Scuba divers experience increasing pressure as they descend because water adds weight above them. At depth, the higher pressure causes more nitrogen from breathing gas to dissolve in the blood and body tissues. If a diver returns to the surface too quickly, the pressure drops faster than the nitrogen can safely leave the body.
This can cause decompression sickness, often called the bends, which can be painful and dangerous.
Decompression stops give the body time to release dissolved nitrogen gradually through breathing. During a slow ascent, nitrogen moves from tissues into the blood, then to the lungs, where it is exhaled. Ships and submarines are designed to handle pressure differences in different ways, but divers must rely on careful ascent planning and pressure control.
Understanding pressure, gas solubility, and bubble formation helps explain why divers ascend slowly.
Understanding Ships and Submarines: Decompression and the Bends
Body tissues do not all take up and release nitrogen at the same rate. Blood-rich tissues, such as the brain, receive gas quickly. Fatty tissues tend to absorb it more slowly and may hold it longer.
Scientists model the body as groups of tissues with different rates of gas exchange. This is why a long, deep dive needs more caution than a short dive at the same depth. It is not only the deepest point that matters.
The time spent there matters greatly. A diver who swims up and down many times can build up extra nitrogen exposure too.
Bubbles are the main danger during a bad ascent. Tiny gas bubbles can form in joints, muscles, skin, blood vessels, or nervous tissue. Joint pain gave the bends its name because early workers with decompression illness often bent over in pain.
More serious cases can cause numbness, weakness, dizziness, trouble walking, or breathing problems. Symptoms may appear soon after a dive, though they can sometimes be delayed. A diver with possible decompression sickness needs urgent medical assessment.
Treatment often uses a pressure chamber, where pressure is raised in a controlled way before being lowered slowly. This can shrink bubbles and help gases leave the body safely.
Dive tables and dive computers estimate how much inert gas a diver has absorbed. They use the depth profile, bottom time, ascent rate, and previous dives. A computer does not measure nitrogen directly inside a person.
It follows a mathematical model based on typical bodies and planned safety margins. Divers must still use it carefully. Staying within limits, ascending at the recommended speed, and making a safety stop near the end of a dive all reduce risk.
Cold water, hard swimming, dehydration, illness, and fatigue can make the situation less predictable. Flying soon after diving is risky because aircraft cabins have lower pressure than sea level, which can encourage remaining dissolved gas to form bubbles.
The same pressure ideas appear in ships and submarines, but the engineering problem is different. A submarine has a strong pressure hull that keeps its crew in a relatively normal internal atmosphere while water presses on the outside. When submariners work outside in pressurised environments, they may need decompression procedures similar to divers.
Surface ships do not face the same deep external pressure, yet they may carry divers, rescue chambers, or hyperbaric equipment. Students should separate two linked ideas when learning this topic. Boyle's law describes how existing gas spaces change size as pressure changes.
Henry's law describes how much gas can enter or leave a liquid. Decompression sickness involves both ideas, but the dangerous part is the formation and movement of bubbles in living tissue.
Key Facts
- Pressure in water increases by about 1 atm for every 10 m of depth.
- Total pressure on a diver is approximately P = 1 atm + depth/10 m.
- Boyle's law: P1V1 = P2V2, so gas volume increases when pressure decreases.
- Henry's law: more gas dissolves in a liquid when the gas pressure above it is higher.
- At 30 m depth, a diver experiences about 4 atm of total pressure.
- Decompression stops slow the pressure change so nitrogen can leave tissues without forming harmful bubbles.
Vocabulary
- Decompression sickness
- A medical condition caused by gas bubbles forming in the body when pressure decreases too quickly.
- Nitrogen narcosis
- A temporary change in thinking and coordination that can occur when breathing nitrogen under high pressure.
- Decompression stop
- A planned pause during ascent that allows dissolved gases to leave the body safely.
- Ambient pressure
- The pressure of the surrounding environment acting on a diver or object.
- Henry's law
- The principle that the amount of gas dissolved in a liquid increases as the pressure of that gas increases.
Common Mistakes to Avoid
- Thinking pressure only comes from the water above the diver is wrong because atmospheric pressure at the surface also adds about 1 atm.
- Ascending quickly after a short deep dive is unsafe because nitrogen can still dissolve into tissues during the dive and form bubbles during rapid pressure decrease.
- Confusing air bubbles in scuba gear with nitrogen bubbles in the body is wrong because decompression sickness involves dissolved gas coming out of body tissues.
- Assuming submarines and divers face the same decompression risk is wrong because submarines have rigid pressure hulls, while a diver's body is directly affected by changing ambient pressure.
Practice Questions
- 1 A diver is at a depth of 20 m. Estimate the total pressure in atmospheres using P = 1 atm + depth/10 m.
- 2 A gas bubble has a volume of 2.0 mL at 30 m depth, where the pressure is about 4 atm. What volume would it have at the surface at 1 atm if temperature stays constant?
- 3 Explain why a decompression stop reduces the risk of the bends even though the diver is still underwater.