Digestion and Absorption Flashcards
what is the role of the intestinal epithelium barrier
- Enable the absorption of nutrients
2. Control the passage of pathogens or toxins
how is the internal epithelial barrier regulated
- Outer microenvironment by the microflora and chyme
- Inner microenvironment by immune cells, fibroblasts or the enteric nervous system and extrinsic nerve fibres)
what is the functional structure of the enterocytes built by
specialised cytoskeletal proteins
how much absorption of nutrients occurs In the small intestine
- 95% of absorption of nutrients occurs in the small intestine
what is the rate of fluid movement in the digestive system
- ingested water (2 liters)
- liver and pancreatic secretions ( 2 liters)
- salivary gland secretions (1.5 liters)
- secretions by glands of the stomach and small intestines (3.5 liters).
- Small intestine absorbs 8.5L
- Colon 400ml
- Faeces 100ml
describe what the role of the stem cells at the bottom of the crypt is
At the base of crypt you have stem cells, replicate all the time, gut is a harsh environment and many of the cells at the top of the crypt is removed therefore the stem cells produced replace them
what are the 3 processes of digestion and absorption
- luminal
- mucosal phase
- post absorptive phase
describe the luminal phase
The ingested food is broken down by acid in the stomach, alkali in the small intestine and substrate-specific enzymes secreted by the gastric and small bowel mucosa and the pancreas
describe the mucosal phase
is when pre-digested nutrients are selectively taken up at the brush border membrane of the enterocytes and then enter the intestinal cells.
describe the post absorptive phase
transport of absorbed nutrients via lympathetics and the portal circulation to the rest of the body
why is water needed in the digestive system
- Hydrolysis reactions of digestion
- Facilitation of absorption (brings products of digestion into close proximity to microvilli)
- Facilitation of propulsion of gut contents
- Combination with mucin granules to make mucus
how do crypt cells assure that water will be in the gut (very important)
- Sodium and potassium and chlorine enter by cotransport
- Chloride enters lumen through CFTR channel
- Sodium is reabsorbed
- Negative CL- in lumen attracts sodium by paracellular pathway
- Water follows
- This is the system by which the crypt cells assure that water will be in the gut
what nutrients are absorbed in the small intestine
- Carbohydrate and complex carbohydrates
- Lipids
- Proteins
- Vitamins
how is carbohydrates digested and absorbed
- Luminal phase
– Split into shorter molecules by salivary (trivial – this is because as soon as you swallow it is inactivated ) and pancreatic (important) enzymes (e.g. maltase, amylase). The amlayse in the salvia does not work at low pH
= disaccharides + limit dextrins - Mucosal phase
- brush border enzymes complete digestion.
- Glucose and galactose enter epithelial cells via sodium-linked secondary active transport across the apical membrane. Fructose enters by facilitated diffusion. - Post absorptive phase – the sugar exits the cells across the basolateral membrane by facilitated diffusion to the portal vein
Potassium enters and sodium leaves
what are the brush border enzymes
Sucrase
- lactase
- maltase
- limit dextrinase
- glucoamylase
describe the digestion and absorption of lipids
- Luminal phase
- Lipid digestion begins in the mouth with lingual lipases and continues in the stomach where gastric lipase is added to the mixture.
- With the help of Bile salts, pancreatic lipase digests triglycerides into free fatty acids, and monoglycerides. - Mucosal phase
- Fatty acids and monoglycerides enter the enterocytes by simple diffusion as the membrane is lipid too
- Inside the enterocytes the molecules are reassembled into triglycerides and are packaged into large particles called chylomicrons. - Post absorptive
- The chylomicrons are secreted across the basolateral membrane by exocytosis. - The chylomicrons enter lymphatic capillaries. The flow of lymphatic fluid then carries the chylomicrons to the bloodstream.
what do micelles contain
fat soluble vitamins and cholesterol
what do bile salts do
- Bile salts enhance the digestive action of liases by breaking down fat globules into smaller droplets
- emulsification,
how are bile salts recycled
- they are absorbed in the ileum and recycled by the liver
what are communal bacteria
- they form the microbiome
- regulates of digestion
- participate in the synthesis of bile salts
biles salts derived from…
commensal bacteria are directly linked to both diet and digestion and can be considered to be diet-dependent microbial products
what is more efficient in emulsifying fats and why
Conjugated bile acids are more efficient in emulsifying fats because at intestinal pH they become more ionized that the unconjugated bile acids
what do microbial enzymes do
deconjugate bile acids
describe the digestion and absorption of proteins
- Luminal phase
Begins in the stomach with the action of pepsin. Pepsin is converted from pepsinogen by other pepsin molecules that are activated by HCl secreted by parietal cells.
