Mastering the Respiratory System

Before You Start, please understand the basic terms that we will use in this chapter. If you are familiar with these terms you can skip to the next section. 

  1. Cartilage: It is a flexible and semi-rigid connective tissue. Cartilage does not contain blood vessels. It is not as hard as bone and provides flexibility but is resistant to compressive forces. 
  2. ATP: Adenosine triphosphate is an organic chemical that provides energy for many processes such as muscle contraction. Our cells produce ATP through cellular respiration in the presence of oxygen and carbon dioxide is produced as a waste product. 
  3. Oral Cavity: Commonly known as mouth, it is mainly part of the digestive system. Additionally, the oral cavity acts as an alternative to nasal passage for entry and exit of air.
  4. Esophagus: A muscular tube that connects the throat to the stomach. 
  5. Ligament: A strong and flexible connective tissue that connects two bones or cartilages or holds together a joint. 
  6. Pathogens: These can adversely affect the respiratory system for example viruses, bacteria, and fungi. 

Fundamentals of the Respiratory System

Can you hold your breath? Try to find out how long you can hold your breath. Breathing is necessary for our life. How long can you go without it? Maybe you started feeling uncomfortable already. Are you? 

A normal person can not hold their breath for more than 3 minutes. How hard you try but you can’t surpass the limit, because you will start losing your consciousness and your nervous system will take over the control of breathing. It’s amazing how the respiratory system works, including its parts, and how it is controlled.

The lungs are an important organ of the respiratory system. Lungs are the primary site for the exchange of oxygen and carbon dioxide. We usually breathe at a rate of 12 to 20 times per minute.

Why do we need to breathe? Because our cells need oxygen to produce ATP; carbon dioxide is produced as a waste product during this process. Using the respiratory system we take in oxygen and exhale carbon dioxide. 

The nasal cavity, pharynx, larynx (voice box), trachea, bronchial tree, and lungs, are parts of this system. It’s a complex system, but in this course, you will learn about the respiratory system, including its structure, the exchange of gases, and everything in between. This is going to be one of the most detailed and easy-to-understand courses on the respiratory system. 

Let’s follow the tradition and start with a definition:

“The human respiratory system is a network of organs and tissues that helps us breathe. The primary function of this system is to introduce oxygen into the body and expel carbon dioxide from the body.”

Division of Respiratory System

The main function of the respiratory system is to provide oxygen to body tissues for cellular respiration and remove carbon dioxide from the body. Functionally we can divide the respiratory system into two zones: 

  1. Conducting Zone: includes organs and structures that are not directly involved in gas exchange. 
  2. Respiratory Zone: includes organs and structures that are responsible for gas exchange. 

Now, starting our interesting journey let’s understand the various structures of the respiratory system.

Conducting Zone

The conducting zone provides a route for air to pass from the external environment to the lungs and vice versa. Additionally, various organs of the conducting zone purify the air, remove dust and pathogens, humidify the air, and warm the air; this is helpful for efficient gas exchange and protection of the respiratory system. Some structures of the conducting zone perform other functions as well like sensing smell. 


The conducting zone primarily includes below structures:

  1. Nose
  2. Pharynx
  3. Larynx
  4. Trachea
  5. Bronchial tree (Bronchi and Bronchioles)

Now, we will introduce each part of the conducting zone one by one.


This is the primary entrance and exit for the respiratory system. Our nose is made of bone and cartilage covered with skin. To understand the structure of the nose, we can divide this into two sections: 

  1. External Nose 
  2. Nasal Cavity or Internal Nose

We will learn internal and external nose in detail in [upcoming] chapter.

Pharynx (Throat)

The pharynx is a muscular tube below the nasal and oral cavities. For detailed understanding, we can divide the pharynx into three regions:

  1. Nasopharynx
  2. Oropharynx
  3. Laryngopharynx

The pharynx is a common passage for food and air, that’s why the pharynx (throat) is part of the digestive system and also of the respiratory system. The epiglottis is an elastic cartilage, that works as a switch between the windpipe and the esophagus. It allows air to go into the larynx and food in the gastrointestinal tract. 

We will discuss all three regions of the pharynx in detail in [upcoming] chapter.

Larynx (Voice Box)

The larynx is positioned below the pharynx. The larynx is an air passageway between the pharynx and the trachea. It controls the volume of air that enters and leaves the lungs. This is also known as a voice box, it contains structures that vibrate and produce voice. 

The air passage must be open always, so the larynx is made of nine cartilages. The cartilages are connected by ligaments. Cartilage is a firm but flexible tissue that prevents the collapse of the larynx and keeps it open. The larynx is formed by several pieces of cartilage:

  1. Thyroid Cartilage
  2. Epiglottis Cartilage
  3. Cricoid Cartilage

The larynx also contains three paired cartilages (6 in total):

  1. Arytenoids
  2. Corniculates
  3. Cuneiforms

We will discuss all nine cartilages and the functioning of the larynx in detail in the [upcoming] chapter.

