The Digestive System (2) Flashcards
How is the bolus processed in the mouth?
There is sensory analysis of the material in the mouth. There is mechanical processing of the food against teeth, tongue, palate and palatine rugate. The bolus is lubricated by mucus and saliva. The saliva produces lysozyme and lactoferrin for sterilisation. Bicarbonate neutralises acidic materials. Amylase is also present in the oral cavity.
State the function of lysozyme.
Lysozyme is an enzyme that acts as an antibiotic.
State the function of lactoferrin.
Lactoferrin binds iron and allows for digestion of Fe.
Describe the oropharyngeal stage of the swallowing reflex.
Swallowing is initiated voluntarily. At the start of the swallow, the bolus is pressed by the tongue against hard palate. The tongue propels the bolus into the pharynx. Swallowing center inhibits the respiratory center in the brain stem. Elevation of the uvula prevents the bolus from entering the nasal passage. The tongue is positioned such that food cannot be regurgitated. The tight alignment of the vocal cords prevents food from entering the trachea. Epiglottis is closed over the glottis. The contraction of the pharyngeal muscles pushes bolus through the pharyngoesophageal sphincter into the oesophagus.
Describe the oesophageal stage of the swallowing reflex.
The pharyngoesophageal sphincter closes, oropharyngeal structures return to resting position, and breathing resumes. Peristalsis propels the bolus down the oesophagus. The gastroesophageal sphincter relaxes as peristalsis pushes bolus down stomach. Swallow is complete, sphincter contracts again.
What are the cells of the superior fundus called?
The interstitial cells of Cajal
Where does most of the mixing of gastric material occur in the stomach?
The antrum
State the function of the interstitial cells of Cajal.
They initiate contractions of the smooth muscle of the stomach
State 2 consequences of peristaltic contraction.
Gastric emptying and gastric mixing
What is the fundus and body of the stomach lined by?
Oxyntic mucosa
What is the antrum of the stomach lined by?
Pyloric gland area
What do mucous cells secrete?
Alkaline mucus
What do chief cells secrete?
Pepsinogen
What do parietal cells secrete?
HCl and intrinsic factor
What do enterochromaffin-like (ECL) cells secrete?
Histamine
What do G cells secrete?
Gastrin
What do D cells secrete?
Somatostatin
How are parietal cells stimulated to secrete HCl?
Gastrin, histamine and ACh stimulate HCl secretion by parietal cells. Neurons release GRP (gastrin-releasing peptide). This is produced by the vagus nerve. This stimulates G cells to release gastrin. Gastrin is secreted into the bloodstream and directly causes HCl production. It indirectly causes this by stimulating ECL cells to produce histamine, which also causes HCl production in parietal cells. ACh released by neurons directly acts on parietal cells for HCl production.
How is HCL secreted by parietal cells?
Inside the parietal cell, carbonic anhydrase (ca) catalyzes the reaction between CO2 and H2O to form H2CO3. Carbonic acid quickly dissociated into H+ and HCO3- ions. The H-K ATPase proton pump actively transports H+ from the parietal cell into the gastric lumen in exchange for K+ ions using ATP. HCO3- is transported out of the parietal cell into the plasma in exchange for Cl- via secondary active transport in a process called chloride-bicarbonate exchange. The Cl- ions passively diffuse into the gastric lumen through Cl channels. The H+ and Cl- combine in the gastric lumen to form HCl. K+ ions that were brought into the cell via the H-K ATPase proton pump are recycled back into the gastric lumen via potassium channels.
Explain the mechanism of gastric secretion during the intestinal phase.
During the intestinal phase of gastric secretion, inhibitory mechanisms originating from the duodenum suppress gastric activity. The presence of fat, acid, hypertonicity, and distension in the duodenum triggers the enterogastric reflex and the release of enterogastrones (such as cholecystokinin and secretin). These mechanisms reduce gastric secretion and motility by inhibiting parietal cells, chief cells, and smooth muscle cells, ultimately reducing antral peristalsis to slow gastric emptying. This phase ensures that the small intestine has sufficient time to process chyme effectively. The accumulation of acid in the stomach stimulates D cells to release somatostatin, which inhibits G cells, parietal cells, and ECL cells, further decreasing gastric secretion.
