Lesson Explainer: The Human Respiratory System Biology

In this explainer, we will learn how to describe the structure of the lungs and explain how they are adapted for efficient gas exchange.

Our lungs are soft, spongy organs that cannot move on their own. Despite this fact, they are constantly in motion, moving air into and out of our bodies for the entire duration of our lives. Your two lungs are located in your chest cavity and protected by the bones of your skeleton. The left lung is actually a little smaller than the right to accommodate the slightly off center bulk of the heart. Not only are your heart and lungs positioned closely together, but also they have closely related functions.

The lungs are the primary organ of the respiratory system. The respiratory system is an organ system composed of several organs working together to carry out the essential life function of gas exchange. Gas exchange in the human body involves taking in oxygen from the atmosphere and removing excess carbon dioxide from the blood. The lungs are the primary organ responsible for this gas exchange, and they work in conjunction with other organs, allowing them to perform this job.

Key Term: Lungs

The lungs are soft, spongy organs located in the chest cavity. They function within the respiratory system to exchange gases between the blood and the external atmosphere.

Key Term: Gas Exchange

Gas exchange describes the movement of gases into and out of the blood. Gas exchange in the lungs involves oxygen entering the blood and carbon dioxide being removed.

The lungs are surrounded on the front, back, and sides by a cage of flat, curved bones called the ribs. In between the ribs, there are layered sheets of muscle called the intercostal muscles. Inter- is a prefix that means “in between,” and costal is a word that means “ribs.”

Beneath the lungs, there is a large, thin, dome-shaped muscle called the diaphragm, which you can see in Figure 1. The diaphragm separates the ribs and other organs of the chest cavity from the stomach, liver, and other organs of the abdominal cavity.

The lungs are connected to the external atmosphere through the openings of the nose and mouth, as you can see in Figure 2. The nose and mouth connect at the back of the throat in a passage called the pharynx. As air passes from the nose to the pharynx, it is filtered of foreign particles and moistened. As we will see later in this explainer, it is very important for the air to be moist when it enters the lungs for gas exchange to occur.

The pharynx opens at the bottom into the larynx and the esophagus, which carries food and water to the stomach when you swallow. The larynx serves important functions in swallowing and protects us from aspirating food. The larynx is also considered our voice box, since it contains our vocal cords, which are so important for us to produce sounds or sing.

The larynx is the upper part of the trachea, which carries air into and out of the lungs when you breathe. The esophagus and the trachea run parallel to each other and have quite different structures. While the esophagus is made of muscles that contract in waves to move food down, the trachea is a more rigid tube that is made up of cartilage, like our nose, so that it does not collapse and remains open at all times.

For flexibility, the cartilage of the trachea is organized like a pile of C-shaped rings. At the opening of each C-ring, bundles of muscles can contract to tighten the ring, which is needed, for example, when you cough. You can see this organization in the cross-sectional diagram shown in Figure 2. The inner surface of the trachea is covered with respiratory epithelium, a lining that forms cilia on its surface and contains glands secreting mucus. It serves to both moisten and capture particles or pathogens in the air coming in. The cilia beat constantly to move the mucus toward the pharynx, where it can be swallowed and eliminated.

Key Term: Epithelium

The epithelium or epithelial tissue serves a function in the lining and protection of body and organ surfaces. The cells of epithelial tissues are closely joined together to form a continuous layer.

Lower in the chest, as you can see in Figure 3, the trachea splits into a left and a right branch called the bronchi (singular: bronchus). The bronchi attach the trachea to each lung. Within the lungs, the bronchi branch further into smaller and smaller vessels called bronchioles.

At the end of these bronchioles are small sac-like structures called alveoli, which act as the site of gas exchange in the lungs. Amazingly, there are around 600 million alveoli per human lung.

Example 1: Identifying the Structures of the Respiratory System in a Diagram

The diagram provided shows a basic outline of the human lungs, with parts magnified.

  1. Which number points to the trachea?
  2. Which number points to the alveoli?

Answer

This question presents us with an anatomical diagram of the human respiratory system and asks us to identify two parts, the trachea and the alveoli. In order to answer this question, we need to first recall the functions of the trachea and alveoli and describe their structures. Then, we will label the corresponding parts of the diagram to match our description to the image.