Do you Know?

It is physically possible to speak while we are inhaling, this is called ingressive speech. But we are not used to it. We are used to egressive speech, where sound is produced when we exhale. With practice, you can learn to produce sound while you inhale. 


We are discussing the conducting zone and introducing various parts of it. Try to visualize various parts in the diagram of organs.


The trachea is an air passway between the larynx and the lungs (see the image). This is a 10 to 13 cm (4 to 5 inches) and 2.5 cm (1-inch) diameter windpipe. It extends from the larynx to the lungs. To prevent the collapse of the trachea the wall of trachea contains 16 to 20 C-shaped stacked cartilages. Any prolonged blockage even for a few minutes can result in death. The trachea divides into primary bronchi at the bottom; one for the left lung and one for the right lung.


We will discuss the trachea and other functions like purifying air in detail in the [upcoming] chapter.

Bronchial Tree

The trachea branches into two primary bronchi (left and right) at the carina. The main function of these bronchi is to provide air passage between the trachea and the lungs. Other functions of bronchi are trapping debris and pathogens. To prevent the collapse of bronchi, both bronchi have a structure of cartilage same as in the trachea. Bronchi is a plural term its singular form is bronchus. Please take a look at the image to visualize the bronchial tree. 

Within the lungs each primary bronchi branches into secondary bronchi leading to the lobes of each lung. The secondary bronchi branch into tertiary bronchi, which further divide into smaller bronchi and this branching continues. A bronchial tree or respiratory tree is a term used for these multi-branched bronchi. Each bronchus divides into smaller and smaller bronchi. These smaller branches are called bronchioles. No cartilage is present in the walls of bronchioles. The smallest bronchioles terminate in a group of alveoli.


We will discuss the bronchial tree in detail in the [upcoming] chapter. Please refer images to understand the structure of the respiratory system.

Do You Know?

The carina (where the trachea branches into primary bronchi) contains special nerve tissues that induce coughing when foreign particles or food is present in the inhaled air. This extreme coughing is very important to prevent the entry of foreign particles into the lungs.


We have discussed all major structures of the conducting zone. Next, you will read about the structures of the respiratory zone.

Respiratory Zone

The respiratory zone includes structures that are involved in gas exchange. The respiratory zone starts from the smallest bronchioles which is known as the respiratory bronchiole. This is the smallest type of bronchiole which leads to an alveolar duct. 


The respiratory zone primarily includes below structures:

  1. Respiratory Bronchiole
  2. Alveolar Ducts
  3. Alveolar sacs
  4. Alveoli (singular: alveolus)

Alveolar Duct, Alveolus and Alveoli

The alveolar duct is a tube that is made of smooth muscle and connective tissues. This tube opens into the alveolar sac (cluster of alveoli). Alveoli is a plural term its singular is alveolus. An alveolus is a tiny air sac where the exchange of oxygen and carbon dioxide takes place. An alveolus is around 200 μm in diameter. It is surrounded by a network of capillaries

Capillaries are tiny blood vessels, blood flows through these capillaries and gas exchange takes place between blood and air sacs. Elastic walls of the alveolus allow it to expand when we inhale thus providing more surface area for gas exchange. 

Alveolar Sac

An alveolar sac is a cluster of alveoli. It is located at the very end of the respiratory tree. Each alveolar sac is a grape-like cluster. 

Alveoli, alveolus, and alveolar sac can be confusing let’s understand these structures again:

  1. Alveolus: A single tiny air sac surrounded by capillaries where gas exchange takes place. 
  2. Alveoli: Alveoli is plural form of alveolus, indicating multiple such air sacs. 
  3. Alveolar Sac: This is a cluster of alveoli. 

An alveolus is a basic unit of gas exchange. 


We will discuss the gas exchange in detail in the [upcoming] chapter.

Anatomical Location-based Division of the Respiratory System

We can divide the respiratory system into upper respiratory tract and lower respiratory tract based on anatomical location. The upper respiratory tract includes parts outside of our chest cavity. The lower respiratory tract includes parts found inside our chest cavity. The chest cavity is also known as the thoracic cavity. The chest cavity is surrounded by ribs and includes pleural membrane, respiratory muscles lungs, heart, and other organs. Have you identified your chest cavity?

The upper respiratory tract includes structures located above the larynx:

  1. Nose
  2. Pharynx
  3. Larynx

The lower respiratory tract includes structures located below the larynx:

  1. Trachea
  2. Bronchi and Bronchioles (Bronchial Tree)
  3. Alveoli


Do not confuse between function-based division (respiratory and conducting zones) and location-based division (upper and lower respiratory tracts) of the respiratory system. We are dividing the same respiratory system based on the location of structures and functioning. 

