Digestive system

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Chapter 21 Digestive System

Villi of the Small Intestine

Function of the Digestive System

• Ingestion– Taking food and water into the mouth

• Break down the food– Mechanical digestion: chewing, mixing, and churning

food – Chemical digestion: digestive enzymes breakdown

food• Absorb nutrients

– Movement of nutrients from the GI tract to the blood or lymph

• Release of waste – Elimination of indigestible solid wastes

Histology of the Digestive Tract

• Two groups of organs within the digestive system– The digestive tract or gastrointestinal (GI)

tract is a muscular tube that winds through the body and is responsible for the digestion and absorption of food

• Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus

– The accessory digestive organs aid in the breakdown of foodstuffs

• Teeth, tongue, gallbladder, salivary glands, liver, and pancreas

Fig. 21.1


• Tunics– Three major functions:

• Secretion of mucus• Absorption of end products of digestion• Protection against infectious disease

– Mucus secretions:• Protect digestive organs from digesting themselves• Ease food along the tract

– Stomach and small intestine mucosa contain:• Enzyme-secreting cells • Hormone-secreting cells (making them endocrine and

digestive organs)


• The digestive tract is composed of four tunics: mucosa, submucosa, muscularis, and serosa or adventitia. – Mucosa

• Innermost layer that lines the lumen of the alimentary canal• Consists of a mucous epithelium, a lamina propria, and a muscularis

mucosae

– Submucosa • Connective tissue layer containing the submucosal plexus (part of the

enteric plexus), blood vessels, and small glands

– Muscularis• Consists of an inner layer of circular smooth muscle and an outer layer

of longitudinal smooth muscle• The myenteric plexus is between the two muscle layers

– Serosa or adventitia• Forms the outermost layer of the digestive tract

Fig. 21.2


• Enteric Nervous System – Consists of nerve plexuses within the wall of

the digestive tract– Serves the digestive tract and regulates

digestive activity. It is composed of two major intrinsic nerve plexuses:• Submucosal nerve plexus: regulates glands and

smooth muscle in the mucosa• Myenteric nerve plexus: major nerve supply that

controls GI tract mobility


• Enteric Nervous System – Three major types of enteric neurons

1. Sensory neurons detect changes in the chemical composition of the digestive tract

2. Motor neurons stimulate or inhibit smooth muscle contraction and glandular secretion

3. Interneurons connect sensory to motor neurons

– Nervous regulation involves local reflexes in the ENS and CNS reflexes

Peritoneum

• The peritoneum is a serous membrane that lines the abdominopelvic cavity and organs

• Mesenteries are peritoneum that extend from the body wall to many of the abdominopelvic organs

• Retroperitoneal organs are located behind the peritoneum

Fig. 21.3

Oral Cavity

• Oral cavity or mouth– Is bounded by lips, cheeks, palate, and tongue – Has the oral orifice as its anterior opening– Is continuous with the oropharynx posteriorly

• Divided into two regions– Vestibule

• Space between the lips or cheeks and the alveolar processes, which contain the teeth

– Oral Cavity Proper• Lies medial to the alveolar processes

• To withstand abrasions: – The mouth is lined with stratified squamous

epithelium

Oral Cavity

• Lips and Cheeks– Involved in facial expression, mastication, and

speech• Palate

– Forms the roof of the mouth. Consists of hard and soft areas

• Hard palate: assists the tongue in chewing • Soft palate: mobile fold formed mostly of skeletal

muscle. Projecting from the soft palate is the uvula– Closes off the nasopharynx during swallowing

• The mouth contains accessory glands (salivary glands) and accessory organs (teeth and tongue) which begin the digestive process

Fig. 21.4

Oral Cavity

• Tongue– Occupies the floor of the mouth and fills

the oral cavity when mouth is closed

– Functions include:• Gripping and repositioning food during chewing• Mixing food with saliva and forming the bolus• Initiation of swallowing, and speech

– Frenulum secures the tongue to the floor of the mouth

Oral Cavity

• Teeth – Tear and grind food

• Born with 20 deciduous teeth– Erupt between about 6 months and 24 months of age

• Deciduous teeth are replaced by 32 permanent teeth– Replacement starts at about 5 years and the process

is complete by about 11 years

– The types of teeth are• Incisors• Canines• Premolars• Molars

Fig. 21.5

Oral Cavity

• A tooth consists of a crown, a neck, and a root – Crown

• Exposed part of the tooth above the gingiva• Dentin covered by enamel

– Enamel: acellular material composed of calcium salts and hydroxyapatite crystals; the hardest substance in the body