The trachea is a rigid tube that connects the lungs to the mouth and nose. It is sometimes commonly referred to as the “windpipe,” since it is a large tubular passage that carries moving air. If you press your hand to the front of your neck, you can easily feel your trachea. In the chest, near the lungs, the trachea branches into two bronchi, which in turn branch into smaller and smaller passages called bronchioles.

The alveoli are tiny complexes of tissue at the end of each of the smallest bronchioles. They have the appearance of a cluster of soap bubbles or a bunch of grapes. The alveoli are thin-walled sacs filled with air and are the site of gas exchange in the lungs. They are surrounded with capillaries. Capillaries are blood vessels that also have extremely thin walls. The carbon dioxide in the blood diffuses into the alveoli to be removed from the body when we exhale. The oxygen from the air diffuses into the blood from the alveoli to be absorbed by red blood cells, which transport it throughout the body.

Here is the diagram from the question. The four indicated structures have been labeled with their names.

Using this information, we can see that number 4 points to the trachea and number 2 points to the alveoli.

Now, let’s have a look at the mechanisms underlying the flow of air into and out of our respiratory system, with the help of Figure 4.

When we breathe in, the intercostal muscles contract, lifting the ribs up and out. The diaphragm also contracts, which makes it flatten and pull downward. This causes the soft, spongy lungs to stretch and expand, as the space within the chest cavity becomes larger. You may recall that gases flow from areas of high pressure to areas of low pressure, just like liquids. As the lungs expand, their volume increases, which causes the pressure within them to decrease. Since the pressure within the lungs is now lower than that outside the body, air from outside the body flows into the nose, down the trachea, and into the lungs. This motion is known as inhalation.

Definition: Inhalation

Inhalation is the inspiration of air from the external atmosphere into the lungs due to the lungs expanding, which decreases their relative pressure.

When we breathe out, the intercostal muscles relax, lowering the ribs downward and inward. The diaphragm also relaxes, which causes it to push upward again. This causes the lungs to become more compact as the chest cavity becomes smaller. The lungs shrink, decreasing their volume, which increases the pressure within them. Since the pressure within the lungs is now higher than that of the air outside the body, this motion pushes air out of the lungs, up the trachea, and out from the nose into the atmosphere. This motion is what we call exhalation. If you place your hand against the bottom of your chest and take a deep breath in and out, you can feel your chest cavity expanding and contracting as you inhale and exhale.

Definition: Exhalation

Exhalation is the expiration of air from the lungs into the external atmosphere due to the lungs contracting, which increases their relative pressure.

Example 2: Describing the Mechanism of Exhalation

The diagram provided shows the basic outline of the lungs when a person is exhaling (breathing out).

  1. Complete the sentence using “upward” or “downward”: As a person exhales, the diaphragm moves .
  2. Complete the sentence using “inward” or “outward”: As a person exhales, the ribs move .
  3. Complete the sentence: As a person exhales, the volume of the chest cavity and the air pressure in the lungs .

Answer

The lungs are soft, spongy organs that cannot move on their own. When we breathe out, or exhale, our respiratory system and related structures work together to push air out of our lungs. In order for this to happen, the lungs must shrink, decreasing their volume, which increases the pressure within them. Air moves from areas of higher pressure to areas of lower pressure. When the lungs shrink, the pressure within them becomes higher than that of the air outside the body. This pushes air out of the lungs, up the trachea, and out from the nose into the atmosphere. In order for the lungs to shrink, the chest cavity must become smaller, which changes the shape of the lungs within. In order for the chest cavity to become smaller, the intercostal muscles between the ribs relax, lowering the ribs downward and inward. The diaphragm also relaxes, which causes it to push upward. This action contracts the chest cavity, shrinking the lungs, increasing the pressure, and causing us to exhale.

  1. As a person exhales, the diaphragm moves upward.
  2. As a person exhales, the ribs move outward.
  3. As a person exhales, the volume of the chest cavity decreases and the air pressure in the lungs increases.

