Now, instead of talking about the idea of pH overall, like in a test tube or beaker, let's talk about pH in our bodies specifically. What molecules and processes in our bodies can cause pH changes?
Your book lists five things on page 824 that affect hydrogen ion concentration in our bodies. I will give the same list here, but in my words.
1. Cellular Respiration (a.k.a., aerobic respiration of glucose)
When we take oxygen and glucose into our cells for the purpose of making ATP, we make carbon dioxide and water as well. Do you remember this figure from A&P 1?
Well, as you learned during the systemic respiration unit, Unit 22, carbon dioxide mainly travels through the blood as bicarbonate ions. And you learned about that fact also back in the blood unit, in the context of maintaining the pH of blood. So, the required production of ATP leads to a carbon dioxide byproduct that affects blood pH. Here's a little more on how that happens...
Carbon dioxide combines with water to make an carbonic acid, H2CO3, a weak acid. This weak acid tends to give off a hydrogen ion when in a neutral-ish pH solution, like in blood. Therefore, it gives off a hydrogen ion as shown to the right in the figure above. You learned about this already. That reaction also produces HCO3-, bicarbonate ion. This reaction is readily reversible, so that when blood reaches the lungs, it quickly reverses and we can expel the carbon dioxide. The reversibility is shown by the arrows that point both ways.
The bicarbonate ion can act as either a weak base or a weak acid. The weak base part is pretty easy to see, because all you have to do is reverse the reaction that just happened, sucking up a hydrogen ion to combine with the bicarbonate ion and re-making carbonic acid. But the bicarbonate ion can also be a weak acid. This is shown more clearly here:
In middle of this figure is the bicarbonate ion. If the reaction goes to the left, the bicarbonate ion acts like a weak base, and if it goes to the right, the bicarbonate ion acts like a weak acid. Because it can act like either, bicarbonate is an excellent buffer, and that will be discussed on the buffer page. For our purposes here, you should be able to see that the production of carbon dioxide can lead to the addition of excess hydrogen ions to solution.
2. Anaerobic respiration of glucose
You learned that sometimes our cells need so much ATP that they must keep making it even when they cannot get enough oxygen. This happens a lot in muscle. When this happens, glucose is only broken down into lactic acid... not all the way to carbon dioxide. But, lactic acid is an acid. It tends to increase the hydrogen ion concentration in solution.
3. Breaking down fatty acids
Have you ever fasted for a little while? When you fast, even if you brush your teeth a lot, you start to get a weird taste in your mouth. Have you ever noticed that? You see, when you fast, your body is forced to break down its supplies of fats, rather than use newly ingested carbohydrates. The fats stored in our bodies are triglycerides, which are molecules made of glycerol and fatty acids. As we break down a lot of fatty acids, we make something called ketone bodies as byproducts. These are acidic, and so they change the concentration of hydrogen ions in our fluids. The weird taste is the taste of the ketone bodies.
4. Breaking down sulfur-containing amino acids
When we break down proteins, we release amino acids. Our bodies try to recycle most of the amino acids (nutrients), but sometimes we end up breaking some of them down. If the amino acids we break down contain the element sulfur in their R groups (there are only 2 of them: methionine and cysteine), the breakdown results in the production of sulfuric acid, H2SO4. Sulfuric acid increases the hydrogen ion concentration in our fluids. It does this by reversibly yielding HSO4- + H+ (by "reversibly yielding," I mean that the arrows in the reaction look like those above for the production of the bicarbonate ion from carbonic acid).
5. Breaking down nucleic acids and "phosphoproteins"
All nucleic acids contain phosphate groups... that is because nucleic acids are made of nucleotide building blocks, and all nucleotides contain at least one phosphate group. "Phosphoproteins" are proteins with phosphate groups attached, since proteins are made of amino acid building blocks, and amino acids don't contain phosphate groups. However, many proteins get modified to have phosphate groups sticking off them. I know that sounds odd, but it is very common. Do you remember what phosphate groups are? They are PO42-. Anyway, the break down of these macromolecules that have phosphate groups hanging off them leads to the production of phosphoric acid, H3PO4.
Like you saw with carbonic acid and sulfuric acid, phosphoric acid can reversibly give off hydrogen ions: H3PO4 <------> H2PO4- + H+ . Phosphoric acid is a weak acid.
All of these five processes that occur in our bodies lead to the production of acids, so they all tend to increase the hydrogen ion concentration in our body fluids. If you re-evaluate them, you'll see that they all fit into one of two categories-- they are either processes that occur in systemic respiration (breathing) or metabolism (nutrients and their breakdown). Because hydrogen ion concentration can increase from either breathing or metabolism, there are two general types of acid-base imbalances that can occur in our bodies:
You already learned a bit about acidosis and alkalosis on the pH webpage. But you didn't learn what caused them. Now you should know. So, for example, respiratory acidosis and respiratory alkalosis have to do with pH imbalances caused by breathing. If one breathes too fast and deeply (hyperventilation), one kicks out excess amounts of carbon dioxide, and that causes respiratory alkalosis. Does that make sense? The metabolic imbalances are a bit more challenging to explain, but they link to the specific causes for pH changes above as well. The easiest metabolic imbalance to explain is that in diabetes mellitis more ketone bodies are made, causing metabolic acidosis.
The next question on your minds should be, now that we see that the pH of body fluids can easily be made more acidic, how do we control this? These are the questions that I hope to answer in the remaining web pages.
© 2011 STCC Foundation Press