Marine Vertebrates: Lecture 12
February 20, 2008
Diving: Effects of Depth and Pressure
I.
Diving depths for selected air-breathing vertebrates (in order of depth, see PowerPoint)
II. Problems of pressure: Overview
A.
Decompression sickness (“the bends”)
B.
Nitrogen narcosis
C.
Barotrauma (air space squeeze, i.e. lungs, middle ear, sinuses)
D.
High pressure neurological syndrome
E.
[Oxygen toxicity, free-radical formation]
F.
Effects on buoyancy
III. Decompression sickness (“the bends”)
A.
What causes decompression sickness?
1.
Henry’s Law: The amount of gas entering solution is proportional to its pressure
a.
Increased PP of nitrogen with depth (essentially increased N2 concentration)
 increased diffusion of N2 into blood, then to tissues.
2.
The problem is on the ascent: As pressure decreases, less N2 can be dissolved in
blood. BUT…It takes time for N2 to diffuse back into bloodstream and then be
offgassed by lungs.
a.
If the trip from depth is too fast, the N2 comes out of solution and forms
bubbles.

In humans, longer dives require longer decompression time because
more air has been breathed over time
B.
What are the effects (in human divers)?
1.
Symptoms depend upon location of bubbles
a.
In skin: rashes or even bubbled appearance of skin
b.
In joints: Extreme pain
c.
In central nervous system

Weakness, paralysis, dizziness  loss of consciousness

“Tiny bubbles, in my brain…”
2.
Long term effects: Deterioration of tissue because oxygen cannot diffuse into tissue
(blocked by nitrogen bubble)
C.
What protects marine mammals from it?
1.
They bring down a limited air supply in their respiratory system!
a.
Decompression sickness is rarely reported by “normal” free divers. It occurs
on SCUBA due to a far-increased air supply and also when humans use
artificial means to get down (i.e. weight sleds) and then rise from extreme
depths quickly.
b.
Note that the deeper diving vertebrates bring down a relatively small supply
of oxygen in their respiratory system.
2.
Exhalation decreases amount of air.
3.
Lung collapse pushes air into spaces that are not in direct contact with blood.
4.
In Weddell seals, the infusion of blood cells from the spleen allows for dilution of
nitrogen gas.
D.
Is there any evidence that marine mammals might experience some of the effects of
the “the bends”?
1.
Bone lesions of sperm whales… (Controversial)
Page 1 of 2
IV. Nitrogen narcosis: “rapture of the deep”
A.
Increased pressure of N2 apparently affects membrane fluidity and function (not well
understood, since N2 is basically “inert”)
B.
Causes mild euphoria  impaired judgment  impaired motor function  hallucination
V. Barotrauma
A.
Middle ear
1.
Humans and some marine mammals can equalize air spaces via creating air pressure
within the Eustachian tube; again, failure to equalize can lead to bursting of
tympanic membrane and blood vessels
2.
Cetaceans have fat-filled middle ears
B.
Lungs
1.
Humans: lung collapse to “residual volume” at 30 m; beyond this is “lung squeeze”
2.
Marine mammals: lungs are collapsible and re-inflatable; air is pushed into
cartilage-reinforced spaces that are strong, shaped to give but not collapse.
a.
Rib cage is flexible
3.
Deep-diving turtles: similar; flexibility of plastron mirrors rib cage flexibility in
mammals.
C.
Sinuses
1.
Humans can equalize sinus spaces via nasal passage; can be problematic, cause
bursting of blood vessels
2.
Marine mammals: no sinuses
VI. High pressure neurological syndrome
A.
Caused by increased excitability of neurons
B.
Marine mammals apparently utilize negative feedback circuits to mitigate this effect.
1.
In Weddell seals, secretion of cortisol from adrenal glands may play a role
(circulation maintained in adrenal gland)
VII. Effects of pressure on buoyancy
A.
Changes of buoyancy with depth
1.
Increased pressure  decreased buoyancy
2.
As the animal moves through the pycnocline, buoyancy decrease due to pressure is
partly offset by increased water density.
VIII. Buoyancy control: The spermaceti organ of sperm whales
Page 2 of 2