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The diaphragm is the main inspiratory muscle. When the diaphragm contracts, its muscle fibres shorten, causing the dome of the diaphragm to descend. This results in an increase in abdominal pressure and a decrease in pleural pressure, leading to the expansion of the rib cage and lungs. The intercostal muscles, parasternals, scalene, and neck muscles also play a role in inspiration, acting on the upper part of the rib cage. During inspiration, the rib cage muscles contract while the abdominal muscles relax, allowing for optimal breathing mechanics.
| Characteristics | Values |
|---|---|
| Diaphragm muscle fibers activated | Shorten, dome descends, pleural pressure falls, abdominal pressure rises |
| Rib cage | Expands |
| Thoracic cavity | Diameter increases, length increases, volume decreases |
| Intercostal muscles | Contraction moves rib cage forward |
| Diaphragm | Becomes flat |
| Intercostal muscles | External, internal (parasternal), scalenes |
| Rib cage muscles | Inspiratory and expiratory |
| Abdominal muscles | Expiratory |
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What You'll Learn
The diaphragm descends, and abdominal pressure rises
The diaphragm is a dome-shaped muscle that separates the thorax and abdomen. It is the primary muscle that is active during inspiration. When the diaphragm contracts, it pulls its central tendon down, increasing the vertical diameter of the thorax. This increases the volume of the thoracic cavity, decreasing the pressure inside it and allowing air to enter the lungs.
When the diaphragm descends, the intrathoracic pressure decreases, and the intra-abdominal pressure increases. This increase in abdominal pressure is further sustained by the co-activation of the diaphragm and abdominal muscles. This compression of abdominal lymph vessels aids their passage upward within the thoracic duct.
The diaphragm also assists in the mechanical stabilisation of the spine via increased intra-abdominal pressure. This is achieved in conjunction with the contraction of the abdominal and pelvic floor muscles. The diaphragm's ability to modulate intrathoracic pressure is essential for respiration and gastric pressure during limb movement.
The diaphragm's contraction also assists the muscles of the anterior abdominal wall in actions such as vomiting, defecation, micturition (urination), and parturition (childbirth). These functions require increased intra-abdominal pressure, which is achieved through the contraction of the diaphragm and abdominal muscles working together.
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The rib cage expands
The diaphragm's descent also causes an additional fall in pleural pressure, which, along with the action of the external intercostals, helps elevate the ribs. This elevation of the ribs by the contraction of the external intercostals and the diaphragm's action results in the expansion of the rib cage.
The intercostal muscles are found between the ribs and are divided into external and internal intercostals. The external intercostals are responsible for moving the rib cage forward during inspiration, while the internal intercostals, specifically the parasternal intercostals, help elevate the ribs.
In addition to the diaphragm and external intercostals, the scalenes and internal intercostals also contribute to rib cage expansion. The scalenes are neck muscles that aid in elevating the ribs and expanding the chest wall. The internal intercostals, particularly the parasternal intercostals, further assist in rib elevation and chest wall expansion.
The expansion of the rib cage during inspiration is a coordinated effort of the diaphragm, external and internal intercostals, and scalenes. This coordination ensures that the rib cage expands effectively during inhalation, allowing for optimal lung expansion and ventilation.
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The thoracic cavity increases in volume
The process of breathing, or respiration, is divided into two distinct phases: inspiration (inhaling) and expiration (exhaling). When the inspiratory muscles contract during inhalation, the diaphragm contracts and pulls downward, and the muscles between the ribs (the intercostal muscles) contract and pull upward. This increases the size of the thoracic cavity, which is the space in which the lungs sit, and decreases the pressure inside. As a result, air rushes in and fills the lungs.
The diaphragm is a muscular partition that separates the thoracic and abdominal cavities. It is slightly asymmetric in humans because the broad liver lies under the right half, making the right half higher up than the left half. The diaphragm contracts and relaxes, and this movement, along with the contraction of the external intercostal muscles, increases the volume of the thoracic cavity in the anteroposterior and dorsoventral axes.
The contraction of the external intercostal muscles pushes the ribs inward and outward, allowing the rib cage to expand, which increases the width of the thoracic cavity. This expansion of the chest wall increases the volume of the lungs. The resulting increase in volume is largely attributed to an increase in alveolar space, as the bronchioles and bronchi are stiff structures that do not change in size.
During exhalation, the diaphragm relaxes, and the volume of the thoracic cavity decreases while the pressure within it increases. As a result, the lungs contract, and air is forced out.
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The intercostal muscles contract
The intercostal muscles are a group of muscles found between the ribs, consisting of 11 muscle trios in humans. They are responsible for helping form and maintain the cavity produced by the ribs, assisting with expansion and contraction during breathing. There are three types of intercostal muscles: the innermost intercostal, which lies inside the ribs; the internal intercostal muscle, which sits between two ribs; and the external intercostal muscles, which sit on the outside of the ribs.
During inhalation, the diaphragm is relaxed, allowing the lungs to expand. The innermost intercostal muscles relax, while the external intercostal muscles contract, causing the chest cavity to expand. This expansion allows the lungs to fill with air, creating negative pressure due to the extra space. Air fills the lungs, gases are exchanged, and it is time to exhale.
To force air out of the lungs, the chest cavity must become smaller to increase pressure on the air. The internal intercostal muscles contract, pulling down on the rib cage, which pushes air out of the lungs. The external intercostal muscles relax during exhalation. The internal intercostal muscles are the most important respiratory muscles for normal speech and singing, as they are responsible for propelling air out through the mouth and nose. The greater the pressure of the escaping air, the louder one's voice.
In addition to their respiratory function, intercostal muscles also play a role in maintaining the shape and size of the body cavity. For example, snakes use their intercostal muscles to create pressure when constricting prey, protecting their internal organs from damage.
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The abdominal muscles relax
The abdominal muscles are composed of three flat muscular sheets, from outside to inside: the external oblique, internal oblique, and transverse abdominis. These are supplemented by the rectus abdominis, which is found in the middle, between the ribs and the pubis at the front of the pelvis.
The abdominal muscles help to keep the body stable and balanced, even when at rest. They also provide tonic, elastic muscular support for the viscera, and by their recoil, pull down the rib cage during expiration. When the glottis is closed and the thorax and pelvis are fixed, the abdominal muscles contribute to the expulsive efforts of urination and defecation.
The abdominal muscles also play a protective role. They contract against blows to form a rigid wall around the viscera.
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Frequently asked questions
When the diaphragm contracts, it expands the abdomen and the lower part of the rib cage. The dome of the diaphragm descends, and the rib cage expands.
The diaphragm is the main inspiratory muscle. Other inspiratory muscles include the rib cage muscles, such as the intercostals, the parasternals, the scalene, and the neck muscles.
The diaphragm helps to expand the chest wall and the lung. It also acts as a flow generator during exercise.
