Animal Breathing: Types and Respiratory Processes

Posted on Mar 4, 2025 in Biology

Usually, the inclusion of oxygen into an animal’s body is called breathing, but breathing encompasses three processes:

  1. Ventilation: Oxygen is taken from the environment and brought to the respiratory organ, and carbon dioxide is expelled.
  2. Gas Exchange: This occurs between respiratory organs and blood. The exchange takes place by diffusion, a process by which molecules pass from a more concentrated region to a less concentrated one without expenditure of energy. These regions are separated by a semipermeable membrane. The diffusion rate of a gas through a membrane is expressed by Fick’s law: the velocity of diffusion (Q) = constant * (surface of the membrane * (C1-C2)) / (membrane thickness (E)). Two conclusions are deduced from this formula: 1) the diffusion speed increases with the membrane surface, and 2) the diffusion speed will be greater the smaller the membrane thickness. Natural selection favored organs with large surface areas and thin membranes.
  3. Cellular Respiration: This is a biochemical process that occurs in the mitochondria, and its function is to obtain energy. It involves the oxidation of energy and nutrients to produce carbon dioxide.

Breathing Patterns and Respiratory Tracts

  1. Direct Diffusion: This occurs in Porifera, Cnidaria, and many worms. A stream of water enters, and gas exchange is performed directly with the cells.
  2. Cutaneous Respiration: Gas exchange occurs through the skin. For this to be effective, the relationship between the animal’s mass and body surface area must be high, and the skin needs to be moist. It occurs in flatworms, adult amphibians, and some fish.
  3. Tracheal Breathing: This is done through tubes called tracheae. Tracheae open to the outside of the body through holes called spiracles. Inside the animal’s body, the tubes branch and become finer; these finer tubes are called tracheoles and reach the cells. Therefore, 50% of the volume of the respiratory system is in the tracheoles, where gas exchange occurs. This is found in insects.
  4. Branchial Respiration: Gills are structures typical of aquatic animal breathing. There are two types:
    • External gills: These are unprotected on the outside of the body, are unevolved, and lack ventilation mechanisms; they only need to move (e.g., Axolotl).
    • Internal gills: These are protected and have a ventilation mechanism, a continuous stream of water that bathes the gills. These are found in fish, mollusks, and crustaceans. In fish, gills are grouped into gill arches; each slide is called a gill filament, and each filament is a set of smaller lamellae called lamellae, where gas exchange takes place.
  5. Pulmonary Respiration: This is characteristic of amphibians, reptiles, birds, mammals, arachnids, and snails.
  • Lungs of Arachnids and Snails: These are simply bags that have no ventilation mechanism. They are called diffusion lungs, and the exchange of substances occurs by simple diffusion.
  • Lungs of Vertebrates: These are highly vascular membranous organs that have a ventilation mechanism. They have evolved from simple bags with lobes and bridge structures to increase the surface area for gas exchange.