Lecture 25 - Secretion

Question 1

Endocrine vs exocrine secretion

Answer 1

Exocrine - lumen gastrointestinal tract secretion as it is actually part of the outside world so you are secreting out of the body onto a body surface which in this case is the lumen of the GI tract

Endocrine - secreted into the blood or into the body, includes the hormones that control the exocrine secretions

Exocrine secretions are produced by epithelial cells that line the gastrointestinal tract

Question 2

Components and functions of exocrine secretion

Answer 2

Mucus
Protection and lubrication
Aids in mechanical digestion

Electrolyte solution (ions and water)
Dilutes food and provides optimal pH (and allows them to be mixed with digestive enzymes)
Essential for chemical digestion (enzymes) of food

Digestive enzymes
Essential for chemical digestion of food
Aids absorption

Question 3

Volume of electrolyte solution that is secreted

Answer 3

Approximately 8L a day secreted from 3L of plasma - reabsorption is therefore important

Question 4

Salivary secretion

Answer 4

3 pairs of salivary glands

Produce 1.5L fluid per day
Basal secretion - 0.3ml/min
Stimulated secretion - 1.5mL/min (when you think about food)

Composition
Mucus - lubrication
Dilute solution of NaHCO3/NaCl - dilution of food, optimal pH for digestive enzymes
Digestive enzymes - lingual lipase, ⍺-amylase

Proportions of total volume
Sublingual glands - 5%
Submandibular glands - 70%
Parotid glands -25%

Question 5

Functions of salivary secretion

Answer 5

Not essential

Aids
Talking
Chewing and swallowing - dissolves food, lubricates

Hygiene - irrigation, xerostomia (dry mouth from reduced or absent saliva)

Digestion 
Dissolves food allows tasting 
Lingual lipase - fats 
⍺-amylase - starch 
Optimal pH of about 7.8 for ⍺-amylase, food gets delivered to the stomach quickly which has a pH of one so the amylase is inactivated and even though there is enough amylase to digest the starch not much of the food actually gets digested by the amylase due to the stomach acid

Question 6

Regulation of salivary secretion

Answer 6

Nervous (because the control has to be fast)
Thought, smell, sight of food
Presence of food in the mouth
These signals give rise to salivary secretion

Autonomic nervous system
Parasympathetic - stimulates the secretion of copious quantities of fluid
Sympathetic - small volumes of viscous fluid

Question 7

Gastric secretion

Answer 7

2-3L per day

Between meals 
Slow rate (15-30ml/hour) of secretion - surface cells-mucus 

When eating
Superimposed on basal rate = mucous cells - mucus, parietal cells - 150mls per hour HCl at pH of 1 and intrinsic factor, chief cells - pepsinogen

Epithelial lining of the stomach is responsible for the secretions of the stomach

Question 8

Functions of gastric secretions

Answer 8

Mucus
Protection from abrasion and acid

Intrinsic factor
Absorption of vitamin B12 in the small intestine

Pepsinogen
Inactive form of pepsin - gastric proteolytic enzyme
Converted to active form pepsin by acid
Starts digestion of proteins

Gastric acid (largest component of the gastric secretion)
Dilutes food - important role in diluting, dissolving the food and mixing it up with secretions
Denatures protein
Activates pepsinogen to pepsin
Creates optimum pH for pepsin action
Protection

Question 9

Secretion of HCl acid process

Answer 9

Secretion of HCl acid by parietal cells - source of acid

1- source of acid H+ - generates the hydrogen ions within the parietal cells by carbonic anhydrase which is an enzyme in the cytoplasm of the parietal cells, combine with CO2 and water to give carbonic acid and this carbonic acid then dissoicates into bicarbonate ions and hydrogen ions and it is the hydrogen ions that we want to secrete
Equations …
Formation of H2CO3 by carbonic anhydrase = CO2 + H2O H2CO3
Dissociation of H2CO3 to give H+ and HCO3- = H2CO3 HCO3- + H+

2- secretion of acid (H+)
H+-K+ ATPase
Apical membrane of parietal cells, pumps H+ ions into lumen in exchange of potassium (K+) ions
K+ return to lumen through membrane channel
Protein that makes use of ATP and pumps H+ out of the apical surface to the lumen of the stomach in exchange for potassium ions

3- Source of chloride (Cl-)
Anion counter transporter
Serosal membrane of parietal cells, ejects HCO3- into interstitial fluid, imports Cl- ions into cells
Bicarbonate ions are transported out of the cell on the basal lateral/serosal membrane which faces the blood in exchange for Cl- ions which brings them into the cell and these chloride ions diffuse across the cell and then out across the apical membrane via a chloride channel

4- Secretion of chloride (Cl-)
Cl- diffuse across cell
Enter lumen via Cl- channel in apical membrane

Question 10

Regulation of gastric secretion

Answer 10

Coordinated with eating and the arrival of food

Named after which region of the body controls secretion
Divided into three phases…
Cephalic phase - head controls secretion
Gastric phase - stomach controls secretion
Intestinal phase - intestine controls secretion

