A&P2 Ch 29 Digestive System
coordination of the different functions of the digestive system
Regulation
some compounds of food resist digestion and are eliminated as feces
Residues of digestion
Mucus lubricates food and, with water, facilitates mixing
Saliva
Sodium bicarbonate increases the pH for optimum amylase function
Saliva
secreted by salivary glands
Saliva
digestive enzyme secretion facilitates chemical digestion
Secretion
mouth to oropharynx
Oral stage
-voluntarily controlled - food bolus in middle of the tongue -tongue press bolus against palate and food is moved to the oropharynx
Oral stage (3)
is the main fat-digesting enzyme
Pancreatic lipase
(secreted as inactive pepsinogen by chief cells)—a protease that begins the digestion of proteins
Pepsin
oropharynx to esophagus
Pharyngeal stage
A small amount of salivary lipase is released; function uncertain
Saliva
Chemical digestion: Chemical changes result from hydrolysis-
process in which compound unites with water and breaks down further
-involuntary movement -contractions and gravity move bolus through esophagus to stomach
(2)Esophageal stage
-wavelike ripple of the muscle layer of a hollow organ -progress motility that produces forward movement of matter along the GI tract
(2)Peristalsis
-2 main types of motility produced by the smooth muscle of the GI tract -can occur together in an alternating fashion
(2)Peristalsis and Segmentation
-mixing movement -digestive reflexes cause a forward-and-backward movement with a single segment of the GI tract -helps break down food particles -mixes food and digestive juices, and brings digested food to intestinal mucosa to absorption
(4)Segmentation
movement of nutrients through the GI mucosa into the internal environment
Absorption
an enzyme that begins digestion of starches
Amylase (Saliva)
all changes in chemical composition of food as it travels thru the digestive tract
Chemical digestion
breakdown of complex nutrients into simple nutrients
Digestion
food is taken in
Ingestion
Reduces size of food particles
Mastication
is functionally an extension of the external
digestive tract
-involuntary movement -propel bolus from the pharynx to the esophagus -the mouth -nasopharynx -larynx must be blocked -combination of contractions and move bolus into esophagus
(6)Pharyngeal stage
Properties of digestive enzymes (5)
-Specific in their action -Function optimally at a specific pH -Most enzymes catalyze a chemical reaction in both directions -Enzymes are continually being destroyed or eliminated from the body and must continually be synthesized -Most digestive enzymes are synthesized as inactive proenzymes
Food in the stomach is churned (propulsion and retropulsion) and mixed with gastric juices to form chyme
Gastric motility
process of swallowing; complex process requiring coordinated and rapid movements
Deglutition
Operate in lumen of digestive tract, outside of any body cells
Digestive enzymes
extracellular, organic (protein) catalysts
Digestive enzymes
excretion of material that is not absorbed
Elimination
esophagus to stomach
Esophageal stage
controlled by hormonal and nervous mechanisms
Gastric emptying
secreted by gastric glands
Gastric juice
-Chyme is ejected every 20 seconds into the duodenum -takes 2 to 6 hours to empty the stomach
Gastric motility
-fats in duodenum stimulate and release of gastric inhibitory peptide, which decreases peristalsis of gastric muscle and slows passage of chyme into duodenum
Hormonal mechanism
(7)Mechanisms used use for the primary function of the digestive system
Ingestion, Digestion, Motility of GI wall, Secretion, Absorption, Elimination, and Regulation
After leaving stomach, pass of chyme to the small intestine take about 5 hours
Intestinal motility
Peristalsis rate picks up as chyme approaches end of jejunum, moving it through small intestine into the large intestine
Intestinal motility
Segmentation in duodenum and upper jejunum mixes chyme with digestive juices from the pancreas, liver, and intestinal mucosa
Intestinal motility
includes peristalsis and segmentation
Intestinal motility
Mixes food with saliva in preparing for swallowing
Mastication
chewing movements
Mastication
Change ingested food from large particles into minute particles, facilitating chemical digestion
Mechanical digestion
Churn contents of the GI lumen to mix with digestive juices and contact w/ the surface of intestinal mucosa, facilitating absorption
Mechanical digestion
Propel food along the alimentary tract, eliminating digestive waste from the body
Mechanical digestion
movements of the digestive tract
Mechanical digestion
physically breaks down large chunks of food materials and moves food along the tract
Motility of the GI wall
-enterogastric reflex -receptors in the duodenal mucosa are sensitive to presence of acid and to distention -impulses over sensory and motor fibers in the vagus nerve cause a reflex inhibition of gastric peristalsis
Nervous mechanism
are hydrolyzed by amylases to form disaccharides
Polysaccharides
Fats must be emulsified by:
bile in small intestine before being digested
material does not truly enter the body until it is absorbed into the internal environment
digestive tract
Main proteases:
pepsin in gastric juice, trypsin in pancreatic juice, peptidases in intestinal brush border
regulated by intrinsic stretch reflexes; stimulated by cholecystokinin (CCK)
peristalsis
Proteases catalyze hydrolysis of:
proteins into intermediate compounds and, finally, into amino acids
Carbohydrates are:
saccharide compounds
Final steps of carbohydrate digestion are catalyzed by:
sucrase, lactase, and maltase, found in the cell membrane of epithelial cells covering the villi that line the intestinal lumen
Primary function of the digestive system:
to bring nutrients into the internal environment so they are available to each cell of the body
Protein compounds are made up of:
twisted chains of amino acids