The gastrointestinal system is organized around a single terminal purpose — absorption, the transfer of macronutrients and micronutrients from the GI lumen across the epithelium into the body proper — with five supporting functions (digestion, secretion, motility, storage, and excretion) existing to make that transfer possible. Because ingested material arrives as complex molecules that cannot cross the epithelial barrier, digestion enzymatically reduces them to transportable form, secretion supplies the substances the lumen requires for that breakdown, motility propels contents along the tract, storage holds contents so material is delivered to the correct segment at the correct time via sphincter control, and excretion eliminates the unusable residue. This framework belongs to gastrointestinal physiology within systems (organ-system) physiology, and it serves as the organizing scheme under which the phases of digestion and the three classes of GI signaling molecules — neurocrine, paracrine, and endocrine (GI hormone) — are subsequently understood.
The gastrointestinal system is organized around a single terminal purpose — absorption, the transfer of macronutrients and micronutrients from the GI lumen across the epithelium into the body proper …