The terms “breathing” and “respiration” are often used interchangeably, but they describe two distinct, yet connected, biological processes. Breathing refers to the mechanical movement of air, which involves the external exchange of gases with the environment. Respiration, specifically cellular respiration, is a biochemical process occurring within every cell to acquire energy.
Breathing: The Mechanical Process
Breathing, or pulmonary ventilation, is the physical movement of air into and out of the lungs, occurring in two phases: inhalation and exhalation. This mechanical act is governed by pressure changes within the thoracic cavity, as air moves from higher to lower pressure. The primary muscle responsible for this movement is the diaphragm, a dome-shaped sheet of muscle located beneath the lungs.
During inhalation, the diaphragm contracts and moves downward, while the external intercostal muscles pull the rib cage upward and outward. This action increases the volume of the chest cavity, causing the pressure inside the lungs to drop below atmospheric pressure. Air then rushes into the lungs to equalize this difference, filling the tiny air sacs called alveoli.
Exhalation is typically a passive process where the diaphragm and intercostal muscles relax, allowing the lungs and chest wall to recoil. The resulting decrease in thoracic volume raises the pressure inside the lungs above atmospheric pressure, forcing the air out. The purpose of this mechanical cycling is gas exchange, taking in oxygen and expelling the waste product, carbon dioxide.
Respiration: The Chemical Energy Reaction
Cellular respiration is an internal, multi-step biochemical process taking place within the cells of living organisms. Its primary function is to break down nutrient molecules, such as glucose, to release stored chemical energy. The main output of this process is adenosine triphosphate (ATP), the energy currency of the cell.
This energy-releasing process occurs predominantly within the mitochondria, specialized organelles found in nearly all eukaryotic cells. Aerobic respiration uses oxygen as a reactant, combining it with glucose to produce energy, carbon dioxide, and water. Glucose is first partially broken down during glycolysis, and the resulting molecules enter the mitochondria for subsequent stages. The majority of ATP is generated during oxidative phosphorylation, where oxygen acts as the final electron acceptor.
The constant supply of oxygen delivered by breathing is necessary for this process. Without biochemical energy production, cells cannot perform fundamental tasks like muscle contraction or nerve impulse transmission.
The Essential Link Between the Two Processes
Breathing and cellular respiration are linked through the continuous exchange of gases and production of energy. The mechanical act of breathing provides the oxygen necessary for aerobic cellular respiration. Air drawn into the lungs diffuses across the alveoli walls and into the bloodstream, where red blood cells pick it up.
The circulatory system transports oxygen-rich blood away from the lungs and delivers it to tissues throughout the body. At the cellular level, oxygen is consumed in the mitochondria to generate ATP, simultaneously creating carbon dioxide as a waste product. This carbon dioxide leaves the cells, enters the bloodstream, and is transported back to the lungs.
Breathing completes the exchange by expelling this carbon dioxide waste back into the atmosphere during exhalation. The rate of breathing adjusts constantly to match the metabolic demand of the cells. For instance, increased physical activity raises the cellular need for oxygen and the production of carbon dioxide, causing the breathing rate to rise.