The processes of cellular respiration and breathing are part of the same process. When you inhale, carbon dioxide is replaced by oxygen in your blood; this oxygen is required by cells to perform aerobic cellular respiration as opposed to anaerobic cellular respiration--a similar process that does not require oxygen.
When you inhale, air is drawn into your lungs by the movement of your diaphragm and other breathing muscles. Air enters your lungs via your nose or mouth and down your windpipe to be divided into each lung by your trachea. As air enters the lung, it is channeled down ever smaller pathways, similar in shape to an upside-down tree. At the end of these pathways are tiny air sacs called alveoli. It is here that oxygen in your inhaled air is transferred into your blood while carbon dioxide, a byproduct of your cells breaking down nutrients, is excreted, replacing the oxygen. As you exhale, this carbon dioxide is expelled into the environment. This process is repeated in every breath you take.
Aerobic Cellular Respiration in Humans
Oxygen drawn into your blood by your lungs is used in the process of aerobic cellular respiration, an "exergonic" process meaning it produces energy for your body. This energy is drawn from glucose in your cells and is released as a substance known as adenosine triphosphate (ATP). ATP is a form of energy that cells can use to perform their basic functions. In simple terms: glucose + oxygen = carbon dioxide, water and ATP (energy).
The Three Stages of Aerobic Cellular Respiration
The first stage of aerobic cellular respiration is glycolysis, which partially oxidizes glucose in the cells and produces two ATP molecules and some pyruvate molecules.
The second stage known as the Krebs or citric acid cycle releases further ATP molecules by processing the pyruvate molecules from the glycolysis stage into ATP.
The third and final stage known as the electron transport chain produces the last ATP molecules as well as water.
Anaerobic Cellular Respiration in Humans
Anaerobic cellular respiration occurs after the glycolysis stage of respiration when there is no oxygen available. This usually happens in muscle tissue cells during periods of physical exertion when the lungs are unable to draw in enough oxygen to create ATP energy. This creates what is known as an "oxygen debt" as lactic acid is produced from the glucose in cells. This lactic acid buildup is what may cause a burning sensation during exercise. Extra oxygen is required to remove this lactic acid and is what causes our body to breathe heavily after exercising.
Anaerobic Cellular Respiration in Yeast
Some cells solely respire anaerobically. Yeast produces carbon dioxide and ethanol when it reacts with glucose; this process is what causes bread to rise in the oven and is also a result of anaerobic cellular respiration.