How does quiet inspiration occur

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 the negative intrapleural pressure; therefore, the level must be closely monitored by the mesothelial cells and drained by the lymphatic system.

Since 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 the intrapleural and intra-alveolar pressures, and it determines the size of the lungs.

A higher transpulmonary pressure corresponds to a larger lung. 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 inspiration 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 inspiration. The recoil of the thoracic wall during expiration causes compression of the lungs.

Contraction and relaxation of the diaphragm and intercostals muscles found between the ribs cause most of the pressure changes that result in inspiration and expiration.

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.

The following formula helps to describe the relationship between airway resistance and pressure changes:. 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 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 inspiration 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. The 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: inspiration and expiration.

Inspiration is the process that causes air to enter the lungs, and expiration is the process that causes air to leave the lungs Figure 3. A respiratory cycle is one sequence of inspiration and expiration. In general, two muscle groups are used during normal inspiration: 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. Figure 3. Inspiration and expiration occur due to the expansion and contraction of the thoracic cavity, respectively. The process of normal expiration 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 inspiration. In turn, the thoracic cavity and lungs decrease in volume, causing an increase in interpulmonary pressure.

The interpulmonary 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 inspiration and expiration.

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, inspiration and expiration both occur due to muscle contractions.

In addition to the contraction of the diaphragm and intercostal muscles, other accessory muscles must also contract. During forced inspiration, 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. The lungs are situated inside the pleural cavity of the thorax and are wrapped in the pleural membrane. The lungs are unable to extend by themselves. Instead, they are inflated by the expansion and contraction movements of the diaphragm and the ribs that give the thorax its shape.

Respiratory movements during quiet breathing are described first. The muscles that contribute to quiet breathing are the external intercostal muscles and the diaphragm. The external and internal intercostals are the muscles that fill the gaps between the ribs. When drawing breath i. This causes the thorax to expand and inflate the lungs by creating negative pressure inside the thoracic cavity.

During expiration, the contraction of these muscles ceases, causing them to relax. The lungs can contract in a manner similar to a deflating balloon.

When the muscles that expand the thorax are relaxed, the lungs contract by their own elastic recoil forces, so that breath is expired. In other words, no muscles are used for expiration in quiet breathing. The movement of the ribs during quiet breathing is described below. Why does the thorax expand when the external intercostal muscles contract? Pulmonary ventilation involves three different pressures:.

Atmospheric pressure is the pressure of the air outside the body. Intraalveolar pressure is the pressure inside the alveoli of the lungs. Intrapleural pressure is the pressure within the pleural cavity. These three pressures are responsible for pulmonary ventilation. A rise in CO2 and a resultant fall in pH will increase the rate and depth of breathing, so that CO2 is blown off and the levels return to normal. These changes seem to act directly on the medullary centres. A fall in blood oxygen levels also sends impulses to the medulla to increase the rate and depth of breathing but usually only when they are very low.

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