Breathing is a necessary mechanism in the human body. A breath occurs in two parts, inhalation (or inspiration) and exhalation (or expiration). The breathing cycle is controlled by the nervous system, particularly the respiratory center of the brain, and respiratory muscles. Inhalation requires contraction of the intercostal muscles and diaphragm, along with neck and abdominal muscles, whereas exhaling is a passive action that requires no energy or exertion under normal conditions.
Breathing Structures
The respiratory tract is responsible for breathing, which is simply a means to an end, the end being gas exchange. The lungs are the organs responsible for gas exchange. Oxygen, which is necessary for cellular processes, is taken in through the upper respiratory tract into the lungs in the lower respiratory tract during inhalation, and carbon dioxide, a cellular waste product, is released during exhalation. Gas exchange occurs in the capillaries, the smallest blood vessels, of the lungs through thin cellular membranes.
The lungs inflate during inhalation and deflate during exhalation because of air sacs, known as the alveoli. Alveoli are the smallest units of lung tissue (pneumocytes) and have thin membranes, allowing the passage of air into the full depth of the lungs. A lipoprotein called pulmonary surfactant is secreted by type II pneumocyte to reduce surface tension in the alveoli, preventing collapse of the air sacs upon exhalation.
Breathing Mechanics
Pressure plays an important role in the mechanism of breathing. Three pressures are particularly important in breathing: atmospheric, intrapleural, and intrapulmonic.
Atmospheric pressure is the pressure of the external environment. The intrapulmonic pressure is the pressure within the lungs. The respiratory muscles contract and expand the chest cavity, reducing the intrapulmonic pressure and drawing atmospheric air in. As the respiratory muscles relax, exhalation occurs and the intrapulmonic pressure increases above atmospheric pressure.
The intrapleural pressure is the pressure within the potential pleural space between the parietal and visceral pleura, the membranes surrounding the lungs. It is called a potential space because there should be no real space between the membranes. The intrapleural pressure should always be slightly below atmospheric pressure, referred to as negative pressure. If an injury creates a space, that is air enters between the membranes, this pressure increases and compresses the lungs, leading to what is known as a collapsed lung. The negative pressure surrounding the lungs keeps them inflated and allows breathing to occur via the fluctuations in the intrapulmonic pressure.
Breathing Capacity and Speed
The capacity of the breathing mechanism to bring in oxygen, called the breathing capacity is affected by a few different factors, including the rate at which the cycle is carried out. First, intrapulmonic pressure determines lung capacity, and this can vary with age, body size, and the health status of the lungs and respiratory tract. In addition, the elasticity of the alveoli diminishes as a person ages. Second, the atmosphere composition and oxygen saturation determines how much oxygen is being brought into the lungs and can trigger a faster respiration rate in the respiratory center of the brain. Thinner air has a lower atmospheric oxygen concentration and occurs at higher altitudes, resulting in faster breathing. Exertion also increases the breathing rate. Lastly, the lungs have reserve air that can be forced out beyond exhalation to increase capacity, but they are never empty of all air. Forced exhalation is beyond the passive exhalation of muscle relaxation, and it involves a squeezing of the chest cavity by the intercostals muscles.
The vital capacity of the lungs, the amount of air that can be inhaled and exhaled in a breathing cycle with the most forceful inhalation and exhalation is 3.5 to 5 liters of air on average in an average person.
