Lecture 27: Absorption Flashcards
What is absorption?
The net passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into the blood or lymph
Where are the sites of low absorption and why is this?
- mouth
- oesophagus
both because the food is not there for very long - stomach
because the pH is too acidic and we don’t want H+ to acidify the blood
Where is the main site of absorption?
in the small intestine
The stomach is permeable/impermeable?
impermeable
90% of the water and sodium absorption occurs in the
small intestine
9% of the water and sodium absorption occurs in the
large intestine
What are the four factors affecting absorption?
- motility
- surface area available for absorption
- transport across epithelium
- removal from the interstitial fluid
What is the effect of motility on absorption?
- it allows the correct rate of propulsion to allow for maximal digestion and absorption
- allows for storage
- segmentation allows for exposure of products of digestion to absorptive surfaces
What is the affect of available surface area on absorption?
the greater the surface area, the faster the rate of absorption
What are four anatomical adaptations of the small intestine that maximise the surface area?
- length
- circular folds (plicae circulares)
- villi
- microvilli
What is the motility pattern in the small intestine that allows the products to be exposed to the absorptive surfaces?
segmentation
The lumen of the intestine is continuous with the _____ _____ and therefore the intestinal epithelium acts as a ________
outside world
barrier
What are the two pathways of transport of molecules across the epithelium?
Paracellular
Cellular
What is the paracellular pathway?
travelling through into the interstitial fluid via the “gap” between the cells
What is the cellular pathways?
across the cell membrane, through the cell cytoplasm
Are amino acids, monosaccharides and salts lipid soluble? Therefore what is needed for them to cross the barrier?
no they are not so they need transport proteins
What is the only barrier to the paracellular pathway?
tight junctions
Are paracellular pathways selective or not?
relatively non-selective which means if the solute is small enough, it can get through
Is transport through the paracellular pathway passive of not? What does this mean?
yes it is passive which means it only requires a concentration gradient
Give examples of things that can travel through via the paracellular pathway
amino acids and monosaccharides
Cell membranes are _____ bilayers
lipid
If the solute is not lipid soluble, it requires a ____ ____ to travel via the cellular pathway
transport protein
How can we maximise absorption across available surface area?
- reduce nutrients into smallest possible unit in chemical digestion
Specific transport proteins absorb what is required and allow for ______ transport ______ the concentration gradient
active
against
Why do we want to remove substances from the interstitial as soon as possible?
to that it can get into the blood fast and go around the rest of the body
What features are there to remove the substances away from the interstitial fluid as soon as possible?
There is a short distance between the epithelium and the blood vessels and lacteal
The large blood flow to the intestine and the short distance between the epithelium and the blood vessels/lacteal prevent what from happening?
a build up of interstitial fluid
How much water do we absorb each day?
about 9L to replace the losses and reabsorb the secreted water
What is the mechanism of water absorption?
osmosis
What is osmosis?
the passive movement of water from the lumen into the blood
How is an osmotic gradient set up?
by the absorption of salts and nutrients
Describe the absorption of Na+
- passive movement via the paracellular pathway
- active transport via the cells
Describe the active transport of Na+ via the cellular pathway
Na+ is not lipid soluble so transporters are required. Since it is moving down its conc gradient to get into the cell, it is passive but to get out of the cell, it is going up its concentration gradient and therefore it requires energy in the K+/Na+ ATPase
How many mechanisms are there to get Na+ across the membrane of the epithelium cell and what are they?
3
- Na+ transport alone
- Na+ transport coupled with monosaccharides
- Na+ transport to amino acids
How does Na+ get across the cell from the lumen to inside the cell by itself?
It moves down its concentration gradient through an Na+ channel or Na+/H+ exchanger
How does Na+ get across the cell from the cell to inside the interstitial fluid?
against its concentration so it requires energy using a Na+/K+ ATPase
What are the two mechanisms of carbohydrate absorption?
Passive or active absorption
Describe the passive absorption of carbohydrates into a cell from the lumen
monosaccharides (glucose, galactose or fructose) diffuse down the concentration gradient via the paracellular pathway
What does the active absorption of carbohydrates involve?
cotransport with Na+ via the cellular pathway
What monosaccharides can travel via the cellular pathway with Na+?
glucose and galactose
Describe the active transport for carbohydrate absorption across the apical membrane
Using the Na+ traveling down its concentration gradient to provide energy, glucose or galactose are transported into the cell over the apical membrane via a Na+/glucose cotransporter
Pancreatic amylase first breaks down _______ to ________ then _____ _____ enzymes break these to __________
polysaccharides
disaccharides
brush border
monosaccharides
Describe the passive transport of glucose and other monosaccharides from the cell across the basal membrane into the interstitial fluid
the monosaccharides simply diffuse across the basal membrane through a glucose carrier
What are the two absorption mechanisms of amino acids?
passive or active absorption
Describe the passive absorption of amino acids
amino acids diffuse down their concentration gradient via the paracellular pathway
Pancreatic enzyme first breaks down _______ to ________ then _____ _____ enzymes break these to __________
proteins
polypeptides
brush border
amino acids
Describe the active transport for amino acid absorption across the apical membrane
Using the Na+ traveling down its concentration gradient to provide energy, amino acids are transported into the cell over the apical membrane via a Na+/amino acid cotransporter
Describe the passive transport of amino acids from the cell across the basal membrane into the interstitial fluid
the amino acids simply diffuse across the basal membrane through a amino acid carrier
Describe the absorption of the products of fat digestion
The products are lipid soluble and can therefore diffuse across the cell membrane without a transporter.
Is the whole micelle absorbed during fat digestion?
No, only the contents (fatty acids and monoglycerides diffuse into the cell)
What four things happen to monoglycerides and free fatty acids when inside the cell
- resynthesised into triglycerides
- packages into chylomicrons
- chylomicrons exit the cell by exocytosis
- chylomicrons enter the lacteals
Bile salts and micelles are eventually absorbed ______ the _____ digestion is complete
after
fat
Where does bile salt and micelle absorption take place?
in the ileum and colon
Describe the absorption of bile salts in the ileum
it an active transport process involving an Na+ dependent bile acid cotransporter in the apical membrane
Is the absorption of bile salts in the colon passive?
yes
Fat soluble vitamins such as vitamin A, D, E, K are absorbed with ______
fats
How are water soluble vitamins absorbed?
through a transported with Na+
Vitamin B12 is absorbed in the _________ bound to ________ _______ which is produced in the ___________
ileum
intrinsic factor
stomach
We eliminate what we don’t absorb via faeces formed in the _______ _______ which is transfered to the rectum via ________
large intestine
peristalsis
How does motility aid in absorption?
by moving the contents to the absorptive surfaces
How does secretion aid in absorption?
by suspending it in solution
