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Breathing Basics - Part 1: Anatomy and Physiology

Updated: Mar 2

Breathing ensures healthy CO2 levels. The main functions of breathing are gas exchange and acid-base (pH) regulation. The respiratory system exchanges oxygen for carbon dioxide (CO2) released by cells during metabolism. CO2 regulates our physiology by increasing nitric oxide and oxygen delivery when tissues are more active. Our body uses 85-88% of CO2 to ensure a healthy acid-base balance, making gas exchange possible through the Bohr effect (Khazan, 2021).

Without sufficient carbon dioxide, the oxygen that we have remains in the bloodstream and does not go into our tissues, where it is most needed (Khazan, 2019, p. 27).

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Dr. Khazan explains internal respiration © Association for Applied Psychophysiology and Biofeedback.


The abbreviation pH refers to the power of hydrogen, which is the concentration of hydrogen ions. Acidic solutions have a low pH (< 7) due to a high concentration of hydrogen ions. A neutral solution of distilled water has a pH of 7. Alkaline or basic solutions have a high pH (>7) due to a low concentration of hydrogen ions. The pH level regulates oxygen and nitric oxide release. Graphic © AlexVector/

pH scale

Hemoglobin molecules on red blood cells transport nitric oxide and oxygen through the bloodstream. Graphic © Designua/


Hemoglobin rarely carries all four nitric oxide and oxygen molecules simultaneously. Red blood cell graphic ©

red blood cells

When nitric oxide reaches the capillaries, it promotes dilation, increased blood flow, and improved vascular function. Increased blood flow delivers more oxygen and nutrients to the tissues and removes waste products more effectively. This is particularly important during exercise when skeletal muscles require additional oxygen and nutrients to maintain performance. After oxygen-rich blood enters the capillaries, oxygen disperses into cells to power aerobic cellular respiration (Fox & Rampolski, 2022). Capillary graphic © hareluya/

Relaxed Breathing

Relaxed breathing increases the carbon dioxide concentration of arterial blood compared to thoracic breathing. At rest, we only excrete 12-15% of blood CO2. Conserving CO2 lowers blood pH, weakens the bond between hemoglobin and oxygen, and increases oxygen delivery to body tissues. This phenomenon is called the Bohr effect. Increased CO2 levels and decreased pH also promote nitric oxide release from hemoglobin in concert with the Bohr effect.

Check out MEDCRAMvideos YouTube lecture Oxygen Hemoglobin Dissociation Curve Explained Clearly! Graphic adapted from Inna Khazan.

Conversely, low CO2 levels due to overbreathing or hyperventilation raise blood pH and reduce oxygen delivery to body tissues since oxygen remains tightly bound to the hemoglobin molecules (Fox & Rompolski, 2022).

Conserve CO2

We do not need more oxygen! (Khazan, 2021). Near sea level, the air healthy clients inhale contains 21% oxygen, while the air they exhale has 15%. We only use ¼ of inhaled oxygen and don’t need more. We need to conserve CO2 by retaining 85-88% of it.

The Respiratory Cycle

We breathe about 20,000 times a day. Typical adult resting breathing rates are 12-14 breaths per minute (bpm; Khazan, 2019a). Disorders that affect respiration may raise rates to 18-28 bpm (Fried, 1987; Fried & Grimaldi, 1993).

The respiratory cycle consists of inhalation (breathing in) and exhalation (breathing out), controlled by separate mechanisms. Animation © weicheltfilm/

The lungs cannot inflate themselves since they lack skeletal muscles. Instead, they passively inflate by creating a partial vacuum by the diaphragm and external intercostal muscles (Gevirtz, Schwartz, & Lehrer, 2016).

During inhalation, contraction by the diaphragm and external intercostal muscles ventilate the lungs.

The dome-shaped diaphragm muscle plays the lead role during inhalation. The diaphragm comprises the floor of the thoracic cavity. When the diaphragm contracts, it flattens, and its dome drops, increasing thoracic cavity volume. Contraction of the diaphragm pushes the rectus abdominis muscle of the stomach down and out.

In the animation below, watch the lungs inflate as the diaphragm descends. Animation © look_around/

In relaxed breathing, a 1-cm descent creates a 1-3 mmHg pressure difference and moves 500 milliliters of air. In labored breathing, a 10-cm descent produces a 100-mmHg pressure difference and transports 2-3 liters of air. The diaphragm accounts for about 75% of air movement into the lungs during relaxed breathing.

The external intercostals play a supporting role during inhalation. External intercostal muscle contraction pulls the ribs upward and enlarges the thoracic cavity. The external intercostals account for about 25% of air movement into the lungs during relaxed breathing.

The contraction of the diaphragm and the external intercostals expands the thoracic cavity, increases lung volume, and decreases the pressure within the lungs below atmospheric pressure. This pressure difference causes air to inflate the lungs until the alveolar pressure returns to atmospheric pressure.

During forceful inhalation, accessory muscles of inhalation (sternocleidomastoid, scalene, pectoralis major and minor, serratus anterior, and latissimus dorsi) also contract (Khazan, 2021). Graphic © Designua/


The dome-shaped diaphragm muscle ascends during normal expiration.

