Chapter 41: Regulation of Respiration

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 41

To read more check out Chapter 24:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-24-circulatory-shock-and-its.html

Respiratory Center

The respiratory center is a specialized group of neurons located bilaterally within the medulla oblongata and pons of the brain stem. Its main purpose is to maintain homeostatic levels of oxygen, carbon dioxide, and hydrogen ions throughout the body's tissues by continuously regulating pulmonary ventilation. This system operates through complex neural circuits that coordinate the contraction of respiratory muscles, automatically adjusting breathing depth and frequency based on metabolic demands and chemical changes in the arterial blood.

The central control of breathing involves an intricate interplay between automatic rhythm generators, chemical sensors, and peripheral feedback loops. The dorsal respiratory group generates the fundamental inspiratory drive, while the pneumotaxic center acts as a rate limiter to fine-tune the breathing cycle. Additionally, the ventral respiratory group serves as an overdrive system activated during heightened physical exertion, and peripheral chemoreceptors in the carotid and aortic bodies monitor rapid arterial gas shifts to protect the body against acute hypoxia.

Organization of the Respiratory Center

Located bilaterally in medulla oblongata and pons.

• Consists of three major functional neuronal collections.
• Dorsal respiratory group controls main inspiratory drive.
• Dorsal group located in dorsal medullary region.
• Ventral respiratory group controls extra respiratory drive.
• Ventral group located in ventrolateral medullary region.
• Pneumotaxic center controls rate and depth.
• Pneumotaxic center located dorsally in upper pons.

Dorsal Respiratory Group of Neurons

• Plays most fundamental role in generating respiration.
• Extends through most length of the medulla.
• Most neurons located within nucleus tractus solitarius.
• Additional neurons lie in adjacent reticular substance.
• Receives sensory input via vagus and glossopharyngeal.
• Receives sensory signals from peripheral arterial chemoreceptors.
• Receives sensory input from systemic vascular baroreceptors.
• Receives sensory feedback from pulmonary neural receptors.

Rhythmical Inspiratory Discharges

• Generates basic automatic rhythm of human respiration.
• Emits repetitive bursts of inspiratory action potentials.
• Functions even when all peripheral nerves sectioned.
• Functions even after brain stem cross-section transection.
• Exact cellular mechanism of rhythmicity remains unknown.
• Reciprocal neuronal excitation and inhibition network hypothesized.
• Neural network operates continuously throughout human lifetime.

Inspiratory Ramp Signal

• Signal to diaphragm transmitted as steady ramp.
• Begins weakly and increases steadily for 2 seconds.
• Ceases abruptly for next 3 seconds duration.
• Abrupt cessation allows passive lung elastic recoil.
• Passive elastic recoil produces normal quiet expiration.
• Prevents sudden gasps by steadily expanding lungs.
• First controlled variable is ramp increase rate.
• Rapid ramp rate fills lungs during exercise.
• Second controlled variable is ramp limiting point.
• Earlier ramp cessation shortens overall inspiration duration.
• Earlier ramp cessation increases overall breathing frequency.

The Pneumotaxic Center

• Located dorsally in upper pons parabrachial nucleus.
• Transmits inhibitory signals to medullary inspiratory area.
• Primary effect controls inspiratory ramp switch-off.
• Limits total duration of lung filling phase.
• Strong signal shortens inspiration to half second.
• Strong signal fills lungs with minimal volume.
• Weak signal prolongs inspiration to five seconds.
• Weak signal fills lungs with excess air.
• Secondary effect increases overall respiratory rate.
• Strong signals increase rate to 30-40 breaths/min.
• Weak signals decrease rate to 3-5 breaths/min.

