Lecture 13: Digestive system part 2 Flashcards
What are the 5 primary functions of the digestive system
- ingestion
- Propulsion
- Digestion
- Absorption
- Defecation
define ingestion
ingestion of food to the alimentary canal
define propulsion
movement of food along the canal
What are the 2 subcategories of propulsion
- voluntary : deglutition or swallowing (oropharynx)
2. Involuntary : Peristalsis (esophagus, stomach, small intestine, large intestine)
define digestion
reduction of food into particles to be assimilated
What are the 2 categories
- Mechanical degestion: physical alteration of food by chewing (mastication), churning (stomach), and segementation (small intestine)
- Chemical digestion: by catabolic enzymes
define absorption
passage of digested end-products of the alimentary canal to (active or passive transport)
- cardiovascular system
- Lymphatic system
define defecation
expulsion of non-digestible or non-absorbed material
What parts are involved in transforming food to a bolus that is swallowed (mastication)
lips cheeks palate tongue salivary glands gums teeth
what controls mastication
voluntary + many stretching reflexes (mechanoreceptors)
What causes the propulsion of the bolus
deglutition (swallowing)
food goes from mouth -> oropharynx -> laryngopharynx ->oesophagus
what coordinates the propulsion / deglutition of the bolus
tongue, soft palate, pharynx, esophagus
- 2 phases
1. buccal
2. pharyngeal-esophageal
What happens in the 1st phase of propulsion the Buccal (oral) phase
- it is a voluntary action
- bolus pushed by tongue against palate & towards the back of oral cavity up to oropharynx; upper esophageal sphincter contracted
What happens in th 2nd phase of propulsion the pharyngeal - esophageal phase
- involuntary action
- bolus in oropharynx stimulate mechanoreceptors
- oropharynx sends an impulse to center of deglutition (medulla oblongata, inferior part of pons)
- medulla sends impulse that provokes elevation of soft palate & uvula to close the nasopharynx & elevation of larynx that cause epiglottis to tip over the larynx (interrupt respiration)
- bolus reaches esophagus (respiration restarts)
What is peristalsis
smooth muscle
- wave-like ripple
- stretch in tract causes reflex contraction of circular muscle -> forward movement
- causes additional contractions
what is segmentation
smooth muscle
- mixing movement
- forward and backwards movement in single region
- helps mix food and digestive fluids
when bolus mixes with gastric juices it turns to
chyme
what happens a few minutes after bolus enters stomach
mixing wave ( churning) which is specific peristaltic movement of stomach (involuntary-propulsion) every 15 - 20 seconds
How does stomach empty chyme into small intestine
a bit of chye is pushed into duodenum via pyloric sphincter while rest returns into stomch (retropulsion)
-this cycle keeps repeating until stomach is empty
Gastric motility: how long does it take to empty stomach and what controls it
- takes ~ 2-6 hours
- controlled by hormonal and nervous mechanisms
What are the hormonal mechanisms that control gastric motility
- fats in duodenum stimulate the release of gastric inhibitory peptide (GIP)
- which decreases peristalsis of gastric muscle and slows passage of chyme into duodenum
what are the nervous mechanisms that control gastric motility
- enterogastric reflex
- receptors in duodenal mucosa are sensitive to presence of acid and distention
- impulses over sensory and motor fibers in the vagus nerve cause a reflex inhibition of gastric peristalsis
- increases in acid and distention in duodenum -> inhibitor of peristalsis -> decreased chyme entering duodenum
Intestinal motility includes what 2 types of movement
peristalsis and segmentation
What does segmentation in duodenum and upper jejunum do
mixes chyme w/ digestive juices frfom the pancreas, liver, and intestinal mucosa
What helps move chyme from small intestine to large
rate of peristalsis picks up as chymes approaches end of jejunum; moving it through rest of s. intest into l. intest
How long does it take chyme to move through small intestine
~ 5 hours
What helps regulate and stimulate peristalsis in intestines
- regulated in part by intrinsic stretch reflexes
- stimulated by cholecystokinin (CCK)
What is chemical digestion
changes in chemical composition of food
changes in chemical composition of food is from
hydrolysis (catabolism)
what is the catalyst in chemical digestion
extracellular, organic (protein)
what are the principles of enzyme action in chemical digestion
- specific in their action
- optimal function at specific pH
- most catalyze a chem reaction in both directions
- enzymes continually destroyed in body and must continually be synthesized
- most digestive enzymes synthesized as inactive proenzymes (need another molecule to be activated)
Enzymes are involved in the breakdown of (3)
- carbohydrates
- proteins
- lipids
what are carbohydrates
saccharide compouds (sugars)
what forms disaccharides
polysaccharides are hydrolyzed by amylases (in saliva and from pancreas)
what is final step of carbohydrate digestion after it is turned to disaccharides
final steps are catalyzed by sucrase, lactase and maltase (found in cell membrane of epithelial cells covering villi lining intestinal lumen)
what are protein compounds composed of
twisted chains of amino acids
stage 2 of protein digestion
proteases catalyze hydrolysis of proteins into intermediate compounds and finally amino acids
stage 3 protein digestion
main proteases: pepsin in gastric juice, trypsin in pancreatic juice, peptidases in intestinal brush border
how is fat digested
- fats are insoluble in water
- must be emulsified by bile in small intestine before being digested
- pancreatic lipase is the main fat-digesting enzyme
what does mucus do
lubricates food and, with water, facilitates mixing
what does amylase do
an enzyme that begins digestion of starches
salivary lipase
a small amount released in saliva
what does sodium bicarbonate do in saliva
increases the pH for optimal amylase function
