Lecture 27: Absorption Flashcards
what is absorption?
absorption is the passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into blood or lymph
what are the sites of absorption?
Mouth, esophagus, stomach
- minimal absorption of lipid soluble substances
Small intestine
- main site of absorption of 90% of water and sodium and all nutrients
Large intestine
- absorption of 9% of water and sodium
what factors affect absorption?
- motility
- surface area
- transport across epithelium
- removal from interstitial fluid
how does motility affect absorption?
Peristalsis
- provides the correct rate of propulsion to allow digestion and absorption
Segmentation
- needed for exposure to products of digestion to absorptive surfaces
how does surface area affect absorption?
the rate of absorption is proportional to surface area
- greater surface area = gaster absorption
what adaptations maximise surface area for absorption?
- length of small intestine
- plicae circulares
- villi
- microvilli
what is the problem with transporting molecules across the epithelium? what is the solution?
- the lumen of the intestine is continuous with the outside world as the intestinal epithelium is a barrier
To overcome there there are 2 pathways:
- the paracellular pathway between cells
- transcellular pathway across the membrane through the cytoplasm
what are the features of the paracellular pathways?
gaps between cells
- solutes don’t cross cell membrane
- the only barrier is the right junctions binding cells together
- relatively non-selective so small solutes can get through
- is passive so requires a gradient
what are the features of the transcellular pathway?
- solutes must cross two cell membranes
- cell membranes are lipid bilayers so if it is not lipid soluble it requires a transport protein
how is absorption maximised across the available surface area?
- chemical digestion reduces nutrients into smallest possible unit
- specific transport proteins absorb what is required and allows active transport against a gradient
how are substances removed from interstitial fluid?
- there is a large blood flow to intestine
- there is an arrangement of blood vessels and lacteals in the villi
- they prevent build up in the interstitial fluid
what specific substances are absorbed?
- water
- sodium
- carbohydrates
- proteins
- lipids
- bile salts
- vitamins
how much water is absorbed each day? how much is divided into each region of the GI tract?
we drink 1-5L/day
- salivary secretion - 1.5L/day
- gastric - 3L/day
- pancreatic - 1.5L/day
- biliary - 0.5L/day
- small intestinal - 1.5L/day
small intestine has 9-10L of water delivered to it everyday
what is osmosis?
- a mechanism for water absorption
- passive movement of water from lumen into blood
- osmotic gradient is set up by absorption of salts and nutrients
what are the mechanisms of sodium absorption?
- passive movement via paracellular pathway
- active transport via the cells
what happens in transcellular absorption of sodium?
- active transport via the cells
- requires transporters to cross the cell membranes
Mechanisms:
- Na+ transport alone (more detail in another card)
- Na+ transport coupled to monosaccharides
- Na+ transport coupled to amino acids
what happens in absorption of sodium alone?
- Na+ moves into cell down its gradient
- goes from high to low concentration
- passes through using transport proteins: Na+ channel and Na+/H+ exchanger
To get out the cell it goes from low concentration to high concentration
- so needs active transport using ATP and Na+/K+ ATPase
What are the mechanisms of carbohydrate absorption?
- passive absorption
- active absorption
what happens in passive absorption of carbohydrates?
involves monosaccharides e.g. glucose, galactose, fructose
- diffuses down concentration gradient via paracellular pathway
what happens in active absorption of carbohydrates?
- involves cotransport with Na+ via cellular pathway
- Na+/K+ ATPase creates driving force for sodium
- Na+ moves into the cell with a monosaccharide via Na+/glucose cotransporter
- Monosaccharides passively diffuse out of the cell via monosaccharide carrier
- Sodium leaves the cell via Na+/K+ ATPase
what are the mechanisms of protein digestion?
- passive absorption
- active absorption
what happens in passive absorption of proteins?
- involves amino acids
- diffuses down concentration gradient via paracellular pathway
what happens in absorption of amino acids?
- active absorption
- involves cotransport with Na+ via cellular pathway
- Na+/K+ ATPase creates driving force for sodium
- Na+ moves into the cell with an amino acid via Na+/amino acid cotransporter
- Amino acids passively diffuse out of the cell via amino acid carrier
- Sodium leaves the cell via Na+/K+ ATPase
what happens in absorption of small peptides?
- active absorption of di and tripeptides
- involves cotransport with H+ via cellular pathway
-Na+/K+ ATPase creates a negative membrane potential which is a driving force for H+ - H+ moves down its electrical gradient and brings a di/tripeptide with it through the H+/peptide cotransporter
- intracellular cytoplasmic peptidases breaks these down into amino acids
- Amino acids leave the cell via amino acid carrier
how are the products of fat digestions absorbed?
In lumen and apical membrane:
- free fatty acids and monoglycerides move by simple diffusion from micelle into cell
- the micelle is not absorbed
- bile salts absorbed in ileum
Intracellular:
- transported to the endoplasmic reticulum
- re-synthesised to triglycerides to maintain gradient for apical diffusion
- secreted from golgi as chylomicrons
- targeted to basolateral membrane
Basolateral membrane:
- exocytosed as chylomicrons
- enters lymph via lacteals of villi
how are bile salts absorbed?
enterohepatic circulation.
- the bile salts in the micelles are eventually absorbed
- this occurs after fat absorption is completed
- occurs in the ileum via an active transport process using an apical Na+ dependent bile acid cotransporter
- then occurs in the colon via passive absorption where 95% of bile salts are absorbed
how are fat soluble vitamins absorbed?
- vitamin A, D, E, K are absorbed with fats
how are water soluble vitamins absorbed?
- Na+ dependent absorption
- e.g. vitamin C
how is vitamin B12 absorbed?
- it is a special case
- it is absorbed in the ileum
- binds to intrinsic factor produced in the stomach
- absorbed via a specific transporter for intrinsic factor vitamin b12 in the ileum
what is vitamin b12 deficiency called?
pernicious anemia
what happens to what is not absorbed?
Elimination
- expulsion of residues of digestion
- feces are formed in the large intestine
- transferred to rectum via peristaltic waves called mass movement
- elimination from body by defecation reflex
