transport along and across the GI tract Flashcards
what is gastric emptying dependent on ?
- Propulsive force generated by the tonic contractions of the proximal stomach
- Stomach’s ability to differentiate the types of meals ingested and their composition
what are the determents of the rate of gastric motility ?
- Type of food eaten: carbohydrates>protein>fatty foods>indigestible solids
- Fatty, hypertonic, and acidic chyme in the duodenum decrease the force and rate of gastric empty
- Osmotic pressure of duodenal contents: hyperosmolar chyme ↓ gastric emptying
- Vagal innervation upon over-distension (e.g., duodenum) ↓ gastric motility
- Hormones (somatostatin, secretin, CCK): inhibit emptying
- Injury to intestinal wall and bacterial infections ↓ motility
describe myogenic control of gastric motility
- Elicited by intrinsic basic electric rhythm (BER) from intestinal cells of cajal (ICC)
- Stomach muscle cells (pacemaker cells) produce electric depolarisations from resting potential
- BER allows the smooth muscle cell to depolarise and contract rhythmically when exposed to hormonal signals
- BER moves ripples towards the antrum
- The rhythm of depolarisation-repolarisation create the slow waves from ICC – mediate regular recurring migrating ripples (3 waves/min) known as BER
give a summary of factors that can effect gastric emptying
what is motility in the small intestine characterized by ?
- Segmentation (mixing contractions): stationary contraction & relaxation
- Peristalsis (propulsive): in stomach (3 waves/min)
- Migrating Motor Complex (MMC)
- Mass movements (evacuation)
what are the phases of motor activity in the alimentary tract ?
- Phase 1: quiescence/quiet period
- Phase II: irregular propulsive contractions
- Phase III: burst of uninterrupted phasic contractions (peristaltic rush)
describe segmentation
- Originates in the pacemaker cells (ICC)
- Segmentation → divisions and subdivisions of chyme, bringing chyme in contact with intestinal walls
- Segmentation causes the slow migration of chyme towards ileum
- Duodenum/jejunum: 10-12 contractions/min
- Distal ileum: 8-9 contractions/min
what are the differences between segmentation and peristalsis ?
- Peristaltic (propulsive) contractions spread the food out allowing digestive enzymes to mix with it, but primarily push the food towards the anus (global movement)
- Segmenting (mixing) contractions primarily churn the food, but also propel it towards the anus - some localisation; allows enzymes to be mixed with the chyme so that digestion can occur; also bring the chyme close to the gut mucosa
describe the migratory motor complex
- Highly organised motor activity
- Cyclically recurring sequence of events
Occurs between meals when the stomach / intestine are “empty” - Initiated by vagus nerve in the upper GIT; prominent in lower portion of stomach
- Only phase III is of interest
- Burst of high frequency, large amplitude contractions * that migrate along the length of the intestine and die out
- Interval between phase IIIs is 90-120min
- Function: “intestinal housekeeper”
- Control: unclear, but may involve motilin, pacemaker cells (ICC, in greater curvature of stomach)
describe the function of the large intestine
- Storage, particularly whilst water is absorbed from contents
- Intensive mixing and slow movement of waste and indigestible material aborally
- Contains “fermenting chambers” which allow the hydrolysis of fibre and indigestible nutrients
→Faeces formation
describe large intestine motility
Features of motility in the large intestine:
* Intensive mixing;
* Slow propagating-slow aboral flow
- Segmental or haustral contractions-mix contents/ key role for taenia coli longitudinal muscle
- Peristalsis: slow in large intestine in comparison to small intestine; moves contents towards the anus; distension initiates contraction
- Mass movement: powerful contraction of mid-transverse colon that sweeps colon contents into rectum (responsible for colonic evacuation).
describe carbohydrate (CHO) digestion and absorption
- Can only be absorbed in the form of monosaccharides
- Complex CHO reduced to disaccharides by amylases
- Specific brush border enzymes convert disaccharides to monosaccharides (e.g. glucose and galactose)
describe protein digestion and absorption
- Polypeptides produced by action of pepsin
- Polypeptides, di- and tri-peptides by action of pancreatic proteases
- Di-peptidases in brush border complete digestion to amino acids
- Amino-acids (AAs) are transported on a sodium-coupled carrier system similar to that for glucose.
- Separate carriers for different types of AAs.
- Some di- and tri- peptides are transported on a carrier system using inwardly directed H+ gradient
describe digestion of lipids
- Triglycerides (TGs) form the majority (90%) of dietary lipids
- Dietary TGs are broken into simpler units to facilitate absorption
- Mouth: salivary lipase digests a small fraction of the TGs
- Most dietary TGs are digested in the small intestine
But TGs are water-insoluble; - Chyme (emulsion of large food particles, including fat, in water);
- Lipase is water-soluble;
- TGs must be dissolved in the aqueous phase before they can be digested -fully
- Digestion and absorption of lipids are facilitated by two important processes:
- Emulsification- bile salts
- Micelle formation
- Gastric lipase breaks down approx. 10-30% of fats (inefficient)
- Remainder of the fat is digested by pancreatic lipase
describe emulsification of fats by bile salts
describe the transport of lipids
- Dietary Lipid Absorption: Lipids from food (mainly triglycerides, cholesterol, and phospholipids) are broken down by bile salts and pancreatic enzymes in the intestine. They are then absorbed by intestinal cells and reassembled into triglycerides.
