Dominic Stone
Quiet breathing, also known as eupnea, is a mode of breathing that occurs at rest and does not require conscious thought. The diaphragm is the primary muscle responsible for quiet breathing, contracting to lengthen and shorten the chest cavity and expand the lungs. The diaphragm works alongside the external intercostal muscles, which elevate the ribs, and the scalenes, which are also physically active during quiet breathing. During quiet exhalation, there is little to no muscle contraction, and the process is driven by the elastic recoil of the lungs.
| Characteristics | Values |
|---|---|
| Predominant muscle of respiration | Diaphragm |
| Secondary muscle of respiration | External intercostals |
| Accessory inspiratory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, inferior fibres of serratus anterior and latissimus dorsi, serratus posterior superior, iliocostalis cervicis |
| Accessory expiratory muscles | Rectus abdominis, external oblique, internal oblique, transversus abdominis, iliocostalis, longissimus, serratus posterior inferior, quadratus lumborum |
| Muscle contraction during exhalation | Little to none; driven by elastic recoil of the lungs |
| Muscle contraction during inhalation | Contraction of the diaphragm and external intercostals |
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What You'll Learn
The diaphragm is the major muscle responsible for quiet breathing
During inhalation, the diaphragm contracts, moving its centre downwards and its edges upwards. This compresses the abdominal cavity, raising the ribs outward and upward, and expanding the thoracic cavity. This expansion draws air into the lungs.
During exhalation, the diaphragm relaxes, and the lungs recoil, causing the thoracic cavity to contract, and air to be gently exhaled. This process is passive and driven by the elastic recoil of the lungs and chest wall, with little or no muscle contraction involved.
The diaphragm works in conjunction with the intercostal muscles, which are also involved in the breathing process. The external intercostal muscles contract during inhalation, raising the ribs and expanding the thorax. During exhalation, the internal intercostal muscles are involved in forcing air out of the lungs, along with the abdominal muscles.
The breathing cycle is controlled by the respiratory centre located inside the medulla oblongata and pons of the brain stem. The dorsal respiratory group within the dorsal portion of the medulla is responsible for the largest part of the breathing cycle, sending impulses to the diaphragm and external intercostal muscles to initiate breathing.
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External intercostal muscles contract during inhalation
The process of breathing is divided into two phases: inspiration (inhaling) and expiration (exhaling). During inspiration, the diaphragm contracts and pulls downward, while the muscles between the ribs, including the external intercostal muscles, contract and pull upward. This increases the size of the thoracic cavity, expanding the chest cavity and lung volume.
The external intercostal muscles are the most superficial muscle of the intercostal muscles, which include the internal intercostal muscles and innermost intercostal muscles. They connect adjacent ribs and slope downward and forward. When the external intercostal muscles contract, the ribs are pulled upward and forward, resulting in an increase in both the anteroposterior and lateral diameters of the thorax. This movement of the ribs during inhalation is due to the relative amount of torque around the centre of rotation (the vertebral articulations) acting on the two points of attachment of the muscle to the respective ribs. The external intercostals run obliquely downward and forward, so their insertion to the lower rib is more distant from the centre of rotation than their insertion to the upper rib. Therefore, when these muscles contract, the torque acting on the lower rib is greater, and its net effect is to raise the ribs.
During quiet breathing, also known as eupnea, the diaphragm and external intercostals must contract. This is a mode of breathing that occurs at rest and does not require conscious thought. The breathing cycle is controlled by the respiratory centre located inside the brain stem, specifically the medulla oblongata and the pons. To initiate breathing, the dorsal respiratory group sends impulses through the phrenic nerve towards the diaphragm and through the intercostal nerves towards the external intercostal muscles.
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The role of accessory muscles in quiet breathing
The process of breathing involves two phases: inspiration, when air moves into the lungs, and expiration, when air leaves the lungs. The muscles of respiration, also known as the breathing pump muscles, are responsible for these inhalation and exhalation processes. The diaphragm and external intercostal muscles are the primary inspiratory muscles, while the internal intercostal muscles and abdominal muscles are involved in forced exhalation.
During quiet breathing, the diaphragm and external intercostals contract, causing the rib cage to expand and move outward, thereby increasing the volume of the thoracic cavity and lung volume. This creates a lower pressure within the lungs, causing air to be drawn into the lungs. During passive expiration, the diaphragm and external intercostal muscles relax, allowing the elastic recoil of the lungs and the decrease in volume of the thoracic cavity, which leads to an increase in pressure inside, facilitating the outward flow of air.
