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Breathing Physiology

Regulation of respiration: 

  • Breathing is automatically controlled by the respiratory center in the brainstem.

  • Normally, the nervous system adjusts the rate of alveolar ventilation almost exactly to the demands of the body so that O2 and CO2 in arterial blood remain:

    • PO2 = ~95 mmHg

    • PCO2 = ~40 mmHg

The level of alveolar ventilation is driven mainly from the input of

specific chemoreceptors to the CNS:

  • Central chemoreceptors:

  1. Neurons located beneath the ventral surface of the medulla. 

  2. Control of respiration by modifying the respiratory drive: 
    o Very sensitive to CO2 level:
    o Acidosis increases respiratory drive: ↑ depth and frequency of breathing to get rid of CO2
    o Alkalosis decreases respiratory drive: ↓ the rate and depth of breaths to retain CO2 

  3. Note: there are no central PO2 receptors!

  • Peripheral chemoreceptors:

They are anatomical collections of chemoreceptors located in:

  1. Carotid bodies - near the carotid sinus, afferents to CNS in glossopharyngeal nerve IX.

  2. Aortic bodies - near the aortic arch, afferents to CNS in vagus nerve X.

                           Detects variation of the oxygen concentration in the arterial blood. 



Changes in blood H+ have a lot less effect in stimulating the chemosensitive neurons than in the blood CO2.

Study guide:

Muscles involved in breathing:

  • All muscles that attach to the rib cage have the potential to generate a breathing action.

    • The main muscle of breathing is diaphragm.

    • It is innervated by the phrenic nerve (C3, 4, 5 keep diaphragm alive!)

Spinal cord injury
Above C3:
Quiet breathing impossible
Below C5:
Quiet breathing not affected 

Muscles of inspiration:

  • External intercostal muscles are the primary inspiratory muscles. 

  • Other muscles in the neck region also have an inspiratory action:
    -    Sternocleidomastoid muscles.
    -    Scalene muscles

Muscles of expiration:

  • Internal intercostal muscles are the primary expiratory muscles.

  • The accessory expiratory muscles are:
    -    Rectus abdominus.
    -    External oblique.
    -    Internal oblique.
    ​-    Transversus abdominus

  • Resting expiration is a passive process brought about by the recoil of the lungs and rib cage at the end of inspiration.

  • Forced expiration: muscles of the abdominal wall contract (obliques, transversus, and latissimus dorsi) to force the air out of the lungs. 


  1. Hall, John E, and Arthur C Guyton. Guyton And Hall Textbook Of Medical Physiology. Print.

  2. Wilson, L. Britt. USMLE Step 1 Physiology. [New York]: Kaplan, 2013. Print.

  3. 'Chemoreceptor Regulation Of Breathing'. Boundless (2015): n. pag. Web. 17 Oct. 2015.

  4.,. 'Integrated Respiratory Control'. N.p., 2015. Web. 17 Oct. 2015.

  5. Breathing, Control. 'Control Of Breathing - Lung And Airway Disorders'. Merck Manuals Consumer Version. N.p., 2015. Web. 17 Oct. 2015.

  6. Human-kinetics,. 'Breathe Strong Perform Bettter - Anatomy And Physiology Of Muscles Involved In Breathing'. N.p., 2015. Web. 17 Oct. 2015.

  7.,. 'Diaphragm And Lungs-Voice Anatomy'. N.p., 2015. Web. 17 Oct. 2015.

  8.,. 'Muscles Of Respiration - Voice Anatomy'. N.p., 2015. Web. 17 Oct. 2015.

Figure 1 breathing muscles

First author: Modhi A.H                                                                                                                          
Reviewed by: Bayan Alzomaili
                               Roaa Amer

Format Editor: Bayan Alzomaili

Audio recording:

- Read by: Bareen Hamoud 

- Directed by: Rana Alzahrani

- Audio production: Bayan Alzomaili

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