– Root• Portion of the tooth embedded in the jawbone• Composed of dentin

– Within the dentin of the root is the pulp cavity, which is filled with pulp, blood vessels, and nerves

– Periodontal ligaments hold the teeth in the alveoli

Molar Tooth in Place in the Alveolar Bone

Fig. 21.6

Oral Cavity

• Salivary Glands – Produce and secrete saliva that:

• Cleanses the mouth• Moistens and dissolves food chemicals • Aids in bolus formation• Contains enzymes that break down starch

– Salivary glands produce serous and mucous secretions

– The three pairs of large salivary glands are the parotid, submandibular, and sublingual

Fig. 21.7

Oral Cavity

• Saliva– Produce ~1 liter of saliva/day– Secreted from serous and mucous cells of salivary

glands• Serous: enzymes, ions, and mucin• Mucous: produce mucous

– 97-99.5% water and slightly acidic solution containing• Electrolytes• Digestive enzyme: salivary amylase• Proteins: mucin, lysozyme, defensins, and IgA• Metabolic wastes: urea and uric acid

Oral Cavity

• Control of Salivation– Primarily by the parasympathetic division of

the autonomic nervous system – Salivary glands secrete serous, enzyme-rich

saliva in response to: • Ingested food which stimulates chemoreceptors

and pressoreceptors • The thought of food

– Strong sympathetic stimulation inhibits salivation and results in dry mouth

Pharynx and Esophagus

• Pharynx – Consists of the nasopharynx, oropharynx,

and laryngopharynx• Food and fluids to the esophagus• Air to the trachea

• Esophagus – Connects the pharynx to the stomach

• The upper and lower esophageal sphincters regulate movement

– Mucous glands produce a lubricating mucus

Tab. 21.1

Swallowing

• During the voluntary phase of swallowing, a bolus of food is moved by the tongue from the oral cavity to the pharynx

Fig. 21.8

Swallowing

• The pharyngeal phase is a reflex caused by the stimulation of stretch receptors in the pharynx – The soft palate closes the nasopharynx, and the epiglottis,

vestibular folds, and vocal folds close the opening into the larynx

– Pharyngeal muscles move the bolus to the esophagus

Fig. 21.8

Swallowing

• The esophageal phase is a reflex initiated by the stimulation of stretch receptors in the esophagus. A wave of contraction (peristalsis) moves the food to the stomach

Fig. 21.8

Peristalsis

Fig. 21.9

Fig. 21.8

Anatomy and Histology of the Stomach

• Temporary “storage tank” where chemical breakdown of proteins begins and food is converted from a bolus to chyme

• The openings of the stomach are the– Gastroesophageal opening to the esophagus– Pyloric orifice to the duodenum

• The major regions are the – Cardiac part– Fundus– Body– Pyloric part which is continuous with the duodenum

through the pyloric sphincter• The lateral sides of the stomach consist of

greater and lesser curvatures


• The wall of the stomach consists of – External serosa– Muscle layer (longitudinal, circular, and oblique)– Submucosa– Simple columnar epithelium

• Surface mucous cells: produce an alkaline mucous with bicarbonate, which coats and protects the stomach lining

• An empty stomach has a volume of 50 mL but can hold about a gallon of food

• When empty one can see the folds called rugae (submucosa and mucosa)


Fig. 21.10


• There are gastric pits with glands that produce juice

• Glands include four secretory cells:– Mucous neck: secrete acid mucus– Parietal cells: secrete HCl and intrinsic factor – Chief cells: produce pepsinogen

• Pepsinogen is activated to pepsin by:– HCl in the stomach– Pepsin itself via a positive feedback mechanism

– Endocrine cells: produce regulatory hormones • Gastrin, histamine, endorphins, serotonin,

cholecystokinin (CCK), and somatostatin into the lamina propria

Fig. 21.10

Secretions of the Stomach

• Chyme is ingested food mixed with gastric juice

• Gastric juice– Mucus protects the stomach lining– Pepsinogen is converted to pepsin, which digests

proteins– Hydrochloric acid promotes pepsin activity and kills

microorganisms– Intrinsic factor is necessary for vitamin B12

absorption– Gastrin and histamine regulate stomach secretions

• A proton pump (H+-K+ exchange pump) moves H+ out of parietal cells

Fig. 21.11

HCl productionby Parietal Cells in the Gastric Glands

of the Stomach

Regulation of Stomach Secretion

• There are three phases of stomach secretion – Cephalic phase

• Initiated by the sight, smell, taste, or thought of food

• Nerve impulses from the medulla stimulate hydrochloric acid, pepsinogen, gastrin, and histamine secretion.

