Usually we are not aware of breathing. The respiratory muscles automatically work to absorb the new air and remove the old one. We exchange breathing gases, without which we cannot normally survive even a few minutes. But why do we breathe in the first place?
Respiration can be defined in humans as the process of exchanging respiratory gases between the external and internal environment. Simply put, we breathe the air from which we take oxygen, and vice versa, we get rid of the product of respiration, carbon dioxide. An organ called the lungs serves for that purpose.
The lungs consist of millions of very small alveoli. The alveoli are very thin and are attached to the blood capillaries, allowing certain substances to be exchanged between the bloodstream and the inside of the lungs. This exchange takes place on the basis of the so-called diffusion of gases in the direction of concentration and pressure gradients. Diffusion is simply the process of spontaneously scattering particles in space. The alveolar wall as well as the vessel wall are a natural barrier to substances such as blood elements but are permeable to gases such as oxygen and carbon dioxide. Through the concentration gradient, oxygen enters the lungs, where there is a lot of it, to the bloodstream, where it is in low concentration, and conversely, carbon dioxide enters the bloodstream, where there is a lot of it, to the lungs, where it is in very little concentration. By exhalation, we get rid of carbon dioxide in the lungs.
After passing into the bloodstream, oxygen binds to hemoglobin, which serves to transport oxygen from the lungs to the tissues and to transport carbon dioxide from the tissues to the lungs. For an idea, hemoglobin is part of the red blood cells, which are the basic components of the blood and gives the blood a specific red color. Oxygen, along with other substances such as sugars, amino acids, and fatty acids, are nutrients that supply chemical energy to cells. Chemical energy is stored in a nucleotide called ATP (adenosine triphosphate) and is later used for cellular processes such as biosynthesis, cell movement or the transfer of molecules across the cell membrane.
Carbon dioxide is a waste product of a complex process of cellular respiration. Upon contact with hemoglobin, which has released an oxygen molecule, carbon dioxide binds to this hemoglobin and travels towards the lungs. However, most of the carbon dioxide enters the lungs in the form of bicarbonate dissolved in blood plasma. In the bloodstream of the lungs, under the influence of the environment, carbon dioxide is again detached from hemoglobin and diffuses into the alveoli by diffusion, which is then transferred from our body to the external environment by exhalation.