In this blog post, we will focus on the structure and principles of respiration and examine the process by which air is transferred from the alveoli to the bloodstream to sustain life.
In order for humans to maintain life and carry out activities, it is essential to supply oxygen to cells and expel carbon dioxide produced by metabolism from the body. This process is called respiration. The movement of air from outside the body to the lungs is called inhalation, and the movement from the lungs to outside the body is called exhalation. This flow of air is closely related to the pressure difference between the lungs and the atmosphere.
To understand this, we first need to look at the components of the respiratory system involved in the movement of air. Air that enters through the nose and mouth passes through the trachea and bronchi and finally enters the alveoli. The air that passes through the trachea and bronchi is warmed to body temperature, humidified by water vapor, and filtered to remove foreign substances. This prevents the air from damaging the alveoli. The alveoli are air sacs attached to the ends of the bronchi in the lungs, like clusters of grapes, and are where gas exchange takes place. Oxygen enters the blood through the thin walls of the alveoli, and carbon dioxide moves from the blood to the alveoli and is exhaled from the body.
The lungs, which consist of the bronchi and alveoli, are located inside the thorax, surrounded by the pleural cavity. The thoracic cavity is made up of bones, including the ribs, and muscles, such as the intercostal muscles, which protect the lungs and are completely separated from the abdomen by the diaphragm. In addition, the pleural cavity is a sac completely enclosed by two thin layers of cells called the pleura, and the space between the pleura is filled with pleural fluid. The inner pleura is attached to the lungs, and the outer pleura is attached to the chest wall, so the pleural fluid ultimately prevents the lungs and chest wall from separating from each other. This is similar to two thin glass plates stuck together by water, which cannot be easily separated due to the cohesive force of the water.
So what is the principle behind the flow of air during breathing? This is related to Boyle’s law, which states that an increase in the volume of a container reduces the pressure of a gas, while a decrease in the volume of a container increases the pressure of a gas. The pressure of the gas inside the alveoli is called alveolar pressure, and the pressure of the air outside the body is called atmospheric pressure. Generally, air flows from places of high pressure to places of low pressure, so when the alveolar pressure is lower or higher than the atmospheric pressure, air enters or exits the lungs. In other words, the volume of the lungs changes during inhalation and exhalation, and this change causes the alveolar pressure to change according to Boyle’s law, causing air to flow in and out of the lungs.
Meanwhile, changes in lung volume are affected by elastic recoil, negative pressure, and intrapleural pressure. First, elastic recoil and negative pressure in the lungs act in opposite directions. Elastic recoil is the force that causes an object to return to its original shape in response to a force that causes it to deform. The lungs have elastic recoil, which is the tendency to shrink like a balloon. At the moment when inhalation ends and exhalation begins, the alveolar pressure is equal to atmospheric pressure, so there is no movement of air. However, even at this time, the lungs are always filled with air, so positive pressure also acts to expand the lungs. At this time, the elastic recoil and positive pressure of the lungs are equal in magnitude but opposite in direction, so the lungs maintain a constant volume without any air flow. Here, negative pressure is the alveolar pressure minus the intrapleural pressure. Therefore, when the intrapleural pressure changes, the negative pressure also changes, causing a difference between the elastic recoil of the lungs and the negative pressure, which causes the volume of the lungs to change.
Intrapleural pressure refers to the pressure of the pleural fluid in the pleural cavity, which always varies within the range of atmospheric pressure. The chest wall, which is closely attached to the outer pleura, tends to move outward in the opposite direction of the force exerted by atmospheric pressure on the human body. This is called the elastic recoil of the chest wall. Therefore, the elastic recoil of the chest wall acts in the opposite direction to the elastic recoil of the lungs, which are closely attached to the inner pleura. As a result, the lungs and thoracic wall are slightly separated from each other, and the intrapleural pressure is maintained at atmospheric pressure. At this time, when the volume of the pleural cavity changes due to muscle movement, etc., the intrapleural pressure changes.
Based on the above, the processes of inhalation and exhalation are as follows. Inspiration begins with the contraction of the diaphragm, which moves downward, and the movement of the intercostal muscles, which causes the ribs to move upward and outward, expanding the thoracic cavity. As a result, the thoracic wall moves slightly away from the surface of the lungs, increasing the volume of the pleural cavity and lowering the intrapleural pressure slightly below the level when there is no air flow. This increases the negative pressure in the lungs, and when this force exceeds the elastic recoil of the lungs, the lungs expand further. As a result, the alveolar pressure decreases relative to atmospheric pressure, and air enters the alveoli due to the pressure difference. The larger the volume of the lungs, the greater the total amount of air that enters the lungs. However, since the alveoli have a limited capacity to expand and are connected to the outside, the alveolar pressure, which had been decreasing, reaches its lowest point at about the middle of inhalation and then begins to increase again. After that, the alveolar pressure becomes equal to atmospheric pressure, so there is no air flow at the end of inhalation, and the volume of the lungs becomes maximum. Exhalation follows the same sequence as inhalation, but the diaphragm moves in the opposite direction and the ribs move in the opposite direction, causing the chest to contract. Changes in pleural pressure and transpulmonary pressure then cause the volume of the lungs to change, allowing air to escape from the alveoli through the airways and into the atmosphere.
This breathing process is essential for sustaining life. Through breathing, oxygen is supplied to the body and carbon dioxide is expelled, enabling us to generate energy and perform various physiological functions. Furthermore, breathing is closely related to psychological stability. Deep, regular breathing helps reduce stress and calm the mind. This is why breathing is considered an important element in activities such as meditation and yoga.
The importance of breathing becomes even more apparent when exercising. During exercise, we need more oxygen than usual and need to expel more carbon dioxide, so our breathing becomes more active. Learning proper breathing techniques improves exercise performance and reduces fatigue. Conversely, improper breathing can lead to a lack of oxygen in the body and a buildup of carbon dioxide, which can have adverse effects on health.
Therefore, we must recognize the importance of normal breathing and strive to learn proper breathing techniques. This will promote both our physiological and psychological health. Breathing is a process that occurs automatically without our conscious awareness, but it is a vital force that sustains life. We must not forget the importance of breathing and continue to pay attention to it so that it can bring positive changes to our lives.