The lungs themselves are complex organs that are adapted to allow oxygen from the air to rapidly and efficiently diffuse into the blood and to allow carbon dioxide from the blood to rapidly and efficiently diffuse out into the air. But why is this gas exchange necessary at all? Well, the cells of our body need oxygen to produce cellular energy, in a process called cellular respiration. This cellular energy is used to carry out all the functions of the cells, keeping them alive, which in turn keeps us alive. Cellular respiration uses lots of oxygen and produces carbon dioxide as a by-product. If this carbon dioxide builds up within our bodies, it can be very harmful, so this carbon dioxide has to be removed from our bodies as waste.

Key Term: Aerobic Cellular Respiration

Aerobic cellular respiration is a process in the cells in which glucose is broken down by reacting with oxygen to release energy in the form of ATP, producing carbon dioxide and water as by-products.

The air that surrounds us is a mixture of gases, as shown in Figure 5. It consists of about 78% nitrogen, 21% oxygen, and 0.03% carbon dioxide. The nearly 1% that remains is a mixture of argon, water vapor, and other gases. While the air we breathe is mostly nitrogen, it has a relatively high concentration of oxygen and an extremely low concentration of carbon dioxide. This is important in the process of breathing because the process of gas exchange in our lungs occurs via diffusion.

You may recall that diffusion is the movement of molecules from areas of high concentration to areas of low concentration. The difference in concentration is called the concentration gradient, and molecules always move down their concentration gradient during diffusion. This concept is illustrated graphically in Figure 6. Diffusion is a passive process and occurs spontaneously, without the necessity of additional energy input.

Key Term: Concentration Gradient

The concentration gradient is the relative difference in concentrations that causes diffusion. Molecules move from areas of high concentration to areas of low concentration down their concentration gradient.

When we inhale, oxygen-rich air flows into the lungs. The blood that enters the lungs has a very little concentration of oxygen and a relatively high concentration of carbon dioxide. Since the concentration of oxygen in the air is higher than that in the blood, oxygen diffuses down the concentration gradient from the air into the blood.

Additionally, the concentration of carbon dioxide in the blood is higher than that in the air within the lungs. This causes the carbon dioxide to diffuse down the concentration gradient out of the blood. When we exhale, the carbon dioxide that diffuses out of the blood is removed from the body as waste.

This exchange of gases, oxygen moving into the blood and carbon dioxide moving out, occurs within the lungs at small, specialized structures called alveoli. Each of the very small bronchioles within the lungs eventually ends in a cluster of bubble-like sacs known as alveoli. These alveoli are made of epithelial tissue that is extremely thin. On the outside, the alveoli are almost completely surrounded by tiny blood vessels called capillaries. These blood vessels also have walls that are extremely thin. The thickness of the blood–air barrier formed by the cells in the walls of the alveoli and blood vessels is about 2 millionth of a meter. The diameter of the capillaries is so small that the red blood cells, or erythrocytes, within them must pass through in a single file.

Definition: Alveoli (Singular: Alveolus)

Alveoli are bubble-shaped structures of thin epithelial tissue surrounded by capillaries. Gases within the alveoli are exchanged with those in the blood within the capillaries.

Since the walls of the capillaries and alveoli are so thin, the carbon dioxide in the blood can diffuse out rapidly. For the same reason, oxygen efficiently diffuses into the blood, where it is absorbed by the red blood cells. Red blood cells possess a special protein called hemoglobin, which helps them carry oxygen from the lungs to the tissues of the body that need it.

Definition: Hemoglobin

Hemoglobin is a protein found in red blood cells that is specialized in carrying oxygen.

Example 3: Outlining the Movement of Oxygen from the Atmosphere into the Body

Which of the following correctly outlines the movement of oxygen from the atmosphere into the body?

