pH Homeostasis and the Arterial Blood Gasses:
The Role of Carbon Dioxide
Making Sense
It is important to realize that, as for all topics in physiology, pH Homeostasis and the Arterial Blood
Gasses make sense. The challenge is that there are things we need to know in order for them to make
sense and we may or may not know these from previous educational experiences. Therefore, I will start
with the basics here. If you are comfortable with this material already, feel free to skip it.
Also, bear in mind that this is a pathophysiology course, not a chemistry course. Even so, there is a little
bit about the chemistry that we need to know. I am including the chemical formulas for the molecules we
are discussing because you may come across those formulas as you review these topics, and you will
want to know “what is what”. Nevertheless, you will not need to become a chemist to understand this
topic.
Carbon Dioxide
Almost everything we need to know about pH homeostasis hinges on the role of carbon dioxide gas (CO2)
in our physiology.
As our bodies convert the chemical energy available from the food we eat into the chemical form our
cells need to function (a molecule called ATP), we produce carbon dioxide gas as a waste product (we
also produce water (H2O), but since our bodies are mostly water anyway, that doesn’t present a problem
for us).
By contrast, CO2 does present a problem.
CO2 is a waste product. There isn’t much our bodies can use it for. Our cells need to get rid of it.
Otherwise, CO2 would build up, cause everything to back up and quit working, we’d quit making ATP, and
we’d die. We must get rid of it.
Unfortunately, our bodies don’t have a system of flues and chimneys to carrie away waste gasses, so our
only option is to use the bloodstream (which is, after all, mostly water).
Like other gasses, CO2 is not very soluble in warm, watery solutions such as our blood and body fluids.
CO2 simply doesn’t dissolve very well. To make matters worse, our bodies produce far more CO2 at any
given time than we could carry away in the relatively small volume of our blood stream.
How can we use the bloodstream if there isn’t enough blood to carry away all the CO2 that we make?
Solving Solubility
The answer is that we convert CO2 into a much, much, much more soluble molecule called carbonic acid
(H2CO3). By converting CO2 into carbonic acid, we can carry away all the CO2 we produce. A complication
is that carbonic acid can release one of its hydrogen atoms as a hydrogen ion (H+).
Page 1 of 8
© R. Rawson, 2025 v. 03052025
, pH Homeostasis and the Arterial Blood Gasses:
The Role of Carbon Dioxide
By the way, an ion (pronounced eye-on) is simply a molecule or atom that has gained or lost
one or more electrons, giving it an electrical charge. This is what the little “+” and “-“ signs
mean when we write the formula for an ion. “+” indicates a positive electrical charge and “-“
indicates a negative charge. This is not something you will be quizzed on but I explain this in
case anyone is curious. Something you WILL want to know is that positively charged ions are
called cations (KAT-eye-ons) and negatively charged ions are called anions (Ann-eye-ons).
Knowing this comes in handy when we discuss the anion gap.
The process of the acid releasing an H+ is known as dissociation. Carbonic acid can dissociate to give us
a hydrogen ion (H+) and a bicarbonate ion1 (HCO3-), both of which are also very soluble in water. This
dissociation is part of what is described in the one chemical equation we cover in this course:
That One Chemical Equation
The details of this chemical reaction are not the key point here. You will want to focus on what this
equation is telling us that is important for understanding pH homeostasis and the ABGs.
Notice the arrows. These indicate the progress of the reaction. Notice too that they point in both
directions. This means the reaction can go either way. This is the first key point to take away from this
equation:
We can run the reaction in either direction (depending on local conditions) and that means that we can
convert CO2 to bicarbonate and H+ and we can convert bicarbonate and H+ to CO2.
1
Technically, “bicarbonate” is the name of the ion. Thus, “bicarbonate ion” is redundant. I use that form here to emphasize
that bicarb is an ion for those who are not familiar with chemical nomenclature. From here forward, I’ll simply refer to this ion
as “bicarbonate” or “bicarb”.
