Chapter 14: Digestion Flashcards
Functions
- ingested food provides energy for the body to use for
1. transportation
2. contraction
3. synthesis
4. secretion - 95% of ingested food is used in the body
Motility
- muscular contraction to mix and move the food in the GI tract
- low level of contraction “tone” to keep a steady pressure
- prevents the digestive wall from permanent stretch and distension
Propulsive Motility
- activity that pushes the contents forward through the system at an appropriate speed
- eg. protein-rich food moves slower through the stomach vs. through the mouth and esophagus
Mixing Movements
- used to mix the food with digestive juices
- absorption exposes all portions so they can be absorbed
- wall of the mouth and anal sphincter involve skeletal muslces
- rest of the digestive tract is controlled by smooth muscles and is involuntary
Secretion
- an appropriate neural or hormonal stimulation causes the release of the secretion
- secretions include: water, electrolytes, specific organic constituents
Digestion
CHO
- ingested in a form of polysaccharides (glucose)
- starch - plants
- cellulose: plant cell wall
- glycogen: body muscles (meat)
- dietary CHO: sucrose, lactose (milk sugar)
Proteins
- proteins break down into amino acids and a few polypeptides
Fats
- triglycerides (glycerol and 3 fatty acids) break down into monoglycerides and free fatty acids
Absorption
- complete digestion and absorption takes place in the small intestine
- from the digestion tract lumen to the blood/lymph
Accessory Organs
- salivary glands
- exocrine pancreas
- gallbladder and liver
Digestive Organs
- mouth
- pharynx
- esophagus
- stomach
- small intestine (duodenum, jejunum, ileum)
- large intestine (cecum, appendix, colon and rectum)
- anus
Tissue Layers
- mucosa (innermost)
- submucosa
- muscularis externa
- serosa (outermost)
Mucosa
- highly folded to increase absorption and surface area. 3 layers
1. mucous membrane - protective surface
- exocrine gland cells: secrete digestive juices
- endocrine gland cells: blood-borne gastrointestinal hormones (secretin, gastrin)
- epithelial cells: absorption
- lamina propria
- houses GALT (gut-associated lymphoid tissue)
- used for immunity - Muscularis mucosa
- little layer of smooth muscle
Submucosa
- thick layer of connective tissue
- allows distensibility and elasticity
- larger blood/lymph vessels
- nerve network: submucosal plexus
Muscularis Mucosa
- major smooth muscle layer
- myenteric plexus lies between the two muscle layers
1. circular layer - inner, contraction decreased diameter of lumen
2. longitudinal layer - outer layer
- contraction shortens the tube
Serosa
- secretes serous fluid
1. lubricates and prevents friction between organs and surrounding viscera - continuous with mesentery (holds intestine to abdominal wall)
1. attachment provides relative fixation
2. supports digestive organs, but allows them freedom for mixing movements
Palate
- forms roof of oral cavity
- uvula seals off nasal passages during swallowing
Pharynx
- common passageway for digestive and respiratory systems
- tonsils on side of pharynx
- lymphoid tissue
Functions of Chewing
- grinds and breaks food down to make swallowing
- mix food with saliva
- stimulate taste buds
Saliva
composition:
- 99.5% H2O
- 0.5% electrolyte and protein (amylase, mucus, lysozyme)
functions:
- salivary amylase: begins carbohydrate digestion
- moistens food to eas swallowing
- mucus provides lubrication
- antibacterial action
- lysozyme destroys bacteria
- saliva rinses away material to prevent bacterial growth - solvent for molecules to stimulate taste buds
- facilitates movements of lips and tongue by lubricating
- cleaning mouth and teeth
- bicarbonate buffer
Amylase
- Salivary amylase breaks down polysaccharides into disaccharides, and then into maltose. Has lysozymes that destroy bacteria.
