General Characteristics of the Digestive System

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Chapter 17
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Use the Visible Body Software
throughout the lecture. Navigate to
the ‘Digestive System Views.’
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17.1 General Characteristics of the Digestive System
Digestion:
•The mechanical and chemical breakdown of foods into forms that cell
membranes can absorb:
•Mechanical digestion breaks down large pieces of food into smaller
ones, but does not change chemical composition
•Chemical digestion uses enzymes to break down food particles, by
changing them into simpler chemicals
Digestive System:
•Organs of the digestive system carry out mechanical and chemical
digestion, as well as ingestion, propulsion, absorption, and defecation
•The digestive system consists of the alimentary canal and accessory
organs
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Organs of the Digestive System (Use the Visible Body
Software. Navigate to ‘Media.’ Under ‘System Overviews’
select ‘Digestive System.’)
The digestive system consists of 2 portions:
•Alimentary canal:
•The food passageway
•Consists of organs that extend from the mouth to the anus
•Mouth, pharynx, esophagus, stomach, small intestine, large
intestine, and anal canal
•Muscular tube, about 8 m long
•Accessory organs:
•Consists of organs that empty secretions into alimentary canal
•Food does not pass through them
•Salivary glands, liver, gallbladder, pancreas
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Figure 17.1 Organs of the Digestive System
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Figure 17.2 The Alimentary Canal (Use the Visible Body
Software. Navigate to ‘Media.’ Under ‘Nutrition and
Elimination’ select ‘Alimentary Canal.’)
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Structure of the Alimentary Canal Wall
Wall of alimentary canal is composed of 4 layers; from innermost to
outermost, the layers are:
•Mucosa:
•Innermost layer, mucous membrane
•Folded in some areas, to increase surface area
•Absorbs dietary nutrients, secretes mucus and enzymes
•Submucosa:
•Connective tissue layer
•Nourishes cells, transports absorbed food molecules
•Muscularis (externa):
•Muscle tissue; contains circular and longitudinal layers
•Moves tube and food materials
•Serosa:
•Outermost layer; serous fluid eliminates friction
•Visceral peritoneum of organs within abdominal cavity
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Figure 17.3 Structure of the Wall (Use the Visible Body
Software. Navigate to ‘Media.’ Under ‘Nutrition and
Elimination’ select ‘Nutrition Absorption.’)
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Movements of the Alimentary Canal Wall
2 types of movement in the alimentary canal:
•Mixing movements:
•Muscle in small sections contracts rhythmically
•Does not move materials in one direction
•Example: segmentation in small intestine, churning in stomach
•Propelling movements:
•Moves materials in one direction
•Peristalsis: Ring of contraction progresses down tube; propels food
particles down the tract in wavelike motion
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Figure 17.4 Movements of the Tube
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17.2 Mouth
The mouth:
•First part of alimentary canal
•Ingests food
•Mastication: Mechanical breakdown of solid particles,
mixes them with saliva
•Functions as an organ of speech and sensory reception
•Surrounded by lips, cheeks, tongue, palate
•Includes oral cavity and vestibule
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Figure 17.5 Mouth
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Cheek, Lips, and Tongue
Cheeks:
•Form the lateral walls of the mouth
•Contain muscles for facial expression and chewing
•Have an inner lining of stratified squamous epithelium (moist)
Lips:
•Highly mobile structures that surround the mouth opening
•Sensory receptors judge temperature and texture of food
•Boundary between skin and mucous membrane inside mouth
Tongue:
•Thick, muscular organ that occupies the floor of the mouth, and nearly
fills the oral cavity when the mouth is closed
•Lingual frenulum: Connects tongue to floor of mouth
•Papillae: Projections that move food, contain taste buds
•Lingual tonsils: Lymphatic tissue masses on root of tongue
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Figure 17.6 Tongue
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Palate
Palate:
•Forms the roof of the oral cavity
•Consists of an anterior portion, the hard palate, and a
posterior portion, the soft palate:
•Hard palate consists of palatine processes of maxillae and palatine
bones
•Soft palate consists of muscular arch, ends in uvula
•Palatine tonsils: Lymphatic masses on sides of tongue
•Pharyngeal tonsils (adenoids): Masses of lymphatic
tissue in posterior wall of pharynx
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Figure 17.7 Mouth, Nasal Cavity, Pharynx
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Teeth (Use the Visible Body Software. Navigate to
‘Microanatomy.’ Select ‘Molar, 2 Roots.’)
