Alimentary Flashcards
The overall layout of the gastro-intestinal tract with an understanding of major functions along the GIT.
○ Overall layout
§ Headgut – salivary gland, mouth & pharynx
§ Foregut – oesophagus & stomach
§ Midgut – small intestine & duodenum (liver, gallbladder & pancreas)
§ Hindgut – large intestine (caecum, colon & rectum)
○ Major Functions
§ Headgut – food prehension & mastication
§ Foregut – digestion
§ Midgut – absorption & initial storage of nutrients
○ Hindgut – expulsion of unabsorbed portions of food
(P1) Describe the cellular arrangement of the GIT, highlighting the common patterns and variations along its length, explaining how these relate to regional function.
○ Salivary Glands
§ Acini – secretory units, clusters of cells
§ Serous cells – watery fluid, parotid gland
§ Mucus cells – sublingual gland
§ Mandibular gland – mix of serous & mucous cells
○ Oesophagus
§ Mucosa – stratified squamous epithelium (lines lumen)
§ Submucosa – elastic & collagen fibres (irregular connective tissue) &
blood/lymph vessels
§ Tunica muscularis – longitudinal superficially, circular muscle fibres deeply
○ Serosa – loose connective tissue
(P2) Describe the cellular arrangement of the GIT, highlighting the common patterns and variations along its length, explaining how these relate to regional function.
○ Stomach
§ Mucosa – gastric glands/pits & muscularis mucosa (smooth muscle layer)
§ Submucosa – blood & lymph vessels
§ Muscularis externa – 3 muscle layers: inner oblique, middle circular,
external longitudinal
○ Liver
§ Hepatocytes – bile synthesis, store energy, biotransformation of
substances
§ Lobule – hexagonal, portal triads & central vein
○ Pancreas
§ Acini – cells form circular shape, produce pancreatic juice (digestive
enzymes)
§ Islets of Langerhans – alpha (glucagon) & beta (insulin) cells in acini
○ Colon § 4 layer: mucosa, submucosa, tunica muscularis (muscle fibres) & serosa
Describe motility of the GIT, explaining how this is regulated, including the role of the enteric nervous system and its modulation by the autonomic nervous system.
○ Motility = controlled muscle stretching & contractions to enable food travel through tract & ensure absorption of nutrients
○ Regulated = by endocrine & paracrine factors, and enteric nervous system (w/ ANS influence)
○ Enteric nervous system
§ Extrinsic control – sympathetic & parasympathetic (dec/inc. GIT activity)
§ Enteric nervous system – myenteric plexus & submucosal plexus
§ Sensory nerve endings – stimuli via GIT mucosa
○ ANS modulation
§ ANS = enteric + sympathetic + parasympathetic
§ Sympathetic = decrease GIT activity via noradrenaline (fight/flight)
○ Parasympathetic = increase GIT activity via acetylcholine (relax)
Motility Regulation - Discuss the concept of regulation of the GIT, through endocrine, paracrine and neural mechanisms.
○ Mechanisms contribute to extrinsic & intrinsic control of GIT motility
§ Endocrine – hormones released into blood (extrinsic)
§ Paracrine – local diffusion of agents into nearby cells (extrinsic)
§ Nervous – intrinsic control via enteric nervous system
□ Myenteric plexus - within muscularis internal & externa layers
□ Submucosal plexus - within submucosal layer of GIT
○ Regulation
1. Stimulus (food) 2. Mechano & chemo receptors a. (CNS) --> Enteric Nervous System --> motility secretion b. Endocrine/Paracrine agents --> blood flow (absorption)
Secretion Regulation - Discuss the concept of regulation of the GIT, through endocrine, paracrine and neural mechanisms
○ Endocrine
§ regulated by stimuli = the presence of food in the duodenum
§ duodenum food presence activates mechano and chemo receptors = pass
on stimuli to the CNS
○ Paracrine
§ regulate motility, secretion & blood flow
§ optimise absorption of nutrients that are digested in small intestine.
○ Neural
§ Homeostasis (w/ endocrine & paracrine mechanisms)
Outline the dynamic and responsive nature of GIT secretions, using the cephalic, gastric and intestinal phases of gastric secretion as an example.