How does the stomach contain its contents without causing injury?
The gastric mucosal barrier allows this. The secretion of mucus and HCO3 protects stomach from low pH and pepsin. The barrier is composed of: a luminal membrane which is impermeable to H+ and HCl and tight junctions between cells which physically prevent the penetration of HCl.
What are the 4 important factors of motility and secretion in the digestive system?
Autonomous smooth muscle function, intrinsic/extrinsic nerves, gastrointestinal hormones
What is the basic electrical rhythm?
The BER allows the smooth muscle cell to depolarize and contract rhythmically when exposed to hormonal signals.
What is gastroparesis?
A collection of disorders involving delayed gastric emptying
What are the membrane-bound enzymes involved in protein digestion at the brush border of the SI?
Enteropetidase and aminopeptidases
What are the membrane-bound enzymes involved in carbohydrate digestion at the brush border in the SI?
Disaccharidases: maltase, sucrase, lactase and isomaltase
Label the diagram of the circular-fold villus.
1=epithelial cell
2=mucous cell
3=central lacteal
4=lymphatic vessel
5=venule
6=arteriole
7=crypt of Lieberkuhn
8=villus
9=capillaries
What is the continuation of the SI? What is the function of each part.
Duodenum, jejunum, ileum
The duodenum adds secretions. The jejunum digests foodstuffs. The ileum is responsible for specialised absorption.
What are the components of the enteric nervous system?
Myenteric plexus (superficial) and submucosal plexus (intrinsic)
State the layers of the SI from superficial to deep.
Serosa, muscularis externa (longitudinal, circular muscle), submucosa, mucosa (muscularis mucosae, lamina propria, mucous membrane), lumen
Describe carbohydrate digestion and absorption.
Carbohydrate digestion begins with salivary amylase, which breaks down starch and glycogen into smaller polysaccharides. In the SI, pancreatic amylase continues this process, producing disaccharides like maltose, sucrose, and lactose. These disaccharides are then broken down by membrane-bound disaccharidases: maltase, sucrase-isomaltase, and lactase which is located on the brush border of the SI, into absorbable monosaccharides: glucose, galactose, and fructose. Glucose and galactose are absorbed via SGLT-1, a sodium-dependent symporter. Fructose enters through GLUT-5 via facilitated diffusion. All three monosaccharides exit the epithelial cell into the bloodstream through GLUT-2 on the basolateral membrane, completing absorption.
Does sucrase or isomaltase exist in the body in isolation?
No
Describe protein digestion and absorption.
Protein digestion begins in the stomach where pepsin breaks down dietary and endogenous proteins into polypeptides. In the SI, pancreatic proteolytic enzymes such as trypsin and chymotrypsin further degrade these into small peptides and amino acids. At the brush border, aminopeptidases convert some small peptides into free amino acids. Some small peptides are absorbed via H⁺-dependent symporters and digested by intracellular peptidases inside enterocytes. Amino acids are absorbed by Na⁺-dependent symporters, transported across the basolateral membrane via facilitated diffusion, and finally enter the blood capillaries, completing absorption.
How is pepsin produced?
Pepsin is produced when pepsinogen is released by chief cells and these come in contact with HCl (secreted by parietal cells). HCl cleaves the pepsinogen into pepsin.
What are micelles?
Micelles are a vehicle for carrying water-insoluble substances through the watery luminal contents. They have a hydrophobic core and a hydrophilic shell.
Describe the roll of cholecystokinin in pancreatic secretion and bile flow.
Cholecystokinin (CCK) is released from the duodenal mucosa in response to fats and proteins in the duodenal lumen. It stimulates pancreatic secretion and regulating bile flow. CCK travels via the bloodstream to pancreatic acinar cells, prompting them to secrete digestive enzymes such as lipase, amylase, and protease. Simultaneously, CCK causes the gallbladder to contract and the sphincter of Oddi to relax, allowing bile to flow into the duodenum, aiding in fat digestion. The enterohepatic circulation is where most bile salts are reabsorbed in the ileum and returned to the liver for reuse, maintaining efficient digestive function.
How are TAGs transported?
