Digestion and Absorption Flashcards
What is the main function of the small intestine?
Major site for digestion and absorption
What is the length of the small intestine?
Approx. 6m long with a 3.5cm diameter
What does the small intestine recieve?
Chyme from the stomach via the pyloric sphincter
Pancreatic juice from the pancreas (sphincter of oddi)
Bile from the gall bladder (sphincter of oddi)
Generally what does the small intestine secrete?
Intestinal juices (succus entericus)
What valve does the small intestine move residues thorough?
The ileocaecal vale which opens in response to proximal pressure and in response to gastrin
How has the small intestine adapted to be good at absorption?
Circular folds
Villi
Microvilli
What peptide hormones are secreted into the blood via endocrine cells within the mucosa of the small intestine?
Gastrin Cholecystokinin (CCK) Secretin Motilin Glucagon-like insulinotropic peptide (GIP) and incretin Glucagon-like peptide-1 (GLP-1) Ghrelin
What cell secretes gastrin?
G cells of the gastric antrum and duodenum
What cell secretes CCK?
I cells of the duodenum and jejunum
What cell secretes secretin?
S cells of the duodenum
What cell secretes motilin?
M cells of the duodenum and jejunum
What cell secretes GIP and incretin?
K cells of the duodenum and jejunum
What cell secretes GLP-1 and incretin?
L cells of the gut
What cell secretes ghrelin?
Gr cells of the gastric antrum, small intestine and elsewhere (pancreas)
What do all the peptide hormones from the small intestine act on except from incretin?
G-protein coupled receptors
What does incretin act on?
Beta cells of the pancreas in essentially a feed-forward manner to stimulate the release of insulin
What are the positive control mechanisms for the release of succus entericus?
Distension/irritation Gastrin CCK Secretin Parasymp ALL enhance
What is the negative control mechanism for the release of succus entericus?
Symp nerve activity
Decreases
What does the succus entericus contain?
Mucus for protection (goblet cells)
Aqueous salt - enzymatic digestion (crypts of lieberkuhn)
NO digestive enezymes
What occurs during segmentation (mixing) of chyme?
The chopping action moves chyme back and forth - very vigorous after a meal
Alternating contraction and relaxation of segments of circular muscle
What initiates segmentation?
Small intestine pacemaker cells causing the BER which is continous. At threshold, it activates segmentation which in the duodenum is primarily due to distension by entering chyme
How is segmentation in the empty ileum triggered?
By gastrin from the stomach (gastroileal reflex)
How is the strength of segmentation altered?
It is enhanced and decreased by parasympathetic and sympathetic activity
What is the migrating motor complex (MMC)?
Occurs between meals every 90-120 mins and is a strong peristaltic contraction passing the length of the intestine (stomach - ileocaecal valve)
What is the function of the migrating motor complex?
Clears the small intestine of debris, mucus and sloughed epithelial cells between meals
What inhibits the MMC?
Feeding and vagal activity
What triggers the MMC?
Motilin, suppressed by gastrin and CCK
What are the endocrine pancreatic secretions?
Insulin and glucagon - secreted to the blood
What are the exocrine pancreatic secretions?
Digestive enzymes (acinar cells), aqueous NaHCO3- solution (duct cells) which are secreted to the duodenum as pancreatic juice
What is the purpose of the duct cells producing alkaline fluid?
To neutralise the acidic chyme entering the duodenum, this provides optimum pH for the pancreatic enzyme function and protects the mucosa from erosion by acid
What are the proteases stored within the acinar cells?
Trypsinogen, chymotrypsinogen and procarboxypeptidae A and B
What are the amylases stored within the acinar cells?
Pancreatic amylase
What are the lipases stored within the acinar cells?
Pancreatic lipase
Where are the enzyme stored within the acinar cells?
In zymogen granules and released in response to elevated calcium
Where are the proteases activated?
Within the duodenum
What are the 3 phases to pancreatic secretion?
Cephalic
Gastric
Intestinal
What occurs during the cephalic phase?
Mediated by the vagal stimulation of mainly the acinar cells
What occurs during the gastric phase?
Gastric distension evokes a vasovagal reflex resulting in parasymp stimulation of acinar and duct cells
What occurs during the neutralisational intestinal phase?
