Ch. 26 - Digestive System Flashcards
Digestive system 6 main functions
Ingestion, motility, secretion, digestion, absorption, elimination
Ingestion
introduction of solid and liquid nutrients into the oral cavity. First step in process of digesting and absorbing nutrients
Motility
voluntary and involuntary muscular contractions that mixes and moves materials through GI tract.
Secretion
process of producing and releasing fluid products facilitating digestion.
Digestion
breakdown of ingested food into smaller structures. Includes mechanical digestion where material is physically broken down by chewing and mixing and chemical digestion that involves specific enzymes to break chemical bond to change large molecules into smaller ones.
Absorption
transport of digested molecules, electrolytes, vitamins, water, etc. from GI tract into blood or lymph.
Elimination
expulsion of indigestible components that are not absorbed.
Enteric Nervous System (ENS)
sensory and motor neurons within submucosal plexus and myenteric plexus. Innervates smooth muscle and glands of GI tract and coordinates mixing and propulsion reflexes.
Autonomic Nervous System (ANS)
Parasympathetic innervation promotes GI tract activity; sympathetic innervation opposes GI tract activity
Baroreceptors
detect stretch in GI tract wall
Chemoreceptors
monitor chemical contents in lumen
Short reflex
local reflex only involving ENS; coordinates small segments of GI tract.
Long reflex
involves sensory input to CNS and autonomic motor output; coordinate GI tract motility, secretions, and accessory digestive organs.
Hormones that help regulate digestion
gastrin, secretin, cholecystokinin, motilin.
Oral cavity overview
where mechanical digestion begins, saliva secreted from salivary glands in response to food; contains salivary amylase to initiate digestion of starch. Saliva mixed with food to form bolus.
Pharynx overview
bolus moved to pharynx during swallowing and mucus secreted to facilitate swallowing.
Esophagus overview
bolus transported from pharynx into stomach, lubricated w/ mucus
Stomach overview
located in LUQ, inferior to diaphragm. chemical and mechanical digestion continue and digestion of protein and fat begins in stomach. Ingested materials spend 2-6 hrs. here. Stomach absorbs small, nonpolar substances and serves as a holding bag for controlled release. bolus mixed w/ gastric secretion by smooth muscle to form chime. Secretions are produced by epithelial cells of stomach.
Salivary glands
1 - 1.5 L secreted daily, most produced during mealtime. Saliva is 99.5% water and mixture of solutes live salivary amylase, mucin, and lysozyme.
Functions of saliva
- moistens food to make bolus
- salivary amylase initiates chem. breakdown of starch
- dissolves food molecules to stimulate taste receptors
- cleanses oral cavity
- antibacterial substances inhibit bacterial growth (lysozyme, IgA antibodies)
Regulation of Salivary secretions
regulated by salivary nuclei in brainstem. basal level of salivation in response to parasympathetic stimulation. Receptors detect stimuli like acids in oral cavity and stomach and send signals to salivary nuclei in brainstem. Signals also received from higher brain centers in response to thought of food. Sympathetic stimulation results in a more viscous saliva
Mastication
mechanically reduces bulk to facilitate swallowing and increases surface area to expose to digestive enzymes. Mastication promotes salivation and is controlled by nuclei in medulla and pons (mastication center) Medications composed of small nonpolar molecules can be placed under tongue where they pass through epithelium by simple diffusion.
Swallowing process
Occurs in 3 phases
- Voluntary phase - occurs after ingestion and bolus is directed posteriorly towards oropharynx.
- Pharyngeal phase - involuntary reflex where tactile sensory receptors around fauces are stimulated. This initiates swallowing center in medulla and signals are relayed to effectors (soft palate, larynx, epiglottis)
- Esophageal phase - involuntary phase when bolus moves through esophagus with sequential waves of muscular contraction. The superior and inferior esophageal sphincters close at rest and relax when bolus is swallowed. Then contract gain to prevent reflux of material.
Effector response of the pharyngeal phase of swallowing
Bolus enters oropharynx and soft palate and uvula elevate to block entry of nasopharynx. The larynx elevates via extrinsic muscles to move epiglottis over glottis. Nerve signals are sent to medulla to ensure breath is not taken during swallowing.
