Question 7

Describe how the respiratory system of a newborn differs from that of an adult.

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College Answer

This question required anatomical detail relating to the upper airway and bronchial tree, which was generally answered well. The functional implications of a highly compliant chest wall in defending FRC and the relationship of FRC to closing volume was less clearly explained. Better answers mentioned the high physiological dead space, oxygen consumption and work of breathing. Additional points were awarded for discussing the immaturity of the respiratory control centre and propensity for apnoea. Common omissions included not providing comparative adult data or a written description of how neonates differed from adults (or the significance of this). Candidates confused chest wall compliance (increased in newborns) with lung compliance (reduced in newborns but rapidly approaches normal adult values as “specific compliance”). Increased oxygen consumption necessitates increased minute ventilation (with tidal volumes equivalent to adults on a weight basis) via respiratory rate. Functional Residual Capacity FRC (equivalent to adults) and Closing Capacity (increased  relative to adults) were often confused. Better answers provided responses often in tabular format. Discussion of cardiovascular responses and response to drugs were not requested and gained no marks.


Differences in Respiratory physiology of the Adult and the Neonate

Domain Differences in the neonate In the adult
  • Small mandible
  • Large tongue
  • Larger tonsils and adenoids
  • Superior laryngeal position
  • Soft, narrow, short trachea
  • Thus, intubation is more difficult
  • Intubation is easier
Airway resistance
  • Respiratory resistance is increased at birth: bronchi arre smaller and lung volumes are smaller
  •  low airway resistance
Lung volumes and spirometry variables
  • FRC is similar to adult
  • VT is similar to adult
  • Minute volume is increased
  • Resp rate is increased
  • ERV is reduced
  • Closing capacity is increased
  • Anatomical dead space is increased (3.0 ml/kg)
  • Normal anatomical dead space is 2.2 ml/kg
  • Lung compliance is decreased (less surfactant)
  • Chest wall compliance is increased (cartilaginous ribs)
  • Good lung compliance
  • Low chest wall compliance

Gas exchange

  • Increased shunt (10-25%, due to patent ductus arteriosus)
  • Foetal haemogloin = left shift of the OHDC (poor affinity for 2,3-DPG) 
  • Postatal increase in 2,3-DPG = right shift of the OHDC
  • Oxygen toxicity includes retinopathy
  • High oxygen-carrying capacity of blood because of higher Hb and haematocrit
  • Normally shunt should be minimal
  • Oxygen is relatively nontoxic
Control of respiration
  • Immature respiratory centre, rhythmogenesis and reflex responses:
  • Decreased response to hypercapnia
  • Parradoxuic resposne to hypoxia
  • Periodic apenoeas and cyclical oscillating respiratory rate 
  • Mature reflxes and rhythmogenesis
Respiratory enegetics
  • The total oxygen consumption of the neonate is increased (6-10ml/kg/min)
  • Work of breathing is increased
  • Ideal efficiency is at a resp rate between 30 and 50
  • Diaphragm is more susceptibel to fatigue (fewer Type 1 fibres)
  • Oxygen consumption is 3ml/kg/min
  • Max efficiency at resp rate 12-14


Davis, Ryan P., and George B. Mychaliska. "Neonatal pulmonary physiology." Seminars in pediatric surgery. Vol. 22. No. 4. WB Saunders, 2013.

Neumann, Roland P., and Britta S. von Ungern‐Sternberg. "The neonatal lung–physiology and ventilation." Pediatric Anesthesia 24.1 (2014): 10-21.

LoMauro, Antonella, and Andrea Aliverti. "Physiology masterclass: extremes of age: newborn and infancy." Breathe 12.1 (2016): 65-68.