The relaxation of the diaphragm and external intercostal muscles, contraction of the internal intercostals, the elastic recoil of the chest wall and lungs, and surface tension produce exhalation during relaxed breathing.

When the diaphragm relaxes, its dome moves upward. When the external intercostals relax, the ribs move downward. These changes reduce the thoracic cavity volume and the lungs and increase the pressure within the lungs above atmospheric pressure. This pressure difference causes air to deflate the lungs until the alveolar pressure returns to atmospheric pressure.

Forceful exhalation during exercise recruits the rectus abdominis, external and internal obliques, and transversus abdominis abdominal muscles (Khazan, 2021; Lorig, 2007; Tortora & Derrickson, 2021). Graphic © Alila Medical Media/

Inspiration and Expiration

The BioGraph ® Infiniti display below shows healthy inhalation and exhalation in which the abdomen gradually expands and then contracts. The purple respirometer waveform shows abdominal expansion and contraction. The pink instantaneous heart rate waveform speeds and slow across each breathing cycle.

Respiration Anatomy Summary

Respiration Anatomy Summary


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accessory muscles: the sternocleidomastoid, pectoralis minor, scalene, and trapezius muscles, which are used during forceful breathing, as well as during clavicular and thoracic breathing.

apnea: breath suspension.

bicarbonates: salts of carbonic acid that contain HC03.

Bohr effect: when the levels of CO2 and hydrogen ions (H+) in the blood increase (resulting in a decrease in pH or increased acidity), hemoglobin's affinity for oxygen decreases. This leads to the release of oxygen from hemoglobin, making it more available to the surrounding tissues.

diaphragm: the dome-shaped muscle whose contraction enlarges the vertical diameter of the chest cavity and accounts for about 75% of air movement into the lungs during relaxed breathing.

end-tidal CO2: the percentage of CO2 in exhaled air at the end of exhalation.

exhalation: the process of releasing air from the lungs.

external intercostals: the muscles of inhalation that pull the ribs upward and enlarge the thoracic cavity. The external intercostals account for about 25% of air movement into the lungs during relaxed breathing.

hemoglobin: a red blood cell protein that carries oxygen throughout the circulatory system.

hypocapnia: decreased CO2 in arterial blood.

inhalation: the process of drawing air into the lungs.

nitric oxide (NO): a gaseous neurotransmitter that promotes vasodilation and long-term potentiation.

overbreathing: a mismatch between breathing rate and depth due to excessive breathing effort and subtle breathing behaviors like sighs and yawns can reduce arterial CO2.

pH: the power of hydrogen; the acidity or basicity of an aqueous solution determined by the concentration of hydrogen ions.

rectus abdominis: a muscle of forceful expiration that depresses the inferior ribs and compresses the abdominal viscera to push the diaphragm upward.

respiratory cycle: an inspiratory phase, inspiratory pause, expiratory phase, and expiratory pause.

respiratory membrane: the site of respiratory gas exchange comprised of alveolar and capillary walls.


Andreassi, J. L. (2007). Psychophysiology: Human behavior and physiological response (5th ed.). Lawrence Erlbaum and Associates, Inc.

Fox, S. I., & Rompolski, K. (2022). Human physiology (16th ed.). McGraw-Hill.

Fried, R., & Grimaldi, J. (1993). The psychology and physiology of breathing. Springer.

Gevirtz, R. N., Schwartz, M. S., & Lehrer, P. M. (2016). Cardiorespiratory measurement and assessment in applied psychophysiology. In M. S. Schwartz and F. Andrasik (Eds.). Biofeedback: A practitioner’s guide (4th ed.). The Guilford Press.

Khazan, I. (2019). A guide to normal values for biofeedback. In D. Moss & F. Shaffer (Eds.). Physiological recording technology and applications in biofeedback and neurofeedback (pp. 2-6). Association for Applied Psychophysiology and Biofeedback.

Khazan, I. (2019). Biofeedback and mindfulness in everyday life: Practical solutions for improving your health and performance. W. W. Norton & Company.

Khazan, I. (2021). Respiratory anatomy and physiology. BCIA HRV Biofeedback Certificate of Completion Didactice workshop.

Khazan, I. Z. (2013). The clinical handbook of biofeedback: A step-by-step guide for training and practice with mindfulness. John Wiley & Sons, Ltd.

Long, K. (2016, February 9). The science behind the sigh. The Wall Street Journal, D3.

Lorig, T. S. (2007). The respiratory system. In J. T. Cacioppo, L. G. Tassinary, & G. G. Berntson, (Eds.). Handbook of psychophysiology (3rd ed.). Cambridge University Press.

Marieb, E. N., & Hoehn, K. (2019). Human anatomy and physiology (11th ed.). Pearson Benjamin Cummings.

Stern, R. M., Ray, W. J., & Quigley, K. S. (2001). Psychophysiological recording (2nd ed.). Oxford University Press.

Tortora, G. J., & Derrickson, B. H. (2021). Principles of anatomy and physiology (16th ed.). John Wiley & Sons, Inc.


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