Ventral Respiratory Group of Neurons

• Located anterior and lateral to dorsal group.
• Extends between nucleus ambiguus and nucleus retroambiguus.
• Inactive during normal quiet resting respiration.
• Resting respiration depends solely on dorsal group.
• Does not participate in basic rhythm generation.
• Receives spillover signals during high ventilatory drive.
• Acts as active overdrive mechanism during exertion.
• Electrical stimulation causes inspiration or expiration.
• Sends strong expiratory signals to abdominal muscles.
• Active during very heavy breathing and exercise.
• Hering-Breuer Inflation Reflex
• Stretch receptors located in bronchial muscular walls.
• Receptors sense excessive stretch in pulmonary tissue.
• Transmit sensory signals via vagi to dorsal group.
• Signals switch off inspiratory ramp during overinflation.
• Prevents excess overexpansion of pulmonary tissues.
• Triggers increased overall rate of breathing.
• Inactive until tidal volume exceeds 1.5 liters.
• Acts primarily as protective pulmonary safety mechanism.

Direct Chemical Control via Chemosensitive Area

• Respiration maintains proper oxygen, carbon dioxide, hydrogen.
• Excess carbon dioxide directly stimulates respiratory center.
• Excess hydrogen ions directly stimulate respiratory center.
• Oxygen lacks direct effect on brain center.
• Chemosensitive area located 0.2 millimeters beneath medulla.
• Chemosensitive neurons excite other respiratory areas.
• Hydrogen ions serve as direct primary stimulus.
• Hydrogen ions cannot easily cross blood-brain barrier.
• Blood hydrogen has weaker acute effect than carbon dioxide.

Role of Carbon Dioxide in Chemosensitive Excitation

• Carbon dioxide has potent indirect stimulatory effect.
• Reacts with tissue water forming carbonic acid.
• Carbonic acid dissociates into hydrogen and bicarbonate.
• Released hydrogen ions directly excite chemosensitive neurons.
• Carbon dioxide crosses blood-brain barrier extremely easily.
• Arterial carbon dioxide increases brain interstitial fluid P_{CO}_2}.
• Higher P_{CO}_2} generates more local brain hydrogen ions.
• Carbon dioxide alters respiration faster than blood hydrogen.
• Stimulatory effect decreases after 1-2 days.
• Kidneys increase blood bicarbonate to buffer hydrogen.
• Bicarbonate diffuses slowly into brain tissue space.
• Bicarbonate neutralizes brain hydrogen back toward normal.
• Carbon dioxide causes strong acute, weak chronic effect.

Quantitative Effects of Carbon Dioxide and pH

• Marked ventilation increase occurs between 35-75 mmHg P_{CO}_2}.
• Demonstrates powerful role of carbon dioxide in regulation.
• Changing pH between 7.3-7.5 causes small effect.
• Carbon dioxide remains primary controller of daily breathing.
• Hemoglobin buffers oxygen supply across wide ranges.
• Tissue P_{CO}_2} varies inversely with pulmonary ventilation.

Peripheral Chemoreceptor System

• Located outside brainstem to monitor arterial blood.
• Primarily detects decreases in arterial oxygen.
• Responds weakly to carbon dioxide and hydrogen.
• Most chemoreceptors situated inside carotid bodies.
• Few chemoreceptors situated inside aortic bodies.
• Carotid afferents travel via Hering's and glossopharyngeal.
• Aortic afferents travel via vagi to dorsal area.
• Receives high blood flow 20 times tissue weight.
• Exposed constantly to true systemic arterial blood.

Stimulation of Chemoreceptors by Hypoxia

• Decreased arterial oxygen strongly stimulates chemoreceptor impulses.
• Impulse rate highly sensitive between 60-30 mmHg P_{CO}_2}.
• Sensitivity matches range of rapid hemoglobin desaturation.
• Increased carbon dioxide and hydrogen excite chemoreceptors.
• Central chemical direct effects seven times stronger.
• Peripheral chemoreceptors respond five times faster.
• Glomus cells synapse with peripheral nerve endings.
• Low arterial P_{O}_2} excites glomus cells or nerve endings.

