Internal Respiration Definition

Internal respiration is the process of diffusing oxygen from the blood, into the interstitial fluid and into the cells. Waste and carbon dioxide are also diffused the other direction, from the cells to the blood. Oxygen is released from blood cells in response to the oxygen concentration in the capillaries of blood vessels, which is usually really low. This enables the exchange of gases and other solutes during internal respiration between the plasma and the interstitial fluid. Cellular respiration refers to the process of converting that oxygen along with glucose into ATP, a molecule that cells use to store usable energy, but creates carbon dioxide. External respiration refers to the process of exchanging oxygen and carbon dioxide in the lungs, gills, or other tissues exposed to the external environment. Breathing is the mechanical process of pulling lungs into or out of the lungs, or moving water over the gills.

  • Respiration – The process of exchanging oxygen, carbon dioxide, and other solutes between the blood and environment to continue producing energy via aerobic pathways.
  • Breathing – A process initiated by muscles to draw air or water into the lungs or over the gills.
  • External Respiration – The process of exchanging solutes with the external environment through special tissues.
  • Cellular Respiration – The process of using oxygen to extract energy from organic molecules.


1. Which of the following scenarios describes internal respiration?
A. A hagfish does not have a closed respiratory system, but rather a system of blood filled sinuses that exchange oxygen and carbon dioxide with the interstitial fluid.
B. A lungfish is one of the only fish that has lungs, and gulps air into the lungs, forcing it down with a gulp.
C. A newt does not need to force water over its gills, because they reside externally and exchange oxygen as the newt moves.

Answer to Question #1
A is correct. Internal respiration is the process of getting oxygen to the cells, and answer A describes that. B describes a primitive form of breathing, which relies on mouth and throat muscles instead of a diaphragm or ribcage. Answer C describes external respiration, through external gills. Remember that external respiration could also happen through internal gills.