- Chylomicron Formation: Inside the intestinal cells, triglycerides, cholesterol, and proteins are packaged into chylomicrons, which are large lipoproteins that transport dietary lipids through the lymphatic system and into the bloodstream.
- Lipoprotein Transport in Blood: Since lipids are insoluble in water, they are transported in the blood via lipoproteins.
- Metabolism and Storage: Once inside the cells, lipids are used for energy production (β-oxidation in mitochondria), membrane synthesis, or stored as fat in adipose tissue.
- Reverse Cholesterol Transport: HDL plays a key role in removing excess cholesterol from cells and delivering it to the liver for excretion in bile.
- Excretion: Cholesterol and bile acids are excreted into the intestine via bile and eliminated from the body, though some bile acids are reabsorbed in a process called enterohepatic circulation.
What are the two responses evoked by vagal efferent fibres in the alimentary tract?
Inhibitory fibres → relaxation of fundic area
Excitatory fibres → contraction of antral wall
Name two mechanisms that move the chyme from the fundic to the antral areas of the stomach.
- Tonic contraction (BER/slow waves from ICC)
- Peristalsis
List two reasons why gastric emptying into the duodenum is regulated.
- Coordinates mechanical/chemical breakdown with absorption
- Reduces swamping of duodenum with acidic chyme (may cause damage to the duodenum)
What is the size limit for particles to pass the pylorus?
> 2mm
Name three substances important in the reciprocal control of gastrointestinal motility.
- Acetylcholine
- NO
- VIP
- CCK
- Somatostatin
What characterizes peristalsis?
Global movement of food/chyme towards the large intestine; propulsive in nature
What characterizes segmentation?
Localized movement of food in the small intestine towards the anus; mixing movements
Name the three inter-digestive cycles.
- Phase I
- Phase II
- Phase III
Which phase of the inter-digestive cycle is most important?
Phase III
What are two functions of Phase III of the inter-digestive cycle?
- Cleanses alimentary tract
- Prevents bacterial overgrowth in the gut
What peptide hormone is important in the control of phase III of the inter-digestive cycle?
Motilin
Which neurotransmitter mediates the receptive relaxation of the fundus of the stomach?
Acetylcholine
What describes the passage of liquids and solids along the alimentary tract?
Liquids pass in spurts without any lag time; solids are ground into ~2mm sizes
What best describes phase III of the motor activity of the alimentary tract?
Marked by a burst of uninterrupted peristaltic contractions
What type of motility is characterized by coordinated contractions and relaxations in the gastrointestinal tract?
Peristalsis
Fill in the blank: The contractions in phase III are generally ________.
loud and gurgling
True or False: Solids have a lag time as they must be broken into smaller sizes before passing along the tract.
True
List one function of the villus cells of the enterocyte
Absorption of nutrients, water, and electrolytes from the gut lumen.
Name 2 enzymes at the brush border of the small intestine which are important in the digestion of carbohydrates; list their substrates and their breakdown products.
Sucrase and Lactase
- Sucrase: acts on sucrose, breaks it down to glucose and fructose
- Lactase: acts on lactose, breaks it down to glucose and galactose
What are the two transporters that work in series to transport glucose into the enterocyte and into blood?
SGLT1 and Glut 2
- SGLT1: apical on brush border, sodium linked glucose/galactose transporter
- Glut 2: basolateral
What effect would the removal of Mrs Paul’s gallbladder have on her ability to digest and absorb fats?
Deprivation of fat-soluble nutrients (A, D, E, K vitamins, omega-3 fatty acids) and production of high-fat content faeces.
Name the breakdown products of sucrose and the enzyme responsible for its breakdown.
Glucose and fructose; enzyme = sucrase
Name 2 side effects of lactose intolerance.
- Excess gas generation
- Intestinal discomfort/discomfort of the stomach
What is the chemical element important for the transport of glucose, amino acids, and dipeptides from the lumen of the intestine into the enterocyte?
Sodium for glucose, amino acids, and galactose; H+ for dipeptides.
Which transporters work in series to enable the movement of glucose from the gut lumen into the bloodstream?
SGLT-1 and Glut 2
Name the milky substance that is formed from the packaging of triglycerides, phospholipids, cholesterol, and proteins.
Chylomicrons
Chylomicrons transport dietary lipids from the intestines to other locations in the body via which lymphatic vessel in the intestinal villus?
Lacteals
What is steatorrhoea and list 2 of its features?
Steatorrhoea = excess fat in the faeces
- Fatty stools that smell and float
- Intestinal discomfort
- Deprivation of fat-soluble nutrients (Vitamins A, D, E, K)
Which enzyme secreted by the pancreas digests fats?
Pancreatic lipase
Is the following statement true? There is strong evidence that the intracellular concentration of sodium outside the cell is low and that this drives the transport of glucose and galactose into the cell.
False
Fill in the blank: The enzyme secreted by the pancreas that digests fats is _______.
Pancreatic lipase
