Gas Exchange Problems

This problem set is designed to help you work through some important gas exchange concepts that will be on Midterm 1. You should view this lab activity as midterm practice. Please work with your lab group to answer these questions in lab; by discussing (and arguing about) your answers, you'll gain a deeper understanding of the ideas. Don't divide up the questions or do them out of order; you'll learn the most by doing them all in order.

Physiology is the nuts and bolts of how organisms work. Typically it involves a quantitative approach to figuring out what organisms can and can’t do. Physiology is critical in medicine, where things like oxygen concentration are matters of life and death. Quantitative approaches are also informative in comparative physiology, in which biologists compare different species in order to figure out general principles that apply to all kinds of organisms.

This lab is an introduction to quantitative physiology, emphasizing gas exchange and circulation. You’ll be given some background information and some equations, and asked to solve some problems. There’s only a little bit of math. Answer the questions during lab. Choose the one best answer for each question. You may discuss your answers with other students in the class, and you may use your textbook. When you’re done, put all your answers on a Scantron sheet and turn it in. Each person should turn in a Scantron. Please write “Gas Exchange” in the subject area of the Scantron.

Basic principles of gases

The two gases we’re concerned with here are carbon dioxide (CO2) and oxygen (O2). The basic problem that oxygen-using heterotrophic organisms need to solve is to have O2 diffuse into the body and CO2 diffuse out of it. The efficiency of this gas exchange is limited by some basic physical and chemical principles. For background information on this, you might want to look at Chapter 42 in Campbell Biology and the Gases & diffusion page on this site.

Composition of air:

  • O2: 20.95%
  • CO2: 0.03%
  • N2: 78.09%
  • Argon: 0.93%
  • Total: 100.00% (give or take a few minor constituents)

Note: the composition above is true for dry air; air may also contain varying amounts of water vapor. Solubility of O2 in seawater at 20°: 5.3 ml O2 per liter seawater when the seawater is in equilibrium with the atmosphere.

Pressure from being underwater:

If you go underwater, you’re subject to pressure from the water above; the deeper you go, the greater the pressure. Ten meters of water above you adds an extra 1 atmosphere to the 1 atmosphere normal air pressure that you’d feel at the surface. Your whole body will experience the pressure – including the air in your lungs. The air in your lungs at sea level is at approximately 1 atm; at 10 meters underwater, the air in your lungs would be at about 2 atm.

It’s not possible for the air in your lungs to stay at a pressure that is significantly different from the environment; whether you're at high altitude or deep below the ocean surface, the air in your lungs will be approximately the same as the ambient (environmental) pressure. If you go SCUBA diving, you'll carry a tank of pressurized air with you, but the regulator on the tank will allow this air to enter your lungs at at ambient pressure.

Questions

Correct answers are highlighted in orange.

Partial Pressures & dissolved gases

Suppose you open a bottle of Pepsi. Sodas like Pepsi have bubbles because they contain a lot of CO2; they are packed with pressurized CO2.

1. What happens to the CO2 gas in the top of the bottle at the moment when you open it?

  1. The PCO2 suddenly increases.
  2. The PCO2 suddenly decreases.
  3. The PCO2 remains unchanged.

2. What happens to the partial pressure gradient of CO2 between the gas-phase CO2 in the bottle and the dissolved CO2 in the Pepsi at the moment when you open the bottle?

  1. The partial pressure gradient increases suddenly, causing bubbles to form.
  2. The partial pressure gradient decreases suddenly, causing bubbles to form.

3. What happens to the dissolved PCO2 in the soda after the bottle has been open for a couple of hours?

  1. The PCO2 of the soda gradually increases as bubbles form.
  2. The PCO2 of the soda gradually decreases as bubbles form.

4. Suppose you’re breathing normally and sitting there staring at a cup of water that’s open on the table in front of you. Which will have a higher PO2 – the water in the cup or the freshly oxygenated blood in your arteries? (Assume that the water is in equilibrium with the air.)

  1. The water.
  2. Your blood.

5. In the above example, which will have a higher concentration of O2 as measured in ml O2 per 100 ml liquid?

  1. The water.
  2. Your blood.

A Diving Bell

A diving bell is an old-fashioned way of allowing people to breathe underwater. It is a big container, shaped roughly like a bell, closed on top and open on the bottom. With air trapped inside, it can be lowered down into the ocean. The air can’t escape, but it is in contact with the water at the bottom. Therefore, the air pressure inside the bell is the same as the surrounding water pressure.

