During inspiration which muscles contract

During inspiration, the diaphragm contracts and the thoracic cavity increases in volume. This decreases the intraalveolar pressure so that air flows into the lungs. Inspiration draws air into the lungs. Expiration exhalation is the process of letting air out of the lungs during the breathing cycle. During expiration, the relaxation of the diaphragm and elastic recoil of tissue decreases the thoracic volume and increases the intraalveolar pressure.

Expiration pushes air out of the lungs. As a result, in these conditions the oxygen consumed by the respiratory muscles is increased [ 4 , 8 ]. The high levels of respiratory muscle work that must be sustained throughout heavy exercise cause respiratory muscle fatigue.

However, whether the respiratory muscle metaboreflex is sufficient to prevail on the local vasodilator effects present in locomotor muscles and redistribute blood flow to respiratory muscles is still an open question.

A further question is whether there is any hierarchy between respiratory and locomotor muscles and which limb muscle group receives a greater or smaller slice of the pie, of the total available cardiac output. It is clear that blood flow is distributed among the different limb muscles. In fact, the addition of arm exercise to leg exercise attenuates blood flow in the legs, while the addition of leg exercise to arm exercise reduces blood flow in the arms [ 10 ].

Conversely, it is not yet fully clear if respiratory muscles have a higher priority than locomotor muscles. Increasing or decreasing the work of breathing had the reciprocal effect on blood flow in the exercising legs, suggesting that the respiratory muscles demonstrate some sort of dominance over the locomotor muscles [ 11 ]. In trained cyclists, however, blood flow to the rib cage muscles intercostals is lower during exercise than when the same level of ventilation is maintained in the absence of limb movement, suggesting that blood flow is controlled in a similar way to other muscles with no evidence of priority over limb muscles [ 12 ].

It is likely that, as several animal studies suggest, blood flow to the diaphragm is less affected by sympathetic stimulation than other skeletal muscles; however, this is still to be confirmed. In addition, reductions in limb blood flow and oxygen transport in response to fatiguing respiratory muscle work would be expected to impair limb locomotor muscle function [ 13 ]. Exercising in hypoxia exacerbates these effects and the increased work of breathing during hypoxia significantly contributes to both limb muscle fatigue and reduction in exercise tolerance.

In fact, at each act of breathing a significant amount of blood, presumably from the splanchnic vasculature, is shifted between the trunk and the extremities contributing to increase cardiac output [ 14 , 15 ].

However, these mechanisms are only valid at moderate levels of exercise [ 16 ]. During heavy exercise, expiratory flow limitation and prolonged expiratory time result in higher average positive intrathoracic pressures that reduce ventricular transmural pressure and act like a Valsalva manoeuver, decreasing the rate of ventricular filling during diastole and reducing stroke volume, venous return and cardiac output.

These effects of respiratory muscles on the cardiovascular system compromise systemic oxygen delivery [ 17 ] and make the limb muscles even more susceptible to fatigue. Although breathing and stepping frequencies are sometimes independent, tuning of locomotor and ventilatory muscles is often seen in humans during activities that involve impact loading with each foot strike, such as walking and running [ 19 ]. This reduces the energy cost of breathing, optimises the action of the muscles that contribute to both functions, allows for body stabilisation during motion, and utilises trunk bending and inertial movements of soft-tissues to augment inspiratory and expiratory flow by passively assisting the action of respiratory muscles, particularly the diaphragm because the abdominal viscera directly attach to this muscle [ 20 ].

Unloading the respiratory muscles during exercise by using low-density gas mixtures such as heliox , mechanical ventilators or supplemental oxygen is neither practicable nor allowed for healthy athletes.

What can be done in order to improve the fatigue resistance and mechanical efficiency of respiratory muscles is training. Although there is still no definitive evidence as to whether it is possible to improve exercise tolerance, reliable recent studies showed that respiratory muscle training has a small but probable and significant effect on endurance exercise performance.

What needs to be determined is the mechanism or combination of mechanisms by which respiratory muscle training improves exercise performance: relief of respiratory muscle fatigue; relief of limb muscle fatigue; attenuation of the respiratory muscle metaboreflex; and relief of the discomfort associated with high levels of respiratory muscle work [ 21 — 23 ].

Women have smaller lungs and airways than height- and age-matched men, and are also likely to develop expiratory flow limitation more often than men.

For a given ventilation, women have a greater absolute oxygen cost of breathing and this represents a greater fraction of the total oxygen uptake compared to men. Although neither men nor women reach their maximal effective ventilation during exercise, women approach this value closer than men. Hence, the greater oxygen cost of breathing in women means that a greater fraction of total oxygen uptake and cardiac output is directed to the respiratory muscles, influencing exercise performance [ 24 ].

Conflict of interest None declared. National Center for Biotechnology Information , U. Journal List Breathe Sheff v. Breathe Sheff. Andrea Aliverti. Author information Copyright and License information Disclaimer. Corresponding author. E-mail: ti.

Breathe articles are open access and distributed under the terms of the Creative Commons Attribution Non-Commercial Licence 4. This article has been cited by other articles in PMC. How does the ventilatory pump work during exercise? Open in a separate window. How do respiratory muscles undertake the increased ventilatory demands of exercise? Are respiratory muscles prone to fatigue? Is there a competition amongst muscles for the available oxygen and blood flow, and which muscle comes first?

Typically, for respiration, other pressure values are discussed in relation to atmospheric pressure. Therefore, negative pressure is pressure lower than the atmospheric pressure, whereas positive pressure is pressure that it is greater than the atmospheric pressure. A pressure that is equal to the atmospheric pressure is expressed as zero.

Intra-alveolar pressure is the pressure of the air within the alveoli, which changes during the different phases of breathing Figure 2. Because the alveoli are connected to the atmosphere via the tubing of the airways similar to the two- and one-liter containers in the example above , the interpulmonary pressure of the alveoli always equalizes with the atmospheric pressure. Figure 2. Alveolar pressure changes during the different phases of the cycle.

It equalizes at mm Hg but does not remain at mm Hg. Intrapleural pressure is the pressure of the air within the pleural cavity, 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 inspiration and expiration, intrapleural pressure remains approximately —4 mm Hg throughout the breathing cycle.

Competing forces within the thorax cause the formation of the 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 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. In addition, accessory muscles primarily the internal intercostals help to compress the rib cage, which also reduces the volume of the thoracic cavity.

Respiratory volume is the term used for various volumes of air moved by or associated with the lungs at a given point in the respiratory cycle. There are four major types of respiratory volumes: tidal, residual, inspiratory reserve, and expiratory reserve Figure 4. Figure 4. These two graphs show a respiratory volumes and b the combination of volumes that results in respiratory capacity. Tidal volume TV is the amount of air that normally enters the lungs during quiet breathing, which is about milliliters.