Muscles of Respiration

What Are the Muscles of Respiration?

The muscles of respiration are the muscles that enlarge and shrink the chest cavity to move air in and out of the lungs, and they fall into three groups: primary muscles of inspiration, accessory muscles of inspiration and muscles of expiration. The lungs have no muscle of their own to inflate themselves, so every breath depends on these muscles pulling on the walls of the thoracic cavity.

The three groups

• Primary muscles of inspiration: the diaphragm and the external intercostal muscles, active in every breath in, even during sleep.
• Accessory muscles of inspiration: neck, chest and back muscles that lift the ribs further when the body needs more air, such as during hard exercise.
• Muscles of expiration: the internal and innermost intercostals and the abdominal wall muscles, which push air out only when breathing out must be fast or forceful.

Quiet breathing out needs no muscle at all. When the diaphragm and external intercostals relax, the stretched, elastic lungs and chest wall spring back on their own, as described on our inhalation and exhalation page.

Explained simply

Your chest works like a pair of bellows. The diaphragm is the floor that pulls down and the rib muscles are the sides that lift out, so the bellows get bigger and air rushes in. When those muscles let go, the bellows spring back to their smaller size and push the air out again.

What Muscles Are Involved in Respiration When You Breathe In?

The diaphragm and the external intercostal muscles are the muscles involved in every normal breath in, with the diaphragm doing most of the work.

The diaphragm

The diaphragm is a thin, dome-shaped sheet of skeletal muscle that separates the chest from the abdomen. When it contracts, its central tendon is pulled down and the dome flattens, making the chest taller. It is responsible for about 75 percent of the air that enters the lungs during quiet breathing. Its dome moves down only about 1 cm in a quiet breath, but up to about 10 cm in a very deep one. It is supplied by the right and left phrenic nerves from the C3, C4 and C5 spinal nerve roots.

The external intercostal muscles

The external intercostals fill the spaces between neighboring ribs, with fibers running downward and forward, like hands in front pockets. When they contract they lift each rib toward the one above, which widens the chest from front to back and from side to side. Each side of the chest has 11 intercostal spaces, and our page on the intercostal muscles describes all three layers in detail.

The parasternal intercostals

Near the breastbone, the front part of the internal intercostal layer between the rib cartilages, called the parasternal or interchondral part, also helps to lift the ribs during inspiration. This is why some textbooks count it among the primary inspiratory muscles, even though the rest of the internal intercostal layer works in expiration.

What Are the Accessory Muscles of Respiration?

The accessory muscles of respiration are muscles of the neck, chest and back whose main job is moving the head, shoulders or arms, but which help lift the ribs and breastbone when the body needs to breathe in more deeply or quickly than the primary muscles can manage alone.

Accessory muscles of respiration location

• Sternocleidomastoid: runs from behind the ear down to the breastbone and collarbone; it lifts the sternum.
• Scalenes (anterior, middle and posterior): run from the neck vertebrae down to the first and second ribs, which they raise and steady.
• Pectoralis minor: lies under the pectoralis major and pulls the third to fifth ribs upward when the shoulder blade is held still.
• Pectoralis major: the large chest muscle; it lifts the ribs and sternum when the arms are fixed in place.
• Serratus anterior: wraps around the side of the chest from the ribs to the shoulder blade and can raise the ribs when the shoulder blade is fixed.
• Serratus posterior superior and levatores costarum: small back muscles that help elevate the ribs.

Accessory muscles of respiration function

Accessory muscles work by reversing their usual pull. Normally the sternocleidomastoid turns the head, but when the head is held steady it pulls up on the breastbone instead. In the same way, a person who is short of breath often leans forward with hands on the knees, fixing the shoulders so the chest muscles can lift the ribs. Accessory muscle use is normal during hard exercise; at rest it is a sign that breathing has become harder work. The scalenes are an exception in being active during quiet breathing in many people, which is why some textbooks list them as primary muscles.

Which Muscles Are Used to Breathe Out?

No muscles are needed to breathe out at rest, but during forced expiration the internal and innermost intercostals pull the ribs down and the abdominal muscles push the diaphragm up.

Quiet expiration is passive

During a relaxed breath out, the diaphragm and external intercostals simply relax. The elastic fibers in the lungs, the surface tension of the fluid lining the alveoli and the spring of the rib cage pull the chest back to its resting size, and air flows out without any muscle effort.