A person can stay in the diving bell, breathing the trapped air. For the following questions, assume that you are in a diving bell that is lowered from the surface to a depth of 40 meters. There is no air supply to the diving bell, so you only have the air that’s trapped in the bell.

6. What would the air pressure in the bell be immediately after it was submerged to 40 meters?

  1. 0.1 atmosphere
  2. 1/4 atmosphere
  3. 1 atmosphere
  4. 4 atmospheres
  5. 5 atmospheres

7. What would the partial pressure of O2 be in the bell immediately after it was submerged to 40 meters? (Assume you haven’t used any of the oxygen yet.)

  1. 0.2 atmosphere
  2. 0.8 atmosphere
  3. 1 atmosphere
  4. 2 atmosphere
  5. 4 atmospheres

8. If the diving bell contained 1000 liters of air at the surface, what volume of air would it contain after it was submerged to 40 meters?

  1. less than 250 liters
  2. 250 liters
  3. 1000 liters
  4. 4000 liters
  5. more than 4000 liters

9. Suppose you spend some time in the diving bell, breathing the air, and then you decide to leave the bell and swim to the surface. What would happen to the partial pressure of oxygen in your lungs as you rapidly ascend? (Assume that your body is using very little oxygen.)

  1. The PO2 in your lungs would decrease as you ascend.
  2. The PO2 in your lungs would increase as you ascend.

10. What would happen to the volume of air in your lungs as you ascend, assuming you don’t allow any air to escape your lungs?

  1. The air in your lungs would expand, dangerously stretching your lungs to several times their normal size.
  2. The air in your lungs would expand somewhat. Since the volume of air in the lungs started out smaller than normal (due to the high pressure), your lungs would expand to approximately normal size as you approach the surface.
  3. The air in your lungs would shrink due to increasing pressure.
  4. The air in your lungs would not change in volume.

11. Now suppose your friend starts at the surface and dives down to where the diving bell is, at 40 meters depth, and then immediately swims back to the surface. She holds her breath the whole time and doesn’t breathe from the diving bell. What would happen to the volume of air in her lungs as she descended?

  1. The air in her lungs would expand, stretching her lungs to several times their normal size, and her lungs might burst like a balloon.
  2. The air in her lungs would shrink due to increasing pressure.
  3. The air in her lungs would not change in volume.

12. In the scenario described in the preceeding question, what would happen to the volume of air in your friend’s lungs as she ascended?

  1. The air in her lungs would expand, stretching her lungs to several times their normal size, and her lungs might burst like a balloon.
  2. The air in her lungs would expand somewhat. Since the volume of air in the lungs started out smaller than normal (due to the high pressure), her lungs would expand to approximately normal size as she approaches the surface.
  3. The air in her lungs would shrink due to increasing pressure.
  4. The air in her lungs would not change in volume.

Gas exchange, circulation, and hemoglobin

The following questions refer to normal breathing in sea-level air. (You might want to look at fig. 42.29 in Campbell Biology, 11th ed.)

13. Where would the PCO2 be highest?

  1. The air you exhale.
  2. The air in the alveoli of your lungs.
  3. The blood entering your alveolar capillaries.

14. Where would the PO2 be the highest?

  1. The air you exhale.
  2. The air in the alveoli of your lungs.
  3. The blood leaving your alveolar capillaries.

15. In a normal person, will the PO2 in active muscles be higher or lower than the PO2 in the blood that flows through the capillaries supplying the muscles?

  1. Higher.
  2. Lower.

16. Comparing figures 42.29 and 42.31 in Campbell (11th ed.), what would the % oxygen saturation be in the systemic veins (assume that blood pH at this point is 7.4).

  1. 90%.
  2. 70%.
  3. 50%.
  4. 30%.

17. If you hold your breath for two minutes, your blood would have ____________? 

  1. Higher pH than normal.
  2. Lower pH than normal.
  3. pH same as normal.

18. What percentage of blood oxygen is released to systemic tissues at a PO2 of 40 mm Hg and pH of 7.4?

  1. 10%.
  2. 30%.
  3. 50%.
  4. 70%.
  5. 90%.

19. What percentage of blood oxygen is released to systemic tissues at a PO2 of 10 mm Hg and pH of 7.2?

  1. 10%.
  2. 30%.
  3. 50%.
  4. 70%.
  5. 90%.

