Why Does The Human Body Release Carbon Dioxide?

Carbon dioxide is produced in the body as a result of cellular respiration, wherein vital nutrients are converted into energy in the presence of oxygen. The carbon dioxide produced is then removed from the body by dissolution in the blood and through binding with hemoglobin to be transported to the lungs, where it’s exhaled out from the nose and mouth.

It’s common knowledge that we breathe in oxygen and breathe out carbon dioxide. We have been reading, learning and hearing about this since we were kids. However, have you ever considered why carbon dioxide is what we exhale?

How Is Carbon Dioxide Produced In The Body?

We all know that carbon dioxide is actually a waste product of various metabolic and biochemical processes that occur inside our bodies, but how exactly is it produced in the first place? The answer to this question lies in…

Cellular Respiration

Talking about respiration, the first thing that comes to mind is the nose and lungs. If you’re more of a nerd, however, you might have a diagram of the entire respiratory tract pop up in your head…

However, cellular respiration is not what you see in the image above. As its name implies, it is something that occurs on a cellular level inside our bodies. More specifically, it’s a bunch of metabolic processes and reactions that go on inside the cells of organisms to convert biochemical energy derived from vital nutrients into a source of energy to fuel cellular activity.

Although many biochemical reactions occur within our bodies all the time, the one that occurs inside our cells and is responsible for giving us energy is probably the most important of all. The reactants involved in this reaction are mainly sugars, carbohydrates, fats and proteins, and since it occurs in the presence of oxygen, it’s known as aerobic respiration.

This biochemical reaction occurs inside our bodies’ cells and produces carbon dioxide gas as a byproduct. In answer to our earlier question, that’s how carbon dioxide is produced inside the body. Since glucose, fats and proteins are all used as fuel sources for this reaction, the ratio of carbon dioxide produced to oxygen consumed (called the respiratory quotient) varies with diet. Burning pure glucose gives a 1:1 ratio (RQ = 1.0), pure fat gives roughly 0.7, and a typical mixed diet falls around 0.8, so on average we exhale slightly less CO₂ than the oxygen we breathe in.

How Is Carbon Dioxide Excreted From The Body?

You might already know that carbon dioxide in high concentrations is poisonous to us. Therefore, it has to be appropriately removed/excreted from within the body.

This is accomplished through three main biological processes: carbon dioxide molecules are dissolved directly into the blood, they bind with proteins (particularly hemoglobin in the blood), or they are buffered with water as carbonic acid (a part of the bicarbonate buffer system). For the scope of this article, we are more interested in the first two methods.

Carbon Dioxide Is More Soluble In The Blood Than Oxygen

Due to certain chemical properties of carbon dioxide, it is much more soluble in human blood than oxygen. This tendency of carbon dioxide molecules to dissolve readily in the blood comes in pretty handy while removing them from within the cells in which they’re produced.

The dissolved carbon dioxide is then carried up to the lungs, where the alveoli take it out of the blood and send it out for exhalation.

Carbon Dioxide Binds With Hemoglobin

Carbon dioxide molecules can also enter red blood cells and bind to hemoglobin – a protein present in the blood that transports oxygen throughout the body.

A molecule called carbaminohemoglobin is formed when carbon dioxide binds with hemoglobin. It accounts for roughly 20–23% of the carbon dioxide carried in the blood. Since such binding of carbon dioxide and hemoglobin is a reversible process, carbon dioxide molecules are separated from hemoglobin once they reach the lungs. Once again, gas exchange takes place at the alveoli (they give oxygen and take carbon dioxide from the blood) and the cargo of carbon dioxide molecules is added to the outgoing breath (i.e., exhalation).

The largest share, about 70%, is carried in the blood as bicarbonate ions (HCO₃⁻). Inside red blood cells, the enzyme carbonic anhydrase rapidly combines CO₂ with water to form carbonic acid, which then dissociates into a bicarbonate ion and a proton. This bicarbonate buffer system also helps maintain blood pH within its narrow physiological range. The remaining ~7–10% travels simply dissolved in the plasma. When the blood reaches the lungs, the reactions reverse, CO₂ is released, and we exhale it.

Where Is Carbon Dioxide Produced During Cellular Respiration?

If you have ever sat through a biology class on cellular respiration, you may remember that it happens in three stages: glycolysis, the Krebs cycle (also called the citric acid cycle) and the electron transport chain. So which of these actually makes the carbon dioxide you breathe out? The answer might surprise you, because the famous first step makes none of it.

