Chapter 23: Digestion Flashcards
Digestion
- > to obtain energy for ATP production through food sources
- > raw materials needed for building and repair
- > needs to be converted to cellular level
- > mechanical and chemical digestion
Two groups of organs
- Alimentary canal (GI tract)
2. Accessory digestive organs
Alimentary canal (GI tract)
- digests and absorbs food
- mouth, pharynx, esophagus, stomach, small intestine, and large intestine
Accessory digestive organs
- teeth, tongue, gallbladder
- digestive glands: salivary glands, liver, pancreas
Digestive processes
- Ingestion-eating
- propulsion-swallowing, peristalsis
- mechanical digestion
- chemical digestion-enzymes
- absorption
- defecation
Accessory digestive organs
Teeth:
-Teeth do much of mechanical work of digestion.
-Human teeth include sharp incisors & cuspids which tear and grasp food. Molars crush and grind
Accessory digestive organs
Tongue:
-Positioning and mixing of food
- Formation of the bolus
- Initiation of swallowing, speech, and taste
- Surface bears papillae for friction, licking, taste buds, secretes lingual lipase
Salivary glands– Intrinsic glands
are scattered in the oral mucosa, keep mouth moist
Salivary glands– Extrinsic
-produce majority of saliva.
Functions:
-cleanses mouth
- moistens and dissolves food chemicals
- contains enzymes (salivary amylase) that begin the breakdown of starch
- contain chemicals (IgA antibodies and defensins) that protects against micro organisms
- The brain stem send impulses along parasympathetic fibers to activation secretions. Sympathetic NS inhibits saliva production
Digestive processes: Mouth
- Ingestion
- mechanical digestion: mastication is partly voluntary, partly reflexive
- chemical digestion: salivary amylase and lingual lipase
- absorption- only certain medications
- propulsion- deglutition (swallowing)
pharynx
- Oropharynx and laryngopharynx (nasopharynx no digestive role)
- Allow passage of food, fluids, and air
- Skeletal muscle layers: inner longitudinal, outer pharyngeal constrictors
Histology of alimentary canal
4 basic layers (tunics)- from esophagus to anal canal
- Mucosa: secretes mucous, enzymes & hormones, absorption and protection
- Submucosa: houses vessels, nerves and lymph vessels, made up of CT
- Muscularis externa: -2 layers of smooth MM (circular/longitudinal), responsible for segmentation and peristalsis
- Serosa: outermost layer, made up of CT
**exception: in esophagus the adventia replaces the serosa layer
Esophagus
- flat muscular tube from laryngopharynx to stomach
- bolus of food (aided by mucous) travels through esophagus by smooth MM contractions -peristalsis
- joins stomach at the cardiac orifice
Deglutition (swallowing)
-> involves the tongue, soft palate, pharynx, esophagus, and 22 muscle groups
- > Buccal Phase:
- voluntary contraction of the tongue, forces bolus into oropharynx
- > Pharyngeal-esophageal phase:
- involuntary
- control center in the medulla and lower pons
- all routes are blocked, soft palate blocks nasoph, epiglottis covers trachea
Deglutition: Buccal Phase
*voluntary contraction of the tongue, forces bolus into oropharynx
Deglutition: Pharyngeal-esophageal phase:
- involuntary
- control center in the medulla and lower pons
- all routes are blocked, soft palate blocks nasoph, epiglottis covers trachea
Peristalsis
- adjacent segments of alimentary tract organs alternately contract and relax, which moves food along the tract distally
- video-flouroscopy
Stomach: Gross anatomy
- Cardial region (cardia): surrounds the cardiac orifice
- Fundus: dome-shaped region beneath the diaphragm
- Body: midportion
- Pyloric Region: pylorus is continuous with the duodenum through the pyloric valve (sphincter)
- greater curvature: convex lateral surface
- Lesser surface: concave medial surface
Cardial region (cardia)
surrounds the cardiac orifice
fundus
dome-shaped region beneath the diaphragm
body
midportion
Pyloric region
pylorus is continuous with the duodenum through the pyloric valve (sphincter)
Greater curvature
convex lateral surface
Lesser curvature
concave medial surface
