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This lesson is on the respiratory system of organisms in the phylum chordata. In this lesson we’ll go over the types of animals in this phylum and the function of the respiratory system in each.

What Is Chordata?

Humans are pretty familiar with their own respiratory system. We know the basics about how we take in air for oxygen, and filter out carbon dioxide. Animals that breathe through their skin or use gills feel pretty foreign to us, but believe it or not, we’re in the same group and have a lot of similarities!The phylum chordata is a group of animals that have a dorsal hollow nerve tube (for us, our spine), a notochord (the nerves inside the spine), pharyngeal slits (which become gills, we only have these in the womb), and a post-anal tail (for us the tailbone, and in other animals an actual tail).

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Classes of chordates
classes of chordates

Let’s look at the varied respiratory systems of different groups of chordates.

Aquatic Chordates

Lower chordates don’t have a true backbone yet and all are aquatic. They have a variety of respiratory tactics. Most use gills as their respiratory system, though there are exceptions, such as the very primitive chordate called the lancelet. These are small eel-like fishes that only use their skin to breathe and use their gills for filtering food.The sessile tunicates use a system of many gills on their surface to filter oxygen and carbon dioxide.

Botrylloides violaceus, a colonial tunicate

As aquatic animals evolved, so did their gill system.

The hagfish (so primitive it’s not usually considered a fish) is a jawless chordate known for their ability to secrete mass amounts of slime to escape predators. This fish burrows into dead carcasses on the sea floor to find food. During this time, the gills on the surface which allow for gas exchange are blocked, so the hagfish is thought to effectively hold its breath while eating.

A fisherman handles hagfish slime from a storage tank

Fish have an even more advanced gill system, with special coverings called an operculum. These coverings prevent dirt from flowing into the gills and aid in ventilation as water is forced through the mouth and pharynx and out the gill slits.

Exposed gills of a Chinook salmon
Gills of a hammerhead shark
shark gills


Amphibians, although they seem small and simple, actually use three types of respiration. Since they start their life as tadpoles in water, they can use gills to breathe like fish, but when they transform to land animals, they can use either lungs or their skin as their respiratory organs.

Amphibians use positive pressure breathing, where they actively move their throats to push air into into the lungs. In the lungs, gas exchange occurs in small sacs called alveoli, which diffuse oxygen into the body and let carbon dioxide escape. During cutaneous respiration oxygen diffuses through the skin directly into their blood vessels and carbon dioxide diffuses out.

Reptiles, Birds and Mammals

More complex animals like reptiles, birds and mammals use negative pressure breathing, where muscle expands the lungs. Humans, reptiles, and birds take in oxygen through the nose and mouth, through the pharynx to the trachea, or wind pipe. For mammals and reptiles, the trachea branches into right and left bronchi, which continue to branch and end in the alveoli, where gas exchange occurs.

Humans can have over 500 million alveoli per lung!

The human respiratory system
human respiratory system

Reptiles use muscles that are attached directly to the rib cage to expand their lungs, which actually limits extensive movement during breathing. Mammals however, use a muscle called the diaphragm that causes the lungs to expand as it contracts, creating the negative pressure necessary for breathing. This is not connected to other skeletal muscle, allowing for long periods of movement.

Zebra tailed lizard
zebra tailed lizard

Birds have an even more amazing respiratory system. Their immense need for energy during flight requires highly efficient respiratory systems.

Unlike mammals, birds expand their rib cage without a diaphragm to take in oxygen, which flows through the lungs and into posterior air sacs. The air sacs are like holding tanks for fresh air.

Bird respiratory system
bird respiratory system

As the bird exhales, the fresh air from the air sacs is pushed into the lungs, where gas exchange occurs in small tubes called parabronchi, instead of the alveoli. During the second inhalation, the stale air filled with carbon dioxide fills the anterior air sacs, where it is expelled during the second exhalation. This method creates a one way flow of air, whereas in other animals the stale and fresh air mix in a two-way pathway.

This flow allows for the most efficient respiratory system of all animals.

Comparison of chordate respiratory systems
respiratory system comparison

Lesson Summary

In summary, chordates are animals with a dorsal hollow nerve tube, gill slits, a post-anal tail and a notochord. All aquatic chordates use gill slits to exchange oxygen and carbon dioxide. Tunicates have the simplest gill system, while sharks have a more complex system which requires constant swimming to filter oxygen in most species. Amphibians have respiratory systems in between aquatic and terrestrial animals.

They use gills, positive pressure breathing and gas exchange through their skin to get oxygen.Mammals, birds, and reptiles use negative pressure breathing. Mammals expand their diaphragm to draw air into the lungs and to the alveoli, where gas exchange occurs.

Reptiles also use alveoli, but they use muscles attached to their rib cage to expand the lungs. Birds have the most efficient respiratory system, which uses air sacs, parabronchi and a two step breathing cycle to create one-way airflow.


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