Initiation of Breathing

Pulmonary Volumes

• Tidal Volume: Volume of air inspired or expired with each normal breath - 500ml.

• Inspiratory reserve volume: Extra volume of air that can be inspired over and above the normal tidal volume when the person inspires with full force - 3000ml

• Expiratory reserve volume: Maximum extra volume of air that can be expired by forceful expiration after the end of a normal tidal expiration - 1100ml

• Residual volume: Volume of air remaining in lungs after the most forceful expiration - 1200ml

Pulmonary Capacities

• Inspiratory Capacity: Tidal Volume + Inspiratory reserve volume

  • 500 + 3000 = 3500ml

• Functional residual capacity: Expiratory reserve volume + Residual Volume.

  • 1100 + 1200 = 2300ml

• Vital Capacity: Inspiratory reserve volume + Tidal volume + Expiratory Reserve volume

  • 500 + 3000 + 1100 = 4600ml

• Total Lung capacity: Vital capacity + Residual Volume

  • 4600ml + 1200ml = 5800ml

Respiratory Center

• Function: Neurons send impulses to respiratory muscles to regulate breathing.

  • Dorsal Respiratory Group (DRG): Medullary rhythmicity.

    • Feeds into DRG; limits inspiration.

    • Active during inspiration with rhythmic "ramped" bursts of neuronal action potentials.

    • Inspires for 2 seconds, then allows for expiration for 3 seconds.

    • Involved in gradual increase of lung volume rather than abrupt respiratory gasps.

    • Receives sensory information from vagal and glossopharyngeal nerves.

    • Nucleus Tractus Solitarius (NTS): Sensory terminal for vagal and glossopharyngeal nerves, transmitting signals from receptors.

• Ventral Respiratory Group (VRG): Located in the medulla.

  • Differs from the DRG; does not play a role in basic rhythmical oscillations during quiet breathing.

  • Becomes active during forced exhalation and modifies inspiration and expiration with exercise or stress.

  • Contains Pre-Bötzinger Complex which may act as a central pattern generator for spontaneous breathing.

  • Controls diaphragm and external intercostal muscles, laryngeal, and pharyngeal muscles.

Efferent Pathway from Respiratory Centers

• Rootlets Exiting the Cervical Spine (C3, C4, C5):

  • Two bilateral phrenic nerves supply the diaphragm, vital for motor control.

  • Intercostal nerves provide motor input to intercostal and abdominal muscles across cervical and lumbar spine.

  • Cranial nerves provide motor output to upper airway dilator muscles.

Apneustic Center

• Located in the lower pons.

  • Promotes long, deep breaths and integrates incoming signals.

  • Involved in prolonging "ramp" action to increase tidal volume.

  • Can be overridden by pulmonary stretch receptors to limit inspiration and prevent over-inflation.

  • Apneustic Breathing: Seen in severe brain injury, characterized by prolonged inspiration interrupted by occasional expiration (gasping).

Pneumotaxic Center

• Located in the upper pons.

  • Controls the ramp duration of the DRG inspiratory signal to regulate respiratory rhythm and rate.

  • Inhibitory to the Apneustic Center, limits the activity of the phrenic nerve.

  • Adjusts the duration of inhalation and reduces tidal volume to prevent over-inflation of the lungs.

  • Damage to the upper pons can increase inspiratory depth.

Breathing Rates and Patterns

• Adult: 12-20 breaths per minute.

• Early Childhood: 20-40 breaths per minute.

• Newborn: 30-60 breaths per minute.

• Late Childhood: 15-25 breaths per minute.

• Patterns of Breathing:

  • Eupnea: Normal breathing cycle.

  • Tachypnea: Rapid, shallow breathing indicative of respiratory distress.

  • Bradypnea: Slow breathing, below normal (less than 12 bpm).

  • Apnea: Temporary halt in breathing, often associated with neurological damage.

  • Dyspnea: Subjective feeling of shortness of breath.