Structure and FUnction of GI tract I Flashcards
What is chyme
mixture of food and secretions
What organs are found within the GI tract
mouth - ingestion of food
{pharynx - swallowing of food
Oesophagus - transport of food to stomach
Stomach - breakdown of food
SI - digestion and absorption
LI - Formation and storage of faecesWhat are the accessory organs of the GI tract
Salivary glands - secretion of lubricating fluid
Liver - secretion of bile, storage of nutrients
Gall bladder - storage and concentration of bile
Pancreas - secretion of enzymes and hormones
Explain the process of digestion
Starts at ingestion of food in oral cavity
Within the oral cavity, chewing and secretion of saliva by salivary glands takes place
Food then swallowed and transported to stomach by oesophagus
Food digested in stomach by mixing food with acids and enzymes to create chyme
Chyme moved into SI via the Pylorus
Most digestion occurs in SI
Unabsorbed chyme passes in LI where it becomes faeces
What regulates what flows through the pylorus
Pyloric valve
Thick band of smooth muscle which allows amounts of chyme into the SI at any one time
What are the layers of the GI tract
Mucosa - epithelium, underlying connective tissue and smooth muscle
Submucosa - dense irregular connective tissue
Muscularis - two layers of smooth muscle
Serosa - simple squamous epithelium and connective tissue
What are the functions carried out by the digestive system
Digestion - chemical and mechanical breakdown of food into smaller pieces that can be moved across the intestinal epithelium
Absorption - movement of substances from the lumen of the GI tract to the ECF
Secretion - movement of water and ions from ECF to digestive tract lumen; release of substances synthesised by GI cells into lumen or ECF
Motility - movement of material in the GI tract as a result of muscle contraction
How does mass balance pose a challenge to the digestive system
Maintaining fluid input with output; people ingest around 2 litres of fluid per day, in addition exocrine glands and cells secrete 7 litres or so of enzymes, mucus, electrolytes and water in the GI tract lumen
If this was not reabsorbed then body would dehydrate
How does autodigestion affect the digestive system
Autodigestion is when digestive enzymes break down the cells of the GI tract itself, causing peptic ulcers to develop
What are digestive enzymes
Breakdown polymeric macromolecules into smaller building blocks
Secreted by exocrine glands or epithelial cells in the stomach and SI
Help facilitate absorption of nutrients into the body
How are digestive enzymes classified
Based on target substances
- amylases = carbohydrates
- proteases = proteins
- lipases = fats
- nuclease = nucleic acids
What are zymogens
Enzymes secreted in an inactive form
Must be activated in the GI lumen before they carry out digestion
What occurs in the mouth during digestion
Secretion of saliva to soften and lubricate food
Mastication mixes food to increase its surface area
Salivary amylase breaks down starch
What are the three salivacry glands
Parotid - back of the mouth
Sublingual - under the tongue
Submandibular - under floor of the mouth
What is saliva
Complex fluid that contains water, ions, mucus and proteins
How does saliva production occur
Initial fluid secreted by acinar cells resembles extracellular fluid composition
Asfluid passes through the duct on its wa to the oral cavity, epithlial cells reabsorb NaCl and secrete K+ and bicarbonate ion until the ion ratio is more like intracellular fluid
Function of saliva
Soften and moisten food
DIgestion of starch with salivary amylase
Taste
Defense via lysozymes and salivary immunoglobulins
Why is colipase needed
Lipase is unable to penetrate the bile salts surrounding the droplets, therfore digestion also requires colipase
What is swallowing and how does it occur
Reflex action that pushes bolus of food or liquid down into oesophagus
As swallowing begins, the soft palate elevates to close of the nasopharynx; muscle contraction move the larynx up and forward which closes off the trachea
as bolus moves down towards the oesophagus, epiglottis fold down completing closure of the upper airway and preventing food and liquid from entering the airway
What triggers swallowing
pressure created when the tongue pushes the bolus against the soft palate and the back of the mouth activates sensory enurons that run the glossopharyngeal nerve to the swallowing centre in the medualla oblongata
What is peristalsis
Contraction of the muscles of the oesophagus to move food down to the stomach
Why is the hydrophobic nature of lipids a complication in digestion
Aqueous chyme leaving the stomach contains large fat droplets with less surface area, decreasing rate of digestion
So bile is secreted to form smaller droplets with larger surface area
What does colipase do
displace bile salts allowing lipase access to the fat droplets
How are lipophilic fats absorbed
Simple diffusion
Once absorbed into enterocytes they move to the smooth endoplasmic reticulum where they recombine into triglycerides
Triglycerides link with cholesterol and protein to form chylomicrons
Exit via exocytosis and then absorbed into lacteals
Pass through lymphatic system then enter venous blood
How are shorter fatty acids absorbed
directly cross basement membrane and enter blood
How is carbohydrate digested
Amylase breaks long glucose polymers into smaller glucose chains and into maltose
Starch digestion starts with salivary amylase and continues with pancreatic amylase
Maltose and other disaccharides are broken down by intestinal brush-border enzymes known as disaccharidases
How does intestinal glucose and galactose absorption occur
uses apical Na+-glucose SGLT symporter and basolateral GLUT2 transporter
Glucose enters on SGLT symporter with Na+ and exits membrane by GLUT2 transporter
How is metabolism of enterocytes different to other cells
Glutamine is main source of energy
Allows absorbed glucose to pass unchanged into blood stream
What are endopeptidases
Commonly known as proteases
Attack peptide bonds in amino aid chains to break them into smaller fragments
secreted as zymogens
What are exopeptidases
Release single amino acids by acting on the terminal amino acid on the protein
How are proteins absorbed
Most free AA are carried by NA+-dependant cotransport proteins
Few AA are transported by H+ dependant cotransporter
Dipeptides and tripeptides are carried into enterocytes on the oligopeptide transporter PepT1 that uses H+ dependant cotransporter
What happens to dipeptides and tripeptides once they are absorbed into cell
Most digested into AA and transported across basolateral membrane into circulation
Those that are undigested are transported across BL membrane on H+ dependant exchanger
How are vitamins absorbed
Fat soluble vitamins like A,D,E and K are absorbed in the SI with fats
Water soluble vitamins B (except B12) and C are absorbed by mediated transport
B12 contains cobalt and is absorbed by a intestinal transporter found in the ileum and only is recognised when it is complexed with protein intrinsic factor
How is dietary iron absorbed
Absorbed by apical transporter on the enterocyte
Fe2+ Actively absorbed by apical cotransport with H+ on a protein called DMT1
Inside cell enzyme converts iron to Fe2+ and both pools of ionised iron leave the cell on transported called ferroportin
How is iron uptake regulated
Hepcidin; when body stores of iron are high, liver secretes hepcidin which binds to ferroportin and destroys the transporter decreasing iron uptake
How does ion and water absorption occur
Absorption of nutrients moves solute from the lumen of the intestine to the ECF creating an osmotic gradient - water can follow
Enterocytes in SI and colonocytes absorb Na+ using apical Na+ channels, Na+-Cl- symporter, Na+-H+ exchanger and
basolateral primary transporter is Na+-K+-ATPase
Chloride uptake via apical Cl- HCO3- exchanger and basolateral Cl channel
What happens in the large intestine
1.5 litres of unabsorbed chyme remain
Bacteria present in the colon break down any undigested complex carbs and protein through fermentation
End products include lactate and short chain fatty acids (which are absorbed by simple diffusion[colonocytes are preferred source of energy])
