Gills are respiratory organs that allow fish to breathe underwater by extracting oxygen from water. However, gills cannot provide adequate oxygen for animals on land. There are several key reasons why gills are not effective for breathing air.
How Gills Work
Gills consist of filaments containing a dense network of blood vessels. As water passes over the gills, oxygen diffuses across the thin membranes of the filaments into the bloodstream. Meanwhile, carbon dioxide passes from the blood into the water. This countercurrent exchange allows efficient gas transfer.
Fish actively pump water over their gills using their mouths and operculum (gill cover). This constant flow brings fresh oxygenated water over the gills. Without water flowing over them, the gills would collapse and stick together, preventing gas exchange.
Problems Breathing Air
When a fish is out of water, its gills begin to dry out. Gills must remain moist in order to function. As they dehydrate, the thin membranes stick together, obstructing oxygen uptake. Gas exchange rapidly becomes impaired, leading to suffocation.
Additionally, air does not flow over stationary gills. Without active pumping, there is no constant replacement of depleted air with fresh oxygen. Diffusion rates are much lower in air versus water. As a result, gills are unable to passively acquire enough oxygen from the air.
Structural Differences in Lungs
In contrast to gills, lungs contain millions of tiny air sacs and dense capillary beds. This provides an enormous surface area for gas exchange. The extensive vascularization also maintains close contact with the alveolar air space.
Lungs are also enclosed in the thoracic cavity. This allows the active ventilation needed to move air in and out. The respiratory muscles change the volume of the chest cavity to drive breathing. Fish lack this anatomical enclosure and dedicated musculature.
Surfactant Allows Lungs to Function
Lungs contain a lipoprotein surfactant that reduces surface tension in the alveoli. This prevents the air sacs from collapsing upon exhalation. Without surfactant, the alveoli would stick together, preventing re-inflation.
Fish lack this surfactant, so their gills have no way to re-open if they collapse out of water. The anatomical structure and biochemistry of lungs is essential for breathing air.
Diffusion Rates in Air vs. Water
The density of air greatly limits its diffusion capacity compared to water. Oxygen diffuses 10,000 times slower in air than in water. This makes passive gas exchange across gills highly ineffective for terrestrial breathing. Lungs compensate with large surface area and specialized structures.
| Medium | Oxygen Diffusion Rate |
|---|---|
| Air | 0.21 cm2/s |
| Water | 2 x 10-5 cm2/s |
As this table shows, oxygen spreads nearly 100,000 times faster in water compared to air. Gills rely on this rapid diffusion, while lungs employ active ventilation mechanisms.
Problems Removing Carbon Dioxide
Gills rely on water flow to remove metabolically-produced carbon dioxide. In air, CO2 accumulates rapidly around stationary gills, slowing oxygen uptake. Lungs use airflow to actively exhale CO2 and prevent this buildup.
No Mechanism for Lung Development
The genes and developmental programs for gill formation and function are distinct from those for lungs. Land animals evolved specific genetic pathways to build and maintain lungs that are not present in fish.
Without lung developmental genes, fish lack the capacity to remodel gills into true lungs, even if they obtain oxygen on land. The morphological change requires major genetic innovations.
Amphibious Animals Use Both
Certain amphibious fish and larval amphibians have structures that enhance cutaneous (skin) gas exchange. This provides supplemental oxygen when out of water. However, these animals still rely heavily on their gills and do not have true lungs.
As they transition to land, amphibians begin expressing new genes that induce lung development. This allows them to metamorphose and adapt to terrestrial life. nevertheless, most amphibians never lose their gills entirely.
The Evolution of Lungs
Lungs likely evolved from outpocketings of the gut that became more complex and specialized for gas exchange. Selection pressure for oxygen in air drove the evolution of lungs over hundreds of millions of years.
This required major changes to respiratory anatomy, physiology, and genetics compared to gills. Fish would require this same lengthy evolutionary process to adapt their gills for breathing air.
Advantages of Gills for Aquatic Life
While gills do not function on land, they are extremely effective and well-adapted for breathing underwater:
- Maximize surface area for gas exchange with dense parallel filaments.
- Thin membranes allow rapid diffusion across short distances.
- Constant unidirectional water flow brings fresh oxygen.
- Open directly to environment so no need to pump water.
- Separate sites for gas exchange and feeding.
Overall, gills are elegant respiratory structures refined by natural selection for life in water. Lungs evolved independently for different conditions on land.
Other Respiratory Adaptations for Land
A few animal groups use alternative respiratory strategies to occupy land habitats. These include:
Insects
– Network of tracheal tubes that branch throughout body.
– Air enters through spiracles and passively diffuses down tracheae.
Arachnids
– Book lungs with stacked hemocoel cavity sheets.
– Air enters chamber and diffuses into hemolymph.
Skin Breathing
– Gas exchange directly across moist skin.
– Seen in many amphibians, worms, insects.
– Provides supplemental oxygen, not enough for full respiration.
These adaptations demonstrate some of the respiratory flexibility that evolved in terrestrial animals. But for vertebrates, lungs became essential, while gills remained suited for aquatic life.
Conclusion
In summary, gills are specifically adapted for breathing underwater, not air. They lack the structures, biochemistry, and evolutionary history required to provide sufficient oxygen on land. While some amphibious animals retain gills for aquatic respiration, only vertebrates with lungs can fully transition out of water. If fish were to colonize land, they would need to evolve complex lungs over many generations in order to breathe. Their gills alone cannot function without being submerged in water and irrigated by constant flows.