Lecture 25: SECRETION Flashcards
How many pairs of salivary glands are there?
3
How much fluid do the salivary glands produce?
1.5L per day
What amount of saliva do each of the salivary glands secrete?
Sublingual (5%), submandibular (70%) and parotid (25%)
What is basal saliva secretion?
0.3 ml/min
What is stimulated saliva secretion?
1.5 ml/min
What is the composition of saliva?
Mucus, a dilute solution of NaHCO3 and NaCl, digestive enzymes
What is the purpose of mucus in saliva?
Lubrication
What is the purpose of a dilute solution of NaHCO3 and NaCl in saliva?
To dilute food and provide an optimal pH for digestive enzymes
What are the digestive enzymes in saliva?
Lingual lipase and alpha amylase (denatured in the stomach)
Is saliva essential?
No
What does saliva aid in?
Talking, chewing and swallowing
How does saliva aid in chewing and swallowing?
Dissolves and lubricates food
What is saliva also involved in?
Hygiene and digestion
How does saliva help in hygiene?
Irrigation (removal of old food)
What is xerostomia?
Dry mouth from limited or absent saliva
How does saliva help in digestion?
Dissolves for allowing tasting
What dies lingual lipase break down?
Fats
What does alpha amylase break down?
Starch
What is saliva secretion regulated by?
Nervous system, autonomic nervous system
What is the nervous system activated by (saliva)?
Thought, smell, sight of food and the presence of food in the mouth
What does the parasympathetic nervous system do (saliva)?
Stimulates secretion of abundant quantities of fluid
What does the sympathetic nervous system do (saliva)?
Small volumes of viscous fluid
How much gastric secretion occurs in a day?
2-3L
What is the rate of gastric secretion between meals?
slow (15-30ml/h)
What secretes gastric fluid between meals?
Surface cells secrete mucus
What is the rate of gastric secretion when eating?
Superimposed on basal rate
What secretes gastric fluid when eating?
Mucous cells - mucus, parietal cells -150 ml/h HCl acid pH=0 and intrinsic factor, chief cells - pepsinogen
What is the function of mucus in gastric secretions?
Protection from abrasion and acid
What is the function of intrinsic factor in gastric secretions?
Absorption of vitamin B12 in the small intestine
What is pepsinogen?
The inactive form of pepsin
What type of enzyme is pepsin?
Gastric proteolytic
How is pepsinogen converted to its active form?
By acid
What does pepsin start digestion of?
Proteins
What is the function of gastric acid in gastric secretions?
Dilutes food, denatures protein, activates pepsinogen to pepsin, creates optimum pH for pepsin and protection
What is the source of acid (H+)?
Formation of H2CO3 (carbonic acid) by carbonic anhydrase (CO2 +H2O <>H2CO3) then dissociation of H2CO3 (H2CO3 <>H+ +HCO3-)
How is H+ secreted into the lumen?
By the H+K+ ATPase
Where is H+K+ ATPase found?
On the apical membrane of parietal cells
What does H+K+ ATPase do?
Pumps H+ ions into the lumen in exchange for K+ ions ( to keep balance) and K+ then returns to the lumen through a membrane channel
What is the source of chloride?
Anion counter transporter
Where is the anion counter transporter?
On the serosal membrane of parietal cells
What does the anion counter transporter do?
Ejects HCO3- into interstitial fluid and imports Cl- ions into the cell
How is Cl- secreted?
It diffuses across the cell and enters the lumen via the Cl- channel in apical membrane
What is regulation of gastric secretion coordinated with?
Eating and arrival of food
What are the phases of gastric secretion?
Cephalic - head/brain controls secretion
Gastric - stomach controls secretion
Intestinal - intestine controls secretion
What is the cephalic phase for?
Preparation for arrival of food
How much secretion associated with meals is in the cephalic phase?
20%
What are the stimuli for the cephalic phase?
Higher centres (brain), thought, smell and sight of food, chewing action, taste
How is secretion regulated in the cephalic phase?
Parasympathetic nervous system acting via enteric nervous system
What is stimulated in the cephalic phase?
Parietal cells, chief cells and goblet cells to secrete acid, pepsinogen and mucus and well as secretion of hormone gastrin
What does gastrin do in the cephalic phase?
It is released into the blood to stimulate parietal cells and chief cells
What does the gastric phase ensure?
