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In this lesson, we will look at the role of chemical buffers in maintaining pH or acid-base balance in the body. The pH of your body fluids, although it may vary from one location to another, is maintained at a fairly constant value. The normal pH of blood stays near 7.4. The pH in your stomach, where protein digestion begins, is very low, typically around 1 to 2 due to the presence of hydrochloric acid. In contrast, the pH just past the stomach, in your small intestine, tends to stay higher, closer to 8. Most cellular activities occur optimally when the pH in the local environment remains optimal at a fairly constant value. Recall that most reactions in the human body require enzymes, which make it much easier for the reactions to occur. To promote a reaction, an enzyme binds very specifically to the substrate, which is the chemical that is about to react. This binding only occurs if the enzyme has a very specific shape that is a unique fit for that substrate. If an enzyme is exposed to a pH that is either higher or lower than the optimal range for that enzyme, bonds in the protein that make up the enzyme are broken and the enzyme's shape changes. It loses its ability to bind its substrate. Think of enzymes as being similar to keys. You likely have several keys and, although they may have similar shapes, the one that starts your car likely can't open the door to your house. And if you break off part of your house key, it will likely no longer open your door. Similarly, the change in pH causes an enzyme's shape to change. It is like changing the shape of a key. It no longer fits with its substrate so that reaction will be severely limited. But many of our body processes produce chemicals that can alter the pH. For example, your stomach secretes strong hydrochloric acid and your muscles can generate lactic acid. Each of those, if left unchecked, would cause your pH to become more acidic. The pH value would drop. Fortunately our bodies contain chemical buffers that act locally to resist or minimize pH changes as they occur. Chemical buffers typically exist in pairs; one is a weak acid and one is a weak base. Recall that a pH value of 7 is neutral. The farther from 7 the pH value is, the stronger the acid or the base, and acids have pH values below neutral while bases have pH values above neutral. Not surprisingly, if a body fluid becomes too acidic, the way to counteract that is to add some base. In other words, if the pH is lower than it should be, it can be raised or brought back closer to neutral with the addition of a base that has a higher pH, above neutral. Adding an acid to an acid would just make the pH drop even more. Right? Think of it this way: If you get a cup of coffee that is way too hot to drink, you don't reheat it, you try to cool it. Similarly, if the pH is too high, or too basic, it can be countered with the addition of something with a pH lower than 7, an acid. Here are the basic equations for some common chemical buffers. A strong acid plus a weak base buffer produces a weak acid and salt. And a strong base plus a weak acid buffer produces a weak base and water. There are numerous buffers in the human body. The classic example, though, involves a weak acid called carbonic acid paired with a weak base called sodium bicarbonate, which, by the way, is the same as baking soda. Let's see how these two buffers work. Hydrochloric acid plus the weak base buffer, sodium bicarbonate, produces the weak acid, carbonic acid, plus salt. And for the other reaction, a strong base like sodium hydroxide reacts with the weak acid buffer, carbonic acid, to produce sodium bicarbonate and water. Note that when one of the buffers from the pair is used, the reaction produces the other buffer. This allows the paired buffers to constantly be restocked. Remember, buffers do not block or prevent pH changes. They merely resist them or try to minimize the amount of the pH shift. If something is wrong in the body, and the pH shifts too low, the resulting condition is called acidosis. Conversely, if the pH shifts too high, the condition is called alkalosis. Either of these conditions will make the person experiencing it feel ill and, if left untreated, can be fatal. Chemical buffers are our body's first defense against such dangerous pH shifts.