Key messages

• The gastrointestinal tract secretes 6 to 9 liters of fluid every 24 h.

• Peristalsis is dependent on intact innervation.

• The splanchnic area receives 25 per cent of cardiac output.

• The small intestine regains postoperative motor function within hours.

• Inhibition of gastric acid secretion may cause bacterial overgrowth.

• Mucosal ischemia is an important pathogenetic factor for stress ulceration.

• Mucosal blood flow is less affected than that of the muscularis layer in critical illness.

• During critical illness villous blood flow may be maintained but oxygenation becomes impaired owing to increased demand and decreased utilization and extraction.

• The villous countercurrent exchanger causes a reduced villous oxygen delivery in hypotensive states. Introduction

The function of the gastrointestinal tract is to serve as a reservoir for ingested food and fluid, to transport and digest ingested material so that it can be absorbed, and to transport non-digestible and non-absorbable material so that it can be removed from the body. The physiology of the gastrointestinal tract is well adapted to these functional needs, and the operation of this system includes co-ordination of several mechanisms such as motility, secretion, active and passive absorption, and blood flow. These separate mechanisms, together with very special structural arrangements, make the gut well adapted to absorptive needs. In this short overview we concentrate on those aspects of gastrointestinal physiology that are likely to have an impact on the care of the critically ill patient rather than attempting to cover all its many special features.

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