Breathing Physiology
Regulation of respiration:
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Breathing is automatically controlled by the respiratory center in the brainstem.
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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:
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PO2 = ~95 mmHg
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PCO2 = ~40 mmHg
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The level of alveolar ventilation is driven mainly from the input of
specific chemoreceptors to the CNS:
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Central chemoreceptors:
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Neurons located beneath the ventral surface of the medulla.
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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 -
Note: there are no central PO2 receptors!
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Peripheral chemoreceptors:
They are anatomical collections of chemoreceptors located in:
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Carotid bodies - near the carotid sinus, afferents to CNS in glossopharyngeal nerve IX.
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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:
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All muscles that attach to the rib cage have the potential to generate a breathing action.
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The main muscle of breathing is diaphragm.
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It is innervated by the phrenic nerve (C3, 4, 5 keep diaphragm alive!)
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Spinal cord injury
Above C3:
Quiet breathing impossible
Below C5:
Quiet breathing not affected
Muscles of inspiration:
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External intercostal muscles are the primary inspiratory muscles.
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Other muscles in the neck region also have an inspiratory action:
- Sternocleidomastoid muscles.
- Scalene muscles
Muscles of expiration:
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Internal intercostal muscles are the primary expiratory muscles.
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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.
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Forced expiration: muscles of the abdominal wall contract (obliques, transversus, and latissimus dorsi) to force the air out of the lungs.
References:
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Hall, John E, and Arthur C Guyton. Guyton And Hall Textbook Of Medical Physiology. Print.
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Wilson, L. Britt. USMLE Step 1 Physiology. [New York]: Kaplan, 2013. Print.
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'Chemoreceptor Regulation Of Breathing'. Boundless (2015): n. pag. Web. 17 Oct. 2015.
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Pathwaymedicine.org,. 'Integrated Respiratory Control'. N.p., 2015. Web. 17 Oct. 2015.
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Breathing, Control. 'Control Of Breathing - Lung And Airway Disorders'. Merck Manuals Consumer Version. N.p., 2015. Web. 17 Oct. 2015.
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Human-kinetics,. 'Breathe Strong Perform Bettter - Anatomy And Physiology Of Muscles Involved In Breathing'. N.p., 2015. Web. 17 Oct. 2015.
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Voiceandalexandertechnique.eu,. 'Diaphragm And Lungs-Voice Anatomy'. N.p., 2015. Web. 17 Oct. 2015.
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Voiceandalexandertechnique.eu,. '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