Bicarbonate Buffer System Equation

Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

Have you ever wondered why you pass out if you hyperventilate? It all boils down to the bicarbonate buffer system of our blood, which you will learn about in this lesson.

Blood is mostly water

What do you picture when you think of blood? You probably don't picture a tall glass of lemonade. However, both of these liquids are composed almost entirely of water. Because of this, the chemistry of blood is very similar to the chemistry of water.

Did you know that gases can dissolve in water just like salt can? Because we can't see molecules like oxygen (O2) and carbon dioxide (CO2) when they exist as a gas, it's hard to imagine them dissolving in water and blood. When gases dissolve, they react with the water molecules (H2 O) and affect the chemistry of the water. Before we talk further about the effect of CO2 on water and the buffering system of blood, let's first review the chemistry of water.

Water, pH and buffers

When we think of liquid water, we picture lots of H2 O molecules moving around. Most of the water is like this, but some of the H2 O molecules disassociate, or split apart, and become a hydrogen ion (H+), which is also called a proton, and a hydroxide ion (OH-). The concentration of H+ in the solution is measured by pH: as H+ increases, the solution's pH decreases. When different molecules dissolve in the water, they can affect the number of H+ molecules and thus affect the pH of the solution.

Many processes in the body are affected by pH, so all living organisms, including humans, have evolved homeostatic mechanisms that resist changes in pH. A molecule that readily binds to or releases a proton can resist pH changes in its solution. This molecule therefore helps to buffer the solution. When protons are added to a solution, the buffer molecule can bind to the free H+ to keep the pH from changing.

Think of a buffer like a ball boy at a baseball game. If any balls get hit into the outfield, it's his job to go pick them up. If a baseball player or umpire needs a ball, the ball boy will give one up from his stock. In this analogy, the balls are protons. If extra protons enter the solution, the buffering molecule will retrieve them. If protons are removed from the solution, the buffering molecule will give up its proton to maintain the proton concentration and the pH.

The bicarbonate buffering system equation

When CO2 dissolves in water it reacts with an H2 O molecule forming carbonic acid. The chemical equation for this reversible reaction is

CO2 reacts with water to form carbonic acid

Carbonic acid is a weak acid, which means that it can release a proton and form bicarbonate. The chemical equation for this reversible reaction is

Carbonic acid dissociates into bicarbonate and a proton

When these two equations are combined, it gives the equation for the major blood buffering system in our bodies.

Blood buffering system equation

This simple-looking chemical equation has two important components.

1) Each of the two reactions that make up the chemical equation is reversible. This means that if there is a lot of CO2, the chemical equation will go toward the right, forming protons and bicarbonate. If the proton concentration, H+, increases, then the chemical equation will go toward the left, forming carbon dioxide. The direction depends on the amount of CO2 and H+ in the solution. The concentration of one molecule can affect the formation of the other.

2) CO2 is a gas, which means that CO2 can enter and leave the blood through the lungs. The rate at which CO2 leaves the blood via the lungs is controlled by how fast we breathe, which is our respiration rate.

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