- Pancreatic breaks polysaccharides into disaccharides and then into monosaccharides
Breakdown of Carbs
- Polysaccharides: starch and glycogen
- Disaccharides: lactose, maltose and sucrose
- Monosaccharides: galactose (from lactose), glucose (from all disaccharides) and fructose (from sucrose)
Neural Regulation of Saliva
food chemoreceptors/pressure receptors afferent nerve fibers salivary center (medulla) impulses via extrinsic autonomic nerve salivary glands increase salivation
Acquired/Conditional Salivary Reflex
thinking about eating elicits a conditioned response that makes us salivate without the presence of food
ANS Influence on Saliva
Parasympathetic
- stimulates a prompt and abundant flow of watery saliva rich in enzymes
Sympathetic
- elicits a smaller volume of saliva, leaves your mouth feeling drier than usual
Swallowing
- all or none response
- most complex reflex in the body
- initiated voluntarily but cannot be stopped once started
- oropharyngeal stage and esophageal stage
Esophageal Sphincters
Pharyngoesophageal sphincter:
- the entrance stays close to prevent large volumes of air from entering the esophagus and stomach during breathing
Gastroesophageal sphincter
- prevents the reflux of gastric contents
Stomach - Protein Digestion
3 sections
- Fundus (top)
- Body (middle)
- Antrum (lower)
3 main functions
- storage of food
- secretes HCL and enzymes that begin protein digestion
- mixing movements convert pulverized food to chyme
pyloric sphincter
- serves a s barrier between stomach and upper part of the small intestine
Stomach - Gastric Motility
4 aspects
- Filing:
- receptive relaxation
- enhances stomach’s ability to accommodate the extra volume of food with a rise in stomach pressure - storage
- body of the stomach - mixing
- antrum of stomach - emptying
- controlled by factors in duodenum
Gastric Emptying
factors in the stomach:
- amount of food/chyme
factors in duodenum
fat
- fat digestion and absorption take place in the lumen of the small intestine
- when fat is already in the duodenum, further emptying of additional fatty stomach contents is prevented
acid
- unneutralized acid in the duodenum prevents emptying of acidic gastric contents until neutralization can be accomplished
hypertonicity
- gastric emptying is reflexily inhibited when the osmolarity of the duodenal contents starts to rise
distention
- too much chyme in the duodenum inhibits more empyting
Vomiting - Progression
- coordinated by a vomiting center in the medulla
- downward contraction of diaphragm and simultaneous contraction of abdominal wall compresses the abdominal cavity
- this increased the abdominal pressure, forcing the abdominal viscera to move upwards
- stomach is squeezed by diaphragm and abdominal cavity
- closed glottis prevents vomit from entering the resp tract
- uvula closes off the nasal cavity
- vomiting is followed by salivation, sweating, rapid heart rate, sensation of nausea. controlled by ANS
caused by
- deep inspiration
- closure of glottis
Vomiting - Causes
- Tactile stimulation (finger back of throat)
- irritation/distention of the stomach and duodenum
- elevated intracranial pressure, caused by a cerebral hemorrhage
- rotation or acceleration of head producing dizziness
- chemical agents
- chemotherapeutic agent used in treating cancer
- psychogenic vomiting induced by emotional factors
Vomiting - Effects and Advantages
effects
- loss of fluids and acids
- dehydration and circulatory problem, loss of acid can cause metabolic alkalosis
advantages
- vomiting triggered by irritation of the GI tract can be useful in
- removing noxious material from the stomach
- emetics (vomiting causing agents) are taken after accidental ingestion fo a poison to quickly remove the offending substance from the body
Factors that Trigger Gastric Motility
- neural response
- mediated through both intrinsic nerve plexuses (short reflex) and autonomic nerves (long reflex)
- called the enterogastric reflex - hormonal response
- involved release of hormones from duodenal mucosa collectively known as enterogastrones
- secretin
- cholecystokinin (CCK) - emotions
a. sadness and fear (decrease)
b. anger and aggression (increase motility)
c. intense pain (inhibit motility)
Gastric Secretion
2 areas of the gastric mucosa secrete gastric juices
- oxyntic mucosa: lines body and fundus
- pyloric gland area (PGA): lines the antrum
3 types of gastric exocrine secretory cells
- mucous cells
- line gastric pits and entrance of glands
- secretes a thin, watery mucus - chief cells
- secretes enzyme precursor, pepsinogen - parietal oxyntic cels
- secretes HCL and intrinsic factor
Functions of HCl
- activates pepsinogen to active enzyme pepsin and provides acid medium for optimal pepsin activity
- aids in the breakdown of connective tissues and muscle fibers
- denatures protein
- kills most of the microorganisms ingested with food
Hormonal Secretion
- control of pancreatic aqueous NaHCO3- secretion
- acid in duodenal lumen
- increase of secretin released from the duodenal mucosa
- pancreatic duct cells are stimulated by secretin
- secretion of aqueous NaHCO3- (neutralizes acid) - control of pancreatic digestive enzyme secretion
- fats and protein products in duodenal lumen
- increase of CCK (cholecystokinin) from duodenal mucosa
- pancreatic acinar cells are stimulated by CCK in the blood
