14 April, 2000

Adapting to Pressure

Question 55: How have the Cape Petrel and Antarctic Prion adapted to take advantage of the Southern Ocean's abundant zooplankton?

Since our bodies are well adapted to living in the conditions found at sea level, we are not usually aware that our bodies are constantly under pressure. Pressure affects many facets of diving. It influences our comfort, our safety, our gear, and our dive plan. Ambient or surrounding pressure at sea level is 14.7 psi or one atmosphere. This measurement comes from the weight of an imaginary column of air 25 mm square going from the top of the atmosphere to the bottom, around 100 km. We have the weight of one atmosphere pressing down on us on the surface of Earth every day. However, water is a liquid, not a gas. This means it is much denser and heavier than air. Pressure increases rapidly as we go underwater. Each 33 ft adds another atmosphere of pressure. 33 feet = 2 atm (don't forget the initial atmosphere of pressure at sea level), 66 feet = 3 atm, 99 feet = 4 atm, etc. Anyone who has swum to the bottom of the deep end of a swimming pool has felt the effects of this quickly-increasing pressure in their ears. Since gasses can be compressed, the areas of our bodies that are most affected are the air spaces: sinuses, ears and lungs. A diver needs to compensate for the compression of these gasses as she descends, or these spaces will be crushed by the strength of the pressure. At 33 feet, where the ambient pressure has doubled, the volume of air in the lungs/sinuses/ears has decreased by half. To solve this problem, more air has to be added to the space (the gas is compressed so the volume of the area stays the same while holding a greater amount of gas). This is called equalizing. If the diver does not equalize the pressure she will feel a squeeze. Continuing to descend without equalizing a squeeze can cause a barotrauma (physical damage). Equalizing our ears works about the same as when we do it going up or down in an airplane. Sinuses are usually open and equalize automatically when the diver breathes in pressurized air from the regulator. The regulators automatically adjust to ambient pressure and deliver the appropriate pressure of air for the diver to fill her lungs as she breathes.

Air spaces in the dive gear (dry suit, mask, and BC) also experience squeezes. We let all the air out of our BC at the start of a dive and do not fill it until we reach the surface again. It is an emergency backup in dry suit diving. Both the dry suit and BC have power inflators that add pressurized air from the tank. As we descend, we add air to the dry suit to relieve suit squeeze and control our descent. During one dive here, the hose for my power inflator got disconnected at the surface, and I dropped down to the bottom at 35 ft unable to add air to the suit. The suit squeeze was painful and gave me long bruises wherever there were wrinkles in my dry suit. It felt like someone pinching my skin hard. The mask is easy to equalize on descent, the diver simply exhales out her nose to add more air.

On the ascent, we have to deal with the problem in reverse. As the pressure on the diver decreases, the air expands. The dry suit has a dump valve on the shoulder that is opened when ascending to allow some of the increasing volume of air to escape. The mask can be burped by pressing it back against the face when it is getting too full of air. The diver's ears also equalize without much assistance on ascent, but the diver now needs to pay attention to her breathing. If the diver's airway isn't open while she ascends, it can result in an overexpansion injury. If the diver doesn't breathe normally or holds her breath, air is trapped in the closed space of the lungs and will find another way to escape.

An overexpansion injury starts with the lung tissue rupturing. Where the air goes after it escapes from the lungs determines what type of overexpansion injury it is. Air bubbles may go into the blood stream, an air embolism, traveling in the blood until they form a blockage in the vessel. Air may enter the space between the lungs near the heart and windpipe; it may travel to the skin forming a raised rash or into the lining between the lungs and the ribcage. These can all be very serious problems.

Answer 54: Skuas are opportunists, just like giant and other large petrels, but they are even more aggressive predators. When attacking penguins, they hit the adult with their feet or pull it by its tail to dislodge it and then grab the egg or chick. Sometimes they work in pairs. They also attack undefended larger chicks. In addition, they eat squid, fish and krill.