Breathing is one of the most natural and fundamental processes of life, but for a long time the nature of breathing was intractable and unknown. The cell was first discovered in the 19th century, revolutionizing our understanding of life. Almost everything about life is centered around the cell.
It may not be self-evident, but breathing is actually an integral part of the metabolic process. Most multicellular organisms on Earth, even those without lungs or lunglike structures, use the abundant supply of oxygen and carbon dioxide in the atmosphere to help fuel the production of energy. This is true for plants and insects and many other forms of life.
When humans breathe in, both lungs on either side of the heart expand outward to allow oxygen to enter. Within the lungs are small sacs composed of clusters of aveoli, which are wrapped in blood vessels. Here oxygen diffuses into the blood in exchange for carbon dioxide, binding to hemoglobin. Four oxygen molecules can bind to a single red blood cell. The oxygen is then pumped to the heart through the pulmonary artery and sent out to the rest of the body.
Soon oxygen enters tissue capillaries and passively diffuses into each cell due to the lower concentration of oxygen inside of the cell membrane. Oxygen is delivered to the mitochondrion, which is kind of like the powerhouse of the cell, at the very end of the metabolic process. Having already driven the production of ATP, the main energy carrier, free electrons and hydrogen ions (charged particles of hydrogen) need something to bind to, or else the entire process will grind to a halt. These particles can freely bind to oxygen, creating water as a byproduct.
Earlier in the metabolic process, carbon dioxide was created as a byproduct due to the constant rearrangement of molecules. Carbon dioxide must then leave the body, taking a journey that is very much the opposite of the one that oxygen took. The gas diffuses out of the cell and directly into the blood plasma via capillaries as a form of bicarbonate ion. When it reaches the lungs, it is exchanged for oxygen and then expelled into the air.
Rate of Breathing
Because energy production is a nearly constant activity in the cells, breathing is nearly constant too (some animals like whales can conserve oxygen for long periods of time). This means that stressful and strenuous activity increases the rate of breathing and blood flow in order to get oxygen into the cells for higher energy production. This rate is carefully regulated by the brain.