The Digestive System (2)

Question 1

How is the bolus processed in the mouth?

Answer 1

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.

Question 2

State the function of lysozyme.

Answer 2

Lysozyme is an enzyme that acts as an antibiotic.

Question 3

State the function of lactoferrin.

Answer 3

Lactoferrin binds iron and allows for digestion of Fe.

Question 4

Describe the oropharyngeal stage of the swallowing reflex.

Answer 4

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.

Question 5

Describe the oesophageal stage of the swallowing reflex.

Answer 5

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.

Question 6

What are the cells of the superior fundus called?

Answer 6

The interstitial cells of Cajal

Question 7

Where does most of the mixing of gastric material occur in the stomach?

Answer 7

The antrum

Question 8

State the function of the interstitial cells of Cajal.

Answer 8

They initiate contractions of the smooth muscle of the stomach

Question 9

State 2 consequences of peristaltic contraction.

Answer 9

Gastric emptying and gastric mixing

Question 10

What is the fundus and body of the stomach lined by?

Answer 10

Oxyntic mucosa

Question 11

What is the antrum of the stomach lined by?

Answer 11

Pyloric gland area

Question 12

What do mucous cells secrete?

Answer 12

Alkaline mucus

Question 13

What do chief cells secrete?

Answer 13

Pepsinogen

Question 14

What do parietal cells secrete?

Answer 14

HCl and intrinsic factor

Question 15

What do enterochromaffin-like (ECL) cells secrete?

Answer 15

Histamine

Question 16

What do G cells secrete?

Answer 16

Gastrin

Question 17

What do D cells secrete?

Answer 17

Somatostatin

Question 18

How are parietal cells stimulated to secrete HCl?

Answer 18

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.

Question 18

How is HCL secreted by parietal cells?

Answer 18

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.

Question 19

Explain the mechanism of gastric secretion during the intestinal phase.

Answer 19

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.

Question 20

How does the stomach contain its contents without causing injury?

Answer 20

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.

Question 21

What are the 4 important factors of motility and secretion in the digestive system?

Answer 21

Autonomous smooth muscle function, intrinsic/extrinsic nerves, gastrointestinal hormones

Question 22

What is the basic electrical rhythm?

Answer 22

The BER allows the smooth muscle cell to depolarize and contract rhythmically when exposed to hormonal signals.

Question 23

What is gastroparesis?

Answer 23

A collection of disorders involving delayed gastric emptying

Question 24

What are the membrane-bound enzymes involved in protein digestion at the brush border of the SI?

Answer 24

Enteropetidase and aminopeptidases

Question 25

What are the membrane-bound enzymes involved in carbohydrate digestion at the brush border in the SI?

Answer 25

Disaccharidases: maltase, sucrase, lactase and isomaltase

Question 26

Label the diagram of the circular-fold villus.

Answer 26

1=epithelial cell 2=mucous cell 3=central lacteal 4=lymphatic vessel 5=venule 6=arteriole 7=crypt of Lieberkuhn 8=villus 9=capillaries

Question 27

What is the continuation of the SI? What is the function of each part.

Answer 27

Duodenum, jejunum, ileum The duodenum adds secretions. The jejunum digests foodstuffs. The ileum is responsible for specialised absorption.

Question 28

What are the components of the enteric nervous system?

Answer 28

Myenteric plexus (superficial) and submucosal plexus (intrinsic)

Question 29

State the layers of the SI from superficial to deep.

Answer 29

Serosa, muscularis externa (longitudinal, circular muscle), submucosa, mucosa (muscularis mucosae, lamina propria, mucous membrane), lumen

Question 30

Describe carbohydrate digestion and absorption.

Answer 30

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.

Question 31

Does sucrase or isomaltase exist in the body in isolation?

Answer 31

No

Question 32

Describe protein digestion and absorption.

Answer 32

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.

Question 33

How is pepsin produced?

Answer 33

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.

Question 34

What are micelles?

Answer 34

Micelles are a vehicle for carrying water-insoluble substances through the watery luminal contents. They have a hydrophobic core and a hydrophilic shell.

Question 35

Describe the roll of cholecystokinin in pancreatic secretion and bile flow.

Answer 35

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.

Question 36

How are TAGs transported?

Answer 36

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.

Question 37

What are the exocrine cells of the pancreas?

Answer 37

Duct (bicarbonate) and acinar (amylase, lipase, protease) cells

Question 38

What are the endocrine cells of the pancreas?

Answer 38

Islet cells of Langerhan (insulin and glucagon)

Question 39

What enzyme triggers trypsinogen?

Answer 39

Enterokinase

Question 40

Why must trypsinogen remain inactive in the pancreas?

Answer 40

Trypsinogen, a precursor to trypsin, would digest pancreatic proteins, leading to pancreatitis.

Question 41

Describe fat digestion in the pancreas.

Answer 41

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.

Question 42

What are zymogen cells?

Answer 42

Storage organelles found in acinar cells in the pancreas.

Question 43

What route do pancreatic exocrine secretions take to the duodenum?

Answer 43

Acinar cells, intercalated ducts, interlobular duct, main pancreatic duct (Wirsung's), common bile duct, hepatopancreatic ampulla, sphincter of Oddi, major duodenal papilla

Question 44

What do duct cells secrete?

Answer 44

NaHCO3- for the neutralization of acidic chyme in the duodenum.

Question 45

What is the functional unit of the liver?

Answer 45

Hexagonal lobules

Question 46

What structure secretes primary bile acids?

Answer 46

Hepatocytes

Question 47

What structure secretes secondary bile acids?

Answer 47

Colonic bateria

Question 48

What are bile acids conjugated with to form functional bile?

Answer 48

Amino acids, either glycine or taurine. Important for their water solubility and pH.

Question 49

What is cholestasis?

Answer 49

Impaired production or secretion of bile

Question 50

State the location and function of the parotid gland.

Answer 50

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.

Question 51

State the location and function of the submandibular gland.

Answer 51

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.

Question 52

State the location and function of the sublingual gland.

Answer 52

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.

Question 53

How does ion transport differ in the jejunum, ileum and colon?

Answer 53

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.)

Question 54

How does cholera toxin cause watery diarrhoea through chloride secretion in the intestinal epithelium?

Answer 54

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.

Question 55

How does oral rehydration therapy work?

Answer 55

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.