What is the definition of digestion and of absorption?
Digestion:
- catabolic process
- breaks down complex food molecules into monomers
Absorption:
- passage of digested end products from lumen of GIT through mucosal cells into blood or lymph
What is the function of digestive enzymes?
- accelerate chemical reactions in digestion
- produced by various organs of the GIT
What are the two mechanisms for regulating bile and pancreatic secretions?
- Hormonal
- chyme in duodenum triggers duodenal enteroendocrine cells to release CCK and secretin
- CCK: initiates secretion of pancreatic enzymes
- secretin: initiates secretion of water and electrolytes from pancreas - Nervous
- vagal stimulation: pancreatic secretion and GB contraction (to eject bile)
What are CCK and secretin, and what is their role?
- released by duodenal enteroendocrine cells when chyme enters duodenum
- CCK (cholecystokinin): initiates secretion of pancreatic enzymes, GB contraction (to squeeze out bile), and relaxation of hepatopancreatic sphincter (to allow bile and pancreatic enzymes to enter D2)
- secretin: initiates secretion of water and electrolytes from pancreas
What are the 4 pancreatic enzymes?
Proteases (digest proteins)
Amylase (digests starch)
Lipases (digest emulsified fats)
Nucleases (digest nucleic acids)
Describe pancreatic juice
- 1200-1500 mL produced daily
- consists of pancreatic enzymes, water and electolytes
- pancreatic enzymes: proteases, amylase, lipases, nucleases
Describe the composition, function and secretion of bile
Composition:
- bile salts
- phospholipids
- bilirubin
- cholesterol and triglycerides
- electrolytes
Function:
- bile salts emulsify large fat particles into small droplets that can be digested by lipases
- bile salts and phospholipids interact with breakdown products of fat digestion to form micelles (ferry free fatty acids and monoglycerides to intestinal mucosa for absorption)
Secretion:
- secreted by liver
- excess stored in gall bladder
- secreted into digestive system via the hepatic and cystic ducts (unite to form the bile duct, enter D2 as the common hepatopancreatic duct via major duodenal papilla)
What is a micelle?
- formed by bile salts, phospholipids and breakdown products of fat digestion
- ferry free fatty acids and monoglycerides to intestinal mucosa for absorption
What is CCK?
Cholecystokinin
- released by duodenal enteroendocrine cells (in response to chyme entering duodenum)
Initiates:
- secretion of pancreatic enzymes
- GB contraction (to squeeze out bile)
- relaxation of hepatopancreatic sphincter (to allow bile and pancreatic enzymes to enter D2)
What causes the secretion of bile?
Stimulated by the return of bile salts from enterohepatic circulation
Where is fat digested?
In the lumen of the small intestine
- broken down by bile salts into fat droplets
- droplets broken down by pancreatic lipase into glycerol, fatty acids and glycerides
Where is protein digested?
Stomach and small intestine
- broken down in stomach by pepsin (into small polypeptides)
- broken down by pancreatic trypsin and chymotrypsin (active components of pancreatic proteases) into and amino acids
Where are carbohydrates digested?
Oral cavity and small intestine
- polysaccharides broken down by salivary amylase into smaller components in oral cavity
- broken down by pancreatic amylases into maltose adn other disaccharides in
- broken down into monosaccharides at brush border
How are fatty acids and monoglycerides absorbed?
- ferried by micelles (formed by bile salts, phospholipids and breakdown products of fat digestion)
- diffuse from micelles into epithelial cells
- repackaged into chylomicrons with other lipids and proteins
- exit epithelial cells via exocytosis
