia) before the person reaches the water sur-
5 11 kPa, equivalent to 112 cm H 2O (peak inspiratory will come too late, and the P O 2 will drop to zero
longer possible at aquatic depths of about 112 cm or face, which can result in unconsciousness and
more due to the risk of hypoxic anoxia (! A). drowning (! C, dotted lines).
Scuba diving equipment (scuba = self-con- Barotrauma. The increased pressure as-
tained underwater breathing apparatus) is sociated with diving leads to compression of
needed to breathe at lower depths (up to about air-filled organs, such as the lung and middle
70 m). The inspiratory air pressure (from pres- ear. Their gas volumes are compressed to /2
1
surized air cylinders) is automatically adjusted their normal size at water depths of 10 m, and
to the water pressure, thereby permitting the to /4 at depths of 30 m.
1
diver to breathe with normal effort. The missing volume of air in the lungs is automati-
However, the additional water pressure increases the cally replaced by the scuba, but not that of the
partial pressure of nitrogen P N 2 (! B), resulting in middle ear. The middle ear and throat are connected
higher concentrations of dissolved N 2 in the blood. by the Eustachian tube, which is open only at certain
The pressure at a depth of 60 meters is about seven times (e.g., when swallowing) or not at all (e.g., in
times higher than at the water surface. The pressure pharyngitis). If volume loss in the ear is not compen-
decreases as the diver returns to the water surface, sated for during a dive, the increasing water pressure
but the additional N 2 does not remain dissolved. The in the outer auditory canal distends the eardrum,
diver must therefore ascend slowly, in gradual stages causing pain or even eardrum rupture. As a result,
so that the excess N 2 can return to and be expelled cold water can enter the middle ear and impair the
from the lungs. Resurfacing too quickly would lead organ of equilibrium, leading to nausea, dizziness,
to the development of N 2 bubbles in tissue (pain!) and disorientation. This can be prevented by
and blood, where they can cause obstruction and pressing air from the lungs into the middle ear by
embolism of small blood vessels. This is called holding the nose and blowing with the mouth
decompression sickness or caisson disease (! B). closed.
Euphoria (N 2 narcosis?), also called rapture of the The air in air-filled organs expand when the
deep, can occur when diving at depths of over 40 to
60 meters. Oxygen toxicity can occur at depths of diver ascends to the water surface. Resurfacing
75 m or more (! p. 136). too quickly, i.e., without expelling air at regular
intervals, can lead to complications such as
When diving unassisted, i.e., simply by holding
one’s breath, P CO 2 in the blood rises, since the lung laceration and pneumothorax (! p. 110)
134 CO 2 produced by the body is not exhaled. Once as well as potentially fatal hemorrhage and air
a certain P CO 2 has been reached, chemosensors embolism.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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