- increased secretion of pancreatic digestive enzymes into the duodenal lumen (digests fats and proteins)
Phases of Gastric Secretion
- Cephalic phase
- increased HCL and pepsinogen secretions that occurs before the food reaches the stomach - Gastric phase
- begins when food actually reaches the stomach
- presence of protein increases gastric secretions - Intestinal phase
- inhibitory phase
- helps shut of flow of gastric juices as chyme begins to empty into the small intestine
Pancreatic Proteolytic Enzymes in Protein Digestion
- Trypsinogen
- Chymotrypsinogen
- Procarboxypeptidase
Pancreatic and Biliary Secretions
- mixed gland, exocrine and endocrine tissues
endocrine functions
islets of langerhans
- found throughout the pancreas
- secretes insulin and glucagon
exocrine functions
secretes pancreatic juice consisting of
- pancreatic enzymes are actively secreted by acinar cells that form the aini
- aqueous alkaline solution is actively secreted by duct cells that line pancreatic ducts
Pancreatic Enzymes
regulated by secretin and CCK
- Proteolytic enzymes
- digests protein
a. trypsinogen: converted to active form trypsin
b. chymotrypsinogen: converted to active form chymotrysin
c. procarboxypeptidase converts to active form carboxypeptidase - pancreatic amylase
- converts polysaccharides into disaccharides - pancreatic lipase
- fat digesting enzyme secreted throughout the entire digestive system
Bile
- actively secreted by the liver and diverted to gallbladder between meals
- stored and concentrated in the gallbladder
- after the meal, bile enters the duodenum
- bile is alkaline
- bile salts are reabsorbed back into the blood via active transport mechanism
consists of:
- bile salts: derivatives of cholesterol, and they convert large fat globules into a liquid emulsion
- cholesterol
- lecithin
- bilirubin
Enterohepatic Circulation of Bile
- recycling of bile salts between the small intestine and liver is called enterohepatic circulation
Hepatitis and Cirrhosis
Hepatitis: an inflammatory disease of the liver that can results from viral infection or exposure to toxic substances such as alcohol, carbon tetrachloride and certain tranquilizers
- can result in acute massive liver damage and death from hepatic failure
Cirrhosis: prolonged hepatic inflammation in association with the chronic alcoholism can lead to cirrhosis
- in this condition the damaged hepatocytes are permanently replaced by connective tissues
- in addition to hepatocytes, small number of fibroblasts are dispersed between the hepatic plates and form a supporting network of the liver
- exposure of the liver to alcohol chronically inhibits new hepatocytes from regeneration
- this results in overgrowth of fibroblast, leaving little space for new hepatocytes to re-grow
- cirrhosis develops gradually over time, active liver tissue is gradually reduced, leading to chronic liver failure
Fat Digestion and Absorption
- pancreatic lipase converts triglycerides into monoglycerides and free fatty acids
- bile salts contain lipid soluble portion that dissolves fat droplets
- facilitate fat absorption through micelle formation
- micelle is water soluble and can dissolve water insoluble substances through a lipid-soluble core
- micelle carries monoglycerides, fatty acids and fat-soluble vitamins
- has a hydrophilic shell (water soluble, and hydrophobic core (lipid soluble)
Absorption of Water and Sodium
- Na+ is absorbed passively due to the electrochemical gradient
- energy dependent process
- Na enters the epithelial cells across the luminal border through Na channels or using glucose/amino acids as a co-transport molecule
- water is absorbed according to the hydrostatic pressure
Absorption of Iron
- only a portion of ingested iron is in a form that can be absorbed
- dietary iron that’s absorbed into the small intestine and is needed for RBC production is transferred into the blood
- in the blood, absorbed iron is carried to the bone marrow bound to transferrin (plasma protein carrier)
- absorbed dietary iron that is not immediately needed is stored in the epithelial cells as ferritin - cannot be transferred into the blood
- excess iron in the blood can be dumped into the ferritin pool
- the unused iron is lost in the feces
- dietary iron not absorbed is also lost in the feces
Segmentation
- refers to the mixing and propelling of the chyme
- consists of oscillating, ring like contraction of circular smooth muscles along the length of the small intestine
- contraction and relaxation of contractile smooth muscles propel the chyme form one end to another within the small intestine
- initiated by pacemaker cells in the small intestine which produce basic electrical rhythm (BER)
- responsiveness is influenced by distension of the intestine, gastrin and extrinsic nerve activity
functions
- mixing chyme with digestive juices
- exposing all chyme to absorptive surfaces of the small intestine mucosa
Migrating Moitlity Complex
sweeps the intestine clean between meals
Small Intestine - Secretion
- juice secreted by the small intestine do not contain any digestive enzymes
- enterokinase, disaccharidases, and aminopeptidases
Small Intestine - Digestion
- pancreatic enzymes from brush-border continue carbohydrate and protein digestion
- fat is digested entirely within the small intestine lumen by pancreatic lipase
Small Intestine - Absorption
- most occurs in the duodenum and jejunum
