Section 12.1 – Objectives
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
• List the three ways the body regulates against increases in free hydrogen ions
• 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
• Describe how the kidneys regulate pH by excreting H and reabsorbing
• Define alkalosis and acidosis.
• Describe the types of alkalosis and acidosis and their causes
Section 12.2 – Introduction
• In this module, we will look at the relationship between hydrogen ions (H ) and
• 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.
Section 12.3 – 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.
Section 12.4 – The HydrogenAtom and Hydrogen Ion
1 • 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.
• Ahydrogen ion (H ): +
It’s a hydrogen atom that has lost its electron, leaving only the positively
In some situations, the hydrogen ion may simply be called a proton.
Section 12.5 – What AreAcids and Bases?
• An acid:
It’s any molecule that will release hydrogen ions when put in a solution.
For example, hydrochloric acid (HCl) will break apart (or
dissociate) into free hydrogen ions (free H ) and chloride ions (Cl )
as shown at right.
o It is the presence o+ the free H that makes a solution acidic
– the more free H , the more acidic the solution, and visa
Will dissociate rapidly and release large amounts of H in solution.
HCl is a strong acid.
Any molecule that will accept a hydrogen ion.
For example, bicarbonate ions (HCO , the most important base in
the body) will bind with H to form carbonic acid (H CO2), a3
shown at right.
Bases lower the concentration of free H in solution by combining with the
With less free H , the acidity of the solution will decrease and become
more basic or alkaline.
Section 12.6 – The pH Scale
• It’s very important to know the concentration of free H in solution – in the body
you don’t want too many H ions or too few.
• The pH scale:
Away of quantifying the concentration of H in any solution.
• pH = 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 versa.
• 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.
Definition: pH = - log [H10 +
Low [H ] Alkaline/Basic High pH
High [H ] Acidic Low pH
Section 12.7 – The pH Scale (cont’d)
• The pH scale:
Goes from 0 to 14 with neutral pH at 7.
Asolution with a pH below 7 is considered acidic (remem+er, the lower
the pH the more acidic the solution, or more free H ).
Asolution with a pH above 7 is alkaline or basic (the higher the 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 7.4.
Note that this is slightly alkaline.Arterial blood has a pH of 7.45
while venous blood has a pH of 7.35.
Describe body fluids when the pH is below 7.4
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.
3 Section 12.8 – The Source of Acid in the Body
• Since maintaining the proper pH in the body is essential for life, we must now
consider the sources of acids in the body – what are they and where do they come
• When cells in the body make energy (ATP) they will produce CO as a byp2oduct.
• As we saw in module 10 – The Respiratory System, this CO can, wi2h the help of
the enzyme carbonic anhydrase, combine with water in red blood cells to produce
carbonic acid, H C2 , 3s shown in the reversible reaction below.
• The carbonic acid will dissociate into free H and bicarbonate ions, HCO . −
• In the lungs, the reaction will then reverse – carbonic acid will reform, it will
convert to CO and H O and the CO will be removed and exhaled.
2 2 2
As a result, there is generally no NET increase in free H in the plasma.
Because the carbonic acid reforms into CO , which 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
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
4 Section 12.9 - Regulation of Hydrogen Ion Concentration
• Because of the various sources of acids mentioned on the previous page and the
negative effects that these acids can have on cellular reactions, the body must
have a way to deal with potential increases in free H .
• The body regulates free H concentrations by the three mechanisms shown below:
2. the respiratory system
5 Section 12.10 – Regulation of H Concentration: Buffers
It’s 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:
X + H XH
• This reaction shows that the buffer, called “X,” combines
with free H to make XH.As a result, with less free H , the
pH of the solution is stabilized.
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 already seen include bicarbonate ions and
Section 12.11 – Regulation of H Concentration: Buffers (con’d.)
• Free H can bind with buffers in both the intracellular and extracellular fluid.
6 • Intracellular buffers include:
Phosphates (which won’t be covered here) and
Intracellular proteins, such as hemoglobin (Hb) inside red blood cells.
• The most powerful extracelluar buffer is the bicarbonate ion, HCO . 3−
Recall that inside red blood cells, Hb can reversibly bind with free H to +
help stabilize the acidity inside the RBCs.
Hemoglobin can also bind CO to reduc2 the potential acidity should the
CO c2mbine with H O to 2orm carbonic acid, H CO . 2 3
• Recall that the buffers mentioned above do not directly remove free H from the
• Buffers only tie up the free H until they can be removed in another manner, in
this case by the respiratory system. Let’s look at this regulatory system now.
Section 12.22 – Regulation o