What is the four-step respiration process?

• A. Inhalation, External respiration, Internal respiration, Cellular respiration
• B. Breathing (ventilation), External respiration, Internal respiration, Metabolic respiration
• C. Breathing (ventilation), External respiration, Internal respiration, Cellular respiration
• D. Inspiration, Expiration, Gas exchange, Cellular respiration

Answer: (C)

The four-step sequence of respiration moves from bringing air into the lungs to using oxygen for energy production in cells. The first step is Breathing (ventilation), the mechanical process that inflates and deflates the lungs to move air to the exchange surfaces. Next is External respiration, the gas exchange between the alveoli and the blood, where oxygen enters the blood and carbon dioxide leaves it. Then Internal respiration occurs, the gas exchange between the blood and body tissues, delivering oxygen to cells and picking up carbon dioxide. The final step is Cellular respiration, the cellular process that uses oxygen to metabolize nutrients and produce ATP, with water and carbon dioxide as byproducts.

This ordering is why Breathing (ventilation) is the correct first term, followed by External respiration, Internal respiration, and Cellular respiration. Other options replace ventilation with Inhalation or use less standard terms like Metabolic respiration or lump gas exchange into a single step, which doesn’t fit the widely taught four-step sequence.