Lecture 25: Secretion Flashcards
What are the types of secretions?
- salivary
- gastric
- pancreatic
- biliary (liver and gallbladder)
- intestinal
define secretion
movement of solutes and water from body to the lumen
define absorption
movement of solutes and water from the lumen into the body
what is the difference between endocrine and exocrine secretion?
- endocrine secretions move into the body (so they aren’t really secretions)
- exocrine secretions are produced by the epithelia and move into the lumen
what are the components of exocrine secretion and their function?
Mucus
- protection and lubrication
- aids mechanical digestion
Electrolyte solution
- dilutes food and provides optimal pH
- essential for chemical digestion of food
Digestive enzymes
- essential for chemical digestion of food
- aids absorption
what volume of electrolyte solution is secreted everyday from each region? what is the pH?
Plasma - 3L/day, 7.4pH
Saliva - 1.5L/day 7.5 pH
Stomach - 3L/day 1
Pancreas - 1.5L/day 7.8
Liver - 0.5L/day 7.5
Small Intestine - 1.5L/day 7.5
In total 8L/day is secreted from 3L of plasma meaning reabsorption is important!
how much salivary secretion is there?
- 3 pairs of salivary glands
- produce 1.5L fluid per day
basal secretion is 0.3ml/min
stimulated secretion 1.5ml/min
what is the composition of the salivary secretion?
- Mucus for lubrication
- Dilute solution of bicarbonate and NaCl for dilution of food and optimal pH of digestive enzymes
- Digestive enzymes: lingual lipase and alpha amylase
what are the functions of salivary secretions?
- aids talking, chewing and swallowing by dissolving food and lubrication
- hygiene by irrigation
- xerostomia (dry mouth)
- digestion by dissolving food to allow taste. lingual lipase digests fats, alpha amylase dissolves starch
what regulates salivary secretion?
Nervous regulation
- thought, smell, sight of food, presence of food in mouth
Autonomic nervous system
- parasympathetic stimulates secretion of copious quantities of fluid
- sympathetic secretes small volumes of viscous fluid which augements parasympathetic response
What is gastric (stomach) secretion per day?
2-3L/day
what are the gastric (stomach) secretion between meals?
- slow rate (15-30ml/h) of secretion
- surface epithelium cells secrete mucous and bicarbonate
what are the gastric (stomach) secretion when eating?
superimposed on basal rate
- surface epithelial cells secrete mucus (goblet cells) and bicarbonate
- parietal cells secrete HCl (gastric acid) (150ml/h, pH 1.0) and intrinsic factor
- chief cells secrete pepsinogen
what is the function of mucus in the gastric (stomach) secretions
- protects against abrasion
- buffers acid with surface bicarbonate
what is the function of intrinsic in the gastric (stomach) secretions
- absorption of vitamin b12 in small intestine
what is the function of pepsinogen in the gastric (stomach) secretions
- pepsinogen is inactive form of pepsin (known as gastric proteolytic enzyme)
- converted to active form pepsin by acid
- starts digestion of proteins
what is the function of gastric acid in the gastric (stomach) secretions
- dilutes food
- denatures protein
- activates pepsinogen to pepsin
- creates optimal pH for pepsin
- protection
what is the secretion of acid by parietal cells?
it is a source of acid
- by the formation of carbonic acid by carbonic anhydrase
- and by the dissociation of carbonic acid to give H+ and bicarbonate
how does the secretion of HCl by parietal cells work? link to diagram
- carbonic anhydrase forms carbonic acid which gives H+ and bicarbonate
- Secretion of acid (H+)
- via the H+K+ ATPase on the apical membrane of parietal cells
- it pumps H+ ions into the lumen in exchange of potassium ions (K+)
- K+ return to the lumen through membrane channel - Source of chloride (Cl-)
- via the anion counter transporter on the serosal membrane of parietal cells
- ejects bicarbonate into interstitial fluid
- then imports Cl- ions into cell - Secretion of chloride (Cl0)
- Cl- diffuses across the cell
- enters the lumen via Cl- channel in apical membrane
what is the regulation of gastric (stomach) secretion coordinated with and named after?
- coordinated with eating and arrival of food
- named after region of body that controls the secretion
what are the 3 phases of gastric (stomach) secretion?
- cephalic phase (head controls secretion)
- gastric phase (stomach controls secretion)
- intestinal phase (intestine controls secretion)
what are the features of cephalic phase, its stimuli and regulation?
