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Saturday, August 10, 2013

Acid Balance Part 1: Important concepts

References - for this post and the following posts.......I will probably add more, but these are the main sources that are a good starting point if you want to do your own reading.

Faubel, S., & Topf, J. (1999). The Fluid Electrolyte and Acid-Base Companion (1st ed.). Alert and Oriented Publishing Co.
- Note: just like Hinchcliff, with some searching on the internet it *may* be possible to find the book for free in an e-version. 

Hinchcliff, K., & Geor, R. (2008). Equine Exercise Physiology: The Science of Exercise in the Athletic Horse. (1st ed.). Saunders Elsevier.

Reynolds, G. (2012). The First 20 Minutes - Surprising Science reveals how we can exercise better, train smarter, live longer. Hudson Street Press.

And now to get started!

I have put more hours into researching this topic than any other subject I have introduced on the blog to date.  And yet, I feel like my understanding is barely sufficient for me to compose a post on it.

In my experience, the more confusing the literature is, the less we actually know about the subject. 

I tend to avoid those subjects on the blog. Because it’s a lot of hard work to sort through the literature, read expert opinions and reviews on the literature since the studies all contradict each other, and then come up with something concise, interesting, and accurate to post here.  But this is an important subject that enough people expressed interest in me doing so, here it goes.

Turns out that acid-base balance, especially in the exercising endurance horse... especially if you are looking at how it changes in response to conditioning..... and especially if you are looking for real world applications to the research is, well.......not exactly clear.  And everyone is contradicting everyone else, and there’s lots of caveats to everything, and it’s quite confusing.

So, here it is.  My best guess at acid and base balance (along with the intimately connected topic of body water) in an exercising horse, and hopefully enough information that you can make your own informed decisions on managing your own endurance horse. 

Important Concepts

We are going to start with “important concepts”.  These are concepts that will probably be familiar if you took chem and bio in school, but you might need a refresher on.  It is very very very important that you are comfortable with these concepts because EVERYTHING that relates to body water and acid base balance are based on these concepts. My hope is that all my upcoming posts are accessible to anyone, no matter what their background, so if this all seems rather simple to you than feel free to skim....

Important Concept 1 - Compartments

Body Water is kept in compartments. 

(All drawings blow up if you click on them)

There are 2 compartments - the intracellular compartment (ICC) which represents all the water inside cells and the extracellular compartment (ECC) which represents all the water outside cells.  The ECC has two “sub compartments” - the interstitial and plasma.  Interstitial is the fluid surrounding cells, that is outside of the blood vessels.  Lymphatic fluid is a type of interstitial fluid.  Plasma is water inside of blood vessels.  Plasma is NOT cells - it is the liquid that all the cells and other “stuff” is dissolved in.  Red blood cells are IN the plasma, but not part of the plasma.  Plasma proteins are IN the plasma, but not part of the plasma. 

Important Concept 2a - Barriers

There are 2 barriers
1. The cell membrane divides the ICC from the ECC. 
2. The blood vessel (BV) wall divides the interstitial fluid from the plasma.  The blood vessels that are represented in the diagrams and that are most relevant to this discussion are mostly capillaries - the really small vessels. 

A simplified diagram that shows the 2 compartments and the 2 barriers looks like this (yes, I know I’m calling the cell membranes “walls”, but once I got it done, I really didn't want to redo it):


The two barriers have very different properties. 

Think about the blood vessel membrane as a decorative fence and the cell membrane as a chain link fence.  The decorative fence allows much more stuff to flow back and forth than the chain link fence. 

The cell membrane and the BV membrane both allow water to flow across them.  So, water moves freely from the ICC to the ECC (both areas) and back and forth as it needs to. 

Important Concept 2b - Barriers and Solutes
Solutes are the “stuff” that is dissolved in the fluid of each body compartment.  This “stuff” is electrolytes (elytes), proteins, non-elytes etc.

Some stuff can go through the dividers, and some can’t. 

Water can go through both dividers. (notice this is the second or third time I’ve mentioned this.  This is because it’s IMPORTANT).

Potassium and Sodium cannot go through the divider between ICC and ECC.  Most of the potassium is trapped in the ICC (99%), most of the sodium is trapped in ECC. 

The sodium and potassium in the ECC can move between ECC compartments (plasma and interstitial). 

Although the “decorative fence” that splits the ECC into plasma and interstitium let’s a lot through.....there are some things like plasma proteins that can’t get through and are trapped in the plasma. 

So, to recap:

ICC: High in Potassium, Low in Sodium
ECC: High in Sodium, low in potassium
ECC/plasma: High in Plasma protein
ECC/interstitial: Low in plasma protein

WATER: can move through ALL barriers and distribute to ALL compartments.  How much water ends up in each compartments depends on how many solutes are in each compartment. 

Which brings us to “important concept 3”

Important Concept 3: Solutes and Water movement

The number of solutes dissolved in the water is called “osmolality”.  Think about this as the “concentration of solutes”. 

Water will follow solutes and distribute in such a way that the concentration of solutes will be equal in the different compartments.

Don’t worry if that hasn’t completely sunk in.  We are going to go over the concept in small bites with pictures. 


Imagine you have 2 containers with the same amount of water.  To each container you try to add the same amount of a substance - we will call our substance “solute”. 

To your horror, you accidentally add TWICE as much to one container as the other one.  The second container has way more solute in it, and thus is more concentrated.  :(

How are you going to fix it? 

You have 2 options.

Option 1 - Move the solute around so that each container has the same number of solutes. 

Option 2 - Move the water around so that each container has the same number of solutes.

In both options, the ending concentration of solutes per water ends up equal!!!

Instead of 2 separate containers, imagine them joined together, and instead of having to “transfer” water and solutes between them, imagine a divider or membrane between the containers that will allow water+solutes or just water through. 


Can you see that water follows solutes?  Water goes where there is a greater concentration of solutes. 

Whenever there is an imbalance, the solutes try to move first.  However, remember that the cell membrane divider between ECC and ICC doesn’t allow K and Na (which are solutes) to pass between freely.  So, the body uses the movement of water to correct concentration imbalances between compartments.

During exercise we aren’t usually adding solutes to the compartments - the problem is that water and solutes are leaving in the form of sweat!!!!!

Here is a simplified example that brings together all the concepts in this section
a. solutes try to equalize across the dividers if they can get through
b. water can move across both dividers to try and equalize concentrations of solutes in all compartments.

Of course, this is simplified, and we will talk later about how elytes and fluids move through the different compartments during exercise.  But hopefully this illustration helps cement the important concepts we have discussed so far. :). 

PS - to EnduranceGranny and anyone else who is having trouble searching the blog - I have no idea what is wrong with the widget on the sidebar.  It used to search the blog and now it doesn't.....I've tried removing and reinstalling it to no avail.....So unless anyone has any bright ideas, it looks like my blog search function will only do that last couple months or so :(, until I have the time to devote to a fix, which doesn't seem likely since school starts Monday.........


  1. Hello,
    I wanted to compliment you on your blog. I am an MD, retired and now teaching basic human physiology to future health professionals. This is one of the most difficult concepts to teach. You have done a remarkable job of making this easier to understand. I plan on using it as a reference to give to my students.

  2. Thank you! What a wonderful compliment. I just had this subject for real in a third year vet school class and I think I could write it even better now (the later posts on this subject) because I understand it better. I feel like if you can't explain it simply, you don't understand it well it's my continuing goal to make physiology fun and understandable to anyone (because I'm convinced that anyone that doesn't find physiology wonderful just hasn't had it explained well enough!). I'd love to teach some day, so thanks again for the kind words.


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