Gastric secretions: 5 types of cells
produced by 5 types of secretory cells; 4 produce gastric juice, 5th type secretes hormones. (surface mucous cells, mucous neck cells, parietal cells, chief cells, and g cells.
surface mucous cells
extend into gastric pits and continuously secrete alkaline product containing mucin. Mucous layer helps to prevent ulceration on stomach lining from gastric enzymes high acidity.
mucous neck cells
produce acidic mucin, helps maintain acidic conditions. Both types of mucous cells (surface and neck) help protect stomach lining from abrasion and injury.
Parietal cells of stomach
adds two substances to stomach: Intrinsic factor that is required for absorption of vit. b12 in ileum and for production of normal erythrocytes and Hydrochloric acid (HCl) responsible for low pH of stomach.
Hydrochloric acid functions
Helps break down plant cell walls and animal CT
denatures protein, facilitating chemical digestion
converts inactive enzyme pepsinogen into active pepsin
kills most microorganisms entering stomach
Chief cells
most numerous secretory cells within gastric glands; produce and secrete packets of zymogen granules that primarily contain pepsinogen that eventually turns into Pepsin. Also produces gastric lipase that plays a limited role in fat digestion (about 10-15% of ingested fat)
Pepsin
released from chief cells in inactive form pepsinogen to prevent destruction of chief cell proteins. Is activated by HCl and other active pepsin and chemically digests denatured proteins into oligopeptides.
Gastric mixing
form of mechanical digestion that changes semidigested bolus into chyme. Material is churned and mixed, leading reduction in size of swallowed particles.
Gastric emptying
movement of acidic chyme from stomach to duodenum. Pressure gradient moves contents towards pylorus which increases force against pyloric sphincter. Sphincter opens and a small amount of chyme enters. Sphincter closes with retropulsion.
retropulsion
reverse flow of some contents back towards the stomach (in the case of gastric emptying)
Pacemaker cells of stomach
spontaneously depolarize to establish a basic rhythm of muscular contraction. Signal spreads through smooth muscle cells in muscularis layer
Regulation of force of stomach contraction and gastric gland secretion overview
regulated by nervous reflexes and hormones; organized in 3 phases: cephalic, gastric, and intestinal.
Cephalic phase - regulation of stomach
first phase that involves cephalic reflex initiated by thought of food. Nerve signals from higher regions of brain sent to hypothalamus, which relays nerve signals to medulla. This increases vagal stimulation, increasing motility and secretory activity - stomach “growls”
Gastric phase - regulation of stomach
second stage; process following bolus reaching stomach. regulated via gastric reflex and through release of gastrin. Gastric reflex is initiated as baroreceptors detect stretch. Chemoreceptors detect protein and increased pH and these signals are sent to medulla. Results in increased stomach motility and secretory activity. Additionally, presence of food, especially protein stimulates release of gastrin to enter blood, circulate back to stomach and further stimulate contracting, HCl release and contraction of pyloric sphincter to slow stomach emptying.
Intestinal phase - regulation of stomach
third stage; once chyme reaches small intestine. Involves intestinal reflex that opposes cephalic and gastric reflexes. Chyme in duodenum signals to medulla to decrease motility and secretion in stomach. Cholecystokinin and secretin also decrease motility and secretion to slow stomach emptying.
Lower GI tract organs
small intestine, liver, gallbladder, pancreas, large intestine, rectum, anus.
Small intestine
most chemical digestion and absorption of nutrients, h20, electrolytes, and vitamins occurs here and receives chyme from stomach. Inferior to stomach, ingested nutrients reside for at least 12 hrs. It is a coiled, thin-walled tube from pylorus to cecum of large intestine and consists of 3 segments.
Duodenum
First segment, originates at pyloric sphincter, is c-shaped around pancreatic head. Receives chyme from stomach and accessory gland secretions from liver gallbladder, and pancreas.
Jejunum
middle region, primary for chemical digestion and nutrient absorption.
Ileum
last region of small intestine, continues absorption of digested material. Distal end terminates at ileocecal valve, sphincter controlling entry of materials into large intestine.
Plicae Circularis
internal folds of mucosal and submucosal tunics of small intestine. “speed bumps” to slow movement of chyme, increases surface area to increase nutrient absorption. More numerous in duodenum and jejunum, fewer in ileum.
Small intestine secretions
intestinal juice, mucin, enteropeptidase, CCK, secretin, alkaline mucus.
