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Questions and Answers
Which factors are involved in the regulation of the enteric nervous system?
Which factors are involved in the regulation of the enteric nervous system?
What is the primary role of the large intestine in the digestive process?
What is the primary role of the large intestine in the digestive process?
What occurs during mass movements in the large intestine?
What occurs during mass movements in the large intestine?
What is a potential consequence of increased absorption in the colon?
What is a potential consequence of increased absorption in the colon?
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Which of the following statements about the colonic phase is true?
Which of the following statements about the colonic phase is true?
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Which of the following correctly lists the six basic processes performed by the digestive system?
Which of the following correctly lists the six basic processes performed by the digestive system?
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What type of secretion is primarily produced by the stomach?
What type of secretion is primarily produced by the stomach?
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Which of the following correctly describes the role of interstitial cells of Cajal in gastrointestinal motility?
Which of the following correctly describes the role of interstitial cells of Cajal in gastrointestinal motility?
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Which of the following hormones, neurotransmitters, or paracrine factors is NOT typically involved in gastrointestinal function?
Which of the following hormones, neurotransmitters, or paracrine factors is NOT typically involved in gastrointestinal function?
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What is the primary function of segmental contractions in the digestive system?
What is the primary function of segmental contractions in the digestive system?
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Which of the following correctly identifies the only secretion of the large intestine?
Which of the following correctly identifies the only secretion of the large intestine?
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What triggers the secretion of digestive juices in the gastrointestinal tract?
What triggers the secretion of digestive juices in the gastrointestinal tract?
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Which enzyme secreted by the pancreas is key for digestion of fats?
Which enzyme secreted by the pancreas is key for digestion of fats?
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Study Notes
General Principles of Gastrointestinal Function
- The alimentary tract has a structure-function relationship.
- Endocrine, paracrine, and neurocrine functions differ in their mechanisms.
- Major hormones, neurotransmitters, and paracrine factors are briefly reviewed.
- The digestive system's organization and functions are broadly understood.
The Digestive System
- Fluid input into the digestive system is about 9 liters. Ingestion (food and drink) makes up 2 liters, saliva is 1.5 liters, bile is 0.5 liters, gastric secretions are 2 liters, pancreatic secretions are 1.5 liters, and intestinal secretions are 1.5 liters.
- Fluid is removed from the digestive system through absorption and excretion. Absorption in the small intestine (7.5 liters), and large intestine (1.4 liters). 0.1 liters is lost in feces.
- Dietary input is about 2000 mL of food and drink.
- Digestive secretions include saliva (1500 mL), gastric secretions (1500 mL), liver (1000 mL), pancreas (1000 mL), and intestinal secretions (2000 mL).
- Colonic mucous secretions are 200 mL.
- Water reabsorption in the small intestine is 7800 mL.
- Water reabsorption in the colon is 1250 mL.
- 150 mL of water is lost in feces.
Basic processes of the digestive system
- The digestive system completes six basic actions: ingestion, secretion, motility, digestion, absorption, and defecation.
- Glands, the GI tract wall, and digestive enzymes mix and propel food and secretions.
- Large food particles are broken down into smaller digestive enzymes and particles.
- Food molecules are sent to blood or lymphatic vessels.
- Feces are expelled.
Basic processes of the digestive system
- External influences on the digestive system include food, local changes in the digestive tract, and receptors within the digestive tract which detect factors like food, osmolarity, and acidity.
- Intrinsic nerve plexuses (short) regulate the gut's muscles.
- Extrinsic autonomic nerves and hormonal pathways (long) outside the GI tract regulate the system.
- Smooth muscles contract for motility.
- Exocrine gland cells secrete digestive juices.
- Endocrine cells secrete gastrointestinal and pancreatic hormones.
- Short and long reflexes are involved in GI function control.
Secretion of digestive juices:
- Digestive juices are produced throughout the system.
- The mouth secretes lingual lipase, salivary amylase, lysozyme, and water/electrolytes.
- The esophagus secretes mucus.
- The stomach secretes HCI, pepsin, mucus, mucin, pepsinogen, gastric lipase, and intrinsic factor.
- The liver secretes bile salts and bicarbonate.
- The pancreas secretes proteases (trypsin, chymotrypsin), pancreatic lipase, amylase.
