Breathing involves the intake of air (inhalation) and its expulsion (exhalation). During normal, quiet breathing, the body primarily relies on the diaphragm and the external intercostal muscles. The diaphragm contracts to draw air into the lungs, while external intercostals expand the rib cage. When the body’s need for oxygen increases beyond what these primary muscles can provide, additional muscles, known as accessory muscles of respiration, are recruited to assist. These muscles enhance the expansion or compression of the thoracic cavity, facilitating greater airflow.
Accessory Muscles for Inhaling
During forced or deep inhalation, several accessory muscles engage to further expand the thoracic cavity, allowing for a larger volume of air to enter the lungs. The sternocleidomastoid muscles, located in the neck, elevate the sternum and clavicle, which significantly increases the chest volume. This action helps to lower the internal pressure within the chest, enabling more efficient airflow into the lungs. The scalene muscles, situated on the sides of the neck, lift the first and second ribs. This elevation contributes to the expansion of the upper chest.
The pectoralis minor, a smaller muscle located beneath the pectoralis major in the upper chest, assists in deep inspiration by elevating the third, fourth, and fifth ribs. The serratus anterior, originating on the surface of the upper ribs, also contributes to chest expansion. These muscles collectively work to maximize air intake when the body requires additional oxygen.
Accessory Muscles for Exhaling
While normal exhalation is typically a passive process driven by the elastic recoil of the lungs and chest wall, forced or active exhalation engages specific accessory muscles. These muscles contract to actively reduce the volume of the thoracic cavity, expelling air more quickly and forcefully. The abdominal muscles, including the rectus abdominis, external obliques, internal obliques, and transversus abdominis, play a significant role in forced exhalation. Their contraction compresses the abdominal contents, pushing the diaphragm upwards and increasing intra-abdominal pressure, which in turn forces air out of the lungs.
The internal intercostal muscles, located between the ribs, also become active during forced exhalation. Unlike the external intercostals, which are primarily involved in inspiration, the internal intercostals depress the ribs. This action reduces the dimensions of the thoracic cavity, aiding in the expulsion of air. Their coordinated contraction with the abdominal muscles ensures a more complete and forceful expulsion of air.
Situations Activating Accessory Muscles
Accessory muscles of respiration are typically recruited during situations that demand increased respiratory effort. During strenuous physical activity, such as intense exercise or heavy lifting, the body’s demand for oxygen significantly increases, prompting the engagement of these muscles to facilitate deeper and more rapid breathing. Their activation ensures adequate gas exchange to meet the metabolic needs of the body.
In clinical settings, the visible use of accessory muscles is often an indicator of respiratory distress or an underlying health condition. Conditions like asthma, chronic obstructive pulmonary disease (COPD), pneumonia, or pulmonary edema can narrow airways or impair lung function, making it difficult to breathe using only primary muscles. The body automatically activates accessory muscles to compensate for the reduced efficiency of normal breathing. Persistent use of these muscles at rest suggests a respiratory challenge that may require medical attention.