Intestinal glands
invaginations between intestinal villi extend to base of mucosa. secrete intestinal juice
Goblet cells
produce mucin (protein), mixes with water to become mucous. Increase in number from duodenum to ileum.
Unicellular gland cells
small intestine; synthesizes enteropeptidase
Enteroendocrine cells
releases hormone, such as CCK and secretin
Submucosal gland
produces alkaline mucus to protect duodenum from chyme.
segmentation
backwards and forwards motion of small intestine motility
Motility of small intestine
segmentation is prevalent early in intestinal phase and mixes chyme, intestinal juice, and accessory secretions. The contraction is initiated by pacemaker cells that spread through muscularis via gap junctions.
Peristalsis is prevalent late in intestinal phase, initiated by motilin that is released from duodenum. The successive waves of contractions are called migrating motility complex that repeats until all content moved to large intestine.
Gastroileal reflex
part of small intestine motility where ileum contracts and ileocecal sphincter relaxes (CCK helps cause this) and cecum relaxes. Moves contents from ileum to cecum in response to food in stomach. Ileocecal valve contracts to prevent backflow.
Liver
main digestive function is production of bile
Bile
produced by liver and contains water, bicarbonate ions, bile salts and pigments, cholesterol, lecithin, mucin. bile salts and lecithin help mechanically digest lipids.
Gallbladder
stores, concentrates, and releases bile produced in liver. Connected to common bile duct by cystic duct. Sphincter valve controls flow of bile into and out of gallbladder.
Pancreas
exocrine function is production of pancreatic digestive juices
pancreatic secretions
pancreatic juice includes formed from secretions of acinar cells and pancreatic duct cells. Has alkaline fluid, mostly water, bicarbonate and digestive enzymes. Pancreatic amylase, pancreatic lipase, proteases, nucleases.
Pancreatic amylase
digests starch
pancreatic lipase
digests triglycerides
proteases
excreted from pancreas, digest proteins when activated
nucleases
excreted from pancreas and digests nucleic acids
Cholecystokinin (CCK)
hormone released from small intestine in response to fatty chyme. Stimulates gallbladder to release bile and pancreas to release pancreatic juice. relaxes smooth muscle within hepatopancreatic amupulla to allow entry of bile and pancreatic juice into small intestine. Also inhibits stomach motility an release of gastric secretions.
Secretin
released from small intestine in response to increased chyme acidity. Causes release of alkaline solution containing HCO3- from liver and ducts of pancreas to help neutralize acidic chyme and inhibit gastric secretions and motility.
large intestine
absorbs water and electrolytes from remaining digested material. Watery chyme is compacted into feces and stored until eliminated through defecation.
Taeniae coli
thin, longitudinal bundles of smooth muscle in the large intestine that acts like an elastic waistband. Bunch up large intestine into many sacs called haustra.
Indigenous microbiota
normal bacterial flora in large intestine. breaks down carbs, proteins, and lipids in chyme. Produce carbon dioxide, H, and B and K vitamins that are absorbed from large intestine into blood.
Feces
final product of large intestine composed of water salts, epithelial cells, bacteria, and undigested material.
Haustral churning
happens in large intestine. relaxed haustrum fills with material and distension stimulates reflex contractions in muscularis. This increases churning and moves material to more distal haustra.
Mass Movements
Powerful contractions in the large intestine taeniae coli. Propels fecal material toward the rectum 2-3 times a day, often after a meal.
Gastrocolic reflex
initiated by stomach distension and causes mass movement in large intestine.
Defecation reflex
filling of rectum initiates urge to defecate. Receptors signal to spinal cord and increases parasympathetic output to sigmoid colon and rectum and decreases output it internal (involuntary) anal sphincter
Voluntary defecation
learned at about 3 and involves Valsalva maneuver. Involves relaxation of external (voluntary) anal sphincter.
Carbohydrates
monosaccharides, disaccharides, and polysaccharides. Starch is broken down into individual glucose molecules and disaccharides are broken into monosaccharides. The main digestion sites are oral cavity and small intestine
monosaccharides
glucose, fructose, galactose
disaccharides
sucrose, maltose, lactose
polysaccharides
starch and cellulose
Carb breakdown in oral cavity
catalyzed by salivary amylase that breaks bonds between glucose molecules within starch. Inactivated by low pH of stomach when bolus is swallowed within 15-20 minutes.
