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Respiration is the process in which oxygen is delivered to cells and carbon dioxide waste is removed from the body. The respiratory system and its components all play vital roles in this process. Cells require oxygen to be able to function, and the circulatory system and respiratory system work together to deliver this oxygen to the cells and remove the carbon dioxide from the body. Oxygen is brought into the body and carbon dioxide removed from the body by pulmonary ventilation, or breathing.
As you previously learned, pulmonary ventilation is the act of breathing, which can be described as the movement of air into and out of the lungs. The major mechanisms that drive pulmonary ventilation are atmospheric pressure, the air pressure within the alveoli (called intra-alveolar pressure), and the pressure within the pleural cavity (called intrapleural pressure). These mechanisms will be explored in this lesson.
The intra-alveolar and intrapleural pressures are dependent on certain physical features of the lung. However, the ability to breathe—to have air enter the lungs during inhalation and air leave the lungs during expiration—is dependent on the air pressure of the atmosphere and the air pressure within the lungs.
At a constant temperature, changing the volume occupied by the gas changes the pressure, as does changing the number of gas molecules.
EXAMPLE
A certain number of gas molecules in a two-liter container has more room than the same number of gas molecules in a one-liter container (see the image below). In this case, the force exerted by the movement of the gas molecules against the walls of the two-liter container is lower than the force exerted by the gas molecules in the one-liter container. Therefore, the pressure is lower in the two-liter container and higher in the one-liter container.Boyle’s law describes the relationship between volume and pressure in a gas at a constant temperature. Boyle discovered that the pressure of a gas is inversely proportional to its volume: If volume increases, pressure decreases. Likewise, if volume decreases, pressure increases. Pressure (P) and volume (V) are inversely related (P = k/V, where k is a constant for a gas sample and depends on its mass and temperature). Therefore, the pressure in the one-liter container (one-half the volume of the two-liter container) would be twice the pressure in the two-liter container. Boyle’s law is expressed by the following formula:
In this formula, P₁ represents the initial pressure, and V₁ represents the initial volume, whereas the final pressure and volume are represented by P₂ and V₂, respectively. If the two- and one-liter containers were connected by a tube and the volume of one of the containers was changed, then the gases would move from higher pressure (lower volume) to lower pressure (higher volume).
As noted above, pulmonary ventilation is dependent on three types of pressure: atmospheric, intra-alveolar, and intrapleural. Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body. Atmospheric pressure can be expressed in terms of the unit atmosphere, abbreviated atm, or in millimeters of mercury (mm Hg). One atm is equal to 760 mm Hg, which is the atmospheric pressure at sea level.
Intra-alveolar pressure (intrapulmonary pressure) is the pressure of the air within the alveoli, which changes during the different phases of breathing (see the image below). Because the alveoli are connected to the atmosphere via the tubing of the airways, the intrapulmonary pressure of the alveoli always equalizes with the atmospheric pressure.
Intrapleural pressure is the pressure of the air within the pleural cavity, which is between the visceral and parietal pleurae. Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing. However, due to certain characteristics of the lungs, the intrapleural pressure is always lower than, or negative to, the intra-alveolar pressure (and therefore also to atmospheric pressure). Although it fluctuates during inhalation and exhalation, intrapleural pressure remains approximately -4 mm Hg throughout the breathing cycle.
Competing forces within the thorax (the region of the chest between the neck and abdomen) cause the formation of negative intrapleural pressure. One of these forces relates to the elasticity of the lungs themselves—elastic tissue pulls the lungs inward, away from the thoracic wall. Surface tension of alveolar fluid, which is mostly water, also creates an inward pull of the lung tissue. This inward tension from the lungs is countered by opposing forces from the pleural fluid and thoracic wall. Surface tension within the pleural cavity pulls the lungs outward.
Too much or too little pleural fluid would hinder the creation of negative intrapleural pressure; therefore, the level must be closely monitored by the mesothelial cells and drained by the lymphatic system. Because the parietal pleura is attached to the thoracic wall, the natural elasticity of the chest wall opposes the inward pull of the lungs. Ultimately, the outward pull is slightly greater than the inward pull, creating the -4 mm Hg intrapleural pressure relative to the intra-alveolar pressure.
Transpulmonary pressure is the difference between intrapleural and intra-alveolar pressures, and it determines the size of the lungs. A higher transpulmonary pressure corresponds to a larger lung, whereas a lower transpulmonary pressure corresponds to a smaller lung. While transpulmonary pressure changes during the breathing cycle, the change is always positive.