Teeth:
•Hardest structures in the body
•Not part of skeletal system
•Human dentition is that of omnivore (eating all types of food)
•20 primary (deciduous) teeth
•32 secondary (permanent) teeth
•Types of teeth: Central incisors, lateral incisors, canines (cuspids),
first premolars (bicuspids), first molars, second molars, third molars
•Break food particles down to smaller pieces, beginning mechanical
digestion
•Chewing mixes food with enzymes
•Incisors used for biting, canines for grasping and tearing, molars and
premolars for grinding
•Consist of crown and root, which meet at neck
•Enamel covers crown, dentin makes up most of tooth and surrounds
pulp cavity
•Periodontal ligament holds root in place, along with cementum
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Figure 17.8 Teeth of Child’s Skull
Rebecca Gray/DonKincaid/McGraw-Hill Education
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Table 17.2 and Figure 17.9 Primary and Secondary Teeth
The first column shows primary
teeth (Deciduous) and the second
column shows secondary teeth
(permanent). Each column shows
type and number.
Primary Teeth
(Deciduous)
Secondary Teeth
(Permanent)
Type Number Type Number
Incisor Incisor
Central 4 Central 4
Lateral 4 Lateral 4
Canine (cuspid) 4 Canine (cuspid) 4
Premolar (bicuspid)
First 4
Second 4
Molar Molar
First 4 First 4
Second 4 Second 4
Third 4
Total Total
(b): Nick Koudis/Getty Images
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Figure 17.10 Section of a Tooth
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17.3 Salivary Glands
Salivary glands secrete saliva:
•Moistens food particles and binds them together
•Dissolves food so it can be tasted
•Contains enzymes (begin chemical digestion of
carbohydrates) and bicarbonate ions (keep pH
favorable for enzyme activity and protect teeth
from acidic foods)
•3 pairs of major salivary glands:
•A . Parotid glands: Largest; secrete serous
saliva with amylase
•B. Submandibular glands: Floor of mouth;
serous + mucous
•C. Sublingual glands: Under tongue;
mainly mucous
•Many minor glands scattered throughout the
mucosa of the tongue, palate, and cheeks; keep
lining of mouth moist
21
A
B
C Image Credit: Visible Body

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Salivary Secretions
The different pairs of salivary glands have varying proportions
of 2 types of secretory cells:
•Serous cells:
•Produce a watery fluid, containing a digestive enzyme
called salivary amylase
•Amylase splits starch and glycogen into disaccharides
•Mucous cells:
•Secrete mucus
•Mucus binds food particles and lubricates food while
swallowing
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Figure 17.11 Major Salivary Glands
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17.4 Pharynx and Esophagus
Pharynx and esophagus do not help in food digestion
Both the pharynx and esophagus have muscular walls,
which function in swallowing
Pharynx:
•Cavity posterior to the mouth
•Extends from nasal cavity to esophagus
•Parts of pharynx:
•A. Nasopharynx: Posterior to nasal cavity; air
passage; contain openings to auditory tubes
•B. Oropharynx: Posterior to oral cavity; air and
food passage
•C. Laryngopharynx: Posterior to larynx;
passageway to esophagus
Esophagus:
•Tubular organ that extends from the pharynx to the
stomach
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A
C
B
Image Credit: Visible Body

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Figure 17.13 Muscles of Pharyngeal Wall
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Swallowing Mechanism
Swallowing can be divided into 3 stages:
•First stage:
•Voluntary stage
•Saliva is mixed with chewed food, forming a bolus
•Second stage:
Starts as food reaches oropharynx, stimulates sensory receptors, and
triggers swallowing reflex:
•The soft palate and uvula rises, to protect nasal cavity from food
• The hyoid bone and larynx elevate
• The epiglottis closes off top of the larynx, to protect trachea
• The longitudinal muscles of pharynx contract
• The inferior constrictor muscles relax and the esophagus opens
• Peristaltic waves force food into esophagus
•Third stage:
•Peristalsis transports food in the esophagus to the stomach
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Figure 17.14 Steps in Swallowing Reflex (Use the Visible
Body Software. Navigate to ‘Media.’ Under ‘Nutrition and
Elimination’ select ‘Chewing and Swallowing.’)