- products of fat digestion undergo transformations that enable them to be passively absorbed
3 adaptations:
- inner surface has permanent circular folds
- microscopic finger-like projections called villi
- brush border (microvilli)
Large Intestine - Structure and Function
- the colon extracts more water and salt from content progression: - cecum - ascending colon - transverse colon - descending colon - sigmoid colon
Taeniae coli:
- longitudinal bands of muscle
Haustra:
- pouches or sac
- actively change location as result of contraction of the circular smooth muscle layer
Haustral Contractions:
- main motility
- initiated by autonomous rhythmicity of colonic smooth muscle cells
Gastrocolic reflex:
- mediated from stomach to colon by gastrin and autonomic nerves
- most evident after first meal of the day
- often followed by the urge to defecate
Defecation Reflex
- distention of rectum stimulates stretch receptors
- initiates defecation reflex
- causes external anal sphincter to relax and defecation take place (voluntary)
Constipation
- retention of colonic contents for too long
- more than the usual amount of H2O is absorbed, so it becomes hard and dry
- delayed in defecation frequencies beyond normal
symptoms:
- abdominal discomfort, dull headache, loss of appetite followed by nausea and mental depression
- symptoms promptly disappear after relief from distension
Appendicitis
- deposition of hardened fecal material in the appendix causes inflammation of the appendix, or appendicitis
- the inflamed appendix can become swollen and filled with pus, and tissue may die as a result of local circulatory interference
Neural Regulation of GI Processes
- neural regulation controls the motility and secretion to maximize digestion and absorption
- influenced by many synergistic interrelated pathways
4 factors
- autonomous smooth muscle function
- intrinsic nerve plexuses
- extrinsic nerve
- gastrointestinal hormones
Enteric Nervous System
Intrinsic Nervous Plexuses
- Myenteric plexus
- Submucous plexus
- both the plexuses are within the digestive tract wall
- digestive system contains intramural nervous system contain many neurons that help digestive tract for self regulation
- influence all facets of the digestive tract activity
- contain both sensory, local neurons that are connected with interneurons
Types of Receptors in the GI Tract
- chemoreceptors: chemical components within the lumen
- mechanoreceptors: sensitive to stretch or tension within the wall
- osmoreceptors: sensitive to the osmolarity or the luminal contents
- stimulation of theses receptors elicit neural reflexes or hormone secretion which then alters the level of activity in the GI tract
Short and Long Reflexes
short:
- all elements are located within the wall of the GI tract
- shrt reflex networks influence local motility or secretion in response to specific local stimulation
long:
- involves pathways between CNS and digestive system
- GI tract activity is also coordinated by the secretion of gastrointestinal hormones
Parasympathetic and Sympathetic Systems
- parasympathetic controls rest-digest system. proceeds optimally. uses Ach
- sympathetic dominates in fight or flight, slows down GI tract
Vagal Nerve Stimulation
Intrinsic plexuses
- promotes increased Ach secretion
- secretory cells
- increased secretion of HCl and pepsinogen
G-cell
- gastrin
- increased secretion of HCl and pepsinogen
Neural/Hormonal Control of the Intestinal Phase
- inhibitory
- this phase is important in helping shut off the flow of gastric juices as chyme is being emptied into the small intestine
Gastrin
- presence of protein in the stomach causes the release
1. increases secretion of HCl and pepsinogen
2. HCl and pepsinogen promotes protein digestion
3. enhances gastric motility, stimulates ideal motility
4. relaxes the ileocecal sphincter and induces mass movements in the colon
5. has trophic effects on the stomach and small intestine to keep the digestive tract lining viable
Secretin
- the presence of acid in the duodenum stimulates the release of secretin into the blood
1. it inhibits emptying to prevent acid from entering the duodenum
2. it inhibits gastric secretion to reduce the normal acid production
3. it stimulates the pancreatic duct cells to produce a large volume of aqueous NaHCO2 secretion in the duodenum to neutralize acid
4. along with CCK, secretin is trophic to exocrine pancreas
Cholecystokinin (CCK)
- presence of fat and other nutrients causes the release of CCK from the duodenal mucosa
- inhibits gastric motility and secretion, allowing more time for the nutrient to be digested and absorbed
- stimulates pancreatic enzyme secretion of a fat digesting enzyme by pancreatic acinar cells
- causes contraction of gall bladder and relaxation of the sphincter of oddi to empty the bile into the duodenum
- involved in long-term adaptive changes by increasing secretion of pancreatic enzymes in response to prolonged changes in diet
- stimulates food intake
Glucose-dependent Insulinotropic Peptide
- is a newly recognized hormone that promotes metabolic processing of nutrients after their absorption
1. stimulates insulin release by the pancreas
2. promotes the use and storage of newly arrived nutrints
3. initiates the release of insulin in anticipation of absorption of the meal in a “feedforward” fashion
4. glucose in the duodenum has been shown to increase GIP secretion
