Have you ever wondered why our bodies exhale carbon dioxide (CO2) when breathing instead of some other gas?
This is basic knowledge that almost everyone learns from a young age: we inhale oxygen and exhale carbon dioxide. However, have you ever questioned why our bodies expel carbon dioxide?
Why Do We Exhale Carbon Dioxide?
Carbon dioxide is a waste product of metabolic and biochemical processes.
The carbon dioxide produced in our bodies is a result of cellular respiration—where essential nutrients are converted into energy in the presence of oxygen. Carbon dioxide is generated and subsequently removed from the body by dissolving in the blood and binding with hemoglobin for transport to the lungs—where it is expelled into the environment through the nose and mouth.
How is Carbon Dioxide Produced in Our Bodies?
Carbon dioxide is a waste product of the metabolic and biochemical processes occurring within our bodies.
Cellular Respiration
When talking about respiration, the first things that come to mind are the nose and lungs. Below is a diagram of the entire respiratory process that we commonly learn in school.
Cellular respiration is not what you can see in this image.
However, cellular respiration is not something you can see in the image above. As the name suggests, it occurs at the cellular level within our bodies. More specifically, it is a series of metabolic processes and reactions within the cells that convert biochemical energy derived from essential nutrients into a source of energy to fuel cell activities.
Although many biochemical reactions continuously occur in the body, most of the energy supplied to the body is metabolized within the cells. The substances involved in this reaction are primarily sugars, carbohydrates, fats, and proteins, and it occurs in the presence of oxygen. This is referred to as aerobic respiration.
This biochemical reaction occurs inside our body cells and produces carbon dioxide as a byproduct.
This biochemical reaction occurs inside our body cells and produces carbon dioxide as a byproduct. Glucose, fats, and proteins are all used as fuel for this reaction. During this process, the ratio of carbon dioxide produced is less than the amount of oxygen we consume.
How is Carbon Dioxide Excreted from the Body?
We all know that high concentrations of carbon dioxide can be harmful to our bodies. Therefore, it must be eliminated from the body.
This is accomplished through three main biological processes: carbon dioxide molecules dissolve directly into the blood, they bind with proteins (especially hemoglobin in the blood), or they exist in the form of water as carbonic acid (part of the bicarbonate buffer system). This article primarily focuses on the first process.
Carbon Dioxide Dissolves in Blood More than Oxygen
Carbon dioxide dissolves in human blood more than oxygen.
Due to certain chemical properties of carbon dioxide, it dissolves in human blood more readily than oxygen. Dissolved carbon dioxide is transported to the lungs, where it separates from the blood in the alveoli and is expelled from the body through the nose and mouth.
Carbon Dioxide Binding with Hemoglobin
Carbon dioxide molecules can enter red blood cells and bind with hemoglobin—a protein involved in transporting oxygen throughout the body.
Carbaminohemoglobin forms when carbon dioxide binds with hemoglobin.
A molecule named carbaminohemoglobin forms when carbon dioxide binds with hemoglobin. It accounts for 30% of the total carbon dioxide in the human body. The binding between hemoglobin and carbon dioxide is a reversible process, where carbon dioxide molecules detach from hemoglobin as they travel to the lungs. Again, gas exchange occurs in the alveoli: taking in oxygen and separating carbon dioxide from the blood, ultimately expelling it from the body through the nose and mouth (exhalation).
60% of carbon dioxide in the body exists in the form of bicarbonate ions.
60% of carbon dioxide in the body exists as bicarbonate ions (hydrogen carbonate)—which help maintain blood pH and support other metabolic activities.
In summary, we release carbon dioxide from our bodies when we breathe. It is produced in the cells as food is broken down when we eat and is converted into energy that helps sustain vital bodily functions.