Carb breakdown in small intestine
pancreatic amylase is a component of pancreatic juice that continues digestion of starch to make shorter strands of glucose, maltose, and individual glucose
Maltase
a brush border enzyme that breaks bonds between two glucose molecules (maltose)
Lactase
a brush border enzyme that breaks bonds between glucose and galactose (lactose)
Sucrase
a brush border enzyme that breaks bonds between glucose and fructose (sucrose)
Digestion of other disaccharides
each disaccharide requires its own enzyme to digest it - named after the substrate it digests
lactose intolerance
reduced amount or lack of lactase and inability to break lactose down.
What happens after monosaccharides are absorbed into blood?
they are transported to liver where fructose and galactose are converted to glucose. Glucose is taken up for energy and converted to glycogen and fat. Cellulose can’t be digested so it is excreted and adds bulk to lumen content.
Proteins
polymers composed of amino acid subunits linked by peptide bonds. digestion releases individual amino acids. Broken down by enzymes that target peptide bonds and must be activated.
Protein breakdown in stomach
begins with pepsin in the stomach lumen and stomach’s low pH activates pepsinogen to pepsin and denatures proteins to facilitate chemical breakdown.
Protein breakdown in small intestine
involves trypsinogen, chymotrypsinogen, and procarboxypeptidase from pancreas.
Enteropeptidase
enzyme synthesized in small intestine that activates trypsinogen to trypsin
Trypsin
inactive form activated by enteropeptidase and trypsin itself. Trypsin also activates chymotrypsinogen to chymotrypsin and procarboxypeptidase to carboxypeptidase.
Trypsin and chymotrypsin function
in small intestine; breaks bonds between specific amino acids and produce smaller stands of peptides
Carboxypeptidase
breaks bonds between two amino acids. in small intestine.
Dipeptidase
breaks final bond between two amino acids
Aminopeptidase
generates free amino acids in small intestine.
Free amino acids
aa’s without bonds, are absorbed across small intestine epithelial lining and used as building blocks for new proteins or converted into glucose.
Lipids
not water soluble, highly variable. Includes triglycerides and cholesterol.
Triglycerides
composed of glycerol and 3 fatty acids. Enzymes are required to break bonds between them.
Cholesterol
type of lipid that does not need to be broken down for absorption
Lipid breakdown in stomach
uses lingual lipase and gastric lipase. Together they digest 30% of all triglycerides to diglycerides and fatty acid. Neither require participation of bile salts.
Lingual lipase is component of saliva in mouth activated in stomach and gastric lipase is produced by chief cells.
Lipid breakdown in small intestine
Pancreatic lipase digests triglycerides into monoglycerides and 2 fatty acids and requires separation of large lipid droplets into smaller droplets (emulsification) via bile salts. During emulsification, nonpolar tails line up around fat droplet with heads towards aqueous fluid (micelle). Micelles allow for greater access of pancreatic lipase to triglycerides.
Lipid absorption
lipids are transported to simple columnar epithelial lining by micelles. In the epithelial cells, triglycerides reform and are wrapped in protein with cholesterol and other lipids. This is called a chylomicron and it is released by exocytosis and enters lacteals (lymphatic capillaries of small intestine) and enter blood and deliver lipids to liver and other tissues. The bile salts remain in lumen and are recovered by active transport in last portion of ileum.
Water absorption
small intestine absorbs 8 out of 9 liters of ingested water. Large intestine absorbs some and the rest is passed in feces. Water is absorbed across epithelia and into blood vessels via osmosis.
Electrolyte absorption
small intestine absorbs almost all electrolytes. Most absorption is unregulated and dependent on diet, except for iron. Hepcidin inhibits iron absorption and decreases when levels are low.
Hepcidin
a hormone released from liver that inhibits iron absorption. Hepcidin levels decrease when iron levels are low.
Vitamin absorption
fat- soluble vitamins (K, A, D, E) are absorbed in small intestine along with lipids within micelles. Water-soluble vitamins (B and C) are absorbed through diffusion and active transport.
B12 absorption
is large, so must be absorbed by receptor-mediated endocytosis. This requires intrinsic factor (formed by parietal cells in stomach)
