Issue 3, December 2001
The Human Body Beneath the Sea
Mechanical Engineering, Princeton University
1 - Pressure and Volume Dependence on Depth
surfaced, kicked a few strokes to the waiting boat, and passed her
scuba gear up on deck. Moments later, like a triumphant fisherman
just returned to shore, she was giving her account of the dive when
the divemaster, Mark, surfaced. "Kara, are you o.k.? You came up
too fast," he hurriedly interrupted her stories as he was climbing
aboard. "Do you feel any tingling? If you feel anything out of sorts,
any pins and needles, or any pain tomorrow, tell me immediately."
Mark was concerned about decompression sickness, which is more commonly
known as 'the bends'. The nickname for this condition comes from
the curled-up shape of those suffering from the severe pain it causes.
Contrary to the belief of many beginning scuba divers, the amount
of air in a diver's tank is not the only factor limiting the length
of a dive. Another limiting factor is the concentration of gases
absorbed by body tissues, which restricts dives even when a diver's
air supply is not exhausted. Most recreational divers breathe air,
a mixture of 79% N2, 20% O2 and 1% all other gases. For these air-breathing
divers, it is the nitrogen that causes decompression sickness.
There are other physiological risks as well, many of which are related
to increased pressure caused by being underwater. The body is primarily
composed of incompressible solid and liquid components, with only
a small percentage of air-filled cavities. As pressure builds, the
bodys solid and liquid systems evenly distribute the stress while
retaining their volume. Internal pressure shifts to equal the pressure
exerted by the body's surroundings. The heart, however, must work
vigorously to produce a pressure greater than the ambient pressure
in the body to circulate blood throughout the tissues.
The remaining components of the body are air-filled cavities, whose
volumes are changeable. Most of these components are in the respiratory
system: the trachea, bronchi, bronchioles and alveoli. Other components
include sinuses and cavities in the middle ear. The tissues around
these cavities are susceptible to barotrauma - damage caused
by a change in the volume of gas that cannot be safely handled by
Boyle's Law helps to explain how the volume of gas in internal cavities
changes while diving. Roughly, the Law states that at a constant
temperature, pressure and volume are inversely proportional. The
increase in pressure and corresponding decrease in volume at a certain
depth of water is shown in Table 1. If a diver holds her breath,
the volume of air in her lungs at the surface is halved at a 10-meter
depth. Conversely, the volume of air in a diver's lungs at a 10-meter
depth is doubled at the surface.
The lungs are especially flexible and capable of expansion. Connective
tissues binding the blood vessels and airways in the lungs, in conjunction
with the diaphragm, allow the lungs to expand. This allows a decrease
in pressure inside the lungs, which allows an increase in volume as
external pressure decreases. The inverse occurs when external pressure
increases: the lungs decrease in volume as internal pressure increases.
However, this system has a structural limit. If a diver ascends through
the water without breathing, the pressure in the lungs could cause
an expansion exceeding the exterior pressure, resulting in a lung
expansion injury and burst alveoli. On the other hand, a 50-meter
descent while breath-holding would decrease the volume of air in lungs
that were full at the surface to below the structural minimum. The
body works to prevent the rib cage from imploding by increasing pulmonary
circulation, causing additional blood to collect in the lung vasculature
and great veins. These pressure and volume increases help to support
the ribs - but injury can still occur.
To avoid these injuries, divers equalize the pressure in their air
cavities through air exchange. In the case of the respiratory cavities,
divers breathe continuously during depth changes. A regulator, which
is a piece of diving equipment that feeds air to the diver, provides
air at ambient pressure during inhalation. Thus, a diver at 40 meters
breathes air at a total pressure of 5 atmospheres (atm) (Table 1).
Following Henry's Law of partial pressures and the relative gas concentrations
of air, at this 40-meter depth, nitrogen is at 4 atm and oxygen is
at 1 atm. Diffusion across membranes ensures that after equilibrium
has been reached, the partial pressure of nitrogen in solution in
the blood and tissues will also be 4 atm. The diffusion process is
time-dependent, and thus most divers at 30 meters will never reach
a uniform concentration of nitrogen that causes 4 atm of pressure
throughout their tissues.
As pressure on a diver decreases, as in during an ascent, gases diffuse
from the tissues into the blood and again from the blood to the lungs
where they are expired. It is unlikely that oxygen, at its lower partial
pressure, could build up to a significantly raised concentration before
being used in the tissues. However, inert nitrogen can be present
in high concentrations, and therefore at high pressure in the tissues.
If the pressure on the diver decreases faster than the supersaturated
nitrogen can diffuse out through the tissue membranes, nitrogen will
form gas bubbles in the blood. These bubbles cause the physiological
symptoms of decompression sickness (the bends), which range from tingling
to body-curling pain. The process of gaseous bubbles leaving a supersaturated
solution can most commonly be observed in soda pop, as the highly-pressurized
carbon dioxide bubbles out. During Kara's quick ascent, her tissues
were essentially at risk of acting like a cork in a recently shaken
Whales, dolphins, seals, aquatic birds, turtles, and even sloths have
all evolved better physiological systems for diving than humans have.
For we humans, exploration of the undersea world for more than the
duration of a single breath is only possible with technical aids,
which allow our versatile bodies to be exposed to conditions for which
they were not designed. Remarkably, with just a basic understanding
of the underlying physiology of scuba diving, millions of people may
safely explore the incredible realm beneath the surface of the water.
G. Principles and Observations on the Physiology of the Scuba Diver.
Office of Naval Research, Arlington, 1970
Deakin, J. Scuba Diving. David & Charles Ltd., London, 1981.
Empleton, E. et al. (Council for National Co-operation in Aquatics)
The New Science of Skin and Scuba Diving. Association Press,
Edmonds, C., Lowry, C. and Pennefather, J. Diving and Subaquatic
Medicine, 3rd ed. Butterworth-Heineman Ltd., Oxford, 1992
Journal of Young
Investigators. 2001. Volume Five.
Copyright © 2001 by Elizabeth Condliffe and JYI. All rights