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Esophagus
Esophagus:
•Muscular food passageway from the
pharynx to the stomach (25 cm)
•Penetrates the diaphragm through the
esophageal hiatus
•Contains mucous glands in submucosa,
which moisten and lubricate lining of
inner wall with mucus
•In muscularis, superior
1
/
3
is skeletal,
middle
1
/
3
is skeletal + smooth, inferior
1
/
3

is smooth muscle
•Lower esophageal (cardiac) sphincter
regulates food passage into stomach,
and closes to prevent regurgitation of
food
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Image Credit: Visible Body

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Figures 17.15 and 17.16 Esophagus
Ed Reschke/Photolibrary/Getty Images
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17.5 Stomach
Stomach:
•J-shaped, pouch-like organ, about 25 to 30
centimeters long
•Inferior to the diaphragm, in the upper-left
portion of the abdominal cavity
•Rugae are folds of mucosa and submucosa that
allow for distention
•The stomach:
•Receives food from the esophagus
•Mixes food with gastric juice
•Initiates protein digestion
•Has limited absorption
•Moves food into small intestine
•Contains the following layers of smooth muscle:
•An inner circular layer
•An outer longitudinal layer
•Some portions have third (innermost) layer
of oblique fibers
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Image Credit: Visible Body

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Figure 17.17 Parts of the Stomach
4 parts of the stomach:
Cardia, fundus, body, and pylorus
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Parts of the Stomach
Portions of the stomach:
•Cardia:
•Region near opening to esophagus
•Contains lower esophageal sphincter
•Fundus:
•Rounded area that rises above cardia
•Temporary food storage, which sometimes contains swallowed air
•Body:
•Main portion
•Lies between fundus and pylorus
•Pylorus:
•Distal portion, closest to small intestine
•Funnel-shaped pyloric antrum narrows to become pyloric canal
•Pyloric sphincter: Circular smooth muscle, controls gastric emptying
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Gastric Secretions
Gastric glands contain 3 types of secretory cells, which produce a mixture
called gastric juice. Components of gastric juice:
•Pepsinogen: Inactive form of pepsin; secreted by chief cells
•Pepsin: Active enzyme that beaks down proteins into polypeptides;
forms from pepsinogen in presence of hydrochloric acid
•Gastric lipase: Fat-splitting enzyme, found in small quantities; action
inhibited by low pH
•Hydrochloric acid: Produced by parietal cells; converts pepsinogen
into pepsin
•Mucus: Secreted by mucous cells; provides lubrication and protects
stomach lining
•Intrinsic factor: Produced by parietal cells; required for absorption of
vitamin B
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Figure 17.19 Lining of the Stomach
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Regulation of Gastric Secretion
Gastric juice is produced continuously
Rate of production varies, and is under neural and hormonal control
Neural regulation:
•Sympathetic impulses decrease gastric activity
•Parasympathetic impulses increase gastric activity; promote release
of histamine, which stimulates gastric secretion
Hormonal regulation:
•Somatostatin: Inhibits hydrochloric acid secretion
•Gastrin: Increases gastric juice secretion
•Cholecystokinin (CCK): Released by small intestine cells when
proteins and fat enter the small intestine; decreases gastric motility
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Vomiting
Vomiting:
•Result of reflex that empties stomach in reverse direction
•Causes include certain drugs, toxins from food,
overstretching of stomach, body motion changes, motion
sickness associated with inner ears
•Controlled by vomiting center in medulla oblongata
•Motor response involves deep breath, raising of soft
palate, closing of nasal cavity and glottis, relaxation of
lower esophageal sphincter, contraction of diaphragm and
abdominal wall muscles; forces food out through mouth
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Figure 17.21 Stomach Movements
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Pancreatic Juice
Pancreatic juice:
•Contains enzymes that digest carbohydrates, fats, proteins, and
nucleic acids
•Components of pancreatic juice:
•Pancreatic amylase: Splits starch and glycogen into disaccharides
•Pancreatic lipase: Breaks down triglycerides
