Lecture 25 Flashcards
What are the secretions of exocrine glands?
Mucus
Electrolyte solution
Digestive enzymes
Function of mucus?
} Protection and lubrication
} Aids mechanical digestion
Function of electrolyte solution
} Dilutes food & provides optimal pH
} Essential for chemical digestion of food
Function of digestive enzymes
} Essential for chemical digestion of food } Aids absorption
composition of saliva?
Mucus
Dilution solution NaHCO3NaCl
Digestive enzymes (lingual lipase and a-amylase)
Functions of salivary secretions?
Not essential.
Aids talking, chewing and swallowing.
Hygiene.
Digestion.
How are salivary secretions regulated?
Nervous (stimulated by food)
Autonomic Nervous system: parasympathetic (copious quantities) Sympathetic (small volumes)
Gastric secretions between meals?
Surface cells secreting mucous at a slow rate.
Gastric secretions when eating?
Superimposed on basal rate.
Mucous cells - mucus
Parietal cells - HCL and intrinsic factor
Chief cells -0 pepsinogen
What is the function of intrinsic factor?
Absorption of vitamin B12 in the SI
Function of pepsinogen?
Converted to active form pepsin.by acid then starts digestion of proteins1`
Functions of gastric acid?
Dilutes food Denatures protein Activates pepsinogen Creates optimum pH for pepsin action Protection
Source of acid (H+)
} Formation of H2CO3 by carbonic anhydrase
} CO2 + H2OóH2CO3
} Dissociation of H2CO3 to give H+ and HCO3-
} H CO óHCO - + H+ 233
Secretion of acid (H+)
H+-K+ ATPase
} Apical membrane of parietal cells
} Pumps H+ ions into lumen in exchange of potassium ions (K+)
} K+ return to lumen through membrane channel
`Source of chloride (CL-) ?
} Anion counter transporter
} Serosal membrane of parietal cells } Ejects HCO3- into interstitial fluid } Imports Cl- ions into cell
Secretion of chloride
} Cl- diffuse across cell
} Enters lumen via Cl- channel in apical membrane
How is gastric secretion regulated?
Cephalic phase - head controls secretion
Gastric phase - stomach controls secretion
Intestinal phase - intestine controls secretion.
Function of the cephalic phase?
Preparing for the arrival of food
What stimulates the Cephalic phase?
Higher senses
Chewing action and taste.
What regulates the Cephalic phase?
Parasympathetic Nervous System:
} Via enteric nervous system
} Stimulates parietal cells, chief cells and goblet
cells
} Stimulate secretion of hormone gastrin
¨ Released into blood
¨ Stimulates parietal cells and chief cells
Function of the Gastric phase?
Ensures sufficient secretion to handle ingested food.
What stimulates the Gastric phase?
Stimuli in the stomach:
} Stretch/distension stomach wall
} Products of digestion stomach lumen
} Elevated pH
How is the gastric phase regulated?
nervous and Hormonal regulation: } Local nervous reflex - Enteric nervous system } External nervous reflex - Parasympathetic nervous system } Stimulate - Secretion- Motility - Gastrin secretion ¨ Further stimulation of secretion and motility
What is the function of the Intestinal phase?
Control delivery to Small intestine.
What stimulates the intestinal phase?
} Distension of duodenum
} Arrival in duodenum of
} Acid chyme
} Lipids and carbohydrates
What regulates the intestinal phase?
Nervous and hormonal regulation } Hormones - GIP, CCK, Secretin } Nerves - Enterogastric reflex } Both inhibit secretion and motility
Pancreatic secretion?
Endocrine and exocrine organ
What are the components of pancreatic secretion?
Enzymes ¨ Acinar cells ¨ Chemical digestion of food Alkaline fluid ¨ Duct cells ¨ Neutralise acid ¨ Optimum pH for enzymes
Why are pancreatic enzymes important?
Most important source of digestive enzymes - work on all classes of food
Pancreatic enzymes?
} Lipolytic – lipase, phospholipase
} Amylytic – pancreatic amylase
} Proteolytic – Trypsin, chymotrypsin, carboxypeptidase
} Nucleolytic – ribonuclease, deoxyribonuclease
What cells secrete pancreatic enzymes?
Acinar cells
What stimulates the release of pancreatic enzymes by acinar cells?
cholecystokinin (CCK)
What stimulates CCKL secretion)?
Arrival of lipids and carbohydrates in duodenum?\
What is the function of pancreatic enzymes?
Luminal chemical digestion of food
What are proteolytic enzymes?
Enzymes that are secreted as inactive
precursors & activated in duodenum
proteolytic enzymes?
} Trypsin – trypsinogen
} Chymotrypsin – chymotrypsinogen
} Carboxypeptidase – procarboxypeptidase
What does activation of proteolytic enzymes involve?
} In small intestine activation involves } Enterokinase (=enteropeptidase) } Bound to duodenal membrane } Converts trypsinogen to trypsin } Trypsin converts other enzymes to active form
Describe the secretion of Alkaline (HCO3 rich) fluid?
Produced by duct cells in the pancreas, stimulated by hormone secretin which is stimulated by the arrival of acid chyme in the duodenum./
Function of alkaline fluid?
} Neutralises acid chyme delivered from stomach } Creates optimum pH (6.7-9.0)for pancreatic and intestinal digestive enzyme
Volume of Biliary secretion?
0.5L per day consisting of products associated with digestion such as bile salts and HCO3 rich fluid As well has excretory products such as waste products, bile pigments and cholesterol.
Functions of Biliary secretions?
Bile Salts for fat digestion.
HCO3 rich fluid to neutralise fluid.
Bile Pigments for excretion.
How are biliary secretions regulated?
} Initial delivery of bile under hormonal control
} Hormone CCK (cholecystokinin)
} Produced in response to products of digestion in duodenum
} Contraction of gallbladder
} Relaxation of hepatopancreatic ampulla
} Hormone secretin
} Mild stimulation of bile by liver
Where is Bile secreted?
Secreted constantly by liver but stored and concentrated in gallbladder. The delivered to duodenum with the arrival of food.
What stimulates bile secretions?
Bile stimulates it’s own secretion - } Enterohepatic circulation
What is } Enterohepatic circulation?
Bile metabolically expensive to produce 95% reabsorbed in ileum Transported back to liver in enterohepatic circulation Reabsorbed and secreted Stimulation of bile secretion
What is secreted by the small intestine?
1.5L per day: Mucus Isosmotic fluid (Alkaline - mixture of NaCl and NaHCO3) Digestive enzymes
zFunction of Isosmotic fluid secretions in the small intestine?
Neutralise food and dilute food to aid digestion.
Function of digestive enzymes in the small intestine.
Shed cells.