Hypoxic Ventilatory Drive and Acclimatization

• Arterial P_{O}_2} below 100 mmHg increases ventilation.
• Ventilation doubles when arterial P_{O}_2} drops to 60 mmHg.
• Ventilation increases fivefold at extremely low P_{O}_2}.
• Normal breathing regulated mainly by carbon dioxide, pH.
• Slow ascent up mountains induces altitude acclimatization.
• Respiratory center loses sensitivity to carbon dioxide chronic.
• Low oxygen drives ventilation without carbon dioxide suppression.
• Alveolar ventilation increases 400-500 percent during acclimatization.
• Regulation of Respiration During Exercise
• Oxygen usage increases up to 20-fold during exercise.
• Carbon dioxide formation increases up to 20-fold.
• Alveolar ventilation increases in step with metabolism.
• Arterial P_{O}_2}, P_{CO}_2}, pH remain nearly normal.
• Brain sends motor signals to active muscles.
• Brain sends simultaneous collateral signals to brainstem.
• Collateral neurogenic signals stimulate respiratory center immediately.
• Ventilation increases instantaneously at exercise onset.
• Anticipatory neurogenic drive briefly lowers arterial P_{CO}_2}.
• Exercise metabolic production balances ventilation within 40 seconds.
• Neurogenic drive shifts ventilation response curve upward 20-fold.
• Ability to shift response curve is learned response.
• Cerebral cortex involvement required for learned response.

Other Factors Influencing Breathing

• Voluntary control overrides automatic brainstem centers temporarily.
• Voluntary hyperventilation causes severe blood gas derangements.
• Airway irritant receptors trigger coughing and sneezing.
• Irritant receptors cause bronchoconstriction in asthma, emphysema.
• Pulmonary J receptors located near alveolar capillaries.
• J receptors excited by capillary engorgement, edema.
• J receptor excitation produces subjective feeling of dyspnea.
• Acute brain edema depresses brainstem respiratory center.
• Brain swelling compresses cerebral blood supply arteries.
• Hypertonic mannitol relieves intracranial pressure and depression.
• Anesthetic or narcotic overdose depresses respiratory centers.
• Sodium pentobarbital and morphine suppress respiratory drive.

Periodic Breathing and Cheyne-Stokes Dynamics

• Periodic breathing involves alternating hyperventilation and apnea.
• Cheyne-Stokes breathing features waxing and waning cycles.
• Cycles repeat approximately every 40 to 60 seconds.
• Overbreathing excessively lowers blood carbon dioxide levels.
• Transport delay postpones altered blood reaching brain stem.
• Overventilated blood eventually depresses brain respiratory center.
• Carbon dioxide accumulates during temporary cessation of breathing.
• Accumulation eventually excites brain center to restart overbreathing.
• Normal blood gas buffering capacity damps periodic oscillations.
• Severe cardiac failure causes long lung-to-brain delay.
• Delayed transport overrides normal blood damping mechanisms.
• Brain damage increases negative feedback sensitivity gain.
• Sudden brain drive shutoff triggers extreme periodic oscillations.
• Respiratory neuron P_{CO}_2} lags behind pulmonary blood P_{CO}_2}.
• Sleep Apnea
• Apnea defined as absence of spontaneous breathing.
• Pathological sleep apnea lasts 10+ seconds hundreds times nightly.
• Obstructive sleep apnea caused by upper airway blockage.
• Pharyngeal muscle relaxation closes narrow airway passages.
• Loud snoring interrupted by silent periods of apnea.
• Apnea causes hypoxia and hypercapnia stimulating breathing.
• Loud snorts and gasps wake patient repeatedly.
• Leads to daytime drowsiness, hypertension, cardiovascular risk.
• Common in older, obese individuals with fat deposition.
• Treated by uvulopalatopharyngoplasty, tonsillectomy, or tracheostomy.
• Treated non-invasively using continuous positive airway pressure (CPAP).
• Central sleep apnea caused by abolished central drive.
• Results from central respiratory center damage or strokes.
• Patients extremely sensitive to sedatives and narcotics.
• Managed with respiratory stimulants or continuous positive pressure.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.