The liver and pancreas are both exocrine and endocrine glands; they are exocrine glands because they secrete products—bile and pancreatic juice—into the gastrointestinal tract through a series of ducts, and endocrine because they secrete other substances directly into the bloodstream.
UPPER GI TRACT
EXOCRINE
Exocrine glands secrete substances on to an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one of two types of glands in the human body, the other being endocrine glands, which secrete their products directly into the bloodstream.
A gastric chief cell is a type of gastric gland cell that releases pepsinogen and gastric lipase. It is the cell responsible for secretion of chymosin in ruminant animals.
UPPER GI TRACT
EXOCRINE
Parietal cells (also known as oxyntic cells) are epithelial cells in the stomach that secrete hydrochloric acid (HCl) and intrinsic factor. These cells are located in the gastric glands found in the lining of the fundus and body regions of the stomach. They contain an extensive secretory network
UPPER GI TRACT
GASTRIC ACID SECRETIONS
Somatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones. Somatostatin inhibits insulin and glucagon secretion.
UPPER GI TRACT
EXOCRINE
Foveolar cells or surface mucous cells are mucus-producing cells which cover the inside of the stomach, protecting it from the corrosive nature of gastric acid. These cells line the gastric mucosa.
LOWER GI TRACT
ENDOCRINE PARACRINE
The endocrine system can be contrasted to both exocrine glands, which secrete hormones to the outside of the body, and paracrine signaling between cells over a relatively short distance. Endocrine glands have no ducts, are vascular, and commonly have intracellular vacuoles or granules that store their hormones. Paracrine signaling is a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behavior of those cells. Signaling molecules known as paracrine factors diffuse over a relatively short distance, as opposed to cell signaling by endocrine factors, hormones which travel considerably longer distances via the circulatory system; juxtracrine interactions; and autocrine signaling. Cells that produce paracrine factors secrete them into the immediate extracellular environment. Factors then travel to nearby cells in which the gradient of factor received determines the outcome. Although paracrine signaling elicits a diverse array of responses in the induced cells, most paracrine factors utilize a relatively streamlined set of receptors and pathways. In fact, different organs in the body – even between different species – are known to utilize a similar sets of paracrine factors in differential development.
LOWER GI TRACT
ENDOCRINE PARACRINE
Cholecystokinin (CCK) is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. CCK is found extensively throughout the central nervous system with high concentrations found in the limbic system. It functions as an anxiogenic, panicogenic and hallucinogenic.
I cells secrete cholecystokinin (CCK), and are located in the duodenum and jejunum. They modulate bile secretion, exocrine pancreas secretion, and satiety.
LOWER GI TRACT
blood sugar
Blood sugar regulation is the process by which the levels of blood sugar, the common name for glucose dissolved in blood plasma, are maintained by the body within a narrow range.
Incretins are a group of metabolic hormones that stimulate a decrease in blood glucose levels. Incretins are released after eating and augment the secretion of insulin released from pancreatic beta cells of the islets of Langerhans by a blood-glucose–dependent mechanism.
LOWER GI TRACT
glucose homeostasis
Gastric inhibitory polypeptide or gastric inhibitory peptide also known as glucose-dependent insulinotropic polypeptide abbreviated as GIP, is an inhibiting hormone of the secretin family of hormones. While it is a weak inhibitor of gastric acid secretion, its main role is to stimulate insulin secretion.
K cells secrete gastric inhibitory peptide, an incretin, which also promotes triglyceride storage. K cells are mostly found in the duodenum.
LOWER GI TRACT
endocrine
APUD cells (DNES cells) constitute a group of apparently unrelated endocrine cells which share the common function of secreting a low molecular weight polypeptide hormone. There are several different types which secrete the hormones secretin, cholecystokinin and the others. The APUD concept is based on calcitonin-secreting parafollicular C cells of dog thyroid.
LOWER GI TRACT
ENDOCRINE PARACRINE
An enterogastrone is any hormone secreted by the mucosa of the duodenum in the lower gastrointestinal tract in response to dietary lipids that inhibits the caudal motion of the contents of chyme. The function of enterogasterone is almost the same as gastric inhibitor peptide, it inhibits gastric secretion and motility of the stomach.
LOWER GI TRACT
ENDOCRINE PARACRINE
Secretin is a hormone that regulates water homeostasis throughout the body and influences the environment of the duodenum by regulating secretions in the stomach, pancreas, and liver. It is a peptide hormone produced in the S cells of the duodenum, which are located in the intestinal glands. In humans, the secretin peptide is encoded by the SCT gene.
S cells are cells which release secretin, found in the jejunum and duodenum. They are stimulated by a drop in pH to 4 or below in the small intestine’s lumen. The released secretin will increase the secretion of bicarbonate (HCO3) into the lumen, via the pancreas. This is primarily accomplished by an increase in cyclic AMP that activates CFTR to release chloride anions into the lumen. The luminal Cl− is then involved in a bicarbonate transporter protein exchange, in which the chloride is reabsorbed by the cell and HCO3 is secreted into the lumen. S cells are also one of the main producers of cyclosamatin.
LOWER GI TRACT
glucose homeostasis
Glucagon-like peptide-1 (GLP-1) is a 30- or 31-amino-acid-long peptide hormone deriving from the tissue-specific posttranslational processing of the proglucagon peptide. It is produced and secreted by intestinal enteroendocrine L-cells and certain neurons within the nucleus of the solitary tract in the brainstem upon food consumption.
L cells secrete glucagon-like peptide-1, an incretin, peptide YY336, oxyntomodulin and glucagon-like peptide-2. L cells are primarily found in the ileum and large intestine (colon), but some are also found in the duodenum and jejunum.