– Gastric phase• Initiated by distention of the stomach, which stimulates

gastrin secretion and activates CNS and local reflexes that promote secretion

– Gastrointestinal phase• Acidic chyme, which enters the duodenum and stimulates

neuronal reflexes and the secretion of hormones (secretin, cholecystokinin) that inhibit gastric secretions

Cephalic Phase

Gastric Phase

Gastrointestinal Phase

Fig. 21.12

Movements of the Stomach

• Waves mix the stomach contents with stomach secretions to form chyme

• Peristaltic waves move the chyme into the duodenum – Hunger contractions– Vomiting

• Reverse peristalsis

Fig. 21.13

Small Intestine

• The body’s major digestive organ• Digestion is completed and absorption occurs• Runs from pyloric sphincter to the ileocecal

valve • Has three subdivisions:

– Duodenum - ~ 10” long. The bile and pancreatic ducts empty here

– Jejunum - ~ 8’ long – Ileum - ~ 12’ long

Fig. 21.14

Anatomy and Histology of the Small Intestines

• Structural modifications of the small intestine wall increase surface area about 600-fold– Circular folds

• Deep folds of the mucosa and submucosa

– Villi• Fingerlike extensions of the mucosa

– Microvilli• Tiny projections of absorptive mucosal cells’ plasma

membranes. This forms a brush border

• The epithelium of the mucosa is simple columnar epithelium. Between the villi the mucosa contains pits called intestinal glands

Anatomy and Histology of the Small Intestines

• The epithelium of the mucosa is made up of:– Absorptive cells

• Produce digestive enzymes

– Goblet cells• Produce a protective mucus

– Granular cells (Paneth cells)• Protect the intestinal epithelium form bacteria

– Endocrine cells• Produce regulatory hormones

• Peyer’s patches (aggregated lymphoid follicles) are found in the submucosa

Fig. 21.15

Secretions of the Small Intestine

• Mucus protects against digestive enzymes and gastric acids

• Digestive enzymes (disaccharidases and peptidases) are bound to the intestinal wall

• Chemical or tactile irritation, vagal stimulation, and secretin stimulate intestinal secretion

Movement of the Small Intestine

• Segmental contractions mix intestinal contents

• Peristaltic contractions move materials distally

• Distension of the intestinal wall, local reflexes, and the parasympathetic nervous system stimulate contractions

• Distension of the cecum initiates a reflex that stimulates contraction of the ileocecal sphincter

Segmental Contractions

Fig. 21.16

Anatomy and Histology of the Liver

• The liver has four external lobes: right, left, caudate, and quadrate

• Internally, the liver is divided into eight segments

• Liver segments are divided into lobules – Hexagonal-shaped liver lobules are the

structural and functional units of the liver– Composed of hepatocyte (liver cell) plates

radiating outward from a central vein– Portal triads are found at each of the six

corners of each liver lobule

Fig. 21.17

Anatomy and Histology of the Liver

• Portal triads consist of– Hepatic duct: conduct bile toward the duodenum– Hepatic artery: supplies oxygen-rich blood to the

liver– Hepatic portal vein: carries venous blood with

nutrients from digestive viscera

• The hepatic cords are composed of columns of hepatocytes separated by the bile canaliculi

• Sinusoids are enlarged spaces filled with blood and lined with endothelium and hepatic phagocytic cells– Kupffer cells: hepatic macrophages found in liver

sinusoids

Fig. 21.18

Functions of the Liver

• Produces bile, which contains bile salts that emulsify fats

• Stores and processes nutrients, produces new molecules, and detoxifies molecules

• Hepatic phagocytic cells phagocytize red blood cells, bacteria, and other debris

• Produces blood components

Blood Flow Through the Liver

• Branches of the hepatic artery and the hepatic portal vein in the portal triads empty into hepatic sinusoids

• Hepatic sinusoids empty into central veins, which join to form the hepatic veins, which leave the liver

Fig. 21.19Blood and Bile Flow Through the Liver

Bile Transport

• Bile canaliculi collect bile from hepatocytes and join the small hepatic ducts in the portal triads