- head controls this secretion
- 20% of secretion associated with meal
- prepares for arrival of food
Stimuli:
- higher centers in CNS detect thought, smell, sight of food
- chewing action, taste
Regulated by parasympathetic nervous system via enteric nervous system:
- stimulates parietal cells, chief cells and goblet cells
- stimulate secretion of hormone gastrin which is released into blood and stimulates parietal cells and chief cells
what are the features of gastric phase, its stimuli and regulation?
- stomach regulates secretion
- 70% of secretion associated with a meal
- ensures sufficient secretion to handle ingested food
Stimuli:
- stretch/distension stomach wall
- products of digestion stomach lumen
- elevated pH
Has nervous and hormonal regulation:
- local nervous reflex by enteric nervous system
- external nervous reflex by parasympathetic nervous system
- stimulates secretion from gastric glands
- stimulates motility
-stimulates gastrin secretion for further stimulation of secretion and motility
what are the features of intestinal phase, its stimuli and regulation?
- small intestine regulates secretion
- 10% of secretion associated with meal
- controls delivery to small intestine
Stimuli:
- distension of duodenum
- arrival of chyme, lipids and carbohydrates in the duodenum
Nervous and hormonal regulation:
- hormones: gastric inhibitory peptide, cholecystokinin, secretin
- nerves: enterogastric reflex
- hormones and nerves inhibit secretion and motility
what type of organ is the pancreas?
endocrine and exocrine organ
what is the volume of pancreatic secretions?
1-1.5L per day
what is the composition of pancreatic secretions?
Enzymes
- secreted by acinar cells
- used for chemical digestion of food
Alkaline fluid
- secreted by duct cells
- used for neutralise acid and optimal pH for enzymes
what is the pancreas important for?
most important source of digestive enzymes
what are the types of pancreatic enzymes? what are they secreted by?
Lipolytic e.g. lipase, phospholipase
Amylytic e.g. pancreatic amylase
Proteolytic e.g. trypsin, chymotrypsin, carboxypeptidase
Nucleolytic e.g. ribonuclease, deoxyribonuclease
all secreted by acinar cells
what stimulates pancreatic secretions?
- secretion of digestive enzymes by acinar cells stimulated by hormone cholecystokinin (CCK)
- arrival of lipids and carbohydrates in duodenum stimulates CCK secretion
what is the function of pancreatic secretions?
luminal chemical digestion of food
how are proteolytic enzymes activated?
- proteolytic enzymes are secreted as inactive precursors and activated in the duodenum (e.g. trypsin/trypsinogen, chymotrypsin/chymotrypsinogen, carboxypeptidase/procarboxypeptidase)
- in the small intestine, activated involves enterokinase (aka enteropeptidase). it is bound to the duodenal membrane and converts trypsinogen to trypsin
- trypsin converts other enzymes to active form
how is alkaline fluid secreted? what is its function?
- produced by duct cells in pancreas
- secretion of alkaline fluid is stimulated by secretin
- secretion of secretin is stimulated by arrival of chyme in duodenum
- alkaline fluid neutralises acid chyme delivered from the stomach. it creates optimal pH (6.7-9) for pancreatic and intestinal digestive enzymes
what is the volume of biliary secretions?
0.5L/day
what is the composition of biliary secretions?
products associated with digestion:
- bile salts
- bicarbonate rich fluid
excretory products
- bile pigments (waste products)
- cholesterol
what are the functions of biliary secretions?
bile salts - fat digestion
bicarbonate rich fluid - neutralise acid
bile pigments - excretion
what is the regulation of biliary secretion?
Bile is secreted constantly by the liver
- stored and concentrated in the gallbladder
- delivered to the duodenum with arrival of food
Initial delivery of bile is under hormonal control
- cholecystokinin is produced in response to products of digestion in the duodenum
- initiates contraction of gallbladder and relaxation of hepatopancreatic ampulla
- secretin has mild stimulation of bile by the liver
what is the enterohepatic circulation?
- bile is metabolically expensive to produce
- 95% is reabsorbed in ileum
- transported back to liver in enterohepatic circulation
- reabsorbed and secreted to stimulate bile secretion
how much small intestinal secretion is there?
1.5L/day from crypts/glands
what are the components of small intestinal secretion? what is their function?
Mucus - lubrication
Isosmotic fluid
- alkaline, mixture of NaCl and sodium bicarbonate
- helps neutralise acid
- dilutes food to aid digestion
Digestive enzymes
- shed cells
what does the large intestine secrete? what is its function?
mucus - lubrication