LOWER GI TRACT
endocrine
The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neural control center for all endocrine systems.
Enteroendocrine cells are specialized cells of the gastrointestinal tract and pancreas with endocrine function. They produce gastrointestinal hormones or peptides in response to various stimuli and release them into the bloodstream for systemic effect, diffuse them as local messengers, or transmit them to the enteric nervous system to activate nervous responses.
LOWER GI TRACT
endocrine
Enterochromaffin (EC) cells are a type of enteroendocrine cell, and neuroendocrine cell. They reside alongside the epithelium lining the lumen of the digestive tract and play a crucial role in gastrointestinal regulation, particularly intestinal motility and secretion.
LOWER GI TRACT
fluids & processes
enteric nervous system
SECOND BRAIN
The enteric nervous system (ENS) or intrinsic nervous system is one of the main divisions of the autonomic nervous system (ANS) and consists of a mesh-like system of neurons that governs the function of the gastrointestinal tract. It is capable of acting independently of the sympathetic and parasympathetic nervous systems, although it may be influenced by them. The ENS is nicknamed the “second brain”. It is derived from neural crest cells. The neurons of the enteric nervous system control the motor functions of the system, in addition to the secretion of gastrointestinal enzymes. These neurons communicate through many neurotransmitters similar to the CNS, including acetylcholine, dopamine, and serotonin.
The submucosal plexus lies in the submucosa of the intestinal wall. The nerves of this plexus are derived from the myenteric plexus which itself is derived from the plexuses of parasympathetic nerves around the superior mesenteric artery. Branches from the myenteric plexus perforate the circular muscle fibers to form the submucosal plexus.
enteric nervous system
SECOND BRAIN
The myenteric plexus provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input, whereas the submucous plexus provides secretomotor innervation to the mucosa nearest the lumen of the gut.
either/both
processes
Peristalsis is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which is preceded by a simultaneous contraction of the longitudinal muscle and relaxation of the circular muscle in the lining of the gut.
Interstitial cells of Cajal (ICC) are interstitial cells found in the gastrointestinal tract. There are different types of ICC with different functions. ICC and another type of interstitial cell, known as platelet-derived growth factor receptor alpha (PDGFRα) cells, are electrically coupled to smooth muscle cells via gap junctions, that work together as an SIP functional syncytium. Myenteric interstitial cells of Cajal (ICC-MY) serve as pacemaker cells that generate the bioelectrical events known as slow waves. Slow waves conduct to smooth muscle cells and cause phasic contractions. They have contractile behaviour in both the cell body and the extended processes. Intramuscular Interstitial cells of Cajal (ICC-IM) are involved in mediating responses to neurotransmission.
The basal or basic electrical rhythm (BER) or electrical control activity (ECA) is the spontaneous depolarization and repolarization of pacemaker cells known as interstitial cells of Cajal (ICCs) in the smooth muscle of the stomach, small intestine, and large intestine. This electrical rhythm is spread through gap junctions in the smooth muscle of the GI tract. These pacemaker cells, also called the ICCs, control the frequency of contractions in the gastrointestinal tract.
either/both
processes
Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells which line the inner surface of the small and large intestines. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase its surface area.
either/both
processes
The gastrocolic reflex or gastrocolic response is a physiological reflex that controls the motility, or peristalsis, of the gastrointestinal tract following a meal. It involves an increase in motility of the colon consisting primarily of giant migrating contractions, or migrating motor complexes, in response to stretch in the stomach following ingestion and byproducts of digestion entering the small intestine. Thus, this ref
either/both
processes
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form of catabolism.
accessory digestive organs
fluids & processes
There are several organs and other components involved in the digestion of food. The organs known as the accessory digestive organs are the liver, gall bladder and pancreas. Other components include the mouth, salivary glands, tongue, teeth and epiglottis.
Bile, or gall, is a yellow-green fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is primarily composed of water, produced continuously by the liver, and stored and concentrated in the gallbladder.
accessory digestive organs
accessory digestive organs
fluids & processes
Enterohepatic circulation allows for recycling of metabolized and non-metabolized compounds, and is of critical importance in toxicologic processes involving the gastrointestinal tract. This circulatory route is active when ingested compounds that are absorbed in the gastrointestinal tract enter the portal circulation, go to the liver, and then return to the gastrointestinal tract via biliary excretion. The enterohepatic circulatory pathway can also be utilized by dermally absorbed or inhaled materials that are excreted in the bile.
accessory digestive organs
fluids & processes
Pancreatic juice is a liquid secreted by the pancreas, which contains a number of digestive enzymes, including trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase, nucleases and amylase. Pancreatic juice secretion is principally regulated by the hormones secretin and cholecystokinin, which are produced by the walls of the duodenum, and by the action of autonomic innervation.
abdominopelvic
The abdominopelvic cavity is a body cavity that consists of the abdominal cavity and the pelvic cavity. The upper portion is the abdominal cavity, and it contains the stomach, liver, pancreas, Spleen, gallbladder, kidneys, small intestine, and most of the large intestine. The lower portion is the pelvic cavity, and it contains the urinary bladder, the rest of the large intestine (the lower portion), and the internal reproductive organs. There is no membrane that separates out the abdominal cavity from the pelvic cavity, so the terms abdominal pelvis and peritoneal cavity are sometimes used. There are many diseases and disorders associated with the organs of the abdominopelvic cavity.
Peritoneal fluid is a serous fluid made by the peritoneum in the abdominal cavity which lubricates the surface of tissue that lines the abdominal wall and pelvic cavity. It covers most of the organs in the abdomen. An increased volume of peritoneal fluid is called ascites.