○ Cephalic
§ Stimuli (5 senses) reflex via vagus nerve (CN X)
§ secrete gastrin & histamine
§ increases – stomach motility, hydrochloride & pepsinogen secretion
○ Gastric
§ Secrete gastrin & histamine via chemo & mechano receptor stimulation
§ Local negative feedback loop – gastric pH less than 3
○ Intestinal phases
§ Inhibits acid secretion & motility – when chyme is present in duodenum
§ Via secretion, GIP & CCK
Describe the breakdown of fats by digestive enzymes in the GIT into simple compounds capable of being absorbed by the small intestine.
○ Emulsification (fat digestion)
1. Fat globule through stomach & sprayed through small intestine into pyloric
sphincter
2. Passage of fat globule in pyloric sphincter into duodenum = encounter bile
salts (hydrophobic & hydrophilic sides)
3. Bile salts bind to fat globule - hydrophobic side touching fat globule,
hydrophilic side touching water
4. Emulsification = bile salts coating fat globule (prevents coalescing)
5. Droplets emulsify with water presence & droplets reduce in size (micelles)
Describe the breakdown of carbohydrates and fats by digestive enzymes in the GIT into simple compounds capable of being absorbed by the small intestine.
○ Carbohydrate digestion
§ Alpha-amylases – enzymes that catalyse hydrolysis of starch into sugars
§ Attacks a-1,4 glycosidic link only & in middle of chain
§ Simple compounds – glucose (fructose & galactose)
○ Salivary a-amylase
§ Begins chemical breakdown process (pH 6.6-6.8)
§ Inactivated at stomach (pH < 3)
○ Pancreatic a-amylase
§ Digests majority of carbohydrates in small intestine
○ Secreted from exocrine pancreas
Explain the breakdown of proteins into amino acids in the stomach and small intestine, including the role of zymogens and how they are activated.
○ Breakdown of proteins
§ Proteins are digested by proteases which attack peptide bonds
§ Most proteases are produced by stomach/pancreas & stored/secreted as
zymogens
§ Pepsinogen secreted in stomach & activated to pepsin by H+.
□ Pepsin is an endopeptidase which attacks nearly all proteins
§ Trypsin, chymotrypsin, and elastase are endopeptidases secreted by the
pancreas as zymogens & activated in the SI.
§ Protein digestion is completed by exopeptidases (carboxy & amino)
secreted by the pancreas or small intestinal glands, which become closely
associated with the brush border.
§ Amino acids are absorbed by mainly by a sodium-dependent active
transport system involving 4 transport proteins
§ Some di/tri peptides are also absorbed by a similar mechanism
○ Zymogens
§ Types – trypsinogen, chymotrypsinogen & pro-elastase
§ Role – precursor substances from peptidases (alkaline optimum pH)
§ Active enzymes – trypsin, chymotrypsin, elastase
§ Activation
□ enterokinase of trypsinogen into trypsin
□ trypsin causes the other 2 zymogens to activate into enzymes
□ trypsin = 1st endopeptidase, chymotrypsin = 2nd, elastase = 3rd
Explain the mechanisms used to transport the products of carbohydrate and protein digestion across the intestinal membrane and into the blood.
○ sodium glucose co transporter-1 (Na+/glucose symport)
§ an active mechanisms that requires energy.
§ Glucose & sodium will then merge into intestinal cell (enterocyte) & stored.
○ Glucose & sodium presence inside intestinal cell
§ create a concentration gradient (differ from conc. gradient in the blood)
○ Glucose & sodium leave cell into the blood by
§ carrier mediated diffusion
○ Sodium potassium pump
Explain the processes involved in the uptake of the products of fat digestion by intestinal cells and their subsequent secretion into lymph as chylomicrons.
§ Products of fat digestion – free fatty acids, 1 & 2 monoacylglycerols & glycerol
§ In lumen – bile acids & phospholipids break down to produce micelles
§ Fatty acids absorbed into intestinal cells & converted into 1 or 2 monoacylglycerols
§ Products combine to form triacylglycerol & turns into chylomicrons
§ Chylomicrons able to move into lymph & blood as TAGs
Lipoprotein lipase breaks chylomicrons into remnant & fatty acids into tissues
