Normal physiology of the gastrointestinal system

Abstract

The gastrointestinal (GI) system is responsible for digestion and absorption, but also has important endocrine, immune and barrier functions. Additionally, the GI system plays a major role in fluid, electrolyte and acid-base balance. The GI system is regulated by complex myogenic, neural and humoral mechanisms, and, in health, these are affected by the presence of luminal nutrient, thereby modulating function of the GI system. Accordingly, GI function varies depending on whether a person is fasted or in the postprandial state. Adequate fasting and postprandial perfusion, motility and exocrine secretion are required for ‘normal’ functioning. The protective mechanisms of the GI system consist of physical (intact gut mucosa), non-immune (gastric acid, intestinal mucin, bile and peristalsis) and immune (gut-associated lymphoid tissue, GALT) elements. Disruption of GI protection is a putative mechanism underlying the development of multiple-organ dysfunction syndrome. Maintenance of GI function is increasingly recognised as an important factor underlying survival in critical illness.

Abdominal pressure and gastrointestinal function: an inseparable couple?

“…Upholding GI functions depends on adequate GI motility, exocrine secretions and local tissue perfusion. GI physiology in detail is beyond the scope of the current review and has been covered elsewhere [14][15][16]. However, the course and role of GI dysfunctions in critical illness is not clearly understood and GI function monitoring tools are lacking.…”

“…The motor activity pattern of the GI system differs between fasting and fed states and both appear to be profoundly disturbed in critical ill patients [14].…”

“…Intact gut mucosa provides a physical barrier while gastric acid, intestinal mucin, bile, and peristalsis are the nonimmunological elements of gut protection against pathogens [14]. The immunological protection gut-associated lymphoid tissue (GALT) takes up orally ingested antigens that activate lymphocytes, able to secrete IgA in response to prolonged antigen exposure and allowing "oral tolerance" [15].…”

“…At the same time, this arrangement makes the tip of the villus highly susceptible to damage from hypoxia and/or hypotension ( Fig. 1) [14,17]. The oxygen extraction rate can be increased remarkably from 20% at rest in order that temporary reductions in blood flow can be tolerated.…”

“…Longer periods of intestinal hypoperfusion may result in mucosal sloughing at the tip of the villi. This sloughing disrupts the barrier, which is a putative mechanism leading to MOF [14]. As the tips of intestinal villi are essential for absorption of nutrients, such injury of the villi can lead to impaired absorption and feeding intolerance [18].…”