The Big Equation

At the beginning of A&P II I usually write the following on the board:

CO₂ + H₂O <-> H₂CO₃ <-> H⁺ + HCO₃^-

While I am writing, I narrate it this way:

"When carbon dioxide dissolves in water, especially in the presence of the enzyme carbonic anhydrase, they combine to form carbonic acid, which readily falls apart into a hydrogen ion and a bicarbonate ion."

This ends up being a stock part of my A&P2 lectures because it comes up in discussion of so many systems this semester.  So I thought I would dedicate a post to the reaction - and its implications for the systems that we study this semester.

The first time we discuss this reaction is often the first day of A&P II.  Discussing the chapter on Blood, we say that red blood cells (RBCs) contain an enzyme called carbonic anhydrase.  The textbook shows the equation in a paragraph of the text, and we say that this enzyme in the RBCs helps form H⁺ and HCO₃^- from CO₂ and H₂O.  This is the first mention of the enzyme and the reaction.

We discuss the reaction a second time in the Respiratory system chapter.  A number of concepts are introduced in the Blood/Cardiovascular chapters that are expanded/built upon in the Respiratory chapter, and this is one of them.  In this chapter we learn that carbon dioxide is carried in the bloodstream mostly as bicarbonate ion (HCO₃^-).  So how does it get converted to bicarbonate (and back)?  By using our reaction!  The carbonic anhydrase enzyme that is in the RBC (which we learned on Day 1...) converts the carbon dioxide to bicarbonate ion and back again.

The gas laws tell us that gases move down their partial pressure gradients.  This is, in fact, the driving force for "our equation" too.  When CO₂ is high, it drives the equation to the right, converting the CO₂ to bicarbonate.  When the partial pressure of CO₂ falls, it drives the reaction to the left and converts bicarbonate back to carbon dioxide to be breathed out.

So we started by learning RBCs have an enzyme called carbonic anhydrase that catalyzes "our equation".  Then we learned that bicarbonate is how the majority of CO₂ is carried in the bloodstream, and it is formed by that enzyme and that equation.  So that's it, right?

Nope.  Eventually we discuss the digestive system.  Parietal cells lining gastric pits also express the enzyme carbonic anhydrase.  So what happens in those cells?  You guessed it .... "when carbon dioxide dissolves in water ... " In the stomach, the point isn't to make bicarbonate, the point is to make the acid.  The hydrogen ion moves down its concentration gradient into the lumen of the stomach where it forms HCl - hydrochloric acid.  So stomach acid forms from the same mechanism, "our equation".

Then once the stomach contents reach the duodenum, the pH needs to be neutralized.  The pancreas secretes bicarbonate to neutralize the stomach acid.  Don't be surprised when I tell you that pancreatic acinar cells express the enzyme carbonic anhydrase which help them produce the bicarbonate that is secreted.

So if you didn't learn it initially, and you didn't learn it with the respiratory or digestive systems, you might think you could get away with not memorizing or understanding the equation.  Then ....

In the renal system chapter, we review a number of figures that appear to be a complicated mess of transporters.  In one set of figures, though, we see something familiar.  We see the carbonic anhydrase enzyme.  We see .... our equation.  And suddenly, it is something familiar in a complicated process.

Renal tubule cells also express carbonic anhydrase.  They also do "the equation".  The bicarbonate ion is important for blood pH homeostasis, so it is reabsorbed into the bloodstream.  The acid is pretty much a waste product and can be excreted in the urine.

By this point is it usually April or November.  Did you ever think you would understand a chemical equation so well?  But wait, there is another chapter to go...and we end up coming full circle.

Because this last chapter is about acid-base balance in the blood.  About respiratory acidosis and alkalosis, caused when the respiratory system doesn't expel CO₂ correctly. About metabolic acidosis and alkalosis, which are compensated for by changes in breathing.  We have come back to the beginning of A&P II, to the Blood and Respiratory chapters.  But we have also come back to the beginning of A&P I, to pH and acids and bases and buffers.

"Our reaction" is reversible.  High P_CO2 drives the reaction to the right, low P_CO2 drives the reaction to the left.  These are facts that we memorized in August or January.  Now we can use them to understand the body's reaction to acid-base imbalance, one of the most complicated topics of the semester.

This post is getting a little long, so I think I will save the details of acid-base balance for another time.  Suffice it to say that we can "blow off" CO2 to lower the partial pressure to restore pH imbalances.

"Our equation" is an efficient little reaction.  It uses one enzyme to convert a waste product into bicarbonate and hydrogen ion.  But it is more than that.  It is how the blood carries carbon dioxide through the bloodstream.  It is how stomach acid gets made, and then neutralized.  It is how kidney tubules retain bicarbonate and dump acid.  It is how we maintain blood pH homeostasis.  One equation, but a multitude of homeostatic and regulatory functions in the human body.

So readers, do you find it fascinating?  What question do you have about The Big Equation?

Resources for Studying "The Bones"

This week begins a period of study in A&P I lab that will last for several weeks.  We are beginning to learn the bones and bone markings of the skeletal system.  Here are some resources to help you with your study.