The trachea and bronchial tree are part of the conducting zone because no gas exchange takes place here. But both the trachea and bronchial tree are located inside the chest cavity (thoracic cavity); so both are part of the lower respiratory tract. 


Lungs are the largest paired organs of the respiratory system. Lungs are located on both sides of the heart in the chest cavity, these are protected by ribs. The lungs house structures of both the conducting and respiratory zones. The left lung has a smaller volume than of right lung, which allows space for the heart. The lungs contain a large network of air sacs and alveoli. Gas exchange is the main function of the lungs, each lung contains millions of alveoli for rapid gas exchange.

The diaphragm is located at the base of the lungs, it is a dome-shaped muscle that contracts and relaxes to help in breathing.

Lobes of Lung

Each lung is composed of smaller units called lobes. The right lung consists of 3 lobes while the left lung consists of 2 lobes. This segmentation allows some sort of independent function among the lobes. 


Lobes of the right lung:

  1. Superior lobe
  2. Middle lobe
  3. Inferior lobe

Lobes of the left lung:

  1. Superior lobe
  2. Inferior lobe

We will discuss the structure and functioning of the lungs in detail in the [upcoming] chapter. 


Now you are familiar with all major structures and organs of the respiratory system. We highly encourage you to read the key terms written in the next section. This will reinforce your learning and solidify your understanding. Each term is a stepping stone towards mastering the respiratory system. 

Key Terms You Learned

  1. Respiratory Zone: The part of the respiratory system where gas exchange takes place. 
  2. Conducting Zone: The section of the respiratory system that provides a route for air to the respiratory zone. 
  3. Upper Respiratory Tract: Part of the respiratory system located outside of the chest cavity. Primarily responsible for purifying, warming, and conducting air. 
  4. Lower Respiratory Tract: Part of the respiratory system located inside of the chest cavity. Primarily responsible for oxygen and carbon dioxide exchange. 
  5. Gas Exchange: The process of moving oxygen from the lungs to the blood and carbon dioxide movement from the blood to the lungs to be exhaled. 
  6. Alveolus: A tiny air sac inside the lung where gas exchange of oxygen and carbon dioxide occurs. Its plural form is alveoli. 
  7. Alveolar sac: A cluster of alveoli, it is a grape-like structure. 
  8. Capillaries: Small blood vessels that surround the alveolus for gas exchange. 
  9. Diaphragm: A large dome-shaped muscle that is located at the base of the lungs. 
  10. Lobes of the Lung: Smaller units of the lung. 

If any term or structure is still confusing; don’t worry continue with the course, each aspect of the respiratory system will become crystal clear.


Images used in this chapter are sourced from Wikimedia Commons and Freepik, and some images are designed by IntakeLearn.

  1. OpenStax College, CC BY 3.0, via Wikimedia Commons
  2. OpenStax College, CC BY 3.0, via Wikimedia Commons
  3. National Heart Lung and Blood Institute, Public domain, via Wikimedia Commons
  4. סתו כסלו, CC BY-SA 4.0, via Wikimedia Commons
  5. OpenStax Anatomy and PhysiologyOpenStax, CC BY 4.0, via Wikimedia Commons
  6. OpenStax College, CC BY 3.0, via Wikimedia Commons


While writing this chapter we took references from multiple sources. We are writing details of sources below:

  1. Gray’s Anatomy The Anatomical Basis of Clinical Practice, 41th Edition
    Editor: Susan Standring
    Publisher: Elsevier
    ISBN: 978-0-7020-5230-9
    Year: 2016
  2. Gray’s Anatomy for Students, 5th Edition
    Author:  Richard L. Drake, PhD, A. Wayne Vogl, PhD and Adam W. M. Mitchell, MB BS, FRCS, FRCR
    Publisher: Elsevier
    ISBN: 9780323934237
    Year: 2023
  3. American Lung Association. “How Lungs Work.” Available at: []
  4. Canadian Lung Association. “Respiratory System.” Available at: []
  5. National Cancer Institute. “Introduction to the Respiratory System.” Available at: []
  6. National Heart, Lung, and Blood Institute. “How the Lungs Work.” Available at: []
  7. American Lung Association. “Warning Signs of Lung Disease.” Available at: []
  8. National Heart, Lung, and Blood Institute. “Chronic Obstructive Pulmonary Disease (COPD).” Available at: []
  9. OpenStax. “Anatomy and Physiology 2e: Organs and Structures of the Respiratory System.” Available at: []
  10. Wikipedia. “Respiratory system.” Available at: []
  11. CK-12 Foundation. “CK-12 Life Science for Middle School: Respiratory System.” Available at: []
Categories: Uncategorized