Protein digestion is completed in the small intestines by enzymes secreted by the pancreas. - Mucosal phase
- Trypsinogen is converted into trypsin by enterokinase, a brush border enzyme.
- Trypsin can then activate:
- chymotrypsinogen -> chymotrypsin
- procarboxypeptidase -> carboxypeptidase
- need to have these proenzymes so it acts as a control mechanism to prevent digesting itself and the gut
- amino acids enter the epithelia cells via sodium linked secondary active transport across the apical membrane - post absorptive phase
- amino acids are then transporterd across the basolateral membrane by facilitated diffusion
what are the two types of vitamins
fat soluble
water soluble
describe how fat soluble vitamins are absorbed
- absorbed with lipids as they readily dissolve in lipid droplets, micelles and chylomicrons
- INTO LYMPH FLUID
describe how water soluble vitamins are absorbed
- most water soluble vitamins B,C follow the flux of water from the gut lumen through the mucosa
what type of vitamin is B12
water soluble
describe how B12 is absorbed in the terminal ileum
- binds to haptocorrin which is produced in the salivary glands
- Must be complexed with stomach-derived intrinsic factor (IF).
- Transported into the portal circulation.
- The vitamin is then transferred to transcobalamin II (TC-II/B12), which serves as the plasma transporter
- Absorbed in the terminal ileum.
- 60-80% IS INGESTED IN FEACES
what circulations do the fat soluble and water soluble vitamins go in
fat soluble - lymph fluid
water soluble - portal vein
describe how vibrio cholera causes secretory diarrhoea
- Gram negative comma shaped bacterium
- Alpha part of the chloera toxin binds to adenylate cyclase
- Activates adenylase cyclase
- Makes cAMP
- This activates the CFTR receptor and causes chlorine to go into the lumen
- Therefore, sodium is excreted to maintain electroneutrality through the paracellular pathway therefore water follow
- Consistent pour of water into the lumen this is called secretory diarrhoea
how do you treat cholera
Oral rehydration
- Drink that contains sodium, glucose and amino acids
- The sodium glucose and sodium amino acid receptors are not effected by cAMP
- The pump pumps the sodium out as you don’t want sodium in the cell and it is pumped out into the extracellular fluid
- Therefore water follows into the extracellular fluid
what causes generalised malabsorption
- commonly results from small intestine
- can also result from pancreatic disease or conditions in other organs such as liver disease
what does small intestinal mucosal disease cause
reduced absorptive area
what are examples of small intestinal mucosal disease
- coeliac disease
- post infections malabsorption
- crohns disease
what is the most common cause of malabsorption in the economically developed world
coeliac disease
what causes malabsorption in the developing world
- actue or chornic infection
- post infectious malabsorption and tropical sprue
- may be initiated by a viral infection
- may be associated with chronic bacterial infection of the upper gastrointestinal tract.
what is specific malabsorption
specific malabsorption is the failure of the processes governing absorption of one class or type of nutrient.
what is an example of specific malabsorption
- For example genetic or acquired failure to absorb dissachride sugars or vitamin B12
disaccharide deficiencies are …
generally genetic and most of the world’s population is lactase deficient
describe how disaccharide deficiency is caused
- Mutation in the gene LCT: Affecting the ‘mucosal phase’ of dissacharides absorption (lactase is in the brush border).
- Glucose cannot be converted to galactose
- Mutation in the gene LCT:
- Mutation in the gene SLC5A1:
- SLC5A1 encodes a member of the sodium-dependent glucose transporter SGLT1 (Sodium dependent GLucose Transporter one). Mutations in SLC5A1 cause Glucose-galactose malabsorption.
what is the consequences of disaccharide deficiency
- Unabsorbed disaccharides reaching the colon adds to the osmotic load and causes watery diarrhoea.
- Fermentation of sugars in the intestine causes gaseous distension and adds to the watery diarrhoea. Produces osmotic diarrhea
