While carbon dioxide is a metabolic waste product, it plays some important physiological roles as well.
This lesson describes how carbon dioxide is transported in our blood, how carbon dioxide is converted into a pH buffer, and how carbon dioxide helps with oxygen transport.
Carbon Dioxide Transport
As we breathe, we take oxygen into our lungs and push carbon dioxide out. Let’s take a moment to consider the physiological significance of these gases before we dive into how carbon dioxide is transported in our body.Our cells need oxygen to make ATP, and ATP is needed for cellular work. Carbon dioxide is a product of this cellular metabolism. Carbon dioxide is a waste product and must be removed from our body, much like trash must be taken out of the house.
While carbon dioxide is metabolic trash, it seems our bodies have found a use for carbon dioxide as it is transported from our tissues to the lungs. In this lesson, we will discuss both the means by which carbon dioxide is transported and how carbon dioxide helps with pH balance and oxygen transport.
Transport in Metabolizing Tissues
Carbon dioxide diffuses out of the tissues and into the blood, where it is dissolved in the plasma. While some of the carbon dioxide remains dissolved in the plasma, most carbon dioxide diffuses into our red blood cells. In the red blood cell, some carbon dioxide binds to hemoglobin, forming what we call carbaminohemoglobin.
Most of the carbon dioxide, however, is converted into bicarbonate. Carbonic anhydrase is an enzyme in the red blood cells that quickly converts carbon dioxide and water into bicarbonate and hydrogen ions. The equation for this reaction is as follows: Carbon dioxide and water are converted to bicarbonate and hydrogen ions, where HCO3- is bicarbonate and H+ is hydrogen ion.CO2 + H2O ;- -; HCO3- + H+While this reaction is reversible, as indicated by the double arrows, the high concentration of carbon dioxide pushes the reaction to the right.
You might be thinking, ‘What does this have to do with carbon dioxide transport?’ Hold on for a moment; this gets interesting fast. Bicarbonate diffuses into the plasma, where it is used as a much-needed pH buffer. Yes, that’s correct; the cellular waste product carbon dioxide plays a vital role in maintaining pH balance. Oh, but that’s not all. Carbon dioxide does even more for us. Remember the hydrogen ion produced along with bicarbonate in the red blood cell? That hydrogen ion binds to hemoglobin, which causes the oxygen to be released from the hemoglobin. That released oxygen can then enter into the cells and be used to make the much-needed ATP.
That’s pretty cool.
Transport in the Lungs
As I said before, most carbon dioxide produced by our tissues is converted into bicarbonate and as such is transported to the lungs. What happens to the bicarbonate once it gets to the lungs? Simply put, bicarbonate is reconverted into carbon dioxide and then exhaled. More specifically, the reaction that occurs in the tissues – that is, carbon dioxide plus water being converted into bicarbonate and hydrogen ion – is reversed in the lungs. The reaction you see below is pulled to the left because the concentration of carbon dioxide in the lungs is decreased as we breathe it off.
Let’s look at this as it occurs in our lungs. As carbon dioxide diffuses out of the blood into the alveolar gas, its concentration decreases in the blood and the reaction is pulled to the left, thus producing more carbon dioxide. More bicarbonate diffuses into the red blood cell and is used up in the reaction.
Therefore, the bicarbonate is reconverted into carbon dioxide and subsequently expired. Oh, but that’s not all! Where does the hydrogen ion come from? As blood flows through the lungs, hydrogen ions are released from hemoglobin, and guess what? That’s right. Oxygen binds to hemoglobin and it is ready for delivery to our metabolizing cells.
In summary, carbon dioxide is a metabolic waste product of ATP synthesis. As our cells produce carbon dioxide, it diffuses into the blood, where it is transported to the lungs for expiration. Some of the carbon dioxide is transported dissolved in the plasma. Some carbon dioxide is transported as carbaminohemoglobin.
However, most carbon dioxide is transported as bicarbonate.As blood flows through the tissues, carbon dioxide diffuses into red blood cells, where it is converted into bicarbonate. The enzyme carbonic anhydrase catalyzes this reaction in the red blood cells, where carbon dioxide and water are converted into bicarbonate and hydrogen ion. While still in the tissues, bicarbonate diffuses out of the red blood cell into the plasma, where it serves as a very important pH buffer.
Additionally, hydrogen ion produced along with bicarbonate causes oxygen to be released from hemoglobin in our tissues.The opposite occurs in our lungs. As blood flows through the lungs, bicarbonate enters the red blood cell, where it is converted into carbon dioxide. Carbon dioxide then diffuses out of the red blood cell and into the lungs, where it is expired.
Additionally, hydrogen ion is released from hemoglobin, thus allowing oxygen to bind to hemoglobin as blood flows through the lungs.
At the conclusion of this lesson, you should be able to:
- Explain the importance of carbon dioxide either being transported as carbaminohemoglobin or as bicarbonate
- Summarize why the equation of carbon dioxide plus water yields bicarbonate plus hydrogen ion is a reversible equation
- Differentiate between how bicarbonate is used in the tissues versus how it is used in the lungs