Stomach: microscopic anatomy
- Mucosa (lining of stomach):
- produces mucus which traps bicarbonate-rich fluid beneath it
*gastric pits lead into gastric glands (produce stomach secretions)
Gastric glands
Cell types:
* Mucous neck cells (secrete thin, acidic mucus)
- parietal cells secrete HCl and intrinsic factor
- chief cells- pepsinogen
- enteroendocrine cells (g cells): gastrin
gastric gland secretions: parietal cell secretions
-Glands in the fundus and body produce most of the gastric juice
- Parietal cell secretions:
- HCl: pH 1.5–3.5 denatures protein in food, activates pepsin, and kills many bacteria
*Intrinsic factor: protein required for absorption of vitamin B 12 in small intestine
Chief cell secretions;
- produce pepsinogen
- activated to pepsin by HCl and by pepsin itself (positive feedback mechanism)
- secrete lipases (fat digestion)
enteroendocrine cells
- Paracrines (act locally)
- Serotonin and histamine
- Hormones
- Somatostatin and gastrin (G cells) (regulates secretions and mobility)
*gastric secretion is controlled by both neural and hormonal mechanisms
Mucosal Barrier
- Layer of bicarbonate-rich mucus
- Tight junctions between epithelial cells
- Damaged epithelial cells are quickly replaced by division of stem cells
- Peptic or gastric ulcers: erosion of the stomach wall
- Most are caused by H pylori bacteria
digestive processes in the stomach
- Physical digestion (3 layers of muscle)
- Denaturation of proteins (HCl)
- Enzymatic digestion of proteins by pepsin
- Secretes intrinsic factor required for absorption of vitamin B12
- *Lack of intrinsic factor-> pernicious anemia
-Delivers chyme (product of its activity) to the small intestine
Neural reflex pathways
-control of digestive activity are both intrinsic and extrinsic
cephalic phase: initiated by stimuli arising inside or outside the GI tract and involve CNS centers. usually from stimuli outside the GI tract
gastric phase: mediated inside GI tract by local stimuli (local distention, pH, presence of substrates). 3-4 hrs after food enters stomach
intestinal phase: controls rate of emptying into intestines
cephalic phase
initiated by stimuli arising inside or outside the GI tract and involve CNS centers. usually from stimuli outside the GI tract
Gastric phase
mediated inside GI tract by local stimuli (local distention, pH, presence of substrates). 3-4 hrs after food enters stomach
intestinal phase
controls rate of empyting into intestines
Response of the stomach to filling
- stretches to accomodate incoming food
- reflex-mediated receptive relaxation: coordinated by the swallowing center of the brain stem
- gastric accommodation: Plasticity (stress-relaxation response) of smooth muscle, can stretch without contracting
Intestinal phase-stimulatory
brief filling of initial part of small intestine of partially digested food
Intestinal phase: inhibitory
enterogastric reflex” – a trio of reflexes that puts “brakes” on gastric activity to protect sm intestine from excessive acidity
dumping syndrome
seen in stomach volume reduction surgeries (nausea/vomiting)
gastric contractile activity
- Peristaltic waves move toward the pylorus
- Basic electrical rhythm (BER) initiated by pacemaker cells (cells of Cajal)
- Distension and gastrin increase force of contraction
- Most vigorous near the pylorus
- Chyme is either:
- Delivered in ~ 3 ml spurts to the duodenum, or
- Forced backward into the stomach
Vomiting (emesis)
- Protective reflex: removes toxic material from GI tract before being absorbed
- triggered by extreme stretching of stomach or irritant such as bacteria, excessive alcohol, spicy foods and certain drugs
- vomiting reflex located in medulla (emetic center)
- Prior nausea, diaphragm & abdominal muscles contract increasing intra-abdominal pressure, gastroesophageal sphincter relaxes, contents are forced out of stomach
Stimuli that regulate gastrointestinal tract activity are generated by
the CNS, chemoreceptors in the gut, the enteric nerve plexus
small intestine: gross anatomy
- major organ of digestion and absorption
- 2-4 m long; from pyloric sphincter to ileocecal valve (6-7 m long in cadaver- height of 2 story building)
