Gas exchange, buoyancy and depth
How gills extract scarce dissolved oxygen, how the swim bladder controls buoyancy, and how pressure and Boyle's law (pV = constant) affect a gas-filled organ as a fish changes depth.
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Fish need oxygen dissolved in water. Surface water in summer typically holds more than 8 mg/L; below 2 mg/L the water is called hypoxic. Cold water holds more dissolved oxygen than warm water.[6]
Because water holds much less oxygen than air, fish ventilate their gills with large volumes of water. Each gill filament carries thousands of lamellae, giving a large surface area with thin membranes across which oxygen diffuses into the blood. In bony fish the gills sit in a chamber covered by the operculum.[4][1]
Fish sense their surroundings with the lateral line, which detects movement and vibration; sharks also have electroreceptors, the ampullae of Lorenzini.[1]
By Archimedes' principle, the upward buoyant force equals the weight of water displaced: Fb = ρwater V g, with ρwater ≈ 1.0 g/cm³ (1000 kg/m³) for fresh water. The swim bladder lets a bony fish adjust its volume and so control its buoyancy.[9][1]
In the sea, pressure rises by about one atmosphere for every 10 m of depth, starting from about 1 bar at the surface. At 10 m the total pressure is about 2 atm, at 20 m about 3 atm.[11]
For a fixed amount of gas at constant temperature, p V = constant (Boyle's law). So if a fish dives from the surface to 10 m without adding gas, the gas in its swim bladder is squeezed to about half its volume, and the fish becomes less buoyant.[10][11]
Try it: Boyle's law with a syringe
You need
- 50 ml plastic syringe with a sealed tip (cap or blocked with silicone)
- Clamp stand, boss and clamp
- Slotted masses (100 g to 1 kg) and a mass hanger or platform
- Vernier calipers or ruler
- Graph paper or spreadsheet
Safety
Clamp the stand firmly and keep feet clear of the masses; add masses gently. Do not point the syringe at anyone. Use only air in the syringe.
What to do
- Pull the plunger to about 40 ml, then seal the tip. Clamp the syringe upright with the plunger at the top.
- Measure the internal diameter of the syringe and calculate the plunger area A in m².
- Record the volume with no extra load. Then add masses in steps of 200 g up to about 1 kg (or until the volume stops changing smoothly), reading the volume each time.
- Calculate the extra pressure from each load: Δp = mg / A, with g = 9.8 N/kg. Add atmospheric pressure (about 100,000 Pa) to get the total pressure p.
- Remove the masses and repeat the whole set two more times; find the mean volume for each load.
- Plot V against 1/p. Is it a straight line through the origin? Use your line to predict the gas volume at 2 atm and relate it to a swim bladder at 10 m depth.
What happens, and why?
Adding mass raises the pressure on the trapped air; for a fixed amount of gas at constant temperature p × V stays constant, so V is proportional to 1/p and the graph is a straight line. Doubling the pressure halves the volume, just as the gas in a swim bladder would be squeezed to about half its volume at 10 m in the sea. Friction in the syringe, leaks and the small dead volume in the tip add uncertainty.[10][11]
Play: Breathing, floating or sensing?
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Other ages: Ages 6–7 · Ages 10–11