20. If the fetal blood and maternal blood are in equilibrium in the placenta, will the blood of the fetus be more or less saturated with oxygen than the blood of the mother?

  1. Fetal blood will be more saturated than the mother's blood.
  2. Fetal blood will be less saturated.

21. Is the PO2 in the systemic tissues of a fetus higher or lower than the PO2 in the systemic tissues of the mother?

  1. Fetal PO2 is higher.
  2. Fetal PO2 is lower.

COPD

Chronic Obstructive Pulmonary Disease (COPD) interferes with breathing. This disease is usually caused by smoking. One aspect of COPD is emphysema, in which the walls of the alveoli break down, allowing the alveoli to fuse and resulting in fewer, larger alveoli in the same volume of lung tissue. (Illustration from National Heart, Lung, and Blood Institute, public domain.)

COPD effect on alveoli

As you might expect, this severely impairs gas exchange. What effects would this have?

22. The blood PO2 of a person with emphysema would tend to be _____ than normal.

  1. Blood PO2 would tend to be higher.
  2. Blood PO2 would tend to be lower.

23. The alveolar air PO2 of a person with emphysema would tend to be _____ than normal.

  1. Alveolar air PO2 would tend to be higher.
  2. Alveolar air PO2 would tend to be lower.

24. The blood pH of a person with emphysema would tend to be _____ than normal.

  1. pH would tend to be higher (less acidic).
  2. pH would tend to be lower (more acidic).

Anemia

The term “anemia” refers to conditions in which the concentration of red blood cells or hemoblobin in blood is lower than normal. In laboratory testing, it is typically measured as grams of hemoglobin per deciliter of blood (g/dl). Anemia can have a variety of causes, but it results in a decreased ability to deliver oxygen to tissues.

Go to the blood gas calculator at prognosis.org, and make sure all the values are at default levels (click “Reset” at the bottom if necessary). The default value for Hb (hemoglobin) concentration is 15 g/dl. In anemia, this value could be much lower. How do you think this would affect the other values in the bar graph? Think about it first, before you change anything. The values shown in the bar graphs are:

Atmos
The oxygen partial pressure (PO2) of atmospheric air. (Hint: anemia isn't going to change this.)
PiO2
PO2 of inspired air (air that has been breathed into the bottom of the trachea). (hint: anemia won't change this, either.)
PAO2
PO2 of air in alveoli.
PcO2
PO2 of pulmonary capillaries. This is normally the same as alveolar gas, so in most cases,PcO2=PAO2.
PaO2
PO2 of oxygenated arterial blood.
PtO2
PO2 of systemic tissue capillaries. This is an "typical value;" actual values could be different among different tissues.
PvO2
PO2 of systemic venous blood. This is an "typical value;" actual values could be different parts of the venous system.

Note that the partial pressure values use units of kPa (kilopascals), while our textbook uses mm Hg. Here is a converter, in case you want to compare the numbers from the book with those on prognosis.org.

25. Using the calculator, change the Hb from 15 g/dl to 7 g/dl. Which of the values listed above will decrease by more than 20%?

  1. PAO2, or alveolar PO2.
  2. PcO2, or pulmonary capillary PO2.
  3. PaO2, or arterial blood PO2
  4. PtO2, or systemic tissue capillary blood PO2
  5. PvO2 or systemic venous blood PO2

Before you move on to the next question, you might want to take a moment to think about why some values change and some don't in the example above.

Hyperventilation and pH

Hyperventilation can be defined simply as breathing a lot more than you need to. How will this affect the levels of O2 and CO2 in your body? Again, go to prognosis.org and look at the bar graphs (called the O2 cascade). Start with all values at default (click Reset). The default value for respiratory rate (RR, or the number of breaths per minute) is 12. Suppose you increase this value to 24 breaths per minute. Before you hit "calculate," take a moment to think about how you expect things to change?

26. How does hyperventilation affect the PCO2 of a person’s blood?

  1. Hyperventilation increases the amount of CO2 in the blood.
  2. Hyperventilation decreases the amount of CO2 in the blood.
  3. Hyperventilation does not affect the CO2 concentration in the blood.

27. How does hyperventilation affect the pH of a person’s blood?

  1. Hyperventilation increases blood pH (more alkaline).
  2. Hyperventilation decreases blood pH (more acidic).
  3. Hyperventilation does not affect blood pH.

