Respiratory Anatomy Part 2 – Accessory Muscles & More

Lesson Overview

In this lesson, we go deeper into respiratory anatomy including how different muscles serve different purposes.


Become familiar with the nasal cycle and lungs, plus the differentiation between primary and accessory respiratory muscles and the muscles used for inspiration and for expiration.


Understand the function of the nose in breathing and explain the nasal cycle. Describe the connections for each nostril. Differentiate primary respiratory muscles from accessory ones and explain the purpose of the latter. Name six muscles involved in exhalation and explain why the fill capacity of the upper lungs is less than the lower. Understand the role of the intercostals in breathing.

The Lungs

  • Lungs have five lobes: three in the right lung (upper, middle and lower lobes) and two in the left (upper and lower lobes).
  • Because lungs are pear-shaped, the fill capacity of the upper lungs is less than the lower.
  • With a full inhalation, air reaches the lower lungs where, according to one source, there is more blood available for oxygen exchange. (Mukunda Stiles quoting John Douillard)

How the Diaphragm & Lungs Work

When relaxed, the diaphragm curves upward like a dome. When it contracts, it shortens and flattens, pushing on the digestive organs below and lengthening the chest cavity above. This expansion of the chest cavity draws air into the lungs. The lungs have no capacity to expand or contract on their own. They simply respond to the size and shape of their container, the chest cavity. When it expands, the lungs inflate and air rushes in to fill the vacuum. When the container shrinks, the lungs are compressed, and air is pushed out. – Julie Gudmestad 

Yoga Practices Support Lung Functioning & More

After the age of 25 or so, the ability of your lungs to function gradually declines. In general, exchange of gases between the air and blood stream happens less efficiently and provides reduced immune functioning… Age-related changes in the muscles that enable you to breathe, including the intercostal muscles and your respiratory diaphragm, can make the muscles weaker and smaller… By practicing yoga poses that address your respiratory muscles, including forward bends, backbends, sidebends, and spinal twists, you can help maintain their strength. Breath practices that lengthen your inhalations and exhalations can also help strengthen your diaphragm and intercostal muscles. – Baxter Bell MD and Nina Zolotow 

The Nose

  • Tubes in the nose moisten air from the in-breath, and fine hairs filter it.
  • Due to the relationship between a nose’s form and its function, people in different climates show a different nose structure.
  • The nasal cycle refers to how humans and other animals breathe in patterns, alternating their breathing through one nostril and then the other.
  • The right nostril corresponds to the left side of the brain (which governs thinking, intellect, and reason) and is connected to the sympathetic nervous system.
  • The left nostril corresponds to the right side of the brain (responsible for feelings and intuition) and triggers the parasympathetic nervous system.
  • See also Nadi Shodhana Pranayama.

Warms, Humidifies & Cleans

Both ways of breathing—through the nose and through the mouth—are possible, and each has its advantages and disadvantages. When you breathe through the nose, the air is warmed up and humidified; cleaned of dust particles; and cleaned of bacteria. Thus, the air that reaches the lungs is warm, purified, and of good quality. From this point of view, it is better to breathe through the nose… It’s easier to do deeper breathing through the mouth… When inhaling or exhaling through the mouth, you can also vary the airflow more easily than when you breathe through the nose. – Blandine Calais-Germain

Are Your Airways Open & Relaxed?

The openings of the nose and nasal passageways, the mouth, the pharynx (back of throat), larynx (the passageway by the vocal cords), trachea and bronchial tree that leads to the air sacks inside your lungs make up the airways that allow oxygen to come into your lungs and carbon dioxide to be released from them. Breathing practices will help you learn whether the upper and lower passages are open and relaxed. If not, you can use calming breath practices or stress management tools to try to reopen them. – Baxter Bell MD 

Nasal Cycle

Ancient yogis detected what scientists now refer to as the nasal cycle. Humans (and other animals) cycle alternately from breathing through one nostril to breathing through the other, for periods ranging from a few minutes to a few hours. This pattern continues even during sleep. In one area of investigation, yogis compared the effects of left-nostril breathing, right-nostril breathing, and breathing through both simultaneously… More and more scientific research is supporting the notion that breathing through different nostrils has very different effects on the body.  – Timothy McCall