Page 2 of 8
© R. Rawson, 2025 v. 03052025
The Role of Carbon Dioxide
Making Sense
It is important to realize that, as for all topics in physiology, pH Homeostasis and the Arterial Blood
Gasses make sense. The challenge is that there are things we need to know in order for them to make
sense and we may or may not know these from previous educational experiences. Therefore, I will start
with the basics here. If you are comfortable with this material already, feel free to skip it.
Also, bear in mind that this is a pathophysiology course, not a chemistry course. Even so, there is a little
bit about the chemistry that we need to know. I am including the chemical formulas for the molecules we
are discussing because you may come across those formulas as you review these topics, and you will
want to know “what is what”. Nevertheless, you will not need to become a chemist to understand this
topic.
Carbon Dioxide
Almost everything we need to know about pH homeostasis hinges on the role of carbon dioxide gas (CO2)
in our physiology.
As our bodies convert the chemical energy available from the food we eat into the chemical form our
cells need to function (a molecule called ATP), we produce carbon dioxide gas as a waste product (we
also produce water (H2O), but since our bodies are mostly water anyway, that doesn’t present a problem
for us).
By contrast, CO2 does present a problem.
CO2 is a waste product. There isn’t much our bodies can use it for. Our cells need to get rid of it.
Otherwise, CO2 would build up, cause everything to back up and quit working, we’d quit making ATP, and
we’d die. We must get rid of it.
Unfortunately, our bodies don’t have a system of flues and chimneys to carrie away waste gasses, so our
only option is to use the bloodstream (which is, after all, mostly water).
Like other gasses, CO2 is not very soluble in warm, watery solutions such as our blood and body fluids.
CO2 simply doesn’t dissolve very well. To make matters worse, our bodies produce far more CO2 at any
given time than we could carry away in the relatively small volume of our blood stream.
How can we use the bloodstream if there isn’t enough blood to carry away all the CO2 that we make?
Solving Solubility
The answer is that we convert CO2 into a much, much, much more soluble molecule called carbonic acid
(H2CO3). By converting CO2 into carbonic acid, we can carry away all the CO2 we produce. A complication
is that carbonic acid can release one of its hydrogen atoms as a hydrogen ion (H+).
Page 1 of 8
© R. Rawson, 2025 v. 03052025
, pH Homeostasis and the Arterial Blood Gasses:
The Role of Carbon Dioxide
By the way, an ion (pronounced eye-on) is simply a molecule or atom that has gained or lost
one or more electrons, giving it an electrical charge. This is what the little “+” and “-“ signs
mean when we write the formula for an ion. “+” indicates a positive electrical charge and “-“
indicates a negative charge. This is not something you will be quizzed on but I explain this in
case anyone is curious. Something you WILL want to know is that positively charged ions are
called cations (KAT-eye-ons) and negatively charged ions are called anions (Ann-eye-ons).
Knowing this comes in handy when we discuss the anion gap.
The process of the acid releasing an H+ is known as dissociation. Carbonic acid can dissociate to give us
a hydrogen ion (H+) and a bicarbonate ion1 (HCO3-), both of which are also very soluble in water. This
dissociation is part of what is described in the one chemical equation we cover in this course:
That One Chemical Equation
The details of this chemical reaction are not the key point here. You will want to focus on what this
equation is telling us that is important for understanding pH homeostasis and the ABGs.
Notice the arrows. These indicate the progress of the reaction. Notice too that they point in both
directions. This means the reaction can go either way. This is the first key point to take away from this
equation:
We can run the reaction in either direction (depending on local conditions) and that means that we can
convert CO2 to bicarbonate and H+ and we can convert bicarbonate and H+ to CO2.
1
Technically, “bicarbonate” is the name of the ion. Thus, “bicarbonate ion” is redundant. I use that form here to emphasize
that bicarb is an ion for those who are not familiar with chemical nomenclature. From here forward, I’ll simply refer to this ion
as “bicarbonate” or “bicarb”.
Page 2 of 8
© R. Rawson, 2025 v. 03052025