- subdivisions:
1. duodenum- chemical digestion
2. jejunum- absorption
3. ileum- vitamin absorption
Duodenum- chemical digestion
-small intestine
- the bile duct and main pancreatic duct
- join at the hepatopancreatic ampulla
- enter the duodenum at the major duodenal papilla
- hepatopancreatic sphincter controls entry of bile and pancreatic juice
structure of small intestine
- increase surface area of proximal part for nutrient absorption
- Circular folds: force chyme to slowly spiral through lumen
- villi: motile fingerlike extensions
- villus epithelium: simple columnar absorptive cells (enterocytes), goblet cells
-microvilli: projections (brush border) of absorptive cells, brush border enzymes (further breakdown of proteins and carbs )
small intestine: Circular folds
force chyme to slowly spiral through lumen
small intestine: Villi
motile fingerlike extensions
*villus epithelium: simple columnar absorptive cells (enterocytes), goblet cells
small intestine: Microvilli
projections (brush border) of absorptive cells, brush border enzymes (further breakdown of proteins and carbs )
intestinal crypts
Intestinal crypt epithelium (like gastric pits)
-Secretory cells:
produce intestinal juice (peptidase, maltase, lactase, sucrase)
- Enteroendocrine Cells – (not to be confused with enterendocrine of stomach)
- secretin & cholecystokinin (CCK)
Lymphocytes:
T-cells that release cytokines that kill infected cells
Paneth cells:
Secrete antimicrobial agents (defensins and lysozyme)
Stem cells:
Dividing cells which become specialized
intestinal crypts: Secretory cells
produce intestinal juice (peptidase, maltase, lactase, sucrase)
intestinal crypts: -Enteroendocrine Cells
(not to be confused with enterendocrine of stomach)
*secretin & cholecystokinin (CCK)
intestinal crypts: Lymphocytes
T-cells that release cytokines that kill infected cells
intestinal crypts: Paneth cells
Secrete antimicrobial agents (defensins and lysozyme)
intestinal crypts: stem cells
Dividing cells which become specialized
liver
- Largest gland in the body (3 lbs)
- Four lobes
Functions:
- Stores glycogen and vitamins (fat soluble)
- Detox of ammonia
- Produces bile
Exceptional regenerative capacity of liver, can regenerate to former size in 6-12 months. This is why live donor transplants are a viable option.
bile
- Missing ingredient to attack fatty foods, contains:
- Bile salts: cholesterol derivatives that function in fat emulsification and absorption
- Bilirubin: waste product from heme
- Facilitate fat and cholesterol absorption
The gallbladder
- Thin-walled muscular green sac on the ventral surface of the liver
- Stores and concentrates bile
- Releases bile via the cystic duct, which flows into the bile duct
Bile extracts cholesterol from body, if too much cholesterol it crystallizes forming gallstones
pancreas
->Endocrine function: Secrete insulin and glucagon
- > Exocrine function
- Secrete pancreatic juice
- Pancreatic juice consisting of enzymes for digestion (digests carbohydrates, fats, proteins)
- Pancreatic juice consisting of bicarbonate ions (neutralizes chyme)
Pancreatic juice
- Watery alkaline solution (pH 8) neutralizes chyme
- Electrolytes (primarily HCO3–)
- Enzymes
- Proteases (trypsin) - proteins
- Amylase - COH
- Lipase - lipids
Nucleases – nucleic acids
regulation of bile and pancreatic secretions
- > Chyme entering duodenum releases:
- Secretin and Cholecystokinin (CCK)
- > CCK induces secretion of pancreatic juice (enzymes) and causes gallbladder to contract
- > Secretin –secretion of watery bicarbonate rich pancreatic juice
- > Bile secretion is stimulated by Bile salts
Motility of the small intestine: Segmentation (massaging)
- after a meal
- mixes and moves contents slowly and steadily toward the ileocecal valve
- intensity is altered by long and short reflexes
motility of the small intestine: peristalsis
- between meals
- occurs late in intestinal phase after absorption has occurs
- meal remnants, bacteria, and debris are moved to the large intestine
This substance secreted by the pancreas helps neutralize chyme.
