GI system: L26 - Secretion

Question 1

Generally speaking, what are the functions of exocrine secretions?

Answer 1

Optimal pH, dilute food, digest food, protection and lubrication.

Question 2

What are the main components of secretion?

Answer 2

Digestive enzymes, mucous and electrolyte (serous) secretions.

Question 3

What are the sources of salivary secretion?

Answer 3

Mainly parotid (amylase), submandibular (amylase and mucous) and sublingual glands (mucous). Buccal glands also maintain epithelium in the mouth.

Question 4

How much saliva is secreted per day?

Answer 4

1.5L, basal rate of 0.3ml/min, stimulated rate 1.5ml/min.

Question 5

What is salivary secretions composed of?

Answer 5

Mucous (for lubrication), dilute solution of NaCl/NaHCO3 which aids in the dilution of food and maintenance of optimum pH. Saliva also contains alpha-amylase which digests starch and salivary lipase which has a small role in fat breakdown in mouth. However, these mouth enzymes both play a small role in chemical digestion.

Question 6

What are gastric secretions composed of?

Answer 6

Mucous cells secrete mucous. Chief cells secrete pepsinogen. Parietal cells secrete HCl and intrinsic factor. Neuroendocrine cells secrete gastrin.

Question 7

What is the volume of gastric secretions?

Answer 7

2-3 litres of gastric secretions per day. Between meals these secretions are mainly mucous and are released slowly at 15-30ml/hour. During eating and when there is food this secretion can go up 10 fold.

Question 8

What are pancreatic secretions composed of?

Answer 8

Enzymes and alkaline fluid. Enzymes are produced by the acinar cells of the pancreas. Alkaline fluids are produced by the ducts of the pancreas.
Some of the digestive enzymes released from the pancreas are proteolytic enzymes (digest proteins) so they are secreted in an inactive precursors (zymogens) so that they do not start digesting the pancreas itself: trypsin and trypsinogen, chymotrypsin and chymotrypsinogen.
In the small intestine, trypsinogen is activated to trypsin by enterokinase. Trypsin can then activate inactive precursors such as trypsinogen and chymotrypsinogen to active enzymes.

Question 9

What is the volume of pancreatic secretions?

Answer 9

1-1.5L/day of pancreatic secretions.

Question 10

Amount of bile secreted per day?

Answer 10

0.5L

Question 11

What does bile comprise of?

Answer 11

Excretory products and products associated with digestion. The excretory products being bile pigments (waste products) and cholesterol (which is secreted by the liver). The products associated with digestion are bile salts and lecithin, and fluid rich in bicarbonate secreted by duct cells. Bile is constantly secreted by the liver and is stored in the gall bladder. Stored bile is released into the small intestine when food is present.

Question 12

How much do the small intestine secretions secrete? What does this comprise of?

Answer 12

1.5L/day. Comprised of mucous and sodium bicarbonate.

Question 13

What does the large intestine secrete?

Answer 13

Mucous.

Question 14

What are the functions of salivary secretions?

Answer 14

Not essential but allows efficiency:
Salivary secretions are involved in lubrication, facilitating chewing and swallowing.
Saliva also neutralises at pH.
Oral hygiene - irrigation (loosening up debris).
Alpha-amylase and salivary lipase begin digestive process.
Allows tasting (stimulus for other GI secretions).

Question 15

What are the functions of gastric secretions?

Answer 15

The mucous secretions are important for protection from abrasion and protection from the acidic environment of the stomach.
Intrinsic factor allows absorption of vitamin B12.
Pepsinogen can be converted to active pepsin in acid to digest proteins.
Gastric acid functions to: continue the process of diluting food, denature proteins, activate pepsinogen, provide optimum pH for operation of pepsin, provide protection from bacteria.

Question 16

What are the functions of pancreatic secretions?

Answer 16

Enzymes produced by acinar cells act on all types of food, allowing them to be broken down into their components. We cannot survive without these enzymes. Pancreatic secretions include lipolytic enzymes (lipase; digests fat), amylolytic enzymes (amylase; digests carbohydrates), and proteolytic enzymes (trypsin, chymotrypsin; digest proteins).
Bicarbonate is also produced in ducts to neutralise acidic chyme. It also helps to provide optimal pH for pancreatic and intestinal enzymes, moving pH back towards 7.

Question 17

What are the functions of bile secretions?

Answer 17

Bile salts and lecithin help facilitate fat digestion and absorption. Bicarbonate is a rich fluid that functions to neutralise acid chyme. Bile serves as the route of excretion pigments such as bilirubin: a breakdown product of Hb.

Question 18

How are salivary secretions regulated?

Answer 18

All regulation occurs via neural pathways. The nervous system is stimulated by the thought, smell and sight of food, the presence of food in the mouth, and chewing. The autonomic nervous system regulates salivary secretion with the parasympathetic pathway being the most important. Parasympathetic pathway releases acetylcholine and secretion of copious volumes of saliva. Sympathetic pathway releases adrenaline and small volume of viscous fluid.

Question 19

What are the 3 phases of gastric secretion?

Answer 19

Cephalic phase, gastric phase, intestinal phase.

Question 20

Explain the cephalic phase of gastric secretion regulation.

Answer 20

30-40% of secretion associated with a meal. Head controlling secretion. This prepares the stomach for receiving food. The brain stimulate acid secretion in response to food related thoughts, smells and sights. Also stimulated by chewing. The parasympathetic nervous system (via the ENS) stimulates the parietal cells in the stomach to release acid. Gastrin production by the endocrine cells is also stimulated, gastrin then circulates in the blood and stimulates parietal cells to secrete acid.

Question 21

Explain the gastric phase of gastric secretion regulation.

Answer 21

60% of secretion associated with a meal released. Ensures that there is enough secretion to deal with food that has been eaten. Stimuli is stretch/distension and products of digestion (e.g amino acids). There are 2 reflexes in response: a local reflex involving the ENS and an external reflex involving the parasympathetic nervous system. These reflexes stimulate parietal cells to release acid, and stimulate production of gastrin by endocrine cells, which stimulates more parietal cells and aids gastric motility.

Question 22

Explain the intestinal phase of gastric secretion regulation.

Answer 22

5-10% of secretion associated with meal. Controls amount of acid delivered to intestines by switching off acid production in stomach. Stimuli for secretion are arrival in duodenum of acid and products of digestion, and the stretch of duodenum. The response is hormones cholecystokinin, gastric inhibitory peptide and secretin are produced by endocrine cells in the duodenum and inhibit acid secretion by the stomach. There is also a neural mechanism which inhibits acid secretion: the enterogastric reflex which involves the vagus nerve.

Question 23

How is the pancreatic secretions regulated?

Answer 23

Between meals there is a slow basal rate of secretion. Release of pancreatic secretions is regulated by hormones. CCK is produced in response to digestive products and stimulates pancreatic acinar cells to secrete digestive enzymes. Secretin is produced in response to increased concentrations of H+ in the intestinal lumen and stimulates duct cells to secrete bicarbonate.

Question 24

How is bile secretion regulated?

Answer 24

Products of digestion in duodenum stimulate endocrine cells to produce CCK causing the gallbladder to contract and the sphincter of Oddi to relax.

Question 25

What are the roles of different components of ENS plexus’.

Answer 25

Submucosal plexus is for regulation of secretion. Myenteric plexus is for regulation of motility.