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11.1: Respiratory Systems of Animals in Aquatic or Moist Environments Definitions: Respiration: Gas Exchange: Earthworms (moist environment) use their entire outer surface fro gas exchange. Oxygen diffuses into capillaries, and carbon dioxide diffuses out. These animals must live in damp or aquatic environments to keep their entire surface moist enough for this gas exchange to occur. Many aquatic organisms take in oxygen through gills, specialized folds in the body surface. Most fish take in water through their mouths and pump it over their gills. As water flows over gills in one direction, blood flows within the gills in the opposite direction. At every point along the surface of the gill the oxygen concentration is higher in the water than it is in the blood, so oxygen flows into the blood. 11.1: Respiratory Systems of Land Animals Definitions Diaphragm: Inhalation: Exhalation: Spiracle: Tracheae: Insects have a system of branching respiratory tubes (spiracles and trachea) that connect body cells directly with environment. Oxygen enters the body through spiracles and diffuses into tracheae. Gas exchange occurs throughout the body, close to all body cells. Carbon dioxide exits the body in the opposite direction. Large active land animals, such as mammals and birds, require more oxygen than could be delivered by tracheal systems. They use muscles to draw air into lungs, where gas exchange takes place across a large moist surface. In inhalation, the diaphragm moves down, and the rib cage moves up and put increasing the volume in the chest cavity. This decreases the air pressure in the chest cavity, the lungs expand and air moves in since the air pressure in the external environment is greater. In exhalation, the diaphragm and rib cage move back to their original positions decreasing the volume, leading to an increase in pressure in the lungs. Air moves from the lungs to the external environment, which has a lower air pressure.