Forced expiration muscles

• Internal intercostals: their fibers run at right angles to the external layer, and they pull each rib down toward the one below.
• Innermost intercostals, subcostales and transversus thoracis: thin inner chest wall muscles that help depress the ribs.
• Rectus abdominis: the straight muscle down the front of the abdomen, which pulls the lower ribs and breastbone down.
• External and internal obliques: diagonal abdominal muscles that compress the abdomen and pull the ribs down.
• Transversus abdominis: the deepest abdominal layer, which squeezes the abdomen like a belt.
• Serratus posterior inferior: a back muscle that pulls the lowest ribs down and back.

When the abdominal muscles contract, they raise the pressure inside the abdomen and push the relaxed diaphragm up into the chest, which squeezes air out quickly. This is the force behind a cough, a sneeze, shouting and blowing out candles, and it is how lung function tests measure the vital capacity.

How Do the Breathing Muscles Move the Rib Cage?

The breathing muscles enlarge the chest in three directions: the diaphragm makes it taller, and the intercostal and accessory muscles lift the ribs to make it deeper from front to back and wider from side to side.

Pump handle and bucket handle movements

The ribs are hinged at the spine and slope downward toward the front. When the upper ribs are raised, the breastbone moves forward and up, like the handle of an old water pump, which increases the front-to-back depth of the chest. When the lower ribs are raised, their middle parts swing outward, like the handle of a bucket being lifted, which increases the width. The structure that makes this possible is described on our rib cage and breathing page.

From muscle movement to airflow

As the chest grows, the lungs follow it because the thin layer of fluid in the pleural cavity holds the outer surface of each lung against the inside of the chest wall. A larger lung volume lowers the pressure inside the alveoli a little below the pressure of the outside air, and air flows in until the two pressures are equal. When the muscles relax, the process runs in reverse. Our page on the mechanics of breathing explains these pressure changes and Boyle's law in detail.

How Are the Respiratory Muscles Controlled?

The respiratory muscles are controlled automatically by breathing centers in the brainstem, which send rhythmic signals through the phrenic and intercostal nerves, but they can also be overridden voluntarily, for example when speaking, singing or holding a breath.

Automatic control

Groups of nerve cells in the medulla oblongata and pons set the basic rhythm of breathing, roughly 12 to 20 breaths per minute in a resting adult. They adjust the depth and rate of each breath in response to signals from chemoreceptors that sense carbon dioxide, oxygen and acidity in the blood, and from stretch receptors in the lungs. This is covered on our control of breathing page.

Voluntary control

Because the breathing muscles are skeletal muscles, the motor cortex can also command them directly, which is how you can take a deep breath on purpose or shape airflow for speech. The automatic centers always win in the end: as carbon dioxide builds up, the urge to breathe becomes too strong to resist.

Nerve supply

The diaphragm is supplied only by the phrenic nerves (C3 to C5), the intercostal muscles by the intercostal nerves (T1 to T11), the abdominal muscles by the lower intercostal, subcostal and first lumbar nerves, and the sternocleidomastoid by the accessory nerve, a cranial nerve.

Which Muscles Keep the Airway Open During Breathing?

Besides the pump muscles of the chest, small muscles in the throat and larynx keep the upper airway open with each breath in, so that the falling pressure inside does not suck the soft walls inward.

• Genioglossus: the main muscle of the tongue, which pulls the tongue forward and keeps it from falling back into the pharynx.
• Muscles of the soft palate and pharyngeal walls: stiffen the throat during inspiration.
• Posterior cricoarytenoid: the only muscle that opens the vocal folds, widening the glottis in the larynx as each breath begins.
• Nasal dilator muscles: flare the nostrils slightly during deep or labored breathing.

These muscles fire just before the diaphragm in each breath. During sleep their tone falls, which is one reason the throat can narrow or close in obstructive sleep apnea.

Common Conditions That Affect the Muscles of Respiration

Conditions that weaken the breathing muscles, damage their nerves or make breathing harder can all affect how well air is moved in and out of the lungs. Our respiratory diseases section covers lung conditions in more depth, and anyone with unexplained shortness of breath should see a doctor.

• Diaphragm paralysis: injury to a phrenic nerve can paralyze one half of the diaphragm, which then moves up instead of down during a breath in.
• High spinal cord injury: damage at or above the C3 to C5 level can stop the diaphragm from working, because its nerve roots arise there.
• Neuromuscular diseases: conditions such as amyotrophic lateral sclerosis, muscular dystrophy and myasthenia gravis can weaken the respiratory muscles over time.
• COPD and emphysema: overinflated lungs flatten the diaphragm, leaving it less effective, so neck and chest accessory muscles are used even at rest.
• Hiccups: sudden involuntary contractions of the diaphragm, explained on our why do we hiccup page.
• Muscle strain: overstretched intercostal muscles can make each breath painful.