In addition to the differences in pressures, breathing is also dependent upon the contraction and relaxation of muscle fibers of both the diaphragm and thorax. The lungs themselves are passive during breathing, meaning they are not involved in creating the movement that helps inhalation and expiration. This is because of the adhesive nature of the pleural fluid, which allows the lungs to be pulled outward when the thoracic wall moves during inhalation. The recoil of the thoracic wall during expiration causes compression of the lungs. Contraction and relaxation of the diaphragm and intercostal muscles (found between the ribs) cause most of the pressure changes that result in inhalation and exhalation. These muscle movements and subsequent pressure changes cause air to either rush in or be forced out of the lungs.
Other characteristics of the lungs influence the effort that must be expended to ventilate. Resistance is a force that slows motion, in this case, the flow of gases. The size of the airway is the primary factor affecting resistance. A small tubular diameter forces air through a smaller space, causing more collisions of air molecules with the walls of the airways.
As noted earlier, there is surface tension within the alveoli caused by water present in the lining of the alveoli. This surface tension tends to inhibit the expansion of the alveoli. However, pulmonary surfactant secreted by type II alveolar cells mixes with that water and helps reduce this surface tension. Without pulmonary surfactant, the alveoli would collapse during expiration.
Thoracic wall compliance is the ability of the thoracic wall to stretch while under pressure. This can also affect the effort expended in the process of breathing. In order for inhalation to occur, the thoracic cavity must expand. The expansion of the thoracic cavity directly influences the capacity of the lungs to expand. If the tissues of the thoracic wall are not very compliant, it will be difficult to expand the thorax to increase the size of the lungs.
As you have learned, difference in pressures drives pulmonary ventilation because air flows down a pressure gradient; that is, air flows from an area of higher pressure to an area of lower pressure. Air flows into the lungs largely due to a difference in pressure; atmospheric pressure is greater than intra-alveolar pressure, and intra-alveolar pressure is greater than intrapleural pressure. Air flows out of the lungs during expiration based on the same principle; pressure within the lungs becomes greater than the atmospheric pressure.
Pulmonary ventilation comprises two major steps: inhalation and exhalation. Inhalation is the process that causes air to enter the lungs, and exhalation is the process that causes air to leave the lungs. A respiratory cycle is one sequence of inhalation and exhalation.
In general, two muscle groups are used during normal inhalation: the diaphragm and the external intercostal muscles. Additional muscles can be used if a bigger breath is required. When the diaphragm contracts, it moves inferiorly toward the abdominal cavity, creating a larger thoracic cavity and more space for the lungs. Contraction of the external intercostal muscles moves the ribs upward and outward, causing the rib cage to expand, which increases the volume of the thoracic cavity. Due to the adhesive force of the pleural fluid, the expansion of the thoracic cavity forces the lungs to stretch and expand as well. This increase in volume leads to a decrease in intra-alveolar pressure, creating a pressure lower than atmospheric pressure. As a result, a pressure gradient is created that drives air into the lungs.
The process of normal exhalation is passive, meaning that energy is not required to push air out of the lungs. Instead, the elasticity of the lung tissue causes the lung to recoil, as the diaphragm and intercostal muscles relax following inhalation. In turn, the thoracic cavity and lungs decrease in volume, causing an increase in intrapulmonary pressure. The intrapulmonary pressure rises above atmospheric pressure, creating a pressure gradient that causes air to leave the lungs.
There are different types, or modes, of breathing that require a slightly different process to allow inhalation and exhalation. Quiet breathing, also known as eupnea, is a mode of breathing that occurs at rest and does not require the cognitive thought of the individual. During quiet breathing, the diaphragm and external intercostals must contract.
A deep breath, called diaphragmatic breathing, requires the diaphragm to contract. As the diaphragm relaxes, air passively leaves the lungs. A shallow breath, called costal breathing, requires contraction of the intercostal muscles. As the intercostal muscles relax, air passively leaves the lungs.
In contrast, forced breathing, also known as hyperpnea, is a mode of breathing that can occur during exercise or actions that require the active manipulation of breathing, such as singing. During forced breathing, inhalation and exhalation both occur because of muscle contractions. In addition to the contraction of the diaphragm and intercostal muscles, other accessory muscles must also contract. During forced inhalation, muscles of the neck, including the scalenes, contract and lift the thoracic wall, increasing lung volume. During forced expiration, accessory muscles of the abdomen, including the obliques, contract, forcing abdominal organs upward against the diaphragm. This helps to push the diaphragm further into the thorax, pushing more air out. In addition, accessory muscles (primarily the internal intercostals) help to compress the rib cage, which also reduces the volume of the thoracic cavity.
