Lecture 32 Flashcards
Where are the sites of absorption in the GI tract?
small intestine and large intestine
What is absorbed in the small intestine?
There is bulk absorption of isosmotic fluid and nutrients
What is absorbed in each part of the large intestine?
- proximal colon: absorption of products of fermentation such as fibre as well as fluid and electrolytes
- distal colon: selective absorption of fluid and electrolytes
What drives water absorption?
Na+
Explain the nutrient dependent Na+ absorption in the small intestine
Na+ is coupled to solutes such as monosaccharides and amino acids. There is also specialised absorption for bile salts and vitamins
Explain the nutrient absorption in the proximal colon
Na+ dependent absorption of short chain fatty acids
In which parts of the body is there nutrient independent Na+ absorption?
small intestine
large intestine
In which parts of the body is there nutrient independent Na+ absorption?
small intestine
large intestine
Describe the nutrient dependent Na+ absorption in the small intestine
This is an Na+-coupled solute absorption through the SGLT1 (this cotransports glucose or galactose with Na+ and water follows
Describe the nutrient independent absorption of Na+ in the small intestine
This uses the Na+/H+ exchanger (NHE3) of the Cl-/HCO3- exchanger in the apical membrane
What happens in the proximal colon?
There is fermentation of indigestible sugars and proteins which generated short chain fatty acids such as acetate, propionate and butyrate
There is also Na+ absorption
Describe the Na+ absorption in the proximal large intestine
This is an electroneutral NaCl absorption (there is paired Na+/H+ and Cl-/HCO3- exchangers in the small intestine). The Na+ absorption is associated with absorption of short chain fatty acids
What is an examples of Na+ transporter in the proximal colon? How does it work?
In the apical membrane there is a sodium monocarboxylate transporter such as SMCT1. This brings in one SCFA and Na+ into the lumen. The SCFAis used by colonocyte for intracellular metabolism and the Na+ leaves the basolateral membrane via the Na+/K+ ATPase and then Cl- and water follow paracellularly
Where does the bulk reabsorption of H2O occur?
In the small intestine
What is the purpose of the distal colon?
Regulation of composition of faeces
Explain the Na+ absorption in the distal colon
This varies depending on fluid and electrolyte status.
Explain the Na+ absorption in the distal colon
This varies depending on fluid and electrolyte status.
How does aldosterone affect the absorption in the distal colon? Why is this?
The distal colon is tight epithelia which means that Na+ must be absorbed transcellularly. If we are in a salt depleted condition, there is high levels of plasma aldosterone which means that ENaC is inserted into the apical membrane to increase the Na+ absorption. If we are in a salt replete condition, we have low levels of plasma aldosterone and we don’t reabsorb as much Na+
Is there secretion in the small and large intestine?
yes
Why do we have secretions into the small and large intestine?
To dilute the contents of the lumen in order to reabsorb the last bit of nutrients
How much do we normally secrete into the small and large intestine?
1-2L
What is secreted into the duodenum?
Cl-and HCO3-
What is secreted into the jejunum?
Cl-
What is secreted into the ileum and proximal colon?
Cl- and HCO3-
What is secreted into the distal colon?
Cl-
Describe the main mechanism for electrogenic Cl- secretion in the small intestine and colon
There is active Cl- secretion which drives the movement of Na+ and water. The result is that there is secretion is isotonic NaCl secretion.
There is NKCC1 cotransporter in the basolateral membrane which accumulates Cl- in the cell above electrochemical equilibrium. There is CFTR in the apical membrane which is activated by cAMP and is an exit pathway for Cl-
What happens in secretory diarrhoea?
This is when vibrio cholerae multiplies in the small intestine. It secretes cholera toxin which binds to the intestine and it enters villous and crypt cells. This stimulates adenylate cyclase irreversibly. The elevation of the cAMP stimulates Cl- secretion in the crypts and inhibits the nutrient independent Na+ absorption in the villi. We lose isotonic fluid
The absorption of Na+ in the small intestine is A. mainly facilitated by channels B. nutrient dependent C. is entirely nutrient independent D. is facilitated by CFTR
B. is nutrient dependent
How much sugar do we ingest per day?
200 - 300 g per day
How much sugar do we ingest per day?
200 - 300 g per day
What are three main types of dietary sugars and what are examples of each of these?
- polysaccharides (glycogen and starch)
- disaccharides (sucrose, lactose and maltose)
- monosaccharides (glucose and fructose)
Only what type of sugar is absorbed?
monosaccharides
What are the main monosaccharides absorbed and how does this happen?
- glucose and galactose are absorbed by Na+-dependant mechanism of SGLT1
- fructose is absorbed by Na_ independent mechanism GLUT5
What are the main monosaccharides absorbed and how does this happen?
