Dominic Stone
The primary muscles involved in ventilation are the diaphragm and external intercostals. The diaphragm is the main muscle used for breathing and is located below the lungs, separating the chest cavity from the abdominal cavity. The intercostal muscles are divided into three types: external, internal, and innermost. These muscles assist in breathing during physical activity. Accessory muscles, such as the sternocleidomastoid, scalenes, and pectoralis major, also play a role in ventilation during exercise or when other inspiratory muscles are impaired. Effective ventilation requires the coordinated contraction and relaxation of these muscles, which is controlled by a complex neural system.
| Characteristics | Values |
|---|---|
| Primary inspiratory muscles | Diaphragm, external intercostals |
| Accessory inspiratory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior, latissimus dorsi, serratus posterior superior, iliocostalis cervicis |
| Accessory expiratory muscles | Rectus abdominis, external oblique, internal oblique, transversus abdominis |
| Muscles involved in forceful expiration | Internal intercostals, intercostalis intimi, subcostals, abdominal muscles |
| Muscles involved in ventilation | Diaphragm, intercostals, parasternals, scalene, neck muscles, abdominal muscles |
| Accessory muscles of respiration | Sternocleidomastoid, scalenes, trapezii, latissimus dorsi, platysma, pectoralis major and minor muscles |
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What You'll Learn
The diaphragm
During inhalation, the diaphragm contracts and moves downward, creating a vacuum around the lungs, which causes air to flow in. The diaphragm is assisted in this process by the intercostal muscles, which are the muscles between the ribs. These muscles contract to pull the rib cage upward and outward, further enlarging the chest cavity and accommodating the expansion of the lungs.
Upon exhalation, the diaphragm and rib muscles relax, reducing the space in the chest cavity. As the chest cavity gets smaller, the lungs deflate and release air through the windpipe and out of the nose or mouth. During physical activity, the abdominal muscles contract and push the diaphragm against the lungs with greater force, rapidly expelling air from the lungs.
Diaphragmatic breathing is a technique that focuses on using the diaphragm correctly during respiration. It involves conscious deep breathing that allows individuals to use their lungs at full capacity, increasing lung efficiency. This technique can be practised by lying on a flat surface or sitting in a chair, with the hands placed on the upper chest and just below the rib cage to feel the diaphragm move. Diaphragmatic breathing offers several benefits, including reduced blood pressure and heart rate, improved relaxation, and decreased oxygen demand.
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Intercostal muscles
The external intercostal muscles, also known as intercostalis externus, aid in quiet and forced inhalation. They originate on ribs 1–11 and insert on ribs 2–12. During inhalation, these muscles elevate the ribs and bend them open, expanding the chest wall and the transverse dimensions of the thoracic cavity. The muscle fibres are directed downwards, forwards, and medially in the anterior part.
The internal intercostal muscles, or intercostalis internus, aid in forced expiration, while quiet expiration is a passive process. They originate on ribs 2–12 and insert on ribs 1–11. These muscles are responsible for depressing the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity. The muscle fibres are directed downwards, forwards, and laterally, forming a right angle with the external intercostal muscle.
The innermost intercostal muscles, also referred to as intercostalis intimus, are the deepest layer of internal intercostal muscles. They originate from the costal groove of one rib, posteriorly to the origin of the internal intercostals, and insert into the superior border of the immediate rib below. The muscle fibres of the innermost intercostals follow the same direction as the internal intercostals. Both the external and internal intercostal muscles are innervated by the intercostal nerves and supplied by the intercostal arteries.
Intercostal muscle strains can occur due to weakened muscles, overexertion, direct trauma from accidents or contact sports, or repetitive torso twisting. Symptoms of a strain include pain, muscle tension and stiffness, and difficulty breathing due to pain.
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Abdominal muscles
The abdominal muscles are one of the three groups of respiratory muscles, the other two being the diaphragm and the rib cage muscles. They are also accessory expiratory muscles, meaning they aid in forceful expiration. The abdominal muscles that aid in expiration include the rectus abdominis, external oblique, internal oblique, and transversus abdominis.
The abdominal muscles act on the abdomen and the abdominal rib cage, which is the lower part of the rib cage. They are expiratory, meaning they aid in breathing out. When the abdominal muscles contract, they cause the abdomen and the lower part of the rib cage to expand. This creates a slight vacuum around the lungs, which causes air to flow into the lungs. When exhaling, the abdominal muscles relax, and the lungs deflate on their own, similar to how an elastic balloon deflates when left open to the air.
The abdominal muscles play a role in regulating the ventilatory response to progressive intensity exercises such as cycling or running. During constant work rate exercises, EMG activities increased in the rectus and external oblique muscles, indicating increased respiratory-related expiratory muscle activity.
Abdominal muscle activity during mechanical ventilation can be an injurious factor in severe acute respiratory distress syndrome (ARDS), and abdominal muscle paralysis may be a strategy to minimize ventilator-induced lung injury. Patients with airway obstruction or chronic obstructive pulmonary disease typically have active abdominal muscles, and their activity may be present in patients with cystic fibrosis or generalized muscle weakness.
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Accessory muscles
The primary inspiratory muscles are the diaphragm and external intercostals. The diaphragm is the main muscle used for breathing and is a dome-shaped muscle that separates the chest and abdominal cavities. The diaphragm contracts and relaxes, altering the volume of the thoracic cavity and the lungs, producing inspiration and expiration. The intercostal muscles are those found between the ribs and they play a role in breathing during physical activity.
The sternomastoid and scalene group of muscles are accessory inspiratory muscles. The abdominal muscles and expiratory intercostal muscles are accessory expiratory muscles. The abdominal muscles involved in expiration include the rectus abdominis, external oblique, internal oblique, and transversus abdominis.
The accessory inspiratory muscles also include the scalenus anterior, medius, and posterior, the pectoralis major and minor, the inferior fibres of serratus anterior and latissimus dorsi, the serratus posterior superior may help in inspiration also the iliocostalis cervicis.
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Muscles of the face, mouth and pharynx
The muscles of the face, mouth, and pharynx are essential for controlling the lips, tongue, soft palate, and other structures to enable breathing.
The pharynx, commonly known as the throat, is a 12-14 cm or 4-5 inch muscular tube that sits behind the nasal and oral cavities, connecting to the voice box (larynx) and the oesophagus. It facilitates the passage of air, food, and liquids from the nose and mouth to the lungs and stomach. The pharyngeal muscles help push food towards the oesophagus during swallowing and assist with speaking.
The pharyngeal muscles can be divided into two layers: an outer circular layer and an inner longitudinal layer. The outer layer includes the superior, middle, and inferior pharyngeal constrictor muscles, which are responsible for propelling food into the oesophagus. The inner longitudinal muscles facilitate peristalsis (constriction), swallowing, and speaking.
The muscles of the face and mouth, including the floor of the mouth, play a role in controlling the lips, tongue, and soft palate, which are essential for breathing, swallowing, and speaking.
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Frequently asked questions
The diaphragm is the main muscle involved in ventilation. It is the primary inspiratory muscle and is located below the lungs, separating the chest cavity from the abdominal cavity.
The accessory or secondary muscles of respiration include the sternocleidomastoid, scalenes, trapezii, latissimus dorsi, platysma, and pectoralis major and minor muscles. These muscles can expand the rib cage and assist in inspiration during situations of increased ventilatory demand, such as exercise.
The abdominal muscles, including the rectus abdominis, external oblique, internal oblique, and transversus abdominis, are the accessory expiratory muscles. They act on the abdomen and the abdominal rib cage to facilitate expiration.
