The Digestive System Lindsey Bily Anatomy & Physiology Austin High School.
Physiology of the Digestive System Chapter 26 Anatomy & Physiology.
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Transcript of Physiology of the Digestive System Chapter 26 Anatomy & Physiology.
Physiology of the Digestive SystemChapter 26Anatomy & Physiology
Mechanical Digestion
• Consists of all motility that brings about:– Change in physical state of food from
large to small– Churning – Propelling food forward
Mastication (chewing)
• Begins in mouth• Tongue, cheeks, lips keep food
between the surfaces of teeth• Reduce particle size & mix with
saliva
Deglutition
•Oral stage: mouth to oropharynx (voluntary control); under control of cerebral cortex•Pharyngeal stage: oropharynx to esophagus (involuntary); deglutition center in medulla•Esophageal stage: esophagus to stomach (involuntary); deglutition center in medulla
Oral stage
• Bolus in the middle of tongue pressed against the palate & moved back into the oropharynx
• Soft palate & uvula prevent food from entering nasopharynx
Pharyngeal stage
• Involuntary reflexes push bolus toward esophagus
• Upward movement of larynx & downward movement of food closes epiglottis
Motility of smooth muscle in GI tract
• Peristalsis• Segmentation
Peristalsis
•Wavelike ripple; always in a forward direction•Bolus stretching GI wall triggers a reflex contraction of circular muscle
Segmentation
•Mixing movement; forward & backward movement
Gastric motility
• Takes 2-6 hours to empty stomach• Food churned with gastric juices:
chyme• Hormonal control: gastric
inhibitory peptide secreted by intestinal mucosa
• Nervous mechanism: triggered by presence of acid & distention-enterogastric reflex
Intestinal motility
• Both segmentation & peristalsis• Segmentation in duodenum &
jejunum mixes chyme w/digestive juices
• Takes 5 hours in small intestine• Peristalsis also stimulated by
cholecystokinin-pancreozymin (CCK) secreted by intestinal mucosa in presence of chyme
Digestive enzymes
• Extracellular, protein catalysts• Enzyme principles
– Specific in action– Function best a specific pH– Catalyze reaction in both directions– Continually being destroyed or
eliminated so continually being synthesized
– Most digestive enzymes are inactive proenzymes
Carbohydrate digestion
•Polysaccharides hydrolyzed by amylases (in saliva & pancreatic juices) to disaccharides•Sucrase, lactase, maltase found in cell membrane of villi cells hydrolyze into monosaccharides, mostly glucose
Protein digestion
•Proteases catalyze hydrolysis into intermediate compounds then finally into amino acids•Main proteases
–Pepsin-stomach–Trypsin-pancreas–Chymotrypsin- “–Peptidase-intestines
Fat digestion
•Lecithin & bile acids in bile emulsify (become small droplets soluble in water) fats•Main fat digesting enzyme: pancreatic lipases
Residues of digestion
• Cellulose: dietary fiber• Undigested connective tissue from
meat
Saliva
• Secretion of salivary glands– Water (mostly)– Mucus: lubricates food– Salivary amylase: begins digestion of
starches– Sodium bicarbonate: increases pH for
optimal amylase activity
Gastric juice
• Secreted by gastric glands– Pepsin (by chief cells as pepsinogen)-
begins digestion of proteins– HCl (by parietal cells) decreases pH
for activation & function of pepsin– Intrinsic factor (by parietal cells)
protect vitamin B12– Mucus & water
Pancreatic juice
• Secreted by acinar cells & duct cells– Proteases (trypsin & chymotrypsin)– Lipases– Nucleases: digest nucleic acids– Amylase– Sodium bicarbonate: increase pH
• All pancreatic enzymes are secreted as inactive proenzymes
Bile
• Secreted by liver, stored & concentrated in gall bladder– Lecithin & bile salts– Sodium bicarbonate for optimum pH– Cholesterol, detoxification products,
bile pigments all eliminated in feces
Intestinal juice
• Secreted by cells of intestinal exocrine cells– Mucus & water: lubricate & aid in
mixing– Sodium bicarbonate
Control of salivary secretions
• Only by reflex mechanisms• Chemical, mechanical, olfactory,
visual stimuli
Control of gastric secretion
•Three phases:–Cephalic phase–Gastric phase–Intestinal phase
Cephalic phase
• Activated by mental factors• Vagus nerve stimulates production
of gastrin which stimulates gastric secretions
Gastric phase
• Products of protein digestion stimulate release of gastrin
• Distension of stomach also stimulates release of gastrin
Intestinal phase
• Fats, carbohydrates & acid in chyme stimulate release of gastric inhibitory peptide (GIP), CCK, secretin which decrease gastric secretion
Control of pancreatic secretion
• Stimulated by hormones secreted by intestinal mucosa– Secretin: production of pancreatic
fluid low in enzyme but high in bicarbonate
– CCK• Increased exocrine secretion by pancreas•Opposes gastrin, thus inhibiting gastric
secretion•Stimulates contraction of gallbladder
Absorption
• Passage of substances through the intestinal mucosa into blood
• Most occurs in small intestines• Water: osmosis• Secondary active transport:
sodium• Sodium cotransport: glucose• With the aid of bile salts in lacteals
Elimination
• Expulsion of the residues of digestion
• Act of expelling feces is defecation• Defecation results as a reflex
stimulated by receptors in rectal mucosa when rectum is distended