- The small intestine secretes enzymes and mucus (salt and water).
- The large intestine primarily secretes mucus.
Ingestion
- Tongue pushes bolus against soft palate.
- Soft palate and hard palate close off nasopharynx.
- Larynx moves upward and forward, epiglottis folds to cover airway Opening.
- Upper esophageal sphincter relaxes.
- Food moves down esophagus by gravity and peristalsis.
Motility
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Slow waves (spontaneous depolarizations) are generated in interstitial cells of Cajal.
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Action potentials fire when slow wave potentials exceed the threshold.
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Force and duration of contraction depend on amplitude and frequency of action potentials.
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Smooth muscles conduct slow waves (short reflexes).
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Peristaltic contractions move food forward.
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Segmental contractions mix chyme.
Regulation of GI Function
- Hormones, paracrines, and neurocrines regulate the GI system.
- Endocrine cells in the GI tract secrete hormones into the portal or systemic circulation.
- Paracrine cells diffuse hormones to neighboring cells.
- Neurocrines involve neurons releasing neurotransmitters.
- The enteric nervous system is a component of the GI system affecting motility.
- Other components include vagal and pelvic nerves and the medulla oblongata.
Regulation of GI Function
- Higher CNS centers (hypothalamus) affect GI function via descending and ascending pathways.
- Brainstem and spinal cord regulate GI function.
Organization of the enteric nervous system
- The enteric nervous system controls the gut.
- Myenteric and submucosal plexuses coordinate activity.
Regulation of GI Function: Cephalic and gastric phase reflexes
- Long reflexes are integrated in the CNS.
- Short reflexes are integrated within the enteric nervous system.
- Cephalic and gastric phase reflexes initiate secretions.
The GI Hormones
- Gastrin stimulates gastric acid secretion and mucosal growth.
- CCK stimulates gallbladder contraction and pancreatic enzyme secretion.
- Secretin stimulates HCO3 secretion and inhibits gastric emptying and acid secretion.
- Motilin stimulates migrating motor complex.
- GIP stimulates insulin release.
- GLP-1 stimulates insulin release, inhibits glucagon release and gastric function.
Digestive Processes
- Oral cavity digests carbohydrates through saliva.
- The stomach digests proteins, lipids, and other nutrients.
- Small intestine digests and absorbs most nutrients.
- The large intestine absorbs H2O and some electrolytes; and contains beneficial bacteria.
Gastric Secretions
- Mucous cells secrete mucus.
- Mucous neck cells secrete bicarbonate.
- Parietal cells secrete gastric acid and intrinsic factor.
- ECL cells secrete histamine.
- Chief cells secrete pepsinogen and gastric lipase.
- D cells secrete somatostatin.
- G cells secrete gastrin.
Integration of cephalic and gastric phase secretion
- Food initiates cephalic and gastric phase reflexes that trigger gastric secretions.
- Gastrin stimulates acid secretion.
- Acid stimulates short reflex secretion.
- Somatostatin functions as negative feedback.
Digestion & Absorption of Fats
- Bile salts emulsify fats.
- Pancreatic lipase breaks down fats.
- Micelles form, and monoglycerides and fatty acids are absorbed.
- Cholesterol and triglycerides are packed into chylomicrons, which are absorbed by lymphatic vessels.
Digestion & Absorption of Carbohydrates
- Amylase digests starch and glycogen.
- Disaccharides are broken down into monosaccharides by specific enzymes.
- Monosaccharides are absorbed into the intestinal cells and transported to the hepatic portal vein.
Digestion & Absorption of Proteins
- Endopeptidases digest internal peptide bonds.
- Exopeptidases digest terminal peptide bonds.
- Di- and tripeptides are absorbed by cotransport with H+.
- Amino acids are absorbed by cotransport with Na+.
The Colonic Phase
- The large intestine absorbs water and electrolytes, and propels feces.
- Mass movements propel feces for elimination.
- Defecation reflex evacuates feces.
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Description
Explore the structure-function relationships of the alimentary tract and understand the diverse roles of hormones, neurotransmitters, and paracrine factors in gastrointestinal function. This quiz delves into the organization of the digestive system and fluid dynamics within it, highlighting the quantities and sources of digestive secretions.
