Biology, The Science of Life

The Respiratory System: Introduction

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Overview: This lesson will consist of the instructor lecturing for 50 minutes in an authoritative fashion on the respiratory system. A summary video will be presented at the end of the lecture to visually summarize the topic. A crossword will be handed out at the end of class to complete for the following class.

Materials: The teacher will need the class computer to project the power point of the lecture onto the screen in the front on the class (file is available for download above). The Power Point presentation will be used as a media teaching tool for the whole duration of the class. Another digital media teaching tool in this lecture is the video which will be presented at the end of class to briefly summarize the topic (video is found at the bottom of the page). The instructor may also use the white board to further emphasize the material when they feel the need to use it. Students will need paper, pen or pencil, or their laptops to take down notes. Also, the Power Point presentation is made available to the students, therefore they are required to take down additional notes that are not presented on the Power Point.

Assessment: A take-home crossword will be distributed at the end of class for a total worth of 2% of the final grade. The crossword provides questions similar to the exam presented at the end of the 10 lectures therefore it is a great practice and review tool. Crosswords are chosen as the method of assessment for these authoritarian lectures because they turn a 50 min lecture into an interactive game. A lot of material is covered in lessons set-up like this one and a crossword is more amusing to complete than a simple questionnaire; it provides clues and to a certain level, a confirmation of your answer. It also covers the material in as much detail as any questionnaire. It is available for download at the top of the page.

Extra Notes: This is an introductory lecture to respiration in humans; more details regarding cell respiration will be examined in lessons 07 and 08.


  • Respiratory Surfaces (3 minutes)
  • Gills in Aquatic Animals (3 minutes)
  • Tracheal Systems in Insects (3 minutes)
  • Lungs (3 minutes)
  • Mammalian Respiratory System (12 minutes)
  • How Amphibians Breathe (5 minutes)
  • How a Mammal Breathes (10 minutes)
  • How a Bird Breathes (7 minutes)
  • Summary Video (2 minutes)

Open Power Point presentation and begin explaining the material.

Respiratory Surfaces
The cells that carry out gas exchange have a plasma membrane that must in contact with aqueous solution. Respiratory surfaces are therefore always moist. The movement of oxygen and carbon dioxide across the moist surface takes place entirely by diffusion. Gas exchange is fast when the area of diffusion is small and slow when the area is small. As a result, respiratory surfaces tend to be large and thin.
In some relatively simple animals, such as sponges, every cell in the body is close enough to the external environment that gases can diffuse quickly between all cells and the environment. In many animals, however, the bulk of the body’s cells lack immediate access to the environment. The respiratory surface in these animals is a thin, moist, epithelium that constitutes a respiratory organ. The skin serves as a respiratory organ in some animals, including earthworms. Just below the skin, a dense network of capillaries facilitates the exchange of gases between the circulatory system and the environment.

Gills in Aquatic Animals
Gills are outfoldings of the body surface that are suspended in the water. Gills often have a total surface area much greater than that of the rest of the body. Movement of the respiratory medium over the respiratory surface, a process called ventilation, maintains the partial pressure gradients of oxygen and carbon dioxide across the gills.
The arrangement of capillaries in a fish gill allows for countercurrent exchange, the exchange of a substance flowing in opposite directions.

Tracheal Systems in Insects
Made up of air tubes that branch throughout the body. The largest tubes, called the tracheae, open to the outside. The finest branches extend close to the surface of nearly every cell, where gas exchange occurs by diffusion across the moist epithelium that lines the tips of the tracheal branches. Because the tracheal system brings air within a very short distance of all body cells in an insect, it can transport oxygen and carbon dioxide without the participation of the animal’s open circulatory system.

Lungs are localized respiratory organs. Representing an infolding of the body surface, they are typically subdivided into numerous pockets. Because the respiratory surface of the lung is not in direct contact with all other parts of the body, the gap must be bridged by a circulatory system, which transports gases from the lungs to the rest of the body (seen in lesson 03).

Ask students if they have questions up to this point. The question period should be between 2-3 minutes.

Mammalian Respiratory Systems
In mammals, a system of branching ducts conveys air to the lungs, which are located in the thoracic cavity. Air enters through the nostrils and is then filtered by hairs, warmed, and sampled for odors as it flows through in the nasal cavity. The nasal cavity leads to the pharynx, an intersection where paths for air and food cross. When food is not being pushed down (seen in lesson 01), the glottis is open, enabling breathing.
From the larynx, air passes into the trachea. In mammals, the larynx also functions as the voice box. Exhaled air rushes by the vocal cords, a pair of elastic bands of muscles in the larynx. Sounds are produced when the muscles in the larynx are tensed.
From the trachea fork two bronchi, one leading to each lung. Within the lung, the bronchi branch repeatedly into finer tubes called bronchioles. The epithelium lining in the major branches of this respiratory tract is covered by cilia and mucus.
Gas exchange occurs in the alveoli, air sacs clustered at the tips of the tiniest bronchioles. Oxygen in the air entering the alveoli dissolves in the moist film lining the inner surfaces and rapidly diffuses across the epithelium into capillaries that surrounds the alveolus.

How an Amphibian Breathes
An amphibian such as a frog ventilates its lungs by positive pressure breathing, inflating the lungs with forced airflow. During the first stage of inhalation, muscles lower the floor of the oral cavity, drawing in air through the nostrils. Next, with the nostrils and mouth closed, the floor of the oral cavity rises, forcing air down the trachea. During exhalation, air is forced back out by elastic recoil of the lungs and by compression of the muscular body wall.

How a Mammal Breathes
Mammals employ negative pressure breathing – pulling rather than pushing air into their lungs. Using muscle contraction at actively expand the thoracic cavity, mammals lower air pressure in their lungs below that of the air outside the body. Because gas flows from a region of higher pressure to a region of lower pressure, air rushes through the nostrils and mouth and down the breathing tubes to the alveoli. During exhalation, the muscles controlling the thoracic cavity relax, and the volume of the cavity is reduced. The increased air pressure in the alveoli forces air up the breathing tubes and out of the body. Thus, inhalation is always active and requires work, whereas exhalation is usually passive.
Expanding the thoracic cavity during inhalation involves the animal’s rib muscles and the diaphragm, a sheet of skeletal muscles that forms the bottom wall of the cavity. Contracting the rib muscles expand the rib cage, the front wall of the thoracic cavity, by pulling the ribs upwards and the sternum outward. At the same time, the diaphragm contracts, expanding the thoracic cavity downward.
Within the thoracic cavity, a double membrane surrounds the lungs. The inner layer adheres to the outside of the lungs, and the outer layer adheres to the wall of the thoracic cavity. A thin space filled with the fluid separates the two layers.

How a Bird Breathes
Ventilation is both more efficient and more complex in birds than in mammals. When birds breathe, they pass air over the gas exchange surface in only one direction. Furthermore, incoming, fresh air does not mix with air that has already carried out gas exchange. To bring fresh air into their lungs, birds use eight or nine sacs situated on either side of their lungs. Instead of alveoli, the sites of gas exchange in birds are tiny channels called parabronchi. Passage of air through the entire system – lungs and air sacs – requires two cycles of inhalation and exhalation.

Ask students if they have any questions. The question period may last the remaining duration of class, however save 2 minutes for the video.

Close the Power Point presentation.

Present the summary video below and then hand out crossword. If there is time remaining after the video, allow students to begin work on the crossword in groups.