L4 - The Digestive System - Topic 1 Flashcards
Chemical digestion in the small intestine depends on 3 accessory organs :
-
Liver
- ONLY digestive function is production of bile -
Gallbladder
- Chief function is storage of bile
- Necessary for digestion of fats, emulsifiers fats to be absorbed -
Pancreas
- Supplies most of enzymes needed to digest chyme, as well as bicarbonate to neutralize stomach acid
Gross Anatomy of the Liver
Largest gland in body ; ~3 lbs
Consists of 4 primary lobes :
anterior view – larger right & smaller left, caudate & quadrate
ligament = form of mesentery
Falciform ligament
- Separates larger R & smaller L lobes
- Suspends liver from diaphragm & anchors to anterior abdominal wall
Round ligament (ligamentum teres)
- Remnant of fetal umbilical vein along free edge of falciform ligament
Lesser omentum anchors liver to stomach
Innervation
Hepatic Portal Vein - Will take oxygen poor, nutrient rich blood absorbed thru GI tarct (stomach/intestine) to liver for processing
Blood flows into liver via hepatic artery –> Bile flows out via
common hepatic duct – collects bile made by hepatocytes, either spewed into duodenum (if consuming meal) or stored in gallbladder
Microscopic Anatomy of the Liver
Liver lobules
- Hexagonal structural & functional units
- Plates of hepatocytes (liver cells) that filter and process nutrient-rich blood
- Central vein located in longitudinal axis
Portal triad in each corner of lobule contains :
- Branch of hepatic artery, which supplies O2 rich blood from arteries to liver
- Branch of hepatic portal vein, which brings nutrient-rich blood from intestine
- Bile duct, which receives bile from bile canaliculi
Liver sinusoids
- Leaky (heavily fenestrated) capillaries between hepatic plates
- Blood flows here to empty into central vein
- Contain hepatic (stellate) macrophages in liver sinusoids
Hepatocytes have :
- Increased rough & smooth ER
- Golgi apparatus
- Peroxisomes
- Mitochondria
Hepatocytes produce about ______ mL of bile per day
900 mL
( this is the ONLY liver’s contribution towards digestion )
What are the functions of hepatocytes ?
-
Process bloodborne nutrients (oxy poor but nutrient rich blood gets processed here before being sent back into venous circulation)
eg. store glucose as glycogen and make plasma proteins -
Synthesis of :
- Most of the plasma proteins / clotting factors
- Lipoproteins
- **Store fat-soluble vitamins ** (can be harmful if too much)
-
Perform detoxification
eg. converting ammonia (waste product) to urea for excretion - Excretion of bilirubin to the bile
- Consist of Stellate macrophages cells for phagocytosis of old RBC, leukocytes and some bacteria
Bile Composition
Liver produces ~500 - 1000 ml bile per day
Yellow - green alkaline solution containing :
1. Bile salts – Cholesterol derivatives that function in fat emulsification & absorption
- Fat insoluble in water, so when we consume a diet heavy in lipids → Fat must be processed / broken down into something that can be absorbed (emulsified) + opens it up to allow digestive enzymes to get into the internal aspects of fat bc it tends to want to form a “globule” (big blob of fat with enzymes only having access to the outside)
-
Bilirubin : from heme of hemoglobin of RBCs
- Bacteria break down in intestine to stercobilin that gives brown color of feces - Cholesterol, triglycerides, phospholipids, & electrolytes
What are gallstones ?
Made up of concentrated bile, there is not enough bile salts which causes it to harden
Bile - Enterohepatic Circulation
Recycling mechanism that conserves bile salts – bile is made constantly but most of them are RECYCLED
Liver – Where bile is synthesized → L & R hepatic ducts will drain bile from liver lobes → depending on whats going on, it can either be stored in gallbladder OR released via bile duct into duodenum [ proximal small intestine ]
!!! If consumed a meal particularly high in fats, will trigger synthesis + release of bile from liver & release of stored bile from gallbladder → Will be mixed together w/ pancreatic juice, thats released into proximal duodenum
Pancreas – Has a duct opening into duodenum = Hepaopancreatic sphincter (ampulla + sphincter)
Bile salts will solubilize fat, make it easier to be processed + absorbed as fat making way through SI
Ileum – By the time we get here (distal part of SI), most of fat would have been absorbed by bile salts (95 %)
Salts will remains within GI tract & reabsorbed into oxy poor blood → diverted into hepatic portal system to be recycled by the liver so that they can be reincorporated into bile
only 5% newly synthesized bile salts each time
The Gallbladder
Thin-walled muscular sac on the ventral surface of the liver
Fundus – Storage area
Neck – Leads into cystic duct to connect it to common bile duct
Common hepatic duct – derived from fusion of 2 ducts → R & L hepatic ducts (coming from liver)
fusion of common hepatic duct (from liver) + cystic duct (gallbladder) becomes common bile duct
- Stores + concentrates bile by absorbing / removing water & ions (helps to reduce AMOUNT of content needed to have the same effect)
- Contains many honeycomb folds that allow it to expand as it fills
- Muscular contraction → Causes releases of bile via the cystic duct, which flows into the common bile duct
Physiology of the Gallbladder
What is the location of the pancreas ?