Question 11

Cephalic phase

Answer 11

Head controls secretion
20% of secretion associated with meal

Preparationfor food arrival

Stimuli
Higher centres - thought, smell, sight of food
Chewing action, taste

Parasympathetic nervous system
Via enteric nervous sytem - stimulates parietal cells and chief cells and goblet cells, stimulate secretion of hormone gastrin - released into blood, stimulates parietal cells and chief cells

Food being prepared, smelt etc then via the CNS and via the vagus nerve, secretion is signalled and the vagus nerve comes down and activates the ENS and in this case the submucosal plexus and gets the three cell types to secrete their products and it also activates the G cells which release gastrin into the blood and this flows through the bloodstream and arrives at the stomach and it to causes the chief cells and parietal cells to secrete

Question 12

Gastric phase

Answer 12

Somach regulates secretion
70% of secretion associated with meal

Ensures sufficient secretion to handle ingested food

Stimuli in stomach
Stretch/distension of stomach wall
Products of digestion stomach lumen
Elevated pH

Nervous and hormonal regulation
Local nervous reflex - ENS
External nervous reflex - parasympathetic nervous system
Stimulate - secretion, motility, gastrin secretion( further stimulation of secretion and motility)

Question 13

Intestinal phase

Answer 13

Small intestine regulates secretion
10% of secretion associated with meal

Controls delivery to the small intestine

Stimuli
Distension of duodenum
Arrival in duodenum of acid chyme, lipids and carbohydrates

Nervous and hormonal regulation
Hormones - GIP,CCK, secretin
Nerves - enterogastric reflex
Both inhibit secretion and motility

Question 14

Pancreatic secretion

Answer 14

Endocrine and exocrine organ

Volume and composition
1-1.5L per day
2 components …
Enzymes - secreted acinar cells, chemical digestion of food
Alkaline fluid - duct cells, neutralise acid, optimum pH for enzymes

Question 15

Pancreatic enzymes

Answer 15

Act on all classes of food

Enzymes
Pancreases most important source of digestion enzymes which are secreted by acinar cells …
Lipolytic = lipase, phospholipase
Amylytic = pancreatic amylase
Proteolytic = trypsin, chymotrypsin, carboxypeptidase
Nucelolytic = ribonuclease, deoxyribonuclease

Stimulation of secretion
Secretion of digestive enzymes by acinar cells stimulated by the hormone cholecystokinin (CCK) - arrival of lipids and carbohydrates in duodenum stimulates CCK secretion

Function
Luminal chemical digestion of food

Question 16

Activation of proteolytic enzymes

Answer 16

Proteolytic enzymes secreted as inactive precursors and activated in duodenum
Typsin - trypsinogen
Chymotrypsin - chymotrypsinogen
Carboxypeptidase - procarboxypeptidase

In small intestine activation involves
Enterokinase (=enteropeptidase) - bound to duodenal membrane, converts trypsinogen to trypsin
Trypsin converts other enzymes to active form

Question 17

Secretion of alkaline (bicarbonate rich) fluid

Answer 17

Produced by duct cells in pancreas

Secretion of alkaline fluid stimulated by hormone secretin
Secretion of secretin is stimulated by the arrival of acid chyme in the duodenum

Alkaline fluid
Neutralise acid chyme delivered from the stomach
Creates optimum pH (6.7-9.0) for pancreatic and intestinal digestive enzyme

Question 18

Biliary secretion

Answer 18

Volume and composition
0.5L per day that consists of …
Products associated with digestion - bile salts, HCO3- rich fluid - ducts
Excretory produces - bile pigments (waste products), cholesterol

Functions
Bile salts - fat digestion
HCO3- rich fluid - neutralises acid
Bile pigments - excretion

Question 19

Regulation of biliary secretion

Answer 19

Unlike the stomach and pancreas that turn on the secretions when food arrives, the liver is constantly secreting bile

Bile secreted constantly by liver
Stored and concentrated in gallbladder
Delivered to duodenum with the arrival of food

Initial deliver of bile is under hormonal control 
Hormone CCK (cholecystokinin) - produced in response to products of digestion in duodenum, contraction of gallbladder, relaxation of hepatopancreatic ampulla (SUMMARY - arrival of food into the lumen of the small intestine, and the hormone CCK is produced, it stimulates the gallbladder to contract and hepatopancreatic ampulla relax and this contraction delivers the bile into the small intestine 
Hormon secretin - mild stimulation of bile by liver 

Bile stimulates its own secretion - enterohepatic circulation

Question 20

Enterohepatic circulation

Answer 20

BIle metabolically expensive to produce

95% is reabsorbed into the ileum

Transported back to the liver in enterohepatic circulation

Reabsorbed and secreted - stimulation of bile secretion

Question 21

Intestinal secretion

Answer 21

Small intestine
1.5L per day
Mucus - lubrication
Isosmotic fluid - alkaline as it is a mixture of NaCL and NaHCO3, helps to neutralise acid, dilutes food to aid digestion
Digestive enzymes - shed cells, every 7 days we shed all the epithelium into the intestinal lumen

Large intestine
Mucus - lubrication