Acid in duodenual lumen, increased secretin release, secretin carried to pancreaetic duct cells resulting in increased secretion of alkaline fluid
What occurs during the digestion part of intestinal pancreatic secretion?
Fat and protein in duodenal lumen
Increased CCK release from I cells which is carried by the blood to the pancreatic acinar cells. This results in an increased secretion of digestive enzymes into duodenal lumen
What are the main constituents of carbohydrates?
Starch (amylose and amylopectin)
Cellulose
Glycogen
Disaccharides (sucrose, lactose)
What are the main constituents of lipids?
Triacylglycerols Phospholipids Cholesterol and cholesterol esters Free fatty acids Lipid vitamins
What are the main constituents of proteins?
Ingested plus some from endogenous sources such as digestive enzymes and dead cells from GI tract
What is digestion?
The enzymatic conversion of complex dietary substances to a form that can be absorbed
What do most digestive proccesses occur in the small intestine as?
Luminal digestion - mediated by pancreatic enzymes secreted into the duodenum
Membrane digestion - mediated by enzymes situated at the brush border of epithelial cells
What is absorption?
The proccesses by which the absorbable products of digestion are transferred across both the apical and basolateral membranes of enterocytes (absorptive cells of the intestinal epithelium)
What are the different types of digestible carbohydrates?
Polysaccharides
Oligosaccharides
Monosaccharides
What are the 2 different types of polysaccharides?
Starch (amylose, amylopectin), glycogen (animal)
What are the different types of oligosaccharides?
Sucrose (glucose and fructose)
Lactose (glucose and galactose)
What are the different types of monosaccharides?
Glucose and fructose and galactose
What do carbohydrates need to be converted to to be digested?
They need to be converted to monosaccharides
How are starch converted to oligosaccharides?
Alpha-amylase by salivary and pancreatic
How are oligosaccharides converted to monosaccharides?
Via oligosaccharidases (lactase, maltase and sucrase-isomaltase)
What is alpha-amylase?
Endoenzyme
What does alpha-amylase break down?
It breaks down linear internal alpha-1,4 linkages but not terminal alpha-1,4 linkages and therefore there is no production of glucose
What are the products when alpha amylases break down amylose or amylopectin?
Products are thus linear glucose oligomers (maltotriose, maltose) and alpha limit dextrins
What are the role of oligosaccharidases?
They are integral membrane proteins with a catalytic domain that faces the lumen on the GI tract
What is the substrate of lactase?
Has only one substrate, breaks down lactose to glucose and galactose
What do all other oligosaccharides do (apart from lactase)?
Cleave the terminal alpha-1,4 linkages of maltose, maltotriose and alppha limit dextrins (to yield glucose)
What is sucrose responsible for?
It is specifically responsible for hydrolysing sucrose to glucose and fructose
How can lactose intolerance result?
Primary lactase deficiency
Secondary lactase deficiency
Congenital lactase deficiency
Where does absoprtion of the final products of carbohydrate digestion occur?
In the duodenum and jejunum via the apical and basolateral membranes.
What receptor absorbs glucose and galactose?
Seconadry active transport mediated by SGLT1
How is fructose absorbed?
Facilitated diffusion mediated by GLUT5
How do all monosaccharides exit the small intestine?
Facilitated diffusino via GLUT
How does SGLT1 work?
2 Na+ bind increasing the affinity for glucose. Na+ and glucose translocate from extracellular to intracellular where the Na+ dissociate and the affinity for glucose decreases
How must protein be digested before it can be absorpbed?
To oligopeptides and amino acids by pepsin and pancreatic proteases
What do pepsidases do?
They further hydrolyse oligopeptides to amino acids at the brush border
How are amino acids transported across the apical membrane?
Via a variety of amino acid transporters, some of which are Na+ dependent and others Na+ independent
How are oligopeptides transported across the apical membrane?
Via H+/oligopeptide co-transporter PepT1
What happens to oligopeptides once they cross the apical membrane/
Within the cytoplasm they are further hydrolysed to amino acids by pepsidases within the enterocyte
How do amino acids exit the enterocyte?
By crossing the basolateral membrane by several Na+ independent transporters
