** See the General Tips notes in Chapter 30 of this Instructor's Manual.
The essence of the respiratory system is that oxygen must enter the individual cell and carbon dioxide must exit. Everything in the respiratory system is geared toward facilitating this gas exchange.
You may find that your students have forgotten what they need oxygen for. If you have done the energy harvesting chapters, you should be able to remind the students that oxygen is the final electron receptor in the electron transport system.
You might wish to copy and distribute to your students the Overview of Chapter Objectives flowchart found at the beginning of this Instructor's Manual Chapter.
Consider beginning this chapter with a non-quiz. Your questions could include: How do insect sprays work? Do fish breathe air or water? Is energy required to breathe in, or to breathe out, or both?
A homemade trachea can be more effective than a laboratory model. Take a piece of white paper and make a lengthwise cylinder. Tape it together. For convenience, designate the seam side as the posterior. Now, take a piece of colored paper and make another smaller cylinder. This is the esophagus. Staple or glue it to the posterior of your trachea. The cranial end of your esophagus should begin about three inches below the cranial end of your trachea. Cut the caudal ends so they are even. The top end of the trachea is the epiglottis. Color this and fold it over from the anterior (front) so it resembles a flap. Draw black lines on the trachea to represent the cartilaginous tracheal rings.
You can now demonstrate how the epiglottis folds over the trachea. You can also demonstrate why the cartilaginous portions of the tracheal rings are necessarily incomplete. (No cartilage on the esophagus side.) These rings are called "c-rings" because they are shaped like the letter "c." Humans usually have 16-20 such rings. Cats have about 33; pigs usually have between 32 and 36. In the pig the edges of the rings overlap but the rings are still incomplete and the function remains the same.
- I. I suggest you reiterate the differences between the different types of respiration and between the scientific and colloquial definitions of respiration.
- II. Use props or models. Make the tracheal system suggested above.
- II.A. Give some practical examples of how organisms use the body surface for respiration. Ask what would happen to a frog if you punctured its thoracic cavity. If frogs use their body surface for gas exchange, what do they use their lungs for?
- II.B. Explain that when you spray insects, you literally clog their tracheae. The insect suffocates. You can make this quite dramatic.
II.C. Although Lewis states that salamanders retain external gills throughout adulthood, it should probably be noted that this is not quite true of all salamander species.
Interestingly, many fish with their continuous supply of fresh water flowing over their gills, do not drink much.
Stress the diagrams and the countercurrent mechanisms. Note the bottom line is always the same -- the tendency toward equilibrium.
III.B. Check what was said about constructing a trachea in the Possible Approaches to the Chapter section above.
The key distinction between a bronchus and a bronchiole is that bronchioles have no cartilage. As you descend the bronchial tree, the cartilage becomes less and less predominant. Toward the end of the tree, the cartilage is like little blocks of tissue. Where it stops, the structure becomes an alveolus.
- IV. If you do not have access to a regular model, you can demonstrate the mechanics of breathing by constructing a model bell jar in this way. Take a glass or plastic container that has a large opening at one end and a small opening (into which you can put a stopper) at the other end. Cover the large opening with a balloon or balloon-like material. Take a stopper with a bore and put a tube through the bore. (Use a stopper that will fit into the smaller opening of your container.) Put a small balloon on the end of the bore and place that end into the small opening. Your set-up might look like this:
Now you are ready to demonstrate what happens when you change the pressure on different parts of the system.
- V. Some students will have difficulty with Figures 36.12 and 36.13. Be certain you yourself understand what is happening before you begin explaining it. If the chemistry is beyond your course objectives, modify your requirements but do not skimp on the basic concept. So often students do not understand what is happening.
Keep in mind that everything happens by diffusion. Every action is a passive movement toward equilibrium.
- VI. It is usually a real revelation when students realize that the control of respiration has to do with the carbon dioxide level rather than the oxygen level. Fortunately, this is not a difficult mind-set to alter.
- VII. There is a great deal you can do with this section on the unhealthy respiratory system. I caution you, however, not to be morbid or preachy. Rather, present facts and encourage your students to look for reasons behind situations. Keep the discussions scientific.
