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📘 How does a fish breathe underwater?

A fish does not split H2O to make oxygen. It takes up molecular oxygen, O2, that is already dissolved between water molecules. The amount available depends on temperature, movement, salinity, pressure, and the activity of organisms in the w

5
lessons
~25 min
to learn
Adults
level
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What you’ll learn

  1. Oxygen is dissolved in the waterDistinguish dissolved molecular oxygen from water molecules and connect water quality to fish respiration.Fish extract scarce dissolved O2 from a moving aquatic environment.
  2. Filaments and lamellae create a huge surfaceIdentify the gill arch, filaments, lamellae, capillaries, and their roles in diffusion.Thin, wet lamellae create a large exchange area close to blood vessels.
  3. Countercurrent exchange keeps the gradient aliveExplain how opposite water and blood flows sustain oxygen diffusion.Countercurrent flow prevents the oxygen gradient from disappearing too soon.
  4. Mouth and operculum drive ventilationDescribe ventilation, oxygen transport in blood, and changing demand during activity.Mouth, operculum, circulation, and metabolism keep oxygen moving through the fish.
  5. Why most fish suffocate in airExplain why gills fail in air and compare ordinary gills with air-breathing adaptations.Gills need water support and flow, although some fish evolved additional air-breathing organs.

Questions this course answers

What oxygen do typical fish gills take up?

Gills extract dissolved O2 from the water; they do not split H2O or swallow air for ordinary respiration.

Match each condition to its effect.

Temperature, mixing, and biological activity all change the oxygen available to gills.

What do lamellae provide?

Lamellae are thin plates on filaments that increase area and shorten the diffusion path.

Why do gills need both a large surface and thin tissue?

Area and distance work together in an efficient respiratory surface.

What is countercurrent exchange in a fish gill?

Opposite flow keeps oxygen moving from water into blood along much of the lamella.

Put oxygen's route in order.

Oxygen must be present in water, reach the exchange surface, cross into blood, and then be transported.

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