Shape Related to the Climate

Individuals from cold, dry climates, such as Greenland or Siberia, had higher and narrower nasal cavities than those from hot, humid climates, such as Papua New Guinea or Gabon… Narrowing of the nasal passage enhances contact between the air and the mucosal tissue, which helps to warm and humidify that air… Cold, dry climate populations also show a relatively longer nasal cavity, giving this population more space in which to bring incoming air in line with body temperature. Microscopic hairs called cilia, which line the nasal passage, help to keep out pathogens and dust that may infect or irritate the lungs, and the cilia work more efficiently when incoming air is moist. “Proper heating and humidification of air in colder climates are important for respiratory health,” says paleoanthropologist Nathan Holton of the University of Iowa.  In warm-climate-adapted populations, inhalations are not directed toward the narrow upper part of the nasal cavity for warming. So “people from warm climates, moving into cold climates, could be more susceptible [to] colds and related diseases,” Noback says. – Joan Ramond

Primary vs Accessory Respiratory Muscles

Primary Respiratory Muscles*

  • Diaphragm – responsible for 75% of all respiratory effort
  • Intercostals (muscles between the ribs)
  • Some sources classify abdominals as primary, and some designate them as accessory (see note below)

Accessory Muscles for Inspiration

The accessory muscles for inhalation are helpers intended to be used for short periods of time. These muscles tire more easily. See Leslie Kaminoff’s video of a person whose diaphragm is paralyzed and therefore uses accessory muscles to breathe.

The accessory muscles for inspiration are:

  • Sternocleidomastoid (SCM) – prominent muscle at front of neck; moves rib cage by pulling it upward at top of sternum
  • Scalenes – these muscles in neck are attached to cervical vertebrae; can raise top ribs
  • Pectoralis Minor – when it contracts, it lifts ribs forward; moves when inhaling and raising top part of chest
  • Pectoralis Major – lifts ribs mostly via lower muscle fibers at ribs 4 through 8; raises sternum by opening lower ribs outward

Muscles for Expiration

The muscles for expiration (whether termed primary or accessory)* are

  • Rectus Abdominis
  • Transversus Abdominis
  • Internal Obliques
  • External Obliques
  • Pelvic Floor
  • Quadratus Lumborum

** Some sources, including Donna Farhi in The Breathing Book 1996 classify the abdominals as primary muscles of respiration (p 51). In the Anatomy of Breathing 2006 Calais-Germain describes the diaphragm as the “primary inspiratory muscle” (p 80) but doesn’t distinguish between primary and accessory muscles of expiration (p 96). In Anatomy of Hatha Yoga 2001, H. David Coulter doesn’t use the terms primary and accessory but in the quote above he includes abdominals in the “main sets of muscles” along with intercostals and diaphragm (p 74).

The Three Main Sets of Muscles

The way in which the muscles of respiration accomplish breathing is more complex than the relatively simple way a muscle creates movements around a joint. Three main sets of muscles are active when you breathe normally: the intercostal muscles, the abdominal muscles, and the respiratory diaphragm. – H. David Coulter

The Intercostals & Abdominals


  • Rib movement is caused by muscles between the ribs, called the internal and external intercostals.
  • With an inhalation, the intercostals open to expand the ribs and increase space for the lungs.
  • They contract during an exhalation.
  • When these muscles are tight, movement of the rib cage is restricted, and therefore so is the breath. Elongating the intercostal muscles improves breathing.

The external intercostal muscles run between the ribs in the same direction as the most external sheet of abdominal muscles; they lift and expand the rib cage for inhalation, like the movement of an old-fashioned pump handle as it is lifted up from its resting position. The internal intercostal muscles run at right angles to the external layer; they pull the ribs closer together as well as down and in for exhalation (usually a forced exhalation). – H. David Coulter

The Abdominals

The [abdominal] muscles shorten concentrically, pressing the abdominal wall inward, which in turn pushes the abdominal organs up against the relaxed (or relaxing) diaphragm. In combination with the action of the internal intercostal muscles, this forcibly decreases the size of the chest cavity and pushes air out of the lungs… In yoga the abdominal muscles are important for what yogis refer to as even breathing, and they are also key elements for many breathing exercises.  – H. David Coulter

See Also

  • Yogic Breathing (Conscious Exhalation: Contracting Abdominals Progressively)