bicarbonate
Large intestine (colon)
- shorter but wider than small intestine
- major digestive function is to absorb remaining water
- stores indigestible food
- houses goof bacteria
- no digestive enzymes, minimal absorption
- takes 12 to 24 hours to move through the large intestine
Large intestine
- Teniae coli: 3 bands of longitudinal smooth muscle in the muscularis
- Haustra: pocketlike sacs caused by the tone of the teniae coli
- epiploic appendages: fat-filled pouches which hang from large intestine
Mesenteries of abdominal digestive organs: omentum, mesocolon, mesentary. membrane sheets that anchor the digestive tract
large intestine
Regions:
*Cecum (pouch with attached appendix)
- Colon (ascending, transverse, descending, sigmoid)
- Rectum
- Anal canal
Rectum
3 rectal valves stop feces from being passed with gas
-goblet cells-produce mucus
anal canal
- the last segment of the large intestine
- superficial venous plexuses
sphincters
internal anal sphincter- smooth muscle
external anal sphincter- skeletal muscle
bacterial flora
- located in colon
- ferment indigestible carbs
- release irritating acids and gases
- synthesize B complex vitamins and vitamin K (needed by the liver to produce clotting proteins)
functions of the large intestine
- vitamins, water, and electrolytes are reclaimed
- major function is propulsion of feces toward the anus
- colon is not essential for life
motility of the large intestine
- > Haustral contractions:
- slow segmenting movements
*occur every 30 min, move material from 1 haustrum to the next
- > Mass movements:
- slow intense peristalsis movement, 3-4/day
- usually after eating
- bulk or fiber in the diet strengthens colon contractions (diverticulosis)
Why is the small intestine well adapted for nutrient absorption?
Large surface area due to the presence of the villi, and microvilli.
In terms of gastrointestinal function, the large intestine’s greatest contribution is:
absorption of water
defecation
- mass movements force feces into rectum
- distension initiates spinal defecation reflex
- parasympathetic signals:
- stimulate contraction of the sigmoid colon and rectum
- relax the internal anal sphincter
-conscious control allows relaxation of external anal sphincter
chemical digestion and absorption of carbs
Digestive enzymes:
*Salivary amylase, pancreatic amylase, and brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)
*Lactose Intolerant – the undigested disaccharides create osmotic gradient that prevents water from being absorbed -> diarrhea.
Treatment – add lactase enzyme
chemical digestion and absorption of proteins
- enzymes: pepsin in the stomach
- pancreatic proteases: trypsin, chymotrypsin, and carboxypeptidase
-brush border enzymes:
aminopeptidases, carboxypeptidases, and dipeptidases
chemical digestion and absorption of lipids
Pre-treatment- emulsification by bile salts
- enzymes: pancreatic lipase
- absorption of glycerol and short chain fatty acids
Malabsorption of nutrients
Gluten-sensitive enteropathy (celiac disease)
- Gluten damages the intestinal villi and brush border
- Results in bloating, diarrhea, pain and malnutrition
- Treated by eliminating gluten from the diet (all grains but rice and corn)
