Physiology of the small intestine 3 Flashcards
LEARNING OBJECTIVES:
*Structure of Villus
*Carbohydrate, Protein, Lipid &
Vitamin Digestion & Absorption
*Role of CCK in Pancreatic Secretion &
Bile Flow
*Destination of Absorbed Materials
*Malabsorption & Deficiencies
Small intestine major function:
Absorption
SLIDE 3 DIAGRAM - Small intestine is made up of?
Duodenum, jejunum and ileum
Most complicated for absorption?
Fats are the most complex in terms of absorption
Villi increase surface area by?
Microvilli increase surface are by?
10 fold
20 fold
slide 4 - structure of a villus see diagram
1. Name the 4 structures?
2. the enzymes are bound by?
3. What kind of fold?
- Epithelial tissue, mucous cell, villi and connective tissue core with vessels
- Membrane bound enzymes
- Villus has a circular fold
Membrane bound enzymes in villus: for carbohydrate and protein digestion
*Enteropeptidase
*Disaccharidases
*Aminopeptidases
=> Collection of enzymes along the surface to break down material on top of diverse capillary bed to pick up nutrients
Length of structures of small intestine?
dodenum is approx 10 inches in length: relatively short
jejunum: 2+1/2 m in length - v long
illeum: 3+1/2 m in length - v v long
Long length to faciliate absorption
Tube is adapted by?
They look like?
What are the adaptions for - explain?
tube is adapted by villi and microvilli
finger like projections
circular folds have villi and within single villus there are brush border: microvilli - mroe finger like projectsion
this adaption realky increases surafe area that is contact with food
Hugely increases surfae area for absorption
600 fold compared to being a flat surface
very well adapted
Centre of each villi contains? - Why? - What else is here?
centre of each villi: fast capillary bed
all digestive contents come into contact with surface and are very close to ciruatory system to move this around the body taht is why villus have capillary beds
**centre of each villi: centre lymphatic villus called LACTEAL -> really important for fat aborption
slide 5 intestinal wall structure - what is it adapted for?
duodenum
jejunum
ileum
digestion and absorption
duodenum - add secretions - site of secretion - where we empty products such as bile and pancreatic juices
jejunum - digest/absorb
ileum - specialised absorption: 2 things -> transport of recycled bile which is key for fat absorption and digetsion body is very economcial and recycles a lot, trasnporters of vitamin b12 which is key for making RBCS
local neuron control
3 major food groups: carbs, proteins, fats
Terminal is?
at major site of absorption jejunum mainly: abundant reserve absorptive capacity
can lose up to 50% of S.I. and can still have good functioning
damage to ileum has significant consequences
Caveat: terminal ileum
Carbohydrate digestion and absorption:
Carbs are chains of sugar, starchy molecules that are joined together in polysaccharides and digestive systems makes them into single chains: monosaccharides
Initially digested by AMYLASES - such as salivary amylase
However they are limited in their ability to digest polysaccharides
Secretions from pancreas: amylase which is emptied into duodenum
-> cannot fully break down into monosaccharides the absorable units but they make them into disaccharides: maltose or dextrins: 2 chains
-> At villus line it is made into single sugars, There is further digestion at the brush border/microvilli on intestinal epithelial cells
slide 8 see diagram - how do we convert materials into units that can be absorbed into our body?
key thing is how do we move digestive contents - how do we convert into absorbable units
Oral cavity seen in this diagram and then to brush border and then to epithelial cell and then transport of monosaccharides across cell into circulatory system via diverse capillary bed: logical approach
What are we digesting?
Why cannot we move these into circulatory systems and what does this instead?
What are these enzymes called?
How are they transported across epithelial cells?
How are they exited - what transporter is used?
Salivary and pancreatic amylases: many sugar units into 2 sugar units: maltose
We also digest 2 chain sugar units such as sucrose sugar and lactose in dairy
=> cannot move these into circulatory systems as too big
=> collection of enzymes at brush border of epithelia at S.I converts to single sugar units
Disaccharidases: lactase, sucrase isomaltase, maltase converts
fructose, glucose and lactose are absorbable units
How are they transported across epithelial cells? primary and secondary active transport mechanism
Focus on ATP pump - basolateral membrane and interstitial fluid and blood capillary in lumen of digestive tract
in basolateral membrane we have a primary sodium potassium ATPase pump which pumps sodium ions out for exchange of potassium ions and reduced intracellular sodium concentration and that facilitates symporter the secondary active trasport which facilitates sodium ions moved down electrical gradient - SG transporter and convert glucose and galatcose into - that is how they gain entry into cell
Fructose enters via gLUT-5 via diffusion
They all exit basolateral membrane via GLUT-2 via diffusion down concentration gradient and into capillaries
BRUSH BORDER DISACCHARIDASES:
Lactose?
Sucrose?
Maltose?
RXNS
Lactose –lactase–> Glucose + Galactose
Sucrose —sucrase-isomaltase –> Glucose + Fructose
Maltose —Maltase—Sucrase-isomaltase —> Glucose
Monosaccharide absorption slide 10
1. Glucose and galactose are absorbed by?
2. Fructose?
3. Glucose, galactose and fructose are transported out by?
lumen on left of diagram
1. Glucose + galactose absorbed by secondary active transport – sodium-glucose transporter 1 (SGLT1)
2. Fructose is transported by GLUT5
3. Glucose, galactose + fructose transported out by GLUT2
Protein digestion: gastric and duodenal proteolysis
What is Pepsin?
What is pepsinogen and what does it do?
Who makes it?
What activates it and converts it?
Why does it have to be normally inactive?
Pepsin breaks down dietary proteins into large peptides and free amino acids
Pepsinogen made by Chief cell in stomach is an inactive enzyme -HCL activates it and converts it to active pepsin which works efficiently in the acid environment
- Has to be inactive otherwise it would break down proteins of actual cell - to protect lining of stomach and the stomach
SLIDE 12
Lumen on left at brush border and moving into the cell into capillary beds
proteins we digest are chains of a.a
Endogenous proteins that we make in our bodies are processed in the same way such as plasma proteins
20-40g of proteins processed this way
exogenous proteins: those we digest
Pepsin in lumen in stomach that acts on these
pancreatic proteolytic enzymes in duodenum
pancreas is associated with stomach just under it - secretion enters duodenum this way
pancreas is a mixed gland: exocrine into ducts and endocrine into circulatory system: looking at this for Digestive systems: acinar cells that make enzymes and duct cells that make bicarbonates
drained into duct and then into duodenum so this is the site for these pancreatic proteolytic enzymes
these enzymes + pepsin convert proteins into single chained amino acids
some are converted to small chains of polypeptides that must be further broken down by aminopeptidases and intracellular peptidases
how do we trasnport over to circulatory system?
teritiary active transport as well as primary and secondary
all depends
sodium out potassium in
lowers Na in cell
stimulates secondary active transport to move amino acids across cell
symporter - 2ndary transport
Only works by ATP pump via basolateral membrane
small peptides move across lumenal membrane by teritiary and that relies on sodium ion exchange
antiporter whereby sodium moves into cell down electrochemical gradient and hydrogen ions move out against electrical gradient
hydrogen ions move separately back in by small peptides
teritary transport moves small peptides into cell
converted intracellularly by amino-peptidases enzymes
then all amino acids move by facilitaed diffusion into capillary beds
What kind of gland is the pancreas?
Exocrine - what does it consist of and it releases?
Endocrine releases?
Mixed gland
Exocrine is the duct and acinar cells that releases enzymes and bicarbonates
also have endocrine portion: islets of Langerhans which releases insulin and glucagon
