Tidal volume refers to the amount of gas, or oxygen, exchanged in and out of your lungs when you breathe. People exchange different amounts of air in different situations. When someone is playing hard, his lungs work harder and move more oxygen in and out of them. When you are asleep, resting or working quietly, less oxygen is needed. Doctors use machines to monitor the tidal volume of patients, especially when the patient has a breathing problem.
Students can build spirometers in the classroom and use them to measure tidal volume. For each spirometer, you will need a clean 3-liter bottle with a cap, 2 feet of plastic tubing, a measuring cup marked with milliliters, a bucket large enough to hold 3 liters of water and a marker. Add 1/2 liter of water at a time to the bottle, marking the height on the bottle after each addition until the bottle is full, and replace the cap. Fill the bucket with enough water to submerge the bottle and invert the bottle into the water. Then remove the cap underwater so no air gets in, and insert one end of the tube into the bottle. When each student breathes out through the tube, her tidal volume will displace the water in the bottle.
Have students work with the spirometer in pairs. One holds the bottle gently to keep it from tipping over while the other inhales and then exhales through the tube. The oxygen from her lungs will fill the bottle, causing it to rise. Her partner can judge her tidal volume by reading the marks on the side of the bottle. Let each student have a chance to test her tidal volume with both normal breaths and the biggest breaths she can manage.
Tidal volume is one aspect of lung function. Students can expand their knowledge of how their lungs work by studying how tidal volume is a part of vital lung capacity. Have each student measure a normal breath, which is his tidal volume, and then the biggest breath he can, which is called inspiratory reserve. Next, have him breathe in normally but exhale as much as possible, which measures expiratory reserve. Adding all of these numbers together will yield an approximation of total lung capacity, or vital capacity. The vital capacity is the greatest amount of oxygen a person's lungs can exchange.