Respiratory system across animals

Definition and overall function of respiratory system

The respiratory system (also called the respiratory apparatus or ventilatory system) is a biological system made up of specific organs and structures that enable gas exchange in animals and plants. In land animals, the respiratory surface is internalized—most notably in the lungs—where air reaches microscopic air sacs (alveoli in mammals and reptiles, and atria in birds) that have rich blood supplies, allowing gases to come into close contact with blood. Overall, the system functions by moving air between the external environment and these gas-exchange surfaces. Air travels through airways (such as the trachea, bronchi, and bronchioles) to reach the alveoli/atria, and breathing—driven by respiratory muscles—pumps air in and out. In aquatic animals, the respiratory system often uses gills, where water flows over thin, highly vascularized structures to support gas exchange; in amphibians, skin can also play a major role. Beyond gas exchange, the lungs contribute to defenses (e.g., cough/sneezing and immune-related secretions), help prevent alveolar collapse via surfactant, and support whole-body functions such as activating hormones and regulating certain substances in circulation.

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Gas exchange surfaces: lungs, alveoli, atria

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