• Small hepatic ducts converge to form the right and left hepatic ducts, which exit the liver

• The left and right hepatic ducts join to form the common hepatic duct

• The cystic duct from the gallbladder joins the common hepatic duct to form the common bile duct

• The common bile duct and pancreatic duct join at the hepatopancreatic ampulla, which opens into the duodenum at the major duodenal papilla

Gallbladder and Bile

• Gallbladder– A small sac on the inferior surface of the liver– Stores and concentrates bile

• Bile– A yellow-green, alkaline solution containing bile salts, bile

pigments, cholesterol, neutral fats, phospholipids, and electrolytes

– Bile salts are cholesterol derivatives that:• Emulsify fat• Facilitate fat and cholesterol absorption• Helps make cholesterol soluble

– Enterohepatic circulation recycles bile salts therefore they are never voided in the feces

– The chief bile pigment is bilirubin, a waste product of heme.• Bilirubin is metabolized by bacteria in the small intestines and

urobilogen is produced, which gives feces its dark color

Liver, Gallbladder, Pancreas, and Duct System

Fig. 21.20

Regulation of Bile Secretion

• Acidic, fatty chyme causes the duodenum to release:

– Cholecystokinin (CCK) and secretin into the bloodstream

• Cholecystokinin causes:– The gallbladder to contract and releases bile– Relaxation of the sphincters of the bile duct and

hepatopancreatic ampulla• Secretin increases bile secretion (water and bicarbonate

ions) • As a result, bile enters the duodenum

• Bile salts and secretin transported in blood stimulate the liver to produce bile

• Vagal stimulation causes weak contractions of the gallbladder

Fig. 21.21

Control of Bile Secretion and Release

Anatomy and Histology of the Pancreas

• Location– Lies deep to the greater curvature of the stomach– The head is encircled by the duodenum and the tail sits against

the spleen• Exocrine function

– Secretes pancreatic juice which breaks down all categories of foodstuff

– The pancreas is divided into lobules that contain acini• Clusters of secretory cells that contain zymogen granules with

digestive enzymes • Connect to a duct system that eventually forms the pancreatic

duct– The pancreatic duct joins the hepatopancreatic ampulla. The

accessory pancreatic duct empties into the duodenum at the minor duodenal papilla.

• The pancreas also has an endocrine function. The islets of Langerhans release of insulin and glucagon

Fig. 21.22

Pancreatic Secretions

• The aqueous component of pancreatic juice is produced by the small pancreatic ducts and contains bicarbonate ions – Water solution of enzymes and electrolytes (primarily

HCO3–)

• Neutralizes acidic chyme• Provides optimal environment for pancreatic enzymes

• The enzymatic component of pancreatic juice is produced by the acini and contains enzymes that digest carbohydrates, lipids, and proteins– Enzymes are released in inactive form and activated in

the duodenum

Regulation of Bile Secretion and Release

• Secretin stimulates the release of the aqueous component, which neutralizes acidic chyme

• Cholecystokinin stimulates the secretion of the enzymatic component and relaxation of the sphincters of the pancreatic duct and hepatopancreatic ampulla

• Parasympathetic stimulation increases and sympathetic stimulation decreases secretion of enzymes

Fig. 21.23

Anatomy and Histology of the Large Intestine

• Extends from the ileocecal valve to the anus • Absorbs water and eliminate the waste via feces• Is subdivided into the cecum, appendix, colon, rectum,

and anal canal– The cecum forms a blind sac at the junction of the small and

large intestines. – The vermiform appendix is a blind tube off the cecum. – The colon has distinct regions: ascending colon, transverse

colon, descending colon, and sigmoid colon– The sigmoid colon joins the rectum – The anal canal, the last segment of the large intestine, opens to

the exterior at the anus• The anal canal contains two sphincters to allow the movement of

feces

Anatomy and Histology of the Large Intestine

• Three unique features– Teniae coli: three bands of longitudinal

smooth muscle in its muscularis– Haustra: pocketlike sacs caused by the

tone of the teniae coli– Epiploic appendages : fat-filled pouches of

visceral peritoneum

• The mucosal lining of the large intestine is simple columnar epithelium with mucus-producing crypts

Large Intestine

Fig. 21.24

Secretions of the Large Intestine

• Mucus protects the intestinal lining • The bacterial flora of the large intestine consist

of:– Bacteria surviving the small intestine that enter the

cecum – Those entering via the anus

• These bacteria: – Colonize the colon– Ferment indigestible carbohydrates– Release irritating acids and gases (flatus)– Synthesize B complex vitamins and vitamin K– Constitute about 30% of the dry weight of the feces

