echoesof.science · Ages 15–16
Gas exchange, buoyancy and depth
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echoesof.science/school/fish/ages-15-16/
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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.
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.
Fish sense their surroundings with the lateral line, which detects movement and vibration; sharks also have electroreceptors, the ampullae of Lorenzini.
Experiment: Boyle's law with a syringe
- 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.
| Load (kg) | Total pressure p (Pa) | Volume, run 1 (ml) | Volume, run 2 (ml) | Volume, run 3 (ml) | Mean V (ml) |
|---|---|---|---|---|---|
| 0 | |||||
| 0.2 | |||||
| 0.4 | |||||
| 0.6 | |||||
| 0.8 | |||||
| 1.0 |
Questions
Explain how the structure of gills makes them efficient at taking in oxygen from water, which holds much less oxygen than air.
A swim bladder holds 9.0 cm³ of gas at the surface. Calculate its volume at 20 m depth if no gas is added, and explain what this does to the fish's buoyancy.
A fish of volume 300 cm³ has a mass of 0.306 kg. Calculate the buoyant force in fresh water and its weight (g = 9.8 N/kg). Does it rise or sink?
Describe your V against 1/p graph and give two sources of uncertainty in the syringe experiment.
Sources for every fact on this sheet: echoesof.science/school/fish/sources/