Barrier Pressure

  • Barrier pressure = LOS pressure - intragastric pressure

Lower Oesophageal Sphincter


  • The LOS is a physiological sphincter, ie anatomically it is indistinguishable from the upper oesophagus
  • Formed by the  lower 2-4cm segment of oesophagus
  • Maintains tonic contraction of circular smooth muscle
  • Its function as a sphincter is contributed to by the following
    • ↑ number of nerve cells
      • providing tonic contraction of circular smooth muscles in the internal layer of the lower oesophagus
    • The pinch-cock mechanism from crural fibres of the diaphragm
      • wraps the oesophagus and pinches it closed during muscular contraction.
    • The flap-valve mechanism at the gastro-oesophageal junction
      • from gastric mucosal folds closing the orifice when the stomach is distended. 
    • Oblique entry of the oesophagus into the stomach
      • prevents retrograde flow
    • Intra-abdominal position of the gastro-oesophageal junction
      • subjects the lower oesophagus to intra-abdominal pressure

Barrier Pressure

  • Barrier pressure = LOS pressure - intragastric pressure
    • ie the pressure gradient that opposes gastric reflux
  • LOS maintains resting intraluminal pressure 25-35cmH2O
  • Intragastric pressure = 10cmH2O at rest
  • Therefore barrier pressure normally 15-25cmH2O
  • Reflux can occur when barrier pressure drops below 13cmH2O

Factors Affecting LOS tone

Hormone Factors

  • ↑ LOS tone
    • gastrin
    • CCK
    • motilin
    • α -adrenergic stimulation
    • oestrogen
  • ↓ LOS tone
    • progesterone
    • PGE2
    • glucagon
    • VIP
    • GIP
    • secretin

Drug Factors

  • ↑ LOS tone
    • metoclopramide
    • anticholinesterases
    • histamine
    • suxamethonium
  • ↓ LOS tone
    • anti-muscarinics
    • dopamine
    • opioids
    • ethanol
    • ganglion blockers
    • beta adrenergics

Physical Factos

  • Cough/sneeze → ↑ intra-abdominal pressure → ↑ LOS tone
  • Swallowing → transient ↓ LOS tone to allow passage of food bolus

Physiological Factors

  • Obesity → ↓ LOS tone
  • Pregnancy → ↓ LOS tone
  • Hiatus hernia → ↓ LOS tone


Physiology of Swallowing

  • Swallowing is a complex reflex controlled by the swallowing centre in the medulla
  • There are 3 phases

Oral Phase (voluntary)

  • Tongue forms a food bolus and pushes it into oropharynx by pushing up and against hard palate
  • Sensory endings of CN IX stimulated → initiates the next 2 involuntary phases

Pharyngeal Phase (involuntary)

  • Respiration is inhibited 1-2 secs, food bolus passes into upper oesophagus
  • Nasopharynx is closed by the soft palate
  • Laryngeal inlet is closed by adduction of vocal cords and aryepiglottis muscle
  • Contraction of pharynx and opening of upper oesophageal sphincter → food pushed into oesophagus

Oesophageal Phase (involuntary)

  • As soon as food enters upper oesophagus, upper sphincter closes and lower sphincter starts to relax
  • Primary, slow peristaltic wave (2-4cm/sec) initiated by swallowing centre via CN X → pressure 20 - 60mmHg
  • Gravity helps fluids to pass faster than solids
  • Presence of food in oesophagus activates stretch receptors in wall → activate intrinsic enteric nervous system → secondary peristaltic wave
  • Lower oesophageal sphincter is fully relaxed by the time food bolus gets there
  • After food passes into stomach, LOS contracts to 1-15mmHg above resting tone for 10-15secs, before returning to baseline