Sufficient secretion to handle ingested food
How much secretion associated with meals is in the gastric phase?
70%
What are the stimuli in the stomach for the gastric phase?
Stretch/distension of stomach walls (receptors), products of digestion in the stomach lumen and elevated pH when food enters (because acidic mixing with ~neutral)
What is the regulation of the gastric phase?
Nervous and hormonal
- local nervous reflex (ENS)
- External nervous reflex (Parasympathetic nervous system)
What does the nervous system stimulate in the gastric phase?
Secretion, motility and gastrin secretion which further stimulates secretion and motility
What does the intestinal phase control?
Deliver to the small intestine
How much secretion associated with a meal is in the intestinal phase?
10%
What are the stimuli for the intestinal phase?
Distension of duodenum and arrival in the duodenum of acidic chyme, lipids and carbohydrates
What is the regulation of the intestinal phase?
Nervous and hormonal
- hormones (GIP, CKK and secretin)
- nervous (eterogastriic reflex)
Both inhibit secretion and motility (in stomach)
What type of organ is the pancreas?
Endocrine and exocrine
How much pancreatic secretion is there in a day?
1-1.5L/day
What are the components of pancreatic secretion?
Enzymes and alkaline fluid
What are enzymes in pancreatic secretion secreted by?
Acinar cells
What is the function of enzymes in the pancreatic secretion?
Chemical digestion of food
What is alkaline fluid in pancreatic fluid secreted by?
Duct cells
What is the function of alkaline fluid in the pancreatic secretion?
To neutralise acid and give optimum pH for enzymes
What is the most important source of digestive enzymes?
Pancreas
What is stimulating of secretion of pancreatic enzymes?
Hormone CCK
What is CCK secretion stimulated by?
Arrival of lipids and carbohydrates in duodenum (stop secretion in stomach, start in pancreas)
What is the function of pancreatic enzymes?
Luminal chemical digestion of food
How are proteolytic enzymes secreted?
As inactive precursors and activated in the duodenum
What are some proteolytic enzymes and their precursors?
Trypsin (trypsinogen), chymotrypsin (chymotrypsinogen and carboxypeptidase (procarboxypeptidase)
What does activation of proteolytic enzymes in the small intestine involve?
Enterokinase/enteropeptidase converting trypsinogen to trypsin and then trypsin converts the other enzymes to their active form
Where is enterokinase/enteropeptidase found?
Bound to the duodenal membrane
What is secretion of alkaline fluid stimulated by?
Hormone secretin which is secreted when acidic chyme arrives in the duodenum
What is the function of alkaline fluid?
To neutralise acidic chyme delivered from the stomach and create optimum pH (6.7-9.0) for pancreatic and intestinal digestive enzymes (increase pH)
How much biliary secretion occurs?
0.5L per day
What does biliary secretion consist of?
Products associated with digestion-bile salts and HCO3- rich fluid (ducts)
Excretory products - bile pigments (waste products) and cholesterol
What is the function of biliary secretin?
Bile salts - fat digestion
HCO3- rich fluid - neutralises acid
Bile pigments - excretion
When is bile secreted?
Constantly by the liver
What is done will bile produced in the liver?
it is stored and concentrated in the gall bladder then delivered to the duodenum with arrival of food
What is initial delivery of bile under control of?
Hormones (CCK)
How is bile delivered?
By contraction of the gall bladder and relaxation of the hepatopancreatic ampulla
What does secretion stimulate (biliary secretion)?
Mild stimulation of bile by the liver
What does bile stimulate?
Its own secretion by the enterophepatic circulation
Bile is ______ to produce?
metabolically expensive
How much bile is reabsorbed into the ileum?
95% and 5% lost in faces
Where is the bile reabsorbed in the ileum transported to?
The liver in enterohepatic circulation. It is reabsorbed and secreted (stimulation of bile secretion)
How much small intestine secretion is there?
1.5L/day
What is in small intestine secretion?
Mucus, isosmotic fluid and digestive enzymes
What is the function of mucus in small intestine secretion?
Lubrication to protect
What is the function of isosmotic fluid in small intestine secretion?
Alkaline - mixture of NaCl and NaHCO3 which helps neutralise acid and dilutes food to aid in digestion
What do digestive enzymes in small intestinal fluid do?
Shed cells
What is in large intestine secretion?
Mucus for lubrication
