Breathing Physiology

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Regulation of respiration: 

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• 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

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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!
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• 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. 

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Muscles involved in breathing:

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• 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!)

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Muscles of inspiration:

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• External intercostal muscles are the primary inspiratory muscles. 

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

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Muscles of expiration:

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• Internal intercostal muscles are the primary expiratory muscles.

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

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• 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. 

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References:

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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. Pathwaymedicine.org,. '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. Voiceandalexandertechnique.eu,. 'Diaphragm And Lungs-Voice Anatomy'. N.p., 2015. Web. 17 Oct. 2015.

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

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

Audio recording:

- Read by: Bareen Hamoud 
 

- Directed by: Rana Alzahrani
 

- Audio production: Bayan Alzomaili