Movement in the Large Intestine

• Haustra mix the contents and moves them slowly toward the anus

• Mass movements are strong peristaltic contractions that occur three or four times a day

• Defecation is the elimination of feces – Distension of rectal walls caused by feces:

• Stimulates contraction of the rectal walls• Relaxes the internal anal sphincter

– Reflex activity moves feces through the internal anal sphincter

– Voluntary activity regulates movement through the external anal sphincter

Fig. 21.25

Digestion, Absorption, and Transport

• Digestion (mechanical and chemical) is the breakdown of organic molecules into their component parts– Chemical digestion is the breaking of

covalent chemical bonds in organic molecules by digestive enzymes

• Absorption is the uptake of digestive tract contents

• Transport is the distribution of nutrients throughout the body

Chemical Digestion• Carbohydrate digestion

– Begins in mouth with salivary amylase digesting starches

– Pancreatic amylase finishes the process in the small intestine. Simple sugars are also broken down here

• Protein digestion– Broken down into amino acids– Begins in the stomach with pepsin– Trypsin and chymotrypsin are also important

protein digesting enzymes.

Chemical Digestion

• Lipid digestion– Small intestine is the sole source of lipid

digestion by lipases– Enter lacteals and are transported to systemic

circulation via lymph

• Nucleic acids– Broken down by pancreatic nucleases in the

small intestine

Tab. 21.2

Carbohydrates

• Include starches, glycogen, sucrose, lactose, glucose, and fructose

• Polysaccharides are broken down into monosaccharides by a number of different enzymes

• Monosaccharides– Taken up by intestinal epithelial cells by symport that is

powered by a Na+ gradient or by facilitated diffusion – Carried to the liver, where the non-glucose sugars are

converted to glucose – Glucose is transported to the cells that require energy – Glucose enters the cells through facilitated diffusion

• Insulin influences the rate of glucose transport

Transport of Monosaccharides Across the Intestinal Epithelium

Fig. 21.26

Lipids

• Include triglycerides, phospholipids, steroids, and fat-soluble vitamins

• Emulsification is the transformation of large lipid droplets into smaller droplets and is accomplished by bile salts

• Lipase digests lipid molecules to form fatty acids and a monoglyceride

• Micelles form around lipid digestion products and move to epithelial cells of the small intestine, where the products pass into the cells by simple diffusion

Lipids

• Within the epithelial cells, free fatty acids are combined with monoglycerides to form triglycerides

• Proteins coat triglycerides, phospholipids, and cholesterol to form chylomicrons

• Chylomicrons enter lacteals within intestinal villi and are carried through the lymphatic system to the bloodstream

• Triglycerides are stored in adipose tissue, converted into other molecules, or used as energy

Transport of Lipids Across the Intestinal Epithelium

Fig. 21.27

Lipoproteins

• Lipoproteins include chylomicrons, VLDL, LDL, and HDL

• LDL transports cholesterol to cells, and HDL transports it from cells to the liver

• LDL are taken into cells by receptor-mediated endocytosis, which is controlled by a negative-feedback mechanism

Fig. 21.28

Proteins

• Pepsin in the stomach breaks proteins into smaller polypeptide chains

• Proteolytic enzymes from the pancreas produce small peptide chains

• Peptidases, bound to the microvilli of the small intestine, break down peptides

• Tripeptides, dipeptides, and amino acids are absorbed by symport that is powered by a Na gradient

• Amino acids are transported to the liver, where the amino acids can be modified or released into the bloodstream

• Amino acids are actively transported into cells under the stimulation of growth hormone and insulin

• Amino acids are used as building blocks or for energy

Amino Acid Transport Across the Intestinal Epithelium

Fig. 21.29

Water and Ions

Fig. 21.30

• About 9 liters of water enters the digestive tract each day– Can move in either direction

across the wall of the small intestine, depending on the osmotic gradients across the epithelium

– Epithelial cells actively transport Na, K, Ca+2, and Mg+2 from the intestine

– Chloride ions move passively through the wall of the duodenum and jejunum but are actively transported from the ileum

Effects of Aging on the Digestive System

• The mucous layer, the connective tissue, the muscles, and the secretions all tend to decrease as a person ages

• These changes make an older person more open to infections and toxic agents

Digestive system

Health & Medicine

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