Print Resources:

Likely students will start with the textbook as a source of information about the bones and bone markings, and this is an excellent place to start.  Your textbook has a number of figures and exhibits that name the bones and label the bone markings.  Remember that you are responsible for the ones on your "Bones List" handed out in lab (and posted on Bb).  If you have the three-ring binder version of the textbook, you can just pull out chapters 7 and 8 to carry with you with the labeled bones.  If you have the bound version of the textbook, that is usually more cumbersome to carry around.  For this reason, the package in the bookstore with the bound version of the textbook also contains an atlas of the skeletal system with a red cover.  That will be easier to carry around to labs, open labs, and study groups.

Your lab manual is another print resource that you have available.  The lab manual also shows photographs of all the bones and bone markings.  However, most of the markings are not labeled.  This is a good way to get active with learning the markings from a print resource, by working on writing in the labels or quizzing yourself to see what you already have mastered.

Online Resources:

Several textbook publishers have online content that includes labeling exercises and other tools for studying bones and bone markings.   Some of these links are also posted in Blackboard.  McGraw Hill has some good labeling exercises associated with one of its A&P textbooks.  And of course, the Wiley Plus access that many of you have purchased will also have lots of labeling activities.

For learning the general terms for bone markings, Quizlet has a series of flashcards for learning those terms.

Apps: (some of these are iOS only and some are also for Android).

Winking Skull is a website that also has an app.  It has quite a bit of anatomy content on topics including the bones.  Thieme, the company that produces Winking Skull, also has their anatomy atlas available on the Epocrates app.  Anatomy and Physiology Revealed has an iPad app.  You can also do a web search for apps and there are several review articles like this one. 


There will be no substitute in the next three weeks for attending labs and as many open lab times as you possibly can.  No amount of studying a photograph or practicing online labeling exercises will replace study time with the actual bones that will be displayed on the practical exam.  You can't pick up a photograph and turn it around like you can the bone models.  Despite the many study tools you have, the best one will be the bone models in the lab.

Twitter - Suggestions for Using Academically

I have already written a post about how to use Twitter to create a “learning network”.  This post will be more helpful if you are brand new to Twitter.

Twitter is a social media tool.  You “follow” people, and the things they “tweet” appear in your timeline.  You can access Twitter from any computer with an internet connection and a web browser.

To get started on Twitter, first create your Twitter account.  Think of a Twitter handle.  Some people do Twitter totally anonymously, others use real names.  Because your handle appears in the tweet when people reply to you, you will want to keep it fairly short.  As with email addresses, if you expect peers or professors to see this, try to make it professional. 

Next: start following people.  It is easy to follow and unfollow people.  In the beginning, I followed tons of people.  Then when I found myself skipping over their tweets in my timeline, I realized that I wasn’t that interested in what they had to say.  As you use Twitter, you can edit the people you follow so that your timeline is filled with things that are interesting to read.  Where to get ideas of who to follow?  Twitter will make suggestions of people similar to who are already in your timeline.  Some of these are more accurate suggestions than others.  One source of good people to follow is to see who other people follow.  If you have come across someone interesting, see who the other people are who follow them, or see the list of people they follow.

Eventually, you figure out hashtags.  These are ways of marking tweets with a subject tag.  For example, I suggest that when you ask questions about lecture content, you mark your question tweets with the hashtag #BIO139.  Then it will be easy for me to search for the tweets that have that hashtag in them, to make sure that I haven’t missed any.  Hashtags start with the pound sign (#) and can’t contain spaces.  So sometimes you see hashtags like #thingsthatareboring

You can "star" a tweet, or mark it as a favorite.  Some people use that function similar to a "like" on Facebook.  I use it to bookmark tweets that have links that I want to come back later and read (see later paragraph on sending those tweets to Read It Later/Pocket).

To be perfectly honest, I find the actual Twitter website to be my least favorite way to access Twitter.  Instead, I use a program called TweetDeck.  Using Tweetdeck, I can manage both my personal and my professional Twitter accounts.  I can view tweets from the #BIO139 hashtag, or “@ replies” directed toward me, or tweets related to other searches.  Tweetdeck displays tweets in columns, so you can see different types of tweets at once.  TweetDeck also has an app for a smart phone, so you can view and send tweets from your phone.

And while I am talking about apps, here is one really cool way that I manage information that I find on Twitter.  By using a website called "If This Then That" and an app called Pocket (used to be Read It Later) you can more easily track and read the articles and links that are in your timeline.  Basically you tell the If This Then That website your Twitter logon information and your Pocket/Read It Later logon information.  That links the two accounts.  The next time you "star" a tweet in your timeline that contains a link, that link will be sent to Read It Later/Pocket.  So when you have some time (waiting in line, etc) you can open Pocket/Read It Later and catch up on the article and links that you have starred.

One aspect of Twitter that I am getting ready to experiment with is the function called Lists.  Lists are a way to organize the various types of people that you follow on Twitter.  For example, I follow a number of higher education organizations.  Perhaps grouping those into a list might be a better way to follow what those organizations are doing.  If I get the lists to work, I might write that into a new post.

So what about you, dear readers?  What questions do you have, or what ways do you use Twitter that you find to be helpful?