TAGs are packaged into chylomicrons and transported through the lymphatic ducts and drain into the thoracic duct. They drain into the bloodstream at the left subclavian vein.
What are the exocrine cells of the pancreas?
Duct (bicarbonate) and acinar (amylase, lipase, protease) cells
What are the endocrine cells of the pancreas?
Islet cells of Langerhan (insulin and glucagon)
What enzyme triggers trypsinogen?
Enterokinase
Why must trypsinogen remain inactive in the pancreas?
Trypsinogen, a precursor to trypsin, would digest pancreatic proteins, leading to pancreatitis.
Describe fat digestion in the pancreas.
Fat digestion begins when bile salts break large fat droplets into smaller ones, forming a lipid emulsion. Pancreatic lipase then acts on these droplets to break TAGs into monoglycerides and FFAs. These products combine with bile salts to form micelles, which transport them to the surface of intestinal cells. The monoglycerides and fatty acids enter the cells, where they are reassembled into TAGs. These TAGs are then packaged into CMs, which are released by exocytosis into the lymphatic system for transport.
What are zymogen cells?
Storage organelles found in acinar cells in the pancreas.
What route do pancreatic exocrine secretions take to the duodenum?
Acinar cells, intercalated ducts, interlobular duct, main pancreatic duct (Wirsung’s), common bile duct, hepatopancreatic ampulla, sphincter of Oddi, major duodenal papilla
What do duct cells secrete?
NaHCO3- for the neutralization of acidic chyme in the duodenum.
What is the functional unit of the liver?
Hexagonal lobules
What structure secretes primary bile acids?
Hepatocytes
What structure secretes secondary bile acids?
Colonic bateria
What are bile acids conjugated with to form functional bile?
Amino acids, either glycine or taurine. Important for their water solubility and pH.
What is cholestasis?
Impaired production or secretion of bile
State the location and function of the parotid gland.
The parotid glands are anterior to either ear. The parotid ducts pierce the buccinator adjacent to the 2nd upper molar tooth. They produce a serous secretion.
State the location and function of the submandibular gland.
The submandibular glands are inferior to the jaw, near the angle of the mandible. It is hook-shaped with superficial and deep arms. Its duct emerges from deep to open onto the sublingual papilla. It produces a mucous and serous secretion.
State the location and function of the sublingual gland.
The sublingual glands are almond-shaped and are inferior to the tongue. Its duct drains into the oral cavity via several minor ducts. It produces a mixed secretion.
How does ion transport differ in the jejunum, ileum and colon?
In the jejunum, Na absorption is rapid and linked to nutrient uptake via co-transporters; chloride follows passively and water moves osmotically. The ileum also absorbs sodium but relies more on Na⁺/H⁺ and Cl⁻/HCO₃⁻ exchangers, helping reclaim bile salts. Potassium is passively absorbed in both. In the colon, sodium is absorbed actively through ENaC channels under aldosterone control, while potassium is secreted. Chloride is absorbed via Cl⁻/HCO₃⁻ exchange, and water follows ion gradients. (Each region fine-tunes electrolytes to match digestive and absorptive needs.)
How does cholera toxin cause watery diarrhoea through chloride secretion in the intestinal epithelium?
Cholera toxin ramps up cAMP, activating CFTR channels to pump Cl⁻ into the gut lumen; Cl⁻ first enters epithelial cells via the Na⁺/K⁺/2Cl⁻ cotransporter (NKCC1), then exits through CFTR. This luminal Cl⁻ creates an electrochemical gradient that pulls Na⁺ paracellularly, and the osmotic gradient draws H₂O the same way—flooding the lumen with salt and water, resulting in profuse diarrhoea.
How does oral rehydration therapy work?
Oral rehydration therapy (ORT) works by exploiting the Na-glucose co-transport mechanism in the small intestine. Glucose and Na are absorbed together via SGLT1 transporters into epithelial cells, even when the body is dehydrated. This co-transport pulls water along osmotically, rapidly restoring fluid balance. Once inside the cell, glucose, galactose, and fructose exit to the bloodstream via GLUT2 transporters, while sodium is pumped out by the Na⁺/K⁺ ATPase, maintaining the sodium gradient for continued absorption.
