Gills
- As fishes' bodies became too big for skin to be sufficient as a respiratory surface, gills evolved as a way to use countercurrent exchange between blood and water to have maximum surface area to diffuse oxygen into the bloodstream, and excrete waste like carbon dioxide and even metabolic wastes.
- Water moves in through the mouth, and goes through the gills, and out the gill slits on the sides of the fish maintaining homeostasis.
- Countercurrent exchange helps fish to maximize the diffusion between their blood and the water allowing fish to obtain the oxygen needed.
- Gills also can be used to diffuse waste into the water, and new studies suggest that the original reason gills evolved was actually for ion diffusion, to keep the fish isotonic to it’s environment.
- Because of the aquatic habitat, gills can be internal or external organs.
- However, gills are not suitable on land because the large surface of wet membrane would evaporate when exposed to air
- Also, the fine filaments of gills would stick together causing it to collapse
Transition from Water to Land
- The beginnings of what we now know as lungs first evolved in fish from their gills, about 375 million years ago in lobe-finned fish.
- Scientists believe lungs evolved from a gas-sac that fish used to help supplement gas exchange from gills.
- This gas-sac or swim bladder help fish to efficiently control buoyancy in the water, thus allowing them to retain enough energy for swimming
- Charles Darwin noted in The Origin of Species that this primitive swim bladder was homologous to today's lungs
- Scientists theorize oxygen depleted waters could have forced fish to seek the air as a new oxygen source.
- Early tetrapods developed adaptations like a rib cage to prevent their lungs from being crushed on land
The Respiration System in Different Animals
- Amphibians use their thin, gas-permeable skin, gills and lungs to breathe.
- As a tadpole transforms into an adult the gills are absorbed into the body and lungs take over as the primary respiration system.
- The lungs of amphibians are simple smooth sac-like structures that lack the complex spongy tissue found in mammals.
- An amphibian like frogs are able to change the volume and air pressure in its mouth while opening and closing its nostrils
- All reptiles have lungs and none go through a larvae stage with gills
- Reptiles primarily use their lungs for breathing but some also are able to use other parts of the body for oxygen absorption
- For example, aquatic turtles use the tissues lining the inside of their mouth to extract oxygen from the water
- The lungs in reptiles are larger than amphibians and are divide into several chambers
- Numerous alveoli are present in their lungs much like in mammals