•Trypsin: Digests proteins; released as inactive trypsinogen, which is
activated by enterokinase in small intestine
•Chymotrypsin: Digest proteins; released as inactive, activated by trypsin
•Carboxypeptidase: Digests proteins; released as inactive, activated by
trypsin
•Nucleases: Digest nucleic acids
•Bicarbonate ions: Make pancreatic juice alkaline; buffer stomach acid
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17.7 Liver and Gallbladder
Liver:
•Largest internal organ
•Located in the upper-right abdominal quadrant, just
beneath the diaphragm
•Reddish-brown organ
•Well-supplied with blood vessels
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Liver Structure
Liver has 4 lobes:
•Right lobe: Largest lobe
•Left lobe: Smaller than right lobe
•Quadrate lobe: Minor lobe, near gallbladder
•Caudate lobe: Minor lobe, near inferior vena cava
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Figure 17.26 Lobes of the Liver
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Liver Functions
The liver carries on many important activities:
•Produces glycogen from glucose
•Breaks down glycogen into glucose
•Converts non-carbohydrates to glucose
•Oxidizes fatty acids
•Synthesizes lipoproteins, phospholipids, and cholesterol
•Converts excess carbohydrates and proteins into fats
•Deaminates amino acids
•Forms urea
•Synthesizes plasma proteins
•Converts some amino acids to other amino acids
•Stores glycogen, iron, and vitamins A, D, and B
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•Phagocytizes damaged RBCs and foreign substances/antigens
•Removes toxins, such as alcohol and certain drugs from the blood
•Acts as blood reservoir
•Role in digestion is to secrete bile
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Composition of Bile
Bile is a yellowish-green liquid that hepatic cells continuously secrete
Components of Bile:
•Water
•Bile salts:
•Produced from cholesterol
•Emulsify fats
•Only bile component that have a digestive function (act as fat emulsifying
agents)
•Bile pigments: Bilirubin and biliverdin, derived from hemoglobin
breakdown
•Cholesterol
•Electrolytes
Jaundice:
•Yellowing of skin, eye sclerae, mucous membranes
•Caused by blockage of bile ducts, diseases, such as cirrhosis or hepatitis,
or rapid red blood cell destruction
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Gallbladder
Gallbladder:
•Pear-shaped sac
•Found on inferior surface of liver
•Stores and concentrates bile
•Releases bile into duodenum of small intestine, via common bile
duct at the hepatopancreatic ampulla
•Hepatopancreatic sphincter regulates release of bile into the
duodenum
•Gallstones: (Us the Visible Body Software. Navigate to
‘Media.’ Under ‘Digestive and Urinary Pathologies’, select
‘Gallstones.’)
•Gallbladder normally concentrates bile salts, bile pigments,
cholesterol
•Sometimes cholesterol precipitates and forms solid crystals
•Crystals can enlarge, forming gallstones
•Causes: Excess bile concentration, too much cholesterol
secretion by liver, or inflammation of the gallbladder
•Bile release under control of hormone Cholecystokinin, which
stimulates contraction of gallbladder
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Image Credit: Visible Body

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Figure 17.29 Gallbladder with Gallstones
Southern Illinois University/Science Source
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Functions of Bile Salts
Aid digestive enzymes through emulsification:
•Break up of large fat globules into smaller droplets (like
soap or detergent)
•Aid in fat digestion by increasing surface area accessible
to the enzyme lipase
Enhance absorption of fatty acids and cholesterol by forming
micelles
Help absorb fat-soluble vitamins A, D, E, and K
Almost all bile salts are recycled
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17.8 Small Intestine
Small Intestine:
•Tubular organ that extends from the pyloric sphincter to the
beginning of the large intestine
•Fills most of abdominal cavity
•Receives chyme from stomach, and liver and pancreatic
secretions
•Completes digestion of the nutrients in chime
•Absorbs products of digestion
•Transports the remaining residue to the large intestine
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Figures 17.35 and 17.36 Villi of Small Intestine (Use the
Visible Body Software. Navigate to ‘Media.’ Under
‘Nutrition and Elimination’ select ‘Nutrient Absorption.’)