Mostly retroperitoneal (outside), head encircled by duodenum; tail next to spleen
What is the exocrine function of the pancreas ?
Acini cells : Clusters of secretory cells that produce zymogen granules containing proenzymes (inactive form of enzyme) → digestive enzymes that will be released into the ducts leading into duodenum
Duct cells : Smaller ducts that secrete water & bicarbonate ions into duodenum via main pancreatic duct
- Combines with acidic chyme of duodenum to neutralize it → Creates environment to allow enzymes to function
What are the main ducts of the pancreas ?
Major pancreatic duct – Joins with bile duct to release their secretions into duodenum together
Accessory pancreatic duct – Releases secretions on its own into duodenum, not joined by bile duct
What is the endocrine function of the pancreas ?
Secretion of insulin & glucagon by pancreatic islet cells as per homeostasis
Hepatopancreatic Ampulla & Sphincter
- Where the bile + major pancreatic duct unite in the wall of the duodenum via major duodenal papilla
Ampulla – “Flask” leading into sphincter
Papilla – Opens up when chyme comes in to allow flow of bile to take care of fats, neutralize it with bicarbonate ions & bring in all needed enzymes for digestion via zymogen granules
Hepatopancreatic sphincter :
- Controls how much bile & pancreatic juice flows into duodenum
- CLOSED, unless digestion is active
- Bile is stored in gallbladder and released to small intestine only with contraction
What is the composition of pancreatic juice ?
1200–1500 ml/day is produced containing :
Watery (aqueous based), alkaline (pH 8, due to bicarbonate ions that are there to neutralize acidity) solution to neutralize acidic chyme coming from stomach
- Electrolytes, primarily HCO3− (bicarbonate ion)
-
Digestive enzymes
− Proteases (for proteins) : Secreted in inactive form as proenzymes, activated after reaching duodenum → If you had them in active form they would chew up / digest cells in pancreas (powerful)
− Amylase (for carbohydrates) mouth
− Lipases (for lipids) starts in mouth
− Nucleases (for nucleic acids)
Proteases
Enteropeptidase (formerly enterokinase) :
- Enzyme bound to plasma membrane of duodenal epithelial cells / mucosa
- When zymogen granules released into duodenum, membrane bound enzyme will activate protease tripsinogen secreted by pancreas first to convert into trypsin
Once trypsin is activated, it can then :
- Work on more tripsinogen to generate more of its cells (pos feedback mechanism) – > Inactive form can then turn around & act on other proenzymes part of zymogen granules to convert / activate them
- Activates procarboxypeptidase to carboxypeptidase
- Activates chymotrypsinogen to active chymotrypsin
Bile & Pancreatic Secretion into the Small Intestine
Bile + pancreatic juice secretions both stimulated by neural (vagus nerve, PNS) & hormonal (enterogastrones) controls
Hormonal controls :
- Cholecystokinin (CCK)
- Secretin
Bile secretion is increased when :
- Large amounts of bile salts in enterohepatic circulation [ seems counterintuitive, BUT their presence signals that we’re digesting a meal high in fat content so we will need more bile for digestion ]
- Secretin (from intestinal cells exposed to HCl and fatty chime) → Gallbladder to release bile
Hormonal Mechanisms Promoting Secretion of Pancreatic Juice
Chyme entering duodenum causes release of cholecystokinin (CCK) & secretin from duodenal enteroendocrine cells into bloodstream
Pancreas → Secretes pancreatic juice
- If chyme is high in fat & protein – CCK will acts on acini cells → Enzyme rich pancreatic juice
- If chyme is acidic – Secretin causes secretion by duct cells →HCO3- rich pancreatic juice[ to neutralize acidity of chyme ]
Whar are the characteristics of the small intestine ?
Major organ of digestion & absorption
Extends from pyloric sphincter to the Ileocecal valve → gate / door opening into colon (demarcation point between SI / LI)
- 7-13 ft during life ; ~20 ft in a cadaver (lose muscle tonicity)
- Small diameter of 2.5 - 4 cm (1.0 -1.6 inches)
BIGGER SA → More area for absorption (major job of small intestine)
What are the subdivisions of the small intestine ?
