Intestinal physiology: fluid, electrolytes and digestion Flashcards
What is important for diffusion?
Shape:
- If the shape is not hollow: greater ratio of volume to exterior surface area than in a single cell
- The cavity or lumen is important for optimal digestion and absorption
What does terrestrial mean?
Not living in an aqueous solution filled with nutrients
What is the function of the gut (tube)?
- Take relatively large solids and digest them into smaller molecules that can be absorbed as nutrients, while still serving as a barrier to toxins, bacteria, parasites etc…
What are some functional anatomy points about the GI system?
- GI system is a hollow organ, a tube through the body.
- The lumen is “outside” the body’s tissues, but its environment is tightly controlled by the body.-
- Specialised organs for secretion of enzymes & bile.
- Epithelial cells line the entire GI tract and serve as the primary barrier.
- Structure maximises surface area for secretion and absorption (folds, villi, and crypts).
What functions do the epithelial cells have?
- Secrete water and electrolytes
- Absorb water and elecrolytes
How much fluid should we ingest a day?
2L/day
How much fluid does the small intestine absorb a day?
7.5 L/day
How much fluid do we excrete a day?
<200ml
How does water move in an osmotic gradient?
- Water moves down the osmotic gradient
- Electrolytes move down electrochemical gradient
How is energy supplied to move up the conc gradient?
Energy is supplied by the sodium gradients (generated by the sodium pump) and by proton gradients
How much fluid does the small intestine ingest a day?
7.5L/day
What are the three segments of the small intestine?
- Duodenum
- Jejunum
- Ileum
How long is the small intestine circa?
6m long
How does Na+ help with water movement?
1st stage: membrane transport protein - Transports glucose from intestinal lumen into epithelial cells via Na+
2nd stage: Na+/K+/ATPase transporter uses ATP to shift sodium into blood/ baso-lateral membrane. This shifts water with it from the gut to body (blood)
This happens because water always follows sodium
How does Cl- help with movement of fluids?
- cAMP (cyclic AMP) shifts Cl from body to intestinal lumen
- This makes the basolateral end of the lumen very negative
- So the Cl- moves across lumen to a positive end and it takes Na+ and K+ with it (as it uses a Na+/K+/2Cl- transporter)
- This therefore takes water with it
What factors affect absorption?
- Number and structure of enterocytes
- Blood and lymph flows
- Nutrient intake
- GI motility (e.g. if take medication that influences movement)
- GI motility also affected by hormonal and neural factors
What factors affect secretion?
- Irritants
- Bile
- Bacterial toxins
- Hormonal and neural factors also affect secretion
What are some clinical conditions that can arise if the intestines can not absorb/secrete properly?
- Diarrhoea
- Weight loss
- Failure to thrive
Why does coeliac disease affect absorption?
- Attacks small bowels and so you lose villi
- Therefore can’t absorb nutrients properly
What toxin induces diarrhoea?
- Cholera toxin is released from bacteria in infected intestine
How does cholera toxin induce diarrhoea?
- Released
- Then binds to intestinal cells
- Stimulates adenylate cyclase to produce cAMP
- Dramatic efflux of ions and water
- Profuse Cl- excretion so lots of water movement into body
- So watery diarrhoea
How do oral rehydration packets work?
- Packet contains lots of SGLT-1 or sodium glucose so lots of sodium ions in the intestinal wall. So Na moves into blood and takes water with it as a co-transporter, therefore rehydrating the body
What is the definition of digestion?
Breakdown of large, complex organic molecules that can be used by the body
- Mechanical (chewing, churning food)
- Chemical (enzymes)
Where is the primary site for digestion and absorption of food?
Small intestine
Where does digestion occur more specifically?
In the GI lumen by secreted enzymes and on the surface of enterocytes (intestinal absorptive cells) by membrane-bound enzymes
Which types of transports facilitate absorption?
Simple diffusion, facilitated diffusion, active transport, endocytosis, paracellular transport
How is the small intestine adapted for absorption?
By extensive folding and the projection of fingerlike villi covered with microvilli
How is CHO (carbohydrate) digested?
- Starts as a glucose polymer then amylase in the mouth and pancreas breaks it down into…
- Disaccharides (e.g. maltose, sucrose)
- These are then broken down further into monosaccharides (e.g. glucose, fructose)
How is digested carbohydrate absorbed?
Enterocytes absorb glucose and galactose through an Na+ dependent secondary active transport process, while fructose is absorbed by facilitated transport.
How are proteins digested?
Only really digested and acted on in the stomach by pepsin and then in the small intestine by trypsin and chymotrypsin breaking them down into amino acids
How are proteins absorbed?
Facilitated diffusion and co-transport into the intestinal mucosa with the help of peptidases and then via Facilitated diffusion and co-transport into the blood stream
How are fats digested?
- Fat and water separate
- So bile (an emulsifier) has an affinity for both fat and water and can thus bring the fat into the water
- This allows fat-digesting enzymes to have access to it and break the fat down
What do bile acids do?
- Gallbladder contracts and releases contents (bile)
- They are released into the small intestine
- After they have emulsified fats, they are reabsorbed in the terminal ileum and recycled back to the liver
How are lipids absorbed?
- Bile salts help absorb from the lumen into the intestinal epithelia
- Then chylomicrons help transport triglycerides and lipids from intestinal epithelia to vessels
- Then they are transported through the lymph system
What enzymes do the salivary glands release?
- Amylase (starch)
- Lipase (triglycerides)
What enzymes does the stomach release?
- Pepsin (proteins)
- Lipase (triglycerides)
What enzymes does the pancreas release?
- Amylase (Starch)
- Lipase and Colipase (Triglycerides)
- Phospholipase (Phospholipids)
- Trypsin (Peptides)
- Chymotrypsin (Peptides)
What enzymes does the intestine release?
- Enterokinase (Activates trypsin)
- Disaccharidases (Complex sugars)
- Peptidases (peptides)
What are the three pairs of salivary glands?
Parotid
Sublingual
Submandibular
What are the functions of saliva?
- Lubricates, cleans oral cavity
- Dissolves chemicals
- Suppresses bacterial growth
- Digest starch by amylase
What regulates the secretion of saliva?
Neural (voluntary and involuntary) secretions