Al Telser/McGraw-Hill Education
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Figure 17.38 Plicae Circulares
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Secretions of the Small Intestine
Mucus:
•Secreted by goblet cells
•Also secreted by specialized Brunner’s glands, that secrete a thick,
alkaline mucus in response to certain stimuli
Watery fluid:
•Secreted by intestinal glands
•Picks up digestion products, and transports them into villi
•Does not contain digestive enzymes
Enzymes in the membranes of the microvilli:
•Peptidases: Break down peptides into amino acids
•Sucrase, maltase, lactase: Break down disaccharides into
monosaccharides
•Lipase: Breaks down fats into fatty acids and glycerol
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Absorption in the Small Intestine 2
Fat digestion and absorption:
•Emulsified by bile salts
•Digested mainly by enzymes from pancreas and small intestine
•Digested into glycerol and fatty acids
•Fatty acids and glycerol are absorbed by a process involving several
steps; absorbed into blood or lymphatic capillaries (lacteals)
Micelles and chylomicrons:
•Micelles: Loose complexes of fatty acids and bile salts, from which
fatty acids can migrate to microvilli and be absorbed
•Fatty acids + glycerol are resynthesized into triglycerides in smooth
ER
•New triglyceride clusters are encased in protein, forming
chylomicrons
•Chylomicrons are absorbed by lacteals
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Figure 17.42 Triglyceride Absorption
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17.9 Large Intestine
Large intestine:
•Named because diameter is greater than that of small
intestine
•About 1.5 m long
•At distal end, opens to outside of body through the anus
•Absorbs some water and electrolytes
•Reabsorbs and recycles water and digestive secretions
•Forms and stores feces
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Parts of the Large Intestine
Large intestine:
•Cecum:
•Pouch, forms beginning of large intestine
•Appendix is attached to cecum; lymph nodules in appendix function in the
immune response
•Colon:
•Ascending, transverse, descending and sigmoid portions
•Contains hepatic (right colic) and splenic (left colic) flexures
•Rectum:
•Extends from sigmoid colon to anal canal
•Lies next to sacrum
•Anal canal:
•Last 2.5 to 4 cm of large intestine; opens to outside as anus
•Internal and external anal sphincters guard anus
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Figure 17.43 Parts of the Large Intestine
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Figures 17.44 and 17.45 Large Intestine, Rectum, and
Anal Canal
BSIP/UIG/Getty Images
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Structure of the Large Intestinal Wall
•Wall has same 4 layers as other tubular organs of the
alimentary canal
•Does not have villi and plicae circulares
•Longitudinal muscle is organized into 3 bands, taeniae
coli, that run down entire length of colon
•Muscle bands create pouches called haustra, which help
form feces
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Functions of the Large Intestine
Large intestine:
•Has little or no digestive function
•Contains tubular glands containing goblet cells; secrete
mucus, the only significant secretion of the large intestine
•Absorbs water (about 90% of water that enters it) and
electrolytes
•Houses intestinal flora, bacteria which break down contents
such as cellulose, and produce vitamins K, B
12
, thiamine
•Forms feces, and carries out defecation
•Intestinal gas: Mixture of nitrogen, oxygen, methane, carbon
dioxide, hydrogen, ammonia, and hydrogen sulfide (causes
unpleasant odor)
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Movements of the Large Intestine
Movements of the large intestine:
•Similar to those of the small intestine
•Slower and less frequent than those of the small intestine
•Types of movements:
•Mixing movements
•Mass movements: Peristaltic waves, 2 to 3 times/day, which usually
follow meals
•Defecation reflex:
•Eliminates feces from body
•Involves holding deep breath, contracting abdominal muscles
•Feces move into rectum
•Peristaltic waves occur in descending colon
•Relaxes internal anal sphincter, and then external anal sphincter
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Feces
Feces are composed of materials not digested or absorbed, and also
contain:
•Water (~75% of feces)
•Electrolytes
•Mucus
•Bacteria
•Bile pigments, which provide the color, after bacterial alteration
The pungent odor is produced by bacterial compounds, including:
•Phenol
•Hydrogen sulfide
•Indole
•Skatole
•Ammonia
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17.10 Life-Span Changes
Changes to the digestive system are slow and slight, and eventually
include the following:
•Tooth enamel thins; teeth may become sensitive
•Gums may recede
•Teeth may loosen, break, or fall out
•GI tract becomes less efficient
•Slowing peristalsis may lead to heartburn
•Gastric secretion slows
•Constipation may become more frequent
•Nutrient absorption decreases, including fat-soluble vitamins
•Incidence of lactose intolerance increases
•Accessory organs age, but typically not necessarily in ways that affect
health
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