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.

Correct answers are highlighted in orange.

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

Crab in a bucket

Suppose you find an interesting crab in the ocean, and you want to bring it in to show the biology class. You want to make sure it stays alive, but you’re concerned that it might not get enough oxygen. The crab uses oxygen at about 5 ml/hour. You’re going to carry it in a straight-sided bucket with some seawater in it. The bucket can hold up to 10 liters, and the surface area of the water in it is 1000 cm2. The crab will have to be in the bucket overnight, and you don’t have a bubbler to put more oxygen into the water.

1. Should you fill the bucket with water, or use only a little water? Why?

  1. Fill the bucket with water; the 10 liters of water will hold enough oxygen to keep the crab alive.
  2. A small amount of water; the water can’t possibly hold enough oxygen to keep the crab alive, so the crab will depend on oxygen diffusing into the water from the air. A smaller volume of water will allow the oxygen to diffuse in faster.

Gas exchange & mountaineering

Suppose you want to climb a mountain that is 19,000 feet tall. The air at that altitude has a total pressure of about half an atmosphere. The chemical composition of the air is the same as at sea level. Suppose you want to breathe the same partial pressure of oxygen that you normally would breathe at sea level – no more and no less. You decide to breathe bottled air from a tank, and adjust the composition of the air in the tank so you get the same PO2 that you’d normally get at sea level.

2. What should the composition of the air in your tank be?

  1. 21% O2, the same as at sea level.
  2. 10.5% O2.
  3. 42% O2.
  4. It depends on the pressure in the tank. The higher the pressure, the lower the O2 concentration should be.

For the question above, start by thinking about what the air pressure in your lungs would be.

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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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. The concentration of dissolved oxygen depends on partial pressure and solubility. The blood may have lower PO2, but much higher solubility due to hemoblobin.

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. 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.

8. 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

9. 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

10. 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. It would contain 1/5 as much volume as at the surface.
  2. 250 liters
  3. 1000 liters
  4. 4000 liters
  5. more than 4000 liters

11. 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.

12. 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. It would expand to 5x normal volume.
  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.

13. 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.

14. In the scenario described in question 13, 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, 10th ed.)

15. 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.

16. 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.

17. 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.

18. Comparing figures 42.29 and 42.30 in Campbell (10th 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%.

19. 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.

20. 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%.

21. 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%.

22. 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. More saturated.
  2. Less saturated.

23. 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?

24. 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.

25. 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.

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

  1. pH would tend to be higher.
  2. pH would tend to be lower.

Hyperventilation and pH

Hyperventilation can be defined simply as breathing a lot more than you need to.

27. How would hyperventilation affect the PCO2 of a person’s blood?

  1. Hyperventilation would increase the amount of CO2 in the blood.
  2. Hyperventilation would decrease the amount of CO2 in the blood.
  3. Hyperventilation would not affect the CO2 concentration in the blood.

28. How would hyperventilation affect the pH of a person’s blood?

  1. Hyperventilation would increase blood pH (more alkaline).
  2. Hyperventilation would decrease blood pH (more acidic).
  3. Hyperventilation would not affect blood pH.

29. How would hyperventilation affect the oxygen concentration in the freshly oxygenated blood leaving the alveolar capillaries?

  1. Hyperventilation would significantly increase the O2 saturation of hemoglobin.
  2. Hyperventilation would significantly decrease the O2 saturation of hemoglobin.
  3. Hyperventilation would not affect blood oxygen concentration very much, because the arterial blood is close to being saturated with oxygen anyway.

Fick's law and exercise

30. 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.

Comparing Gas Exchange in Plants & Animals

Plants must also do gas exchange, both to take in CO2 for photosynthesis and to take in O2 for respiration. The two processes generally don't balance each other out. If a plant is growing, its rate of photosynthesis must be greater than its rate of respiration. With plants, as with animals, the rate of gas exchange must be proportional to the metabolic rate. In both cases, as gas exchange increases, so does water loss. One key difference is the lower partial pressure of CO2 compared to O2 in the atmosphere.

31. Suppose a plant is doing a certain amount of photosynthesis, and it needs to take in 100 ml of CO2 (assume standard temperature & pressure). The plant can only absorb CO2 from the air that actually enters the airspace in the leaf. What is the approximate minimum amount of air the plant would have to take in to meet its needs?

  1. 100 ml.
  2. 300 ml
  3. 0.03 ml
  4. 300,000 ml, or 300 liters.

32. Suppose an animal is doing a certain amount of respiration, and it needs to take in 100 ml of O2 (assume standard temperature & pressure). The animal can only absorb O2 from the air that actually enters the lungs. What is the approximate minimum amount of air the animal would have to process to meet its needs?

  1. 100 ml.
  2. 21 ml
  3. 0.21 ml
  4. 476 ml
  5. 476,000 ml

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

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