Glycolysis, which splits a molecule of glucose into two molecules of pyruvate in the fluid of the cell, releases no carbon dioxide at all. The CO₂ appears later, and only after the pyruvate has moved into the mitochondria, the cell’s power plants. First comes a short link reaction called pyruvate oxidation, in which a carboxyl group is stripped from each pyruvate and released as one molecule of carbon dioxide. Then comes the Krebs cycle, which throws off two more molecules of carbon dioxide for every acetyl group it burns.

Add it up and the numbers are tidy. A single glucose molecule yields two pyruvates, so pyruvate oxidation gives off 2 molecules of CO₂, and two turns of the Krebs cycle give off 4 more. That makes six molecules of carbon dioxide for every molecule of glucose, all of them produced inside the mitochondrial matrix. So when a question asks when and where CO₂ is made during respiration, the honest answer is: during pyruvate oxidation and the Krebs cycle, deep inside your mitochondria, and not during the headline-grabbing glycolysis step.

Which Organ Removes Carbon Dioxide From The Body?

This one trips up a lot of people, partly because the kidneys, the skin and even the gut get name-checked in trivia questions. The clean answer is the lungs. Almost all of the carbon dioxide your body offloads leaves through the lungs in the breath you exhale.

Here is how it works. Carbon-dioxide-rich blood returning from your tissues is pumped to the lungs, where it flows through a dense mesh of capillaries wrapped around millions of tiny air sacs called alveoli. Because the blood arriving is loaded with CO₂ while the air inside the alveoli is not, the gas simply diffuses out of the blood, through the thin capillary and alveolar walls, and into the air sac. From there it rides out on your next exhale. As StatPearls puts it, once venous blood returns to the lungs the carbon dioxide diffuses out of the bloodstream and into the alveoli, from where it is expelled.

What about the kidneys? They do help, but with a different job. The kidneys manage the longer-term acid-base balance of the blood by adjusting how much bicarbonate they keep or excrete in urine, and they do not breathe out CO₂ gas. So if an exam asks which organ excretes most of your carbon dioxide, the lungs win comfortably.

What Does Carbon Dioxide Do In The Body?

It is tempting to write carbon dioxide off as pure waste, a fume to be vented and forgotten. In reality it is one of the most important signaling molecules you carry, and your body watches its level far more closely than it watches oxygen.

Here is the part most people get backwards. The main thing that makes you feel the urge to breathe is not a shortage of oxygen, but a build-up of carbon dioxide. When CO₂ dissolves in blood it forms carbonic acid, which lowers blood pH. Sensors called central chemoreceptors in the medulla of your brainstem are exquisitely tuned to that pH change. According to StatPearls, a drop in the pH of the cerebrospinal fluid ultimately increases minute ventilation, meaning you breathe faster and deeper to blow off the excess gas. In healthy people, CO₂ is the dominant driver of breathing, while the oxygen sensors only take over when oxygen falls a long way below normal.

That same chemistry is why carbon dioxide turns dangerous at high levels. When CO₂ piles up in the blood faster than the lungs can clear it, a condition called hypercapnia, the blood tips toward respiratory acidosis. The symptoms climb with severity, from headaches, flushed skin, a racing heart and confusion, through to drowsiness and, in extreme cases, seizures. So carbon dioxide is not just a passenger heading for the exit. It is the gas that sets the pace of every breath you take and helps hold your blood chemistry steady.

How Much Carbon Dioxide Do We Exhale Each Day?

If carbon dioxide is leaving with every breath, how much actually adds up over a day? More than you might guess for an invisible gas. An average adult at rest breathes out roughly 1 kilogram (about 2.2 lb) of carbon dioxide a day, which works out to around 500 litres of the gas, according to BBC Science Focus.

That figure is not fixed, though, because it tracks how hard your cells are working. The moment you start exercising, your muscles burn more fuel, churn out more CO₂, and your breathing has to ramp up to clear it. A classic physiological study found that the amount of air your lungs move (your alveolar ventilation) rises in almost direct proportion to how much carbon dioxide your body is producing, whether that extra CO₂ comes from exercise or from any other cause. In other words, the harder you push, the more carbon dioxide you make, and the faster you breathe it out, which is exactly why a hard run leaves you panting while a nap does not.

One reassuring footnote: the carbon dioxide you exhale does not add net new carbon to the atmosphere the way burning fossil fuels does. The carbon in your breath came from the food you ate, which ultimately came from plants pulling that same CO₂ out of the air in the first place, so human breathing is part of a closed, short-term carbon loop.

In a nutshell, we release carbon dioxide when we exhale because it’s produced in the cells of our body in order to break down the food that we eat and subsequently produce energy for sustaining life.