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Biology LibreTexts

39.1B: Skin, Gills, and Tracheal Systems

  • Page ID
  • Respiration can occur using a variety of respiratory organs in different animals, including skin, gills, and tracheal systems.


    Describe how the skin, gills, and tracheal system are used in the process of respiration


    Key Points

    • Some animals, such as amphibians and earthworms, can use their skin (integument) to exchange gases between the external environment and the circulatory system due to the network of capillaries that lie below the skin.
    • Fish and other aquatic organisms use gills to take up oxygen dissolved in the water and diffuse carbon dioxide out of the bloodstream.
    • Some insects utilize a tracheal system that transports oxygen from the external environment through openings called spiracles.

    Key Terms

    • coelom: a fluid-filled cavity within the body of an animal; the digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum
    • gill: a breathing organ of fish and other aquatic animals
    • spiracle: a pore or opening used (especially by spiders and some fish) for breathing

    Skin and Gills

    There are various methods of gas exchange used by animals. As seen in mammals, air is taken in from the external environment to the lungs. Other animals, such as earthworms and amphibians, use their skin (integument) as a respiratory organ. A dense network of capillaries lies just below the skin, facilitating gas exchange between the external environment and the circulatory system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes.

    Organisms that live in water also need a way to obtain oxygen. Oxygen dissolves in water, but at a lower concentration in comparison to the atmosphere, which has roughly 21 percent oxygen. Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water. Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in the water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans.


    Common carp: This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen from water.

    The folded surfaces of the gills provide a large surface area to ensure that fish obtain sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration). Similarly, carbon dioxide molecules diffuse from the blood (high concentration) to water (low concentration).


    Oxygen transport and gills: As water flows over the gills, oxygen is transferred to blood via the veins.

    Tracheal Systems

    Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role in oxygen transport. Insects have a highly-specialized type of respiratory system called the tracheal system, which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system, the most direct and efficient respiratory system in active animals, has tubes made of a polymeric material called chitin.

    Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, allowing oxygen to pass into the body, regulating the diffusion of CO2 and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements.


    Insect respiration: Insects perform respiration via a tracheal system, in which openings called spiracles allow oxygen to pass into the body.