Chordata Respiratory System

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

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).

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
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
hagfish

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
Chinook salmon

Sharks and rays do not have the operculum, as other fish do. Sharks use ram jet ventilation, where through swimming, water is forced through the mouth and pharynx and out five sets of gill slits. Sharks can forcefully open and close their jaws to pump water through their gills, but many have to keep swimming to maintain the current.

Gills of a hammerhead shark
shark gills

Amphibians

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

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