Animals need a steady supply of oxygen for cellular respiration and must remove carbon dioxide waste before it builds up in body fluids. Different environments create different challenges for gas exchange because water contains less oxygen than air, and dry air can cause water loss. Respiratory strategies such as gills, lungs, tracheal tubes, and skin all solve the same basic problem in different ways.
Each system works best when it matches the animal’s habitat, body size, and activity level.
Gas exchange happens by diffusion across thin, moist surfaces, so respiratory organs are built to maximize surface area and maintain a concentration gradient. Fish gills use flowing water and blood vessels to pull oxygen from water, while lungs use internal air spaces to reduce drying on land. Insects deliver air directly to tissues through tracheal tubes, and some amphibians and worms exchange gases through moist skin.
Comparing these systems shows how structure, environment, and energy demand shape animal evolution.
Key Facts
- Gas exchange depends on diffusion from high concentration to low concentration.
- Cellular respiration uses oxygen and releases carbon dioxide: C6H12O6 + 6O2 = 6CO2 + 6H2O + energy.
- Fick’s law idea: diffusion rate increases with surface area and concentration difference, and decreases with membrane thickness.
- Gills work well in water because they are thin, moist, and highly branched, but they often collapse or dry out in air.
- Lungs work well on land because respiratory surfaces are inside the body, which helps reduce water loss.
- Insect tracheal systems move oxygen through air-filled tubes directly to body cells rather than relying mainly on blood transport.
Vocabulary
- Gills
- Gills are thin, feathery respiratory organs that exchange gases with water as it flows across a large surface area.
- Lungs
- Lungs are internal air-filled organs where oxygen diffuses into blood and carbon dioxide diffuses out.
- Tracheal system
- A tracheal system is a network of air tubes in insects and some other arthropods that carries gases directly between the outside air and body tissues.
- Cutaneous respiration
- Cutaneous respiration is gas exchange through the skin, which must stay thin and moist for diffusion to occur.
- Diffusion gradient
- A diffusion gradient is a difference in gas concentration that causes oxygen or carbon dioxide to move across a membrane.
Common Mistakes to Avoid
- Thinking all animals breathe with lungs is wrong because many animals use gills, tracheal tubes, skin, or a combination of systems.
- Forgetting that respiratory surfaces must be moist is wrong because oxygen and carbon dioxide must dissolve before they diffuse across cell membranes.
- Assuming gills work equally well in air is wrong because gill filaments can collapse and dry out when they are not supported by water.
- Confusing ventilation with gas exchange is wrong because ventilation moves water or air over a surface, while gas exchange is the diffusion of oxygen and carbon dioxide across that surface.
Practice Questions
- 1 A fish moves 900 mL of water across its gills each minute. If the gills remove 6 percent of the dissolved oxygen from that water and the water contains 8 mg of oxygen per liter, how many mg of oxygen does the fish take up each minute?
- 2 An amphibian gets 70 percent of its oxygen through its skin and 30 percent through its lungs. If it uses 12 mL of oxygen in 10 minutes, how many mL came through the skin and how many came through the lungs?
- 3 A small worm lives in damp soil and has no lungs or gills. Explain why a thin, moist body surface can meet its respiratory needs, and why the same strategy would not work well for a large dry-land animal.