Module 12: Acid-Base Balance
By the end of this section, you should be able to:
• Define acid and base.
• Describe pH, how it is calculated, and the normal pH of the body.
• Describe and understand the relationship between free hydrogen ion concentration,
acid, and pH.
• Explain what a volatile acid is, what a nonvolatile acid is, and where they come from.
• List the three ways the body regulates against increases in free hydrogen ions (H ).
• Explain how buffers work to help maintain pH when the levels of H change.
• Explain how the respiratory system regulates the levels of volatile acids in the body.
• Describe how the kidneys regulate pH by excreting H and reabsorbing bicarbonate ions.
• Define alkalosis and acidosis.
• Describe the types of alkalosis and acidosis and their causes. Introduction
In this module, we will look at the relationship between hydrogen ions (H ) and acidity.
We will also define what acids and bases are, what the pH scale is, and why it is
important to maintain H at precise levels.
We will also look at the different mechanisms in the body for regulating H
concentrations (and thus pH) and we will also learn about some of the things that can
alter the pH of the body.
Why is this important?
Many chemical reactions that take place in the body, and most of the machinery inside
all the cells, are very sensitive to the presence of hydrogen ions.
Hydrogen ions (H ) can alter the shape of proteins that act as enzymes that speed up
chemical reactions. As a result, any change in the concentration of H will affect the
activity of almost every cell.
Therefore, it is essential that H concentration be very carefully regulated.
The Hydrogen Atom and Hydrogen Ion
In order to better understand what an acid is we must first look at the hydrogen atom
and hydrogen ion.
The hydrogen atom has a single proton, which is positively charged, and a single
electron, which is negatively charged. The result is an electrically neutral element.
A hydrogen ion (H ), on the other hand, is a hydrogen atom that has lost its electron,
leaving only the positively charged proton. In some situations, the hydrogen ion may
simply be called a proton.
What are Acids and Bases?
An acid is any molecule that will release hydrogen ions when put in a solution.
o For example, hydrochloric acid (HCl) will break apart (or dissociate) into free
hydrogen ions (free H ) and chlo+ide ions (Cl ). +
o It is the presence of the free H that makes a solution acidic—the more free H ,
the more acidic the solution, and visa versa.
o A strong acid will dissociate rapidly and release large amounts of H in solution.
HCl is a strong acid.
On the other hand, a base is any molecule that will accept a hydrogen ion.
o For example, bicarbonate ions (HCO , 3he most important base in the body) will
bind with H to form carbonic acid (H CO ), as shown at right.
2 +3 +
o Bases lower the concentration of free H in solution by combining with the H .
o With less free H , the acidity of the solution will decrease and become more
basic or alkaline The pH Scale
The pH scale is a way of quantifying the
concentration of H in any solution.
The definition of pH is very complicated—pH
is the negative logarithm (to the base 10) of
the hydrogen ion concentration.
Essentially, because it is a negative logarithm,
the more free H , the lower the pH and vice
It is very important not to get confused
between the terms H concentration, acidity,
and pH. Keep in mind that the more free H
there are in solution, the more acidic the
solution and the LOWER the pH.
The pH scale, shown at right, goes from 0 to
14 with neutral pH at 7.
A solution with a pH below 7 is considered
acidic (remember, the lower the pH the more
acidic the solution, or more free H ).
A solution with a pH above 7 is alkaline or basic
(the higher th+ pH the more basic the solution,
or less free H ).
The normal pH of body fluids varies slightly
between 7.35 and 7.45 and has an average of
o Note that this is slightly alkaline.
Arterial blood has a pH of 7.45 while venous blood has a pH of 7.35.
Acidosis is a term used to describe body fluids when the pH is below 7.4, while alkalosis
occurs when the pH is above 7.4. When the pH of body fluids is below 6.8 or above 7.8
for long periods of time, death will occur.
The Source of Acid in the Body
When cells in the body make energy (ATP) they will
produce CO as2a byproduct. As we saw in module
10, this CO 2an, with the help of the enzyme
carbonic anhydrase, combine with water in red
blood cells to produce carbonic acid, H CO2, a3
shown in the reversible reaction at right.
The carbonic acid will di−sociate into free H and
bicarbonate ions, HCO . 3
In the lungs, the reaction will then reverse—
carbonic acid will reform, it will convert to CO 2nd
H O and the CO will be removed and exhaled.
2 2 +
As a result, there is generally no NET increase in free H in the plasma. Because the
carbonic acid reforms into CO , 2hich is then removed at the lungs, carbonic acid is
known as a volatile acid.
The metabolic breakdown of various
proteins will produce a number of acids
including sulphuric acid, phosphoric acid,
lactic acid, and other organic acids.
Also, the stomach is a large source of
hydrochloric acid. These acids cannot be
removed by the lungs and are therefore
called nonvolatile acids. These acids are a
significant source of free H and are
constantly being produced throughout
the body. As a result, they must be dealt
with in order to maintain a constant pH.
Regulation of Hydrogen Ion Concentration
The body regulates free H concentrations
by the three mechanisms shown at
right—they include buffers, the
respiratory system, and the kidneys.
Regulation of H+ Concentration – Buffers
A buffer is any molecule that can reversibly bind (or release) free H . +
Because buffers bind free H , and thereby reduce the amount of free H in solution+ they
help to stabilize the pH. The general reaction between a buffer and a free H is:
This reaction shows that the buffer, calle+ “X,” combines with free H
to make XH. As a result, with less free H , the pH of the solution is
It is very important to note that buffers do not prevent the pH from
changing, they only help to minimize any pH change until the free H +
can be removed from the body by either the lungs or kidneys.
Some of the buffers we have seen include bicarbonate ions and
Free H can bind with buffers in both the intracellular and
extracellular fluid. Intracellular buffers include phosphates (which won’t be covered
here) and intracellular proteins, such as hemoglobin (Hb) inside red bloo