  1. Atmosphere trachea alveoli capillaries bloodstream bronchi bronchioles
  2. Atmosphere trachea bronchi bronchioles alveoli capillaries bloodstream
  3. Atmosphere alveoli trachea bronchi bronchioles capillaries bloodstream
  4. Atmosphere trachea bronchi bronchioles bloodstream alveoli capillaries
  5. Atmosphere bronchi bronchioles trachea alveoli capillaries bloodstream

Answer

Our respiratory system serves the function of transferring oxygen from the air of the outside atmosphere into our bodies. The air we breathe into our lungs is rich in oxygen. The oxygen from the air diffuses out of thin-walled sacs called alveoli and into the bloodstream through tiny blood vessels called capillaries. The air in the atmosphere is inhaled into the lungs to reach the alveoli. From outside the body, the air passes first through the nose and/or the mouth and then down a rigid tube called the trachea. The trachea branches into two large bronchi. Each bronchus is attached to one lung. The bronchi then split into smaller vessels and eventually into tiny tubes called bronchioles. At the end of each bronchiole are the alveoli, which are the site of oxygen diffusion into the bloodstream.

Therefore, the correct sequence of the movement of oxygen into the body is atmosphere trachea bronchi bronchioles alveoli capillaries bloodstream.

Besides the thin walls that shorten the distance for diffusion, alveoli have other adaptations that allow the efficient transfer of gases between the blood and the atmosphere.

First, the number of alveoli is extremely large. Their bubble-like structure increases the surface area available for gas exchange within the lungs. If you spread the surfaces of all the alveoli from both lungs out on a flat surface, they would cover an area the size of a tennis court. Such a large surface area allows more diffusion of gases.

Next, the blood in the capillaries is constantly moving into and out of the lungs with every beat of your heart. Blood that has been oxygenated is removed from the lungs quickly and replaced with deoxygenated blood. This allows the concentration gradient between the blood and the air to stay as steep as possible. This steep concentration gradient between the blood and the atmosphere increases the rate of diffusion of gases into and out of the blood.

Finally, the insides of our airways and the inner surfaces of our lungs are all moist in order to saturate the air with water vapor. This water vapor plays an important protective role on the very thin surface in the alveoli. It prevents it from drying up and lowers the tension surface, which prevents the alveoli from collapsing. This water vapor is partly exhaled, which causes our bodies to lose one-fourth of our daily water intake every day through breathing.

Example 4: Describing the Adaptations of the Alveolus

An alveolus has a large surface area relative to its volume. Which of the following best describes the advantage of this?

  1. A large surface area allows the temperature to be regulated more easily.
  2. A large surface area allows more diffusion to occur.
  3. A large surface area prevents the entry of microbes or pathogens.
  4. A large surface area provides more space for the enzyme-controlled reactions to take place.

Answer

Gas exchange, oxygen moving into the blood and carbon dioxide moving out, occurs within the lungs at small, specialized structures called the alveoli. Each of the very small bronchioles within the lungs eventually ends in a cluster of bubble-like sacs known as alveoli. The alveoli are almost completely surrounded by tiny blood vessels called capillaries. The number of alveoli is extremely large. Their bubble-like structure increases the surface area available for gas exchange within the lungs. If you spread the surfaces of all the alveoli from both lungs out on a flat surface, they would cover an area the size of a tennis court. Because the alveoli offer such a large surface area for contact between the lungs and the tiny capillaries, large quantities of carbon dioxide in the blood can diffuse out rapidly to be removed from the body when we exhale. For the same reason, large quantities of oxygen can quickly diffuse into the blood, where it is absorbed by the red blood cells.

So, the advantage of the large surface area relative to volume in an alveolus is that such a large surface area allows more diffusion to occur.

As a result of cellular respiration, the cells of our bodies constantly need oxygen and are constantly producing carbon dioxide. The action of the lungs allows our bodies to efficiently remove the carbon dioxide waste generated by our cells and replenish the oxygen as quickly as it is being used.

Let’s review what we have learned about the human respiratory system in this explainer.

Key Points

  • The lungs are the major organ within the respiratory system and the primary organ responsible for gas exchange.
  • The lungs expand and contract, causing air to move in and out.
  • Blood is pumped into the lungs, where it becomes oxygenated by absorbing oxygen from the air.
  • Blood also carries carbon dioxide to the lungs to be removed from the body.
  • The alveoli have several adaptations that allow the lungs to efficiently and continuously exchange gases between the blood and the atmosphere.

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