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Acids and Bases

Acids and Bases

The Theme of Chemistry of Blood in Acids and Bases

Blood as a Buffer

Everyone knows that aliens have extremely corrosive and acidic blood, which is why you would never want to harm an alien should you ever run into one. What you probably didn't realize was that Earthlings (yes, you) also have acids and conjugate bases in their blood.

For your body and cells to function properly, your blood must be maintained at a very specific pH value: pH 7.4, which is slightly basic. Several buffers contained in the blood maintain this pH. One of the most interesting buffers in blood is the bicarbonate buffer system consisting of the acid H2CO3 and its conjugate base HCO3-. How does the body get this buffer into the blood? Take a deep breath because you're about to learn the shocking truth.

Did you take a deep breath? If you did, you just lowered the concentration of your blood's biocarbonate buffer system by exhaling CO2. CO2 levels control the bicarbonate buffer because gaseous CO2 in the lungs is in equilibrium with dissolved CO2 in the blood (see figure below). This equilibrium is linked to the bicarbonate buffer equilibrium because dissolved CO2 can form H2CO3 by taking up H2O in the blood stream. A third equilibrium between H2CO3 and HCO3- forms the buffer system. H2CO3 is a weak acid because it can release its proton to form HCO3-.

When you exercise, the lactic acid produced by muscle tissue causes a build up of H+ in the blood. Without the biocarbonate buffer system this process would eventually turn your blood very acidic and that would obviously have dire consequences for your fragile little blood vessels. They are not built to withstand the corrosive and toxic properties of acidic solutions. Thankfully, when H+ begins to build up, HCO3- acts like all good buffers do—by absorbing the free H+. This shifts the equilibrium to the H2CO3 side. As a result, the second equilibrium between H2CO3 and CO2 shifts toward the CO2 side. The build up of CO2 in the blood gets released into the lung air space making you breath heavier.

One reason exercise results in heavy exhalation is that exhalation is needed to keep the blood pH balanced. The opposite process happens when you're sitting around watching YouTube videos. This causes minimal exhalation, keeping plenty of CO2 around in the lung air space to supply the bicarbonate buffer system. Ultimately, the balance of CO2 by your exhale and inhale rate serves to balance the buffering system that keeps the pH of your blood very near 7.4 at all times.

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