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39.1A: The Respiratory System and Direct Diffusion

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  • Respiratory processes that help organisms exchange O2 and CO2 range from simple direct diffusion to complex respiratory systems.


    Review an overview of the functions of the respiratory system


    Key Points

    • Respiration ensures that cells, tissues, and major organs of the body receive an adequate supply of oxygen and that the carbon dioxide, a waste product, is efficiently removed; the exchange of oxygen and carbon dioxide occurs via diffusion across cell membranes.
    • The mechanisms, processes, and structures used for respiration are dictated by the type, size, and complexity of the organism.
    • Direct diffusion of gases through the outer membranes can be used by organisms such as flatworms as a means of respiration due to their small size and simplicity.

    Key Terms

    • deoxygenated: having removed the oxygen atoms from a molecule
    • diffusion: The passive movement of a solute across a permeable membrane
    • aerobic: living or occurring only in the presence of oxygen


    Breathing is an involuntary event. How often a breath is taken and how much air is inhaled or exhaled are tightly regulated by the respiratory center in the brain. Under normal breathing conditions, humans will breathe approximately 15 times per minute on average. A respiratory cycle consists of an inhalation and an exhalation: with every normal inhalation, oxygenated air fills the lungs, while with every exhalation, deoxygenated air rushes back out. The oxygenated air crosses the lung tissue, enters the bloodstream, and travels to organs and tissues. Oxygen (O2) enters the cells where it is used for metabolic reactions that produce ATP, a high-energy compound. At the same time, these reactions release carbon dioxide (CO2) as a by-product. CO2 is toxic and must be eliminated; thus, CO2 exits the cells, enters the bloodstream, travels back to the lungs, and is expired out of the body during exhalation.

    The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove carbon dioxide. The main structures of the human respiratory system are the nasal cavity, the trachea, and the lungs. All aerobic organisms require oxygen to carry out their metabolic functions.

    Along the evolutionary tree, different organisms have devised different means of obtaining oxygen from the surrounding atmosphere. The environment in which the animal lives greatly determines how an animal respires. The complexity of the respiratory system correlates with the size of the organism. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. In unicellular (single-celled) organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell. Diffusion is a slow, passive transport process. In order to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick, diffusion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore, dependence on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small organisms or those with highly-flattened bodies, such as flatworms (platyhelminthes). Larger organisms have had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages, accompanied by a complex circulatory system to transport oxygen throughout their entire body.


    Direct diffusion: This flatworm’s process of respiration works by diffusion across the outer membrane.

    Direct Diffusion

    For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist so that gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which ‘breathe’ through diffusion across the outer membrane. The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen.