Respiratory System in Vertebrate Animals

All animals need oxygen from the environment. They use oxygen to transform the nutrients they get from their food into energy. This energy is used to carry out the different functions of living, such as moving, learning, and digesting. When food combines with oxygen, energy is produced. This process is calledmetabolismmetabolism.

During metabolism, carbon dioxide is produced as a waste product. Animals must be able to get rid of carbon dioxide for proper metabolism to happen. The respiratory system is the organ system animals use to bring in oxygen and get rid of carbon dioxide. We call the process of bringing in oxygen and releasing carbon dioxide breathing.

Mammalian Respiratory System

Breathing starts with the movement of air through your mouth and nostrils. The human respiratory system begins at the trachea. This is a stiff tube reinforced by rings of cartilage. It starts at the back of your mouth and nose and then splits into two tubes called bronchi.

You can feel the cartilage rings in your trachea by running your fingers down the front of your neck. Can you feel the ridges? Those ridges are the cartilage rings! Eachbronchusbronchus splits off many times to form smaller tubes called bronchioles. These bronchioles form a complex network of millions of little tubes that lead to sacs called alveoli. This complex network of tubes and sacs forms the lungs.

Did you know?

Your left lung is smaller than your right lung. This is because the left lung has to leave space for your heart! 

Your lungs, as well as your heart, are located in your chest, also known as your thorax. These organs are protected by a frame of bones called the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity, where the stomach and intestines are found. It is a large, dome-shaped muscle located below the rib cage.

Ventilation

Ventilation is the process by which air is pulled in and pushed out of the lungs. This is done by controlling the volume of the thoracic cavity, which is the space where the lungs are found. The main muscle responsible for ventilation is the diaphragm. It is helped by the intercostal muscles, which are muscles between our ribs.

When you breathe in, the diaphragm and intercostal muscles contract. This increases the volume of the thoracic cavity and causes air to rush into the lungs. This phase of ventilation is called inhalation.

Exhalation is when you breathe air out. During this phase, the diaphragm and intercostal muscles relax. This causes the space in the thoracic cavity to get smaller so air is forced out of the lungs.

Try taking a deep breath and imagining you are filling your lungs completely full of air. If you put your hands on your stomach, you can feel how far it moves out each time you breathe in.

The Gas Exchange Pathway

Let’s zoom in at a microscopic view and see how oxygen and carbon dioxide are transported and used in our bodies.

When an oxygen molecule is inhaled, it moves into the respiratory system through the mouth or nose. It then passes through the trachea, bronchi and bronchioles and into an alveolus. In the alveolus, the oxygen diffuses into the blood in the surrounding capillariescapillaries. Now the oxygen is in the circulatory system.

Red blood cellsRed blood cells are specialized cells that carry the gases in the blood. They do this using a specialized protein called hemoglobin. Within red blood cells, hemoglobin binds with molecules of oxygen and carbon dioxide. Hemoglobin relies on an iron molecule to make this happen. That’s why it's important to always have enough iron in your diet. A deficiency in iron is called anemia. This can cause fatigue as well as other more serious health problems.

You probably know that when iron rusts it looks reddish-orange. The same happens when hemoglobin binds to oxygen molecules. This is the reason why our blood looks red.

Did you know?

Some animals use other molecules with their gas exchange proteins. Horseshoe crabs have a protein that binds with copper called hemocyanin. This makes their blood look blue. And it doesn’t end there! Some animals have purple or even transparent blood! 

Misconception Alert

You might see blue veins through your skin. But the blood in our veins is not blue. This is an effect of blue light being the only one that can travel through our skin to our eyes! 

The oxygenated blood is then sent from the lungs to the heart which pumps it to the rest of the body through the arteries. Arteries branch out into all parts of the body and get smaller, until they become capillaries. Here oxygen diffuses from the blood to the body cells that need it. 

Diffusion of oxygen from the blood to the body cells happens because the concentration of oxygen is higher in the blood than in the cells. Since molecules move from high to low concentration, oxygen moves from blood to cells.

Inside any body cell, oxygen is taken up by mitochondria. Some people call this organelleorganelle the powerhouse of the cell. There's a good reason for this. In the mitochondria, oxygen and sugars are used and produce energy for bodies. A waste product of this metabolism is carbon dioxide.

Now, let's follow a carbon dioxide molecule as it leaves the body. The concentration of carbon dioxide in the blood increases as cells produce it as waste. This causes it to diffuse into the blood. The carbon dioxide is carried by the red blood cells through the veins to the lungs. The lungs have a low concentration of carbon dioxide compared to blood. This is why carbon dioxide diffuses into the alveoli of the lungs. From there it passes through bronchioles, bronchi, and the trachea to leave the body through the nose or mouth. This is the process of exhalation.

The process of absorbing oxygen and releasing carbon dioxide we call gas exchange. All animals use a gas exchange to breathe. There are some differences, though, in the organs and molecules that make it happen.

Did you know?

There is a group of tiny bottom-dwelling marine organisms that live in an environment with no oxygen. It is believed that the Loricifera do not have mitochondria, and instead have a different type of organelle that can metabolize sugar without oxygen.

