What Gas Do We Breathe Out? Understanding Our Vital Exhale

The human body is an incredibly complex system, with many different processes going on at any given moment. Breathing is one of the most essential of these processes, as it allows us to take in oxygen and expel carbon dioxide. In this article, we will explore the question: what gas do we breathe out? We’ll look at the science behind the exhale, the role of the lungs, and factors that can affect the composition of our exhaled air. By the end of this article, you’ll have a better understanding of how breathing works and the importance of maintaining healthy lung function.

The Science of Breathing

Breathing is controlled by the respiratory system, which includes the lungs, trachea, bronchi, and diaphragm. When we inhale, the diaphragm contracts and the intercostal muscles expand the ribcage, creating a negative pressure that draws air into the lungs. This air contains oxygen, which is used by the body to fuel metabolic processes. When these processes occur, carbon dioxide (CO2) is produced as a waste product. This CO2 then diffuses from the bloodstream into the lungs, where it is exhaled back out of the body during the exhale phase of breathing.

The Role of the Lungs

The lungs are responsible for the exchange of gases during breathing. They are made up of millions of tiny air sacs called alveoli, which are surrounded by a network of capillaries. When we inhale, air enters the lungs and travels down the bronchioles, eventually reaching the alveoli. Here, oxygen moves from the air into the capillaries, where it binds to hemoglobin and is transported throughout the body. At the same time, CO2 moves from the capillaries into the alveoli, where it is exhaled out of the body during the exhale phase of breathing.

Factors Affecting the Composition of Exhaled Air


During exercise, the body’s need for oxygen increases, and the rate of breathing also increases in response. As a result, more CO2 is produced as metabolic processes ramp up. This increased rate of CO2 production means that the composition of exhaled air changes during exercise, with higher levels of CO2 and lower levels of oxygen compared to resting levels. This is why athletes are often seen gasping for air after a strenuous workout – they need to expel the excess CO2 that has built up in their bodies.


Several respiratory diseases can affect the composition of exhaled air. For example, chronic obstructive pulmonary disease (COPD) is characterized by damage to the lungs, which impairs their ability to exchange gases effectively. This can lead to higher levels of CO2 in the bloodstream and an altered composition of exhaled air. Similarly, asthma is a condition that causes inflammation and narrowing of the airways, making it harder to breathe. This can lead to a buildup of CO2 and other gases in the lungs, which can alter the composition of exhaled air.


In conclusion, understanding the composition of exhaled air is important for maintaining healthy lung function. Breathing is a vital process that allows us to take in oxygen and expel waste products like CO2. By knowing what gases we breathe out, we can better understand the functioning of our respiratory system and the factors that can affect it. Whether you’re exercising, dealing with respiratory disease, or simply going about your day-to-day activities, being aware of your breath can help you maintain optimal lung health.


  • What gas do we breathe out? We breathe out carbon dioxide (CO2).
  • Why do we breathe out CO2? CO2 is produced by the body as a waste product of metabolic processes, and must be removed from the bloodstream to prevent acidosis.
  • Can the composition of exhaled air change? Yes, the composition of exhaled air can change depending on factors such as exercise and respiratory disease.
  • How does exercise affect the composition of exhaled air? During exercise, more CO2 is produced by the body, which can lead to higher levels of CO2 in exhaled air.
  • What respiratory diseases can affect the composition of exhaled air? Diseases like COPD and asthma can lead to altered compositions of exhaled air due to impaired lung function.


1. American Lung Association. (2021). How Lungs Work. Retrieved from https://www.lung.org/lung-health-diseases/how-lungs-work

2. National Heart, Lung, and Blood Institute. (2021). How the Lungs Work. Retrieved from https://www.nhlbi.nih.gov/health-topics/how-lungs-work

3. WebMD. (2021). The Respiratory System. Retrieved from https://www.webmd.com/lung/picture-of-the-respiratory-system

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