Chemistry: Biological Buffers

Understanding Chemistry: Biological Buffers

A buffer works because its two chemical forms can trade hydrogen ions in either direction. This is an equilibrium, not a one-time reaction. If extra hydrogen ions enter a fluid, the base form captures some of them.

If hydrogen ions are removed, the acid form supplies some. The pH still changes slightly, but far less than it would in pure water. Buffering has limits.

Once most of one form has been used up, the system can no longer resist change well. Buffer capacity is greatest when the acid form and base form are present in similar amounts. This is why their relative amounts matter more than the amount of either form alone.

The bicarbonate system is especially useful in blood because one part of it can leave the body through the lungs. Carbon dioxide produced during cellular respiration enters the blood and moves into red blood cells. An enzyme called carbonic anhydrase speeds up its conversion into carbonic acid.

This rapid reaction helps blood respond quickly to changing conditions. During faster breathing, more carbon dioxide is exhaled. Fewer hydrogen ions remain in the blood, so pH rises.

During slow or shallow breathing, carbon dioxide builds up. More hydrogen ions form, so pH falls. This link explains why severe lung disease can disturb body chemistry, not only oxygen supply.

Other buffers work most strongly in particular places. Proteins contain side groups that can gain or lose hydrogen ions. Hemoglobin is important because it buffers hydrogen ions inside red blood cells while carrying gases.

When hemoglobin releases oxygen in active tissues, it can bind more hydrogen ions. This supports the transport of carbon dioxide back to the lungs. Phosphate buffers are common inside cells and in urine.

They are less important in blood plasma, yet they matter where chemical reactions occur every second. Cells need these local buffer systems because substances cannot always move quickly enough to rely only on blood.

The kidneys provide slower, longer-term control. They can remove hydrogen ions in urine and return bicarbonate to the blood. In prolonged acid buildup, the kidneys can make new bicarbonate, although this takes hours to days.

Doctors use blood pH, carbon dioxide pressure, and bicarbonate concentration to identify the source of an imbalance. A breathing problem usually changes carbon dioxide first. A problem such as severe diarrhea, uncontrolled diabetes, or kidney failure often changes bicarbonate first.

When learning this topic, keep track of time scale and location. Chemical buffers act within seconds, breathing adjusts within minutes, and kidneys respond much more slowly. It is important to distinguish a cause from compensation, since the body may change one system to reduce the effect of a problem that began elsewhere.