Gastrointestinal system is composed of digestive tract and the accessory organs, salivary glands, pancreas, gallbladder, and liver. The upper gastrointestinal (GI) tract consists of buccal cavity (mouth), pharynx, esophagus, stomach, and duodenum. Duodenum can be divided in four segments: bulb, and descending, horizontal, and ascending duodenum. The lower GI tract includes the rest of the small intestine, and large intestine. The small intestine can be divided into three segments: duodenum (part of the upper intestine), jejunum, and ileum. The large intestine consists of cecum; ascending, transverse, descending, and sigmoid colon; rectum, and anal canal.
The layers of the GI tract surrounding lumen are
- mucosa, containing: epithelium, lamina propria, and muscularis mucosae;
- submucosa, containing: vasculature, lymphatic vessels, and Meissner’s plexus;
- (tunica) muscularis, containing: circular muscle, myenteric (Auerbach’s) plexus, and longitudinal muscle; and
- (tunica) serosa (serous layer), containing: areola connective tissue, and epithelium.
The smooth muscles of the GI tract are regulated by sympathetic and parasympathetic autonomous nervous system, and by an extensive network of intrinsic neurons of the gut, located in the submucosal plexus and in the myenteric plexus with glial cells.
Vasculature of the GI tract belongs to splanchnic circulation, which includes gastric and intestinal, and hepatic, pancreatic, and splenic circulations. Splanchnic circulation is supplied by celiac, superior mesenteric, and inferior mesenteric arteries, and is drained via portal vein to the liver. Chylomicrons (chylomicra), synthesized by enterocytes in the intestines, do not enter the splanchnic circulation and liver, but are released into the intestinal lymphatics, and enter the circulation via thoracic duct.
Most of the blood flow of GI tract is directed to the mucosa and submucosa, where perfusion can be ∼0.2-1.2 mL/(mL*min), while perfusion in outer layers is ∼0.3 mL/(mL*min) or less (Hultén et al., 1976a, 1976b, and 1977; Ivarsson et al., 1982). Perfusion in GI tract can increase substantially after a meal, and reduce during exercise. Oxygen consumption is relatively constant, as oxygen extraction changes according to perfusion changes.
Enterocytes absorb luminal contents and deliver some of that as chyle into lymphatic capillaries of the GI tract (lacteals). Lacteals connect to the submucosal lymphatic vessels, and with lymphatic vessels from the muscular layer drain into collecting lymphatic vessels, and from there to lymph nodes. Lymph nodes from the intestinal and lumbar trunks drain into a dilated sac, cisterna chyli, and from there into the thoracic duct. Chyle contains lymph, lymphocytes, immunoglobulins, albumin, and chylomicrons. Most of the lymph produced in the body is derived from the GI tract, ∼2 L/day, especially after a fat-containg meal (Alexander et al., 2010). Lymph flow is increased also during acute and chronic inflammation, partly because venous outflow may be blocked.
During a meal, glucose sensing activates sympathetic nervous system, which stimulates glucose uptake and glycogenesis in skeletal muscle, and synthesis and storage of lipids in white adipose tissue (WAT). Ghrelin, peptide YY (PYY), and GLP-1 are secreted from entero-endocrine cells in response to meal and nutrients in intestinal lumen. Gut hormones can directly modulate triglyseride metabolism in adipocytes; for instance, PYY inhibits lipolysis, and secretin stimulates lipolysis. All macronutrients elicit meal-associated thermogenesis. Cholecystokinin (CKK) and GLP-1 stimulate sympathetic innervation in brown adipose tissue (BAT), activating meal-associated thermogenesis. Secretin directly stimulates BAT thermogenesis (Li et al., 2018).
PET can be used to dynamically follow fluid distribution in the GI tract by giving orally a nonabsorbable radiotracer, such as [18F]deoxyfluoropoly(ethylene glycol) (Takashima et al., 2013).
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Created at: 2018-09-04
Updated at: 2018-11-29
Written by: Vesa Oikonen