Good Answers to Essay Questions on Exam III - Bio 242 Spring 2010
Explain how hyperventilation, hypoventilation effect the pH of the blood. Be sure to include any
important chemicals and equilibria in your answer. Continue on the reverse side if more space is
needed (but indicate with an arrow that the reader should turn the page).
Hyperventilation is breathing deeply and rapidly. Hypoventilation is breathing slowly and
shallowly. both affect the pH of the blood because they affect the CO2 levels in the blood. CO2 travels in
the blood plasma as bicarbonate (HCO3-) and hydrogen ions (H+). When it leaves the tissue it diffuses
into the red blood cell where it is converted from CO2 and H2O to carbonic acid (H2CO3) and then to
bicarbonate and hydrogen ions by the carbonic anhydrase enzyme. The equilibrium is as follows:
CO2 + H2O <--> H2CO2 <--> HCO3- + H+.
When you hyperventilate you blow off a lot of CO2 and LeChatellier's Pinicple states that when
you increase the reactants on one side of an equation you tend to push the equilibrium towards the other
side, and when you decrease the reactants one side, you pull the equilibrium towards that side. So a
decrease in CO2 levels with hyperventilation would result in an decrease of H+ concentration in the blood
and an increased blood pH (alkalosis). Hypoventilation would have the opposite effect. You would
conserve CO2 which would cause the hydrogen ion concentration to increase and the blood pH would be
more acidic (acidosis).
[Why this is a good answer: The student clearly stated what happens to blood CO2 levels as a result of
hyperventilating and hypoventilating (half of the credit - 3 pts). The student also explicitly wrote out or
described the equilibrium between CO2 and H+, and identified how an increase or decrease in CO2 led to
increases or decreases in H+ (other half credit- 3 pts). As a bonus, the student included the location and
name of the enzyme that catalyzes this conversion.]
-------------------------------------------------What is the Bohr Effect? What chemicals, molecules, or structures are involved in creating this
effect? What conditions must be present to see or not see this effect? Be as thorough and complete
as possible to get full credit.
The Bohr Effect states that a decrease in pH weakens the hemoglobin-oxygen bond, allowing for the offloading of O2 into tissues. A decrease in pH can be attributed to a high concentration of CO2 entering the
bloodstream in the tissues. Increased CO2 releases H+ ions due to the reaction: H2O <--> H2CO2 <-->
HCO3- + H+. An increase in H+ ions leads to a decrease in pH and allows O2 to unbind from heme and to
travel into the tissues.
OR
In areas where the blood is very acidic and large amounts of CO2 are present, the hemoglobin-oxygen
bond weakens (known as the Bohr Effect). Because increased amounts of CO2 and acidic conditions
cause a modification of the hemoglobin molecule, it decreases its affinity to oxygen. As a result,
unloading of oxygen is increased in areas where it is mostly needed (in the tissues). In areas such as the
lungs, oxygen unloading by hemoglobin is decreased [or, hemoglobin has a higher binding affinity]
because of less acidic conditions there (less CO2).
[These are good answers because they identify the Bohr Effect as a decrease in oxygen-binding to
hemoglobin when the pH is low. Temperature change and BPG concentrations are not part of the Bohr
Effect. These answers also identify a change in hemoglobin structure when the pH is low, and the low pH
is linked directly to high CO2 concentrations. Finally, both students said that low pH (high CO2)
conditions occur in the tissues where oxygen off-loading is most needed.