Duodenum ( retroperitoneal )
~ 25.0 cm (10.0 in) long; curves around head of pancreas
- Has most features involved in digestion
- Sets up chyme for absorption
Jejunum
~ 2.5 m (8 ft); attached posteriorly by mesentery
Ileum
~3.6 m (12 ft) attached posteriorly by mesentery); joins large intestine at ileocecal valve
Blood & Nerve Supply of Small Intestine
Blood supply :
- Superior mesenteric artery brings blood supply (oxygenated) - - comes from abdominal aorta
- Veins (carrying nutrient-rich blood) drain into superior mesenteric veins, then into hepatic portal vein, and finally into liver (blood will exit from hepatic vein back into vena cava)
Nerve supply :
- Para NS innervation via vagus nerve, + Sympa NS innervation from thoracic splanchnic nerves
Microscopic Anatomy of Small Intestine
SI length & other structural modifications → HUGE SA, increased 600× to ~200 m2 (size of a tennis court)
Circular folds
- Permanent folds (~1 cm deep) of mucosa + submucosa that provides extra SA for dig/absorption & force chyme to slowly spiral through lumen, allowing more time for nutrient absorption to take place
Villi
- Fingerlike projections of mucosa (~1 mm high) with a core that contains dense capillary bed + lymphatic capillaries called lacteals for absorption
- Increases SA for digestion + absorption
Microvilli
- Cytoplasmic extensions of mucosal cell that give fuzzy appearance called the brush border; contains membrane-bound enzymes – brush border enzymes used for last step carbohydrate + protein digestion into smallest possible components to able to be absorbed
Histology of the SI wall
4 tunics but mucosa & submucosa modified for digestion
Intestinal Crypts : Tubular glands scattered between villi, have additional microvilli on each villi
- ↓ in number along the length of the SI
- Produce intestinal juice - watery mixture of mucus that acts as carrier fluid for chyme
5 major types of cells found in villi & crypts
Enterocytes : Make up bulk of epithelium
- Simple columnar epithelial absorptive cells bound by tight junctions with many microvilli (stratified would slow down absorption)
- Function in villii to absorb nutrients + electrolytes, in crypts to produce intestinal juice [ watery mixture of mucus that acts as carrier fluid for chyme ]
Goblet cells : Mucus-secreting cells in epithelia of villi & crypts
Enteroendocrine cells : Source of enterogastrones (CCK /secretin) & GIP
- Found scattered in villi but some in crypts
Paneth cells : Deep in crypts (not found in villus), specialized secretory cells that fortify SI’s defenses
- Secrete antimicrobial agents (defensins & lysozyme) that can destroy bacteria [ MALT ]
Stem cells : Continuously divide to produce other cell types as they go up
- Villus epithelium renewed every 2–4 days
Mucosa
Mucosa-associated lymphoid tissue (MALT = reticular tissue + immune cells WBCs, macrophages, T cells) dot areas along SI to protect intestine against microorganisms & includes :
- Individual lymphoid follicles & Peyer’s patches(aggregated lymphoid nodules) in lamina propria
- ↑ numbers in distal part of small intestine – As we get closer to colon, it has a colony of bacteria (micriobiome) which we dont want to escape / invade small intestine
Lamina propria : Large numbers of plasma cells (antibody secreting cells) that secrete IgA
Submucosa
- Areolar CT
- Duodenal glands secrete alkaline mucus to neutralize acidic chyme
Muscularis
Circular & longitudinal muscle
- Most of the duodenum (retroperitoneal) → Adventitia (more robust outer covering)
- Visceral peritoneum (serosa) covers the external intestinal surface
Intestinal Juice
- 1–2 L secreted daily by the intestinal glands in response to distension or irritation of mucosa
Major stimulus : Hypertonic or acidic chyme
- Slightly alkaline and isotonic (same osmolarity / concentration of solutes as blood plasma)
- Consists largely of water but also contains mucus secreted by duodenal glands + goblet cells of mucosa
- Absorptive cells synthesize digestive (brush border) enzymes
Sources of enzymes for digestion
- Substances such as bile, bicarbonate ion, digestive enzymes (not brush border enzymes) are from liver & pancreas [ bicarb is made by neck cells in pancreatic acinar cells ]
- Brush border enzymes bound to plasma membrane of mucosa (outer surface) to perform final digestion of chyme before absorption
Regulating Chyme Entry
Chyme from stomach contains :
- Partially digested carbohydrates, *proteins & undigested fats
- Usually hypertonic (higher conc of solutes) ; therefore, delivery of chyme must be slow to prevent osmotic loss of H2O from blood
- Low pH must be adjusted upward (rlly acidic)
- Must be mixed with bile & pancreatic juice to continue digestion
Enterogastric reflex & enterogastrones control movement of food into duodenum to regulate duodenal filling
[ stomach talks to small I, telling stuff coming in so that it prepares itself ]
Mobility of the SI
After a Meal
- Segmentation is most common motion of SI
- Initiated by intrinsic pacemaker cells
- Mixes/moves contents toward ileocecal valve
- Intensity is altered by long /short reflexes & hormones
- Parasympathetic ↑ motility; sympathetic ↓
- Goes for as long as there is chyme to be processed + digested
Between Meals
- Peristalsis ↑, initiated by ↑ in hormone motilin in late intestinal phase
- Wave every 90–120 minutes
- Each starts distal to previous one → Migrating Motor Complex (MMC)
- Meal remnants, bacteria & debris are moved toward large intestine
- Complete trip from duodenum to ileum: ~2 h
- Housekeeping function to move waste (whats left behind from meal) towards colon to get it ready for secretion
Ileocecal Valve Control
[ endpoint for MMC ]
Relaxes & admits chyme into large intestine when :
- Gastroileal reflex enhances force of segmentation in ileum
- Gastrin comes from stomach to increase motility of ileum
- Ileocecal valve flaps close when chyme exerts backward pressure to prevent regurgitation into ileum
when P one side → remnants of chyme forces open sphincter → chyme makes way into colon → valve slams shut