28. How does hyperventilation affect the arterial oxygen saturation (SaO2)?

  1. Hyperventilation increases the O2 saturation of arterial blood by more than 5%.
  2. Hyperventilation increases the O2 saturation of arterial blood by less than 5%.
  3. Hyperventilation decreases the O2 saturation of arterial blood.

29. How does hyperventilation affect the P50Hb in systemic tissue capillaries (PtO2)? (Click on "numerical output" to see this. P50Hb is the oxygen partial pressure at which hemoglobin (Hb) is 50% saturated.)

  1. Hyperventilation increases the P50Hb of systemic tissues.
  2. Hyperventilation decreases the P50Hb of systemic tissues.
  3. Hyperventilation has no effect on the P50Hb of systemic tissues.

30. How do you explain your answer to the previous question?

  1. Hyperventilation changes the O2 concentration, thereby affecting the P50Hb.
  2. Hyperventilation changes the pH, thereby affecting the P50Hb.

High altitude

Suppose you decide to climb Mont Blanc, the highest mountain in western Europe, with an altitude of approximately 5000m. To see how this altitude will affect gas exchange, go to prognosis.org and start by resetting all values to the default. Then set the altitude at 5000 meters.

31. How is the PO2 in arterial blood affected by increasing the altitude from 0 meters to 5000 meters?

  1. At 5000 m, the arterial blood PO2 is decreased by no more than 15%.
  2. At 5000 m, the arterial blood PO2 is decreased by more than 50%.
  3. At 5000 m, the arterial blood PO2 is increased,

Since there is less oxygen available at high altitude, you would probably compensate by breathing deeper and faster. Increase the tidal volume (VT) to 3000 ml, meaning that you would breathe in and out 3 liters of air with each breath, compared to the default 475 ml. Also increase the respiratory rate (RR), breathing faster to see if you can compensate for the altitude.

32. The default value for tissue capillary PO2 (PtO2) is around 6 kPa. If you set the tidal volume to 3000 ml and the altitude to 5000 m, how fast would you need to breathe to bring your PtO2 up to at least 5 kPa?

  1. Breathing at 50 breaths per minute would bring PtO2 above 5 kPa.
  2. Breathing at 100 breaths per minute would bring PtO2 above 5 kPa
  3. No matter how fast you breathe, you can never bring PtO2 above 5 kPa in this situation.

33. Suppose you are climbing the mountain, at an altitude of 5000 m, breathing 60 breaths per minute with a tidal volume of 3000 ml. Working hard at high altitude, you would hope that your hemoglobin would release plenty of O2 to your tissues.  Click on the O2-Hb dissociation button to see the dissociation curve for this high altitude/high exertion situation compared to the standard default values. At a PaO2 of 5 kPa, which of the following is true?

  1. The high altitude/high exertion curve shows more O2 being released to tissues at this PO2 (in other words, a greater decrease in saturation).
  2. The high altitude/high exertion curve shows less O2 being released to tissues at this PO2.
  3. Both curves show the same O2 being released to tissues at this PO2.

34. How do you explain your answer to the previous question?

  1. High altitude reduces the amount of O2 available, so hemoglobin releases more O2.
  2. Hyperventilation raises the pH, increasing hemoglobins's binding affinity so it remains more saturated at the same PO2.

Fick's law and exercise

35. According to Fick's law of diffusion, the rate of gas diffusion across a barrier (such as the surface of the lungs) is proportional to surface area, concentration gradient, and the diffusion constant, and is inversely proportional to the diffusion distance. If you start to exercise, you're going to need more oxygen. Which of these factors can change in order to allow you to absorb the O2 you need into your blood?

  1. Surface area.
  2. O2 concentration gradient.
  3. Diffusion constant.
  4. Diffusion distance.

Write "Gas Exchange," today's date, your lab section, and your name on your Scantron.

References & Further Reading

You don't need to read these references to answer the questions on this page, but you may find them helpful if you want to learn more.

COPD (Chronic Obstructive Pulmonary Disease)

Pulmonary Emphysema: When More is Less. David G. Morris, Dean Sheppard, Physiology Dec 2006, 21 (6) 396-403; DOI: 10.1152/physiol.00027.2006. A good review of the medical and physiological aspects of emphysema and COPD.

Anemia

Anemia. National Heart, Lung, and Blood Institute.

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