|MadSci Network: General Biology|
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 death. Thanks for an interesting question. Good luck with your studies. Regards, 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 y.about.com/library/weekly/aa090601a.htm Human Physiology ĘC Respiration. Course web page, Eastern Kentucky University. http://www.biolog y.eku.edu/RITCHISO/301notes6.htm Carbon Monoxide Poisoning: U.S. Environmental Protection Agency. http://www.epa.gov/iaq/co.html
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