- glucose and galactose are absorbed by Na+-dependant mechanism of SGLT1
- fructose is absorbed by Na_ independent mechanism GLUT5
Digestion of sugars to release monosaccharides is a two step process. Describe them including the products of each step
There is luminal digestion: eg. in the mouth there is amylase, in the small intestine there is pancreatic amylase. The products are α-limit dextrins and short oligo-saccharides and some disaccharides.
There is also contact or brush border digestion which is where there is enxyes attached to the apical membrane of the epithelial cells and this forms monosaccharides such as galactose, glucose and fructose
What breaks down lactose into _______ and _________. What absorbs these products?
lactase breaks down lactose into glucose and galactose to be absorbed via SGLT1
What breaks down sucrose into _______ and _________. What absorbs these products?
Sucrase breaks down sucrose into fructose and glucose. The fructose is absorbed via the GLUT5 and the glucose is absorbed via the SGLT1 transporter
Describe the absorption of glucose, galactose and fructose
Glucose (or galactose) and Na+ are cotransported via the SGLT1 into the cell and fructose is absorbed via the GLUT5 (all of these are in the apical membrane) and then glucose, fructose and galactose are released basolaterally via GLUT2. Cl- and water follow the Na+ which is pumped out basolaterally via Na+/K+ ATPase
The absorption of glucose via SGLT1 is against the concentration gradient, BECAUSE basolateral glucose absorption is mediated by facilitated diffusion.
A: both statements true and causally related
Is protein a source of energy?
Only when we are starving
We need proteins for amino acids
What are the three sources of protein and what percentage do each of these make up?
50% diet
25% sloughed cells
25% digestive cells
What are the two steps involved in digestion of proteins?
luminal digestion
contact digestion
Describe the absorption of proteins
In the gastric lumen, pepsin and H+ break down proteins into proteins and oligopeptides then enzymes such as trypsin break these down into oligopeptides in the intestinal lumen
Then there is the contact digestion where the oligopeptides are broken down into di and tri-peptides and the amino acids to be absorbed by amino acid transport proteins. The di and tripeptides can be absorbed by a peptide transport protein and then broken down into the amino acids
Describe the Na+ dependent amino acid absorption
Na+ and AA are contransported into the cell and them AA leave. Na+ leaves via the Na+/K+ ATPase and Cl- and water follow which results in the absorption of isosmotic fluid
Describe the di and tripeptide absorption
H+ and diand tri peptides are cotransported into the cell lumen and then are broken down into amino acids by cytoplasmic peptidases. Na+ enters the cell apically via the Na+/H+ exchanger and leaves basolaterally via the Na+/K+ ATPase. Cl- and H2O follow paracellularly
What are the 6 stages of fat digestion and absorption?
- emulsification and stabilisation
- digestion
- formation of micelles
- absorption
- re-esterification
- formation of chylomicrons
Describe the emulsification and stabilisation of fat
There is reduction in the size to droplets by motility in the stomach. This is is important because it increases the surface area available for digestion.
In the small intestine there is emulsification of fats that are stabilised by the emulsification agents. What are these?
Bile salts
phospholipids
cholesterol
lecithin
What are the enzymes are digesting fat after the droplets are stabilised by bile salts?
The main enzymes are the pancreatic lipase and colipase which are secreted by the pancreas into the small intestine. The gastric and lingual lipases also play a minor role
How is fat absorbed
You have a fat droplet and then this is broken down into emulsion droplets stabilised by bile acids. Pancreatic lipase breaks them down into micelles and then these get broken down into free molecules of fatty acids and monoglycerides which diffuse into the cell. In the cell they are reformed into triglycerides and then they are packaged into vesicles called chylomicrons which go into the lymphatic system
Vitamins are either ______ soluble or water soluble. Give examples of each
fat eg. vitamins A,D and E
water eg. vitamin C (which is Na+ dependent) and vitamin B12
Describe the absorption of vitamin B12
IF is released in stomach
in duodenum B12 binds to IF
IF/B12 complex attaches to specific receptors in terminal ileum
complex absorbed by endocytosis and degraded to release B12 into portal blood bound to transcobalamin II
How can anaemia result from a gastric ulcer?
These patients have a problem synthesising IF which means that we can’t absorb B12 which means that we can’t synthesise RBC properly which means that you could have anaemia
Protein
A. digestion starts in the mouth.
B. digestion occurs mainly in the stomach.
C. digestion is reduced during the gastric phase.
D.
absorption is facilitated by brush border membrane peptidases.
absorption is facilitated by brush border membrane peptidases.
