Robert W. Mellette
Topic
Human respiration
Rationale
In any closed environmental system, whether aboard a spacecraft or on the planet earth, humans require an atmosphere with an oxygen content sufficient to support life. The atmosphere of the Space Shuttle closely approximates that of earth, both in regards to its air pressure and oxygen content. As a result of human respiration carbon dioxide (CO2) is exhaled into the atmosphere. One earth this gas is absorbed by green plants and does not accumulate in dangerous levels. Since there are no plants aboard the Space Shuttle to absorb carbon dioxide, chemical means are employed. Canisters of granular lithium hydroxide are used to absorb carbon dioxide from the cabin atmosphere.
Objective
After instruction the student will be able to construct and use the apparatus described to test for the presence of the gas carbon dioxide (C02).
Materials Required
small mouth bottle or flask
two hole stopper
25 ml flint glass tubing
calcium hydroxide
test tubes
straws
water
figure available in print form
Procedure
Construct the apparatus pictured in the diagram. This apparatus is designed to be used by the teacher for demonstration purposes. If supplies are adequate, students should construct similar apparatus for desk top use. It is most effective to have the students work in pairs for this activity. If the resources are not available to construct the type apparatus pictured, students can simply use a test tube filled with limewater and then exhale into it using an ordinary drinking straw.
Distribute student activity sheets to the class and discuss the process of human respiration. Focus attention to the exchange of gases that occurs as a result of this process. One way to raise awareness to the differences between inhaled and exhaled gases is to provide students with circle graphs that they can color with crayons or colored pencils. Using different colors for carbon dioxide and oxygen will help students visualize the changes in the gases.
To use the breathing apparatus, place tube A between your lips, draw outside air from tube B in through limewater by inhaling on tube A. Continue this process for a one-minute time period. Direct the students to observe any changes that may occur in the appearance of the limewater solution. (Inhaled air has a negligible amount of CO2, so there will be no noticeable change in the appearance of the limewater solution.) Now exhale into tube B for a period of one minute. As exhaled air containing CO
2
passes through the limewater solution students should observe a noticeable change in the appearance of the limewater. A milky color indicates the presence of the gas carbon dioxide (CO2).
Background Information:
The primary function of the human respiratory system is to deliver oxygen to the bloodstream. Oxygen is carried via the bloodstream to all the cells of the body, where it is used to produce heat and energy. In the process of using oxygen, carbon dioxide is produced which the respiratory system disposes of through the lungs during exhaling. This exchange of oxygen for carbon dioxide within the lungs is called
RESPIRATION
.
Note: To prepare limewater solution add powdered calcium hydroxide to a beaker of water. Stir constantly and continue to add the powdered lime (calcium hydroxide) until the lime can no longer be dissolved into the water and falls to the bottom of the beaker. Filter this solution through filter paper and it is ready for use.