The accessory muscles of breathing support the primary respiratory muscles and become crucial during periods of increased oxygen demand or compromised lung function. They aid in forced inspiration and facilitate more efficient expiration. These accessory muscles include the sternocleidomastoid, scalene muscles (anterior, middle, and posterior), and abdominal muscles. The sternocleidomastoid helps to lift the sternum and increase the anteroposterior diameter of the chest, allowing for greater lung expansion. The scalene muscles assist in lifting the first two ribs and contribute to the anteroposterior expansion of the chest. The abdominal muscles, including the rectus abdominis and obliques, contract to push the diaphragm upwards, increasing intra-abdominal pressure and aiding in expelling air from the lungs.
While the accessory muscles are generally not prominent during quiet breathing, they become increasingly active as the demand for oxygen rises or when lung function is impaired. This recruitment of accessory muscles occurs during exercise or physical exertion, as well as during certain medical conditions that affect respiration. Their involvement ensures that the respiratory system can adapt to varying oxygen requirements and maintain effective breathing.
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Intra-abdominal pressure and its impact on quiet breathing
The diaphragm is the primary muscle involved in quiet breathing, also known as eupnea, which occurs at rest and does not require conscious thought. During quiet breathing, the diaphragm lengthens and shortens the chest cavity, allowing air to enter and leave the lungs.
Intra-abdominal pressure (IAP) is known to affect the function of the respiratory system. During inhalation, IAP increases, and during exhalation, it decreases. The magnitude of intra-abdominal pressure is influenced by breath control and load, with the inhalation-hold form of breath control producing significantly higher peak intra-abdominal pressure. This pressure has been shown to increase consistently during lifting tasks, and breath control is a significant factor in managing it.
In healthy individuals, intra-abdominal pressure and abdominal muscle activation during breathing vary depending on the breathing pattern and posture. For example, the external oblique muscle exhibits higher activity in the elbow-toe posture than in the supine posture during quiet nasal breathing.
In certain situations, such as exercise or playing an instrument, expiration can become active, and intra-abdominal pressure increases further. The abdominal muscles, including the rectus abdominis, contract and force the abdominal organs upward against the diaphragm, pushing it further into the thorax and reducing the volume of the thoracic cavity. This helps to push air out of the lungs during forced exhalation.
Additionally, during general anaesthesia, increased IAP affects lung mechanics and function by causing a cranial shift of the diaphragm and a reduction in lung volume. Increased IAP can also lead to lung oedema, ventilator-induced lung injury, and reduced lymphatic flow. Therefore, monitoring IAP is crucial for interpreting respiratory mechanics and ensuring appropriate ventilator settings in ARDS patients.
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The respiratory centre in the brain stem controls breathing
The act of breathing is controlled by the respiratory centre located inside the brain stem. This centre consists of three major collections of neurons. The dorsal respiratory group within the dorsal portion of the medulla is responsible for the largest part of the breathing cycle. The ventral respiratory group in the ventrolateral part of the medulla plays a role in forced expiration. The third group is the pneumotaxic centre located dorsally in the superior portion of the pons, which controls the rate and depth of breathing.
During inspiration, the diaphragm and external intercostal muscles contract, causing the rib cage to expand and move outward, and expanding the thoracic cavity and lung volume. This creates a lower pressure within the lung than that of the atmosphere, causing air to be drawn into the lungs. During quiet breathing, also known as eupnea, the diaphragm lengthens and shortens the chest cavity.
Expiration can be 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. The air pressure within the lungs increases to above the pressure of the atmosphere, causing air to be forced out of the lungs. However, during forced expiration, the internal intercostals and abdominal muscles may be involved in forcing air out of the lungs.
The respiratory rate and depth are controlled by the respiratory centres of the brain, which are stimulated by factors such as chemical and pH changes in the blood. These changes are sensed by central and peripheral chemoreceptors, which are located in the brain, and the aortic arch and carotid arteries, respectively. A rise in carbon dioxide or a decline in oxygen levels in the blood stimulates an increase in respiratory rate and depth.
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Frequently asked questions
The diaphragm is the predominant muscle of respiration during quiet breathing. The diaphragm contracts, lowering alveolar pressure, which draws air in from the mouth to the alveoli.
Quiet breathing, also known as eupnea, is a mode of breathing that occurs at rest and does not require conscious thought.
During quiet breathing, exhalation is a passive process. The diaphragm and intercostal muscles relax, allowing the lungs to contract due to their own elastic recoil.
Intercostal muscles fill the gaps between the ribs. When they contract, the ribs are elevated, causing an expansion in the thoracic cavity. They are involved in both inhalation and exhalation.
