 Explain
oxygen dissociation for adult
and foetal haemoglobin and
myoglobin
 Know
the structure of haemoglobin amd
myoglobin
 Know what is dissociation curve
 Investigate dissociation curves for
haemoglobin and myoglobin
 Describe
the structure of haemoglobin
and myoglobin
 Write the differences between two of
them
Haemoblobin
Structure
Description
Myoglobin

The oxygen dissociation curve is a graph that plots
the proportion of haemoglobin in its oxygen-laden
saturated form on the vertical axis against the partial
pressure of oxygen on the horizontal axis. The curve
is a valuable aid in understanding how the blood
carries and releases oxygen and it is a common theme
that is tested on in many medical examinations.

At high partial pressures of oxygen, haemoglobin
binds to oxygen to form oxyhaemoglobin. All of the
red blood cells are in the form of oxyhaemoglobin
when the blood is fully saturated with oxygen. Each
gram of haemoglobin can combine with 1.34 mL of
oxygen. At low partial pressures of oxygen (e.g.
within tissues that are deprived of oxygen),
oxyhaemoglobin releases the oxygen to form
haemoglobin.

The oxygen dissociation curve can be shifted
right or left by a variety of factors. A right
shift indicates decreased oxygen affinity of
haemoglobin allowing more oxygen to be
available to the tissues. A left shift indicates
increased oxygen affinity of haemoglobin
allowing less oxygen to be available to the
tissues.

A decrease in the pH shifts the curve to the
right, while an increase in pH shifts the curve
to the left. This occurs because a higher
hydrogen ion concentration causes an
alteration in amino acid residues that
stabilises deoxyhaemoglobin in a state (the T
state) that has a lower affinity for oxygen.
This rightwards shift is referred to as the
Bohr effect.

A decrease in CO2 shifts the curve to the left,
while an increase in CO2 shifts the curve to the
right. CO2 affects the curve in two ways. Firstly,
accumulation of CO2 causes carbamino
compounds to be generated, which bind to
oxygen and form carbaminohaemoglobin.
Carbaminohaemoglobin stabilizes
deoxyhaemoglobin in the T state. Secondly,
accumulation of CO2 causes an increase in H+
ion concentrations and a decrease in the pH,
which will shift the curve to the right as
explained above.

An increase in temperature shifts the curve to
the right, whilst a decrease in temperature
shifts the curve to the left. Increasing the
temperature denatures the bond between
oxygen and haemoglobin, which increases
the amount of oxygen and haemoglobin and
decreases the concentration of
oxyhaemoglobin. Temperature does not have
a dramatic effect but the effects are
noticeable in cases of hypothermia and
hyperthermia.
Factor
Decrease
Increase
pH
CO2
Temperature
2,3-DPG
Right shift
Left shift
Left shift
Left shift
Left shift
Right shift
Right shift
Right shift

Fetal haemoglobin (HbF) is the main oxygen transport
protein in the human fetus during the last 7 months
of development. It persists in the newborn until
roughly 6 months of age. HbF has different globin
chains to adult haemoglobin (Hb). Whereas adult
haemoglobin is composed of two alpha and two beta
subunits, fetal haemoglobin is composed of two
alpha and two gamma subunits. This change in the
globin chain results in a greater affinity for oxygen
and allows the fetus to extract blood from the
maternal circulation. This increased affinity for
oxygen means that the oxygen dissociation curve for
fetal haemoglobin is shifted to the left of that of
adult haemoglobin.

The curve for myoglobin lies even further to
the left than that of fetal haemoglobin and
has a hyperbolic, not sigmoidal, shape.
Myoglobin has a very high affinity for oxygen
and acts as an oxygen storage molecule. It
only releases oxygen when the partial
pressure of oxygen has fallen considerably.
The function of myoglobin is to provide
additional oxygen to muscles during periods
of anaerobic respiration.