1. Gas Exchange Part 2: Gas Exchange and Oxygen Dissociation
Topic 6.1
&
Option D.6 (cont.)

2. Oxygen-Carrying Molecules
Hemoglobin
-Composed of FOUR polypeptide chains (quaternary structure!)
-Each polypeptide chain has an iron-containing heme group that is able to REVERSIBLY bind
with oxygen
-Binding of oxygen to hemoglobin is cooperative – one molecule (of O2) binding changes the
shape so it’s easier for another oxygen to bind, which changes the shape again so it’s even
easier for another molecule to bind and so forth (up to four per hemoglobin).
-The opposite of this is also true! As O2 molecules are released from hemoglobin, the shape
changes, making it easier for other O2 molecules to be unloaded as well
Myoglobin
Composed of only ONE polypeptide chain with an iron-containing heme group (able to bind reversibly with oxygen – NO cooperative binding though b/c only one heme group, so binds only one oxygen molecule at a time)
Binds to O2 and stores it IN muscle cells (acts as an “oxygen reserve”)
Able to provide O2 to muscle cells when O2 in blood is very low; delays anaerobic respiration

3. Oxygen Dissociation Curves
An oxygen dissociation curve shows the relationship between the partial pressure of oxygen and the % saturation of oxygen-carrying molecules in the body (hemoglobin and myoglobin)
Note: a partial pressure is the pressure of a gas when it is found in a mixture of gasses
In general:
When the partial pressure (concentration)
of oxygen is LOW (at the cells/ tissues in
the body), the saturation of O2 carrying
molecules is also LOW (they are releasing
their oxygen to respiring cells/ tissues –
they are not holding onto O2, so they are
NOT very saturated!)
of oxygen is HIGH (at the alveoli/ in the lungs), the saturation of O2 carrying molecules is also HIGH (they are taking up oxygen from the alveoli in the lungs)

4. Oxygen Dissociation Curves for Adult Hemoglobin, Fetal Hemoglobin and Myoglobin
Shows a sigmoidal (S-shaped) curve (due to its cooperative binding of oxygen molecules)
Low saturation at low pressures (pO2); High saturation at high pressures (pO2)
Fetal hemoglobin:
Hemoglobin molecules have slightly different shape (molecular structure) than adult hemoglobin, making them have a HIGHER AFFINITY for oxygen (they bind it more readily/ easily)
Sigmoid shape dissociation curve but shifted LEFT (b/c it has a higher oxygen affinity)
Higher affinity for oxygen ensures that oxygen moves from adult (mom’s) hemoglobin to fetal hemoglobin in the capillaries of the uterus
Myoglobin:
Higher affinity for oxygen than hemoglobin (saturated at extremely low O2 concentrations, so able to store oxygen in muscle cells no matter what concentrations are in body) – NOT an S-shaped curve b/c NO cooperative binding, like in hemoglobin. Only binds ONE O2 molecule at a time (only one heme group per myoglobin)
Releases oxygen to muscle cells when levels of O2 in blood are extremely low (from intense exercise), allowing aerobic respiration to continue/ delaying anaerobic respiration!

5. Carbon Dioxide and the Blood
Carbon dioxide is carried in the blood (from the tissues to the lungs) in one of three ways:
1. Bound to hemoglobin
2. Dissolved in the blood plasma
3. In erythrocytes (RBC’s) as carbonic acid (85% of CO2 carried this way!):
a. CO2 diffuses into erythrocyte
b. CO2 combines with water to form carbonic acid, which is more soluble (H2CO3) – this reaction is
catalyzed by carbonic anhydrase
c. Carbonic acid dissociates into H+ ions and bicarbonate ions (HCO3-)
d. Chloride shift: Bicarbonate ions are pumped OUT of erythrocytes and Cl- ions are pumped in
(overall charge remains the same)
e. Bicarbonate ions combine with sodium ions in the blood plasma(NaHCO3) – these are carried
to the lungs
f. H+ ions in the erythrocyte lower the pH, causing hemoglobin to release oxygen (to respiring
cells/ tissues)
g. Hemoglobin binds excess H+ ions to
restore pH in the erythrocyte (H+
released in lungs)

6. The Bohr Shift and Oxygen Dissociation Curves
Respiring tissues release MORE CO2 into the blood, increasing the partial pressure of CO2 (pCO2) in the blood
Increased partial pressures of CO2 in the blood shift the oxygen dissociation curve to the RIGHT (the Bohr shift!) – MORE O2 is released to respiring cells/ tissues!
This shift (the Bohr shift/ effect) promotes the release of more oxygen to respiring cells/ tissues
HOW?!?
More CO2 in the blood LOWERS the pH of the blood (it becomes more acidic)
CO2 diffuses into RBC’s and is converted into
carbonic acid
Carbonic acid dissociates into H+ ions and HCO3-
ions
H+ ions in the RBC bind to hemoglobin (causing it to
have a LOWER affinity for oxygen, so O2 is released)
When one oxygen molecule is released, hemoglobin
changes shape, making it easier for other O2
molecules to be released as well
The Bohr shift ENSURES that respiring tissues have enough O2 when they need it the most!

7. Gas Exchange at High Altitudes
At higher altitudes there is lower air pressure (less O2
in air = lower partial pressure of O2)
Lower pO2 makes it more difficult for hemoglobin to take up oxygen (lower pO2 = lower % saturation in the lungs/ alveoli) – tissues and cells receive LESS oxygen!
Symptoms of low oxygen intake:
Breathlessness, headache, fatigue, rapid pulse, nausea
Over time, the body IS able to acclimate to lower O2 in the air! HOW?
1. RBC production increases (more RBC’s = more hemoglobin = more O2 transport)
2. RBCs are produced with more hemoglobin molecules in them (these also have a
higher affinity for oxygen)
3. Ventilation rate increases (more gas exchange)
4. Muscles produce more myoglobin (capillaries become more dense too – more O2
diffusion into cells and binding by myoglobin)
5. Increased heart rate (available O2 circulates around body faster)
6. Greater lung surface area (larger chest size – if living permanently at high alt.)