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