Respiratory Systems of Non-Mammalian Animals

Birds

The lungs of birds are organized very differently than those of humans. Unlike in human lungs, where the air moves both in and out, in bird lungs, the air only flows in one direction. To help make this possible, birds have nine air sacs that surround the lungs. The air sacs allow for a continuous stream of air to flow through the lungs. It is important to note that gas exchange does not happen inside the air sacs. 

For a complete respiratory cycle, birds require two inhalations and two exhalations.

1. On the first inhalation air flows from the trachea to the rear air sacs.
2. On the first exhalation air flows from rear air sacs to lungs.
3. On the second inhalation air flows from lungs to front air sacs.
4. On the second exhalation air flows from the front air sacs out through the trachea. 

The bird respiratory system is far more efficient than that of mammals. The continuous flow of oxygen is important to birds as they need a lot of energy to fly. Another advantage of the air sacs is that they make birds less heavy!

Reptiles

The respiratory system of reptiles is similar to that of humans. One major exception is that most reptiles, except for members of the crocodile family, do not have a diaphragm. They have evolved different ways to inflate their lungs.

Many reptiles and amphibians use their throat muscles to “gulp” air in a process called buccal pumping.

There are four steps in buccal pumping.

1. First, air is let in through the nostrils. The buccal cavity, which is connected to the mouth, expands.
2. When the buccal cavity is full, the nostrils close and airways to the lungs open. The opening to the airway is called the glottis. Then, the animal uses muscles to push air into the lungs.
3. When exhaling, first the lungs contract which pushes air into the buccal cavity. The buccal cavity once again expands.
4. Finally, the glottis closes, the buccal cavity contracts and the nostrils open. 

Turtles use many different methods to breathe. Some move their legs into and out of their shells to help inflate and deflate their lungs. Others have large muscles that wrap around the lungs to control the volume of the thoracic cavity. Finally, some aquatic species can even use their cloaca to breathe. The cloaca is the external opening used for solid, liquid excretions and access to the reproductive system. In mammals, there are different openings for these things. Cloacal breathing is mostly used when turtles hibernate underwater. It is considered to be a type of cutaneouscutaneous respiration, which is gas exchange that happens through the skin.

Did you know?

Sea turtles can hold their breath for up to 10 hours! 

Another interesting detail of most reptiles is that they lack a separation between their nose and mouth. This forces them to stop breathing to be able to swallow. To deal with the long time it takes to swallow their prey, snakes have a long extension of the trachea.

Amphibians

Amphibians are animals that have adapted to living both on land and in water. This group includes frogs, toads, salamanders and newts. Amphibians are interesting because they start off as aquatic larvae before they move on land as adults. As they grow and move from living in water to living on land, their respiratory systems change.

In their juvenile forms, amphibians have gills that let them breathe underwater, just like fish. As they grow to their adult form, the gills disappear and lungs develop to replace them. Some species of salamanders remain aquatic, however, and keep their gills all through their lives. We call keeping characteristics of the larval stage neoteny. 

Neoteny is easily seen in axolotl (pronounced axe-o-lot-ill), a popular aquarium species from Mexico. In Canada, our largest salamander species is the mudpuppy. It is easy to recognize because it has reddish gills on each side of its head.

In addition to buccal pumping, amphibians also use cutaneous respirationcutaneous respiration. This means that the gas exchange happens through the animal’s skin. Many other animals have cutaneous gas exchange, even humans, but in small quantities! Amphibians rely on this process much more. The lungless salamander, as its name implies, almost exclusively uses its skin to breathe. This breathing method requires having thin skin that can be kept wet, which many amphibians have. Since their breathing happens through their skin, it is best not to handle amphibians. Not only can you affect their breathing, you can also transfer germs from your skin to theirs.

Fish

The respiratory system in fish is very different from the respiratory systems ofterrestrialterrestrialanimals. Like frog larvae and mudpuppies, fish have gills. Gills are similar to lungs in that they have branches that split. Gill arches branch to form gill filaments. On the gill filaments are gill lamellae, which is where gas exchange occurs. Here oxygen dissolved in water diffuses into the blood and carbon dioxide diffuses into the water.

In bony fishesbony fishes, a bony plate called an operculum covers and protects the fragile gills. In cartilaginous fishes, such as sharks and rays, the gills are located near their 5 to 7 gill slits. 

During the inhalation phase, fish pull water in through their oral valve to their buccal cavity. From there the water is pushed towards their gills. The opercular valve is closed at this point. During the exhalation phase, the oral valve is closed and the opercular valve is opened. Water passes over the gills and out through the opercular valve. 

Some fish have specialized muscles that allow them to pump the water over their gills. Fish that are very active use a type of ventilation called ram ventilation. Moving forward forces water to move over the gill arches so that oxygen and carbon dioxide can be exchanged. Sharks are a type of fish that use ram ventilation. This is why some species of sharks can actually drown if they stop moving! 

Did you know?

Sharks and rays bring water into their buccal cavities through an extra opening behind the eye called a spiracle. Spiracles are also used by insects for respiration.