MadSci Network: General Biology

Re: Why is Carbon Dioxide Harmful to us But we breathe it out?

Date: Mon Jun 13 18:20:34 2005
Posted By: Shannon DeVaney, Grad student, Ecology and Evolutionary Biology, University of Kansas
Area of science: General Biology
ID: 1113244691.Gb

Hi Melissa,

That's an intriguing question. Let's proceed in two steps: first, we'll 
look at how and why our bodies produce carbon dioxide (CO2) and second, 
we'll look at why carbon dioxide can be bad for us. 

You may have already discussed cellular respiration in your science 
classes. To review, cellular respiration is the process by which cells in 
our bodies convert "fuel" from the food we eat (molecules like 
carbohydrates, proteins, and fats) to energy. Oxygen is a reactant in the 
process, and carbon dioxide is a by-product. The process, which is rather 
complicated, can be summarized like this:

Organic fuel + Oxygen --> Carbon dioxide + Water + Energy

So, our bodies are constantly converting molecules from the food we eat 
into usable energy, using the oxygen we breathe in and creating carbon 
dioxide and water as by-products. Because carbon dioxide is always being 
produced, it would quickly build up in our bodies if we did not get rid 
of it in some way. This alone could explain why we breathe out carbon 
dioxide -- why carry around all these molecules that you don't really 
need? However, as you mentioned, carbon dioxide can actually be bad for 
our bodies. 

To understand how and why carbon dioxide is bad for us, we first have to 
know that much of the body's excess carbon dioxide ends up in the blood. 
Now, our blood also contains a molecule called hemoglobin, and other 
molecules a lot like it. The job of these molecules is to carry oxygen 
throughout the body. As blood passes by the lungs, where oxygen is 
plentiful, oxygen molecules bind to hemoglobin molecules. Later, as the 
blood travels through oxygen-poor regions of the body, oxygen is 
released, where it can be taken up by cells and used. 

The net effect of increased carbon dioxide in the blood is lowered blood 
pH (that is, the blood becomes more acidic). The ability of hemoglobin to 
bind with oxygen decreases with decreasing pH in a phenomenon called the 
Bohr effect. Because of the Bohr effect, increasing CO2 concentrations 
indirectly reduce the oxygen carrying capacity of the blood. 

Carbon dioxide can also react with parts of the hemoglobin molecule to 
form carbamino compounds. The formation of these compounds directly 
reduces the ability of hemoglobin to bind with oxygen and therefore also 
reduces the oxygen carrying capacity of the blood. 

So, in these two ways (indirectly by reducing blood pH and directly by 
reacting with hemoglobin) carbon dioxide can reduce the ability of our 
blood to carry oxygen to tissues throughout the body where it is needed. 
It's a good thing, then, that the excess carbon dioxide in our blood 
diffuses into our lungs, where it leaves the body when we exhale. 

As an added note, you may also be interested in carbon monoxide (CO), a 
toxic molecule found in automobile exhaust, tobacco smoke, and many home 
heating systems. Carbon monoxide also binds with hemoglobin in the same 
way oxygen does, but the ability of hemoglobin to bind with CO is 200 
times greater than its ability to bind with oxygen. So, in the presence 
of carbon monoxide, most of the hemoglobin binding sites are taken up by 
CO, leaving no place for oxygen to bind. People who breathe air 
containing significant levels of CO don't get enough oxygen to their 
tissues, including the brain; this can lead to serious illness and even 

Thanks for an interesting question. Good luck with your studies. 


Shannon DeVaney
Ph.D. Candidate, Ecology & Evolutionary Biology
University of Kansas

References -- For further information (just a start; there's lots of 
information out there)

Eckert Animal Physiology: Mechanisms and Adaptations, 4th Edition. 
Randall, Burggren, and French. 1997. W.H. Freeman and Co., New York. 

Cellular Respiration.  http://biolog

Human Physiology ĘC Respiration. Course web page, Eastern Kentucky 
University. http://www.biolog

Carbon Monoxide Poisoning: U.S. Environmental Protection Agency.

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