digestive system

Unit 15

Ingestion—taking in food
• Digestion—breaking food into nutrient molecules
• Absorption—movement of nutrients into the bloodstream
• Defecation—excretes to rid the body of indigestible waste

Anatomy of the Digestive System
• Two main groups of organs
• Alimentary canal (gastrointestinal, or GI, tract)—continuous, coiled,
hollow tube
• Organs ingest, digest, absorb, defecate
• Accessory digestive organs
• Include teeth, tongue, and several large digestive organs
• Assist digestion in various ways

Organs of the Alimentary Canal
• The alimentary canal is a continuous, coiled, hollow tube that
runs through the ventral cavity from stomach to anus
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus

Mouth (1 of 4)
• Anatomy of the mouth
• Mouth (oral cavity)—mucous membrane–lined cavity
• Lips (labia)—protect the anterior opening
• Cheeks—form the lateral walls
• Hard palate—forms the anterior roof
• Soft palate—forms the posterior roof
• Uvula—fleshy projection of the soft palate

Anatomy of the mouth
• Vestibule—space between lips externally and teeth and gums
internally
• Oral cavity proper—area contained by the teeth
• Tongue—attached at hyoid bone and styloid processes of the skull,
and by the lingual frenulum to the floor of the mouth
• Tonsils
• Palatine—pair of tonsils located at posterior end of oral cavity
• Lingual—single tonsil located at the base of the tongue

Recall that papillae containing taste buds, or taste receptors,
are found on the tongue surface (see Chapter 8, p. 293). So,
besides its food-manipulating function, the tongue allows us to
enjoy and appreciate the food we eat.

Serves as a passageway for foods, fluids, and air
• Food passes from the mouth posteriorly into the

Food is propelled to the esophagus by two skeletal muscle
layers in the pharynx
• Longitudinal outer layer
• Circular inner layer
• Alternating contractions of the muscle layers (peristalsis)
propel the food

Anatomy
• About 10 inches long
• Runs from pharynx to stomach through the diaphragm
• Physiology
• Conducts food by peristalsis (slow rhythmic squeezing) to the
stomach
• Passageway for food only (respiratory system branches off after the
pharynx)

• Summary of the four layers (tunics) from innermost to
outermost, from esophagus to the large intestine (detailed
next)
1. Mucosa Lines the cavity (known as the lumen)

2. Submucosa Just beneath the mucosa

1. Muscularis externa
   smooth muscle
   • Inner circular layer
   • Outer longitudinal layer
   4. Serosa  outermost layer of the wall; contains fluid-
   producing cells

Regulate mobility and secretory activity of the GI tract organs

Stomach (1 of 6)
• C-shaped organ located on the left side of the abdominal
cavity
• Food enters at the cardioesophageal sphincter from the
esophagus
• Food empties into the small intestine at the pyloric sphincter
(valve)

Regions
• Cardial (cardia)—near the heart and surrounds the
cardioesophageal sphincter
• Fundus—expanded portion lateral to the cardiac region

 Stomach can stretch and hold 4 Li(1 gallon) of food when full
• Rugae—internal folds of the mucosa present when the stomach is
empty
• Lesser omentum

Small Intestine (1 of 5)
• The body’s major digestive organ
• Longest portion of the alimentary tube (2–4 meter s, or 7–13 feet, in
a living person)
• Site of nutrient absorption into the blood
• Muscular tube extending from the pyloric sphincter to the
ileocecal valve
• Suspended from the posterior abdominal wall by the
mesentery

• Subdivisions
• Duodenum
• Jejunum
• Ileum
• Ileocecal valve

Chemical digestion begins in the small intestine
• Enzymes produced by intestinal cells and pancreas are carried to the
duodenum by pancreatic ducts
• Bile, formed by the liver, enters the duodenum via the bile duct
• Hepatopancreatic ampulla is the location where the main
pancreatic duct and bile ducts join

Large Intestine (1 of 5)
• Larger in diameter, but shorter in length at 1.5 meter s, than the
small intestine
• Extends from the ileocecal valve to the anus
• Eliminates feces
• Subdivisions (detailed next)
• Cecum
• Appendix
• Colon
• Rectum
• Anal canal

Cecum—saclike first part of the large intestine
• Appendi

 Sigmoid colon, rectum, and anal canal are located in the
pelvis

 Anal canal ends at the anus
• Anus—opening of the large intestine

• The large intestine delivers indigestible food residues to the
body’s exterior

Goblet cells produce alkaline mucus to lubricate the passage
of feces
• Muscularis externa layer is reduced to three bands of muscle,
called teniae coli
• These bands of muscle cause the wall to pucker into haustra
(pocketlike sacs

• Teeth • Teeth masticate (chew) food into smaller fragments
• Humans have two sets of teeth during a lifetime

• Teeth are classified according to shape and function
• Incisors—cutting
• Canines (eyeteeth)—tearing or piercing
• Premolars (bicuspids)—crushing and grinding
• Molars—crushing and grinding

• Two major regions of a tooth
1. Crown
2. Root

. Crown—exposed part of tooth above the gingiva (gum

• Salivary glands

• Three pairs of salivary glands empty secretions into the mouth
1. Parotid glands
• Found anterior to the ears
• Mumps affect these salivary glands
2. Submandibular glands
3. Sublingual glands

• Saliva
• Mixture of mucus and serous fluids
• Helps to moisten and bind food together into a mass called a bolus
• Pancreas
 Soft, pink triangular gland
• Found posterior to the parietal peritoneum
• Mostly retroperitoneal
• Extends across the abdomen from spleen to duodenum

• Liver Largest gland in the body
• Located on the right side of the body under the diaphragm
• Consists of four lobes suspended from the diaphragm and
abdominal wall by the falciform ligament

Digestive role is to produce bile
• Bile leaves the liver through the common hepatic duct and enters
duodenum through the bile duct

• Gallbladder Green sac found in a shallow fossa in the inferior surface of
the liver

Function

enzymatic reactions are called hydrolysis
reactions because a water molecule is added to each bond
to be broken (see Chapter 2, p. 38). Water is also necessary as
a dissolving medium and a softening agent for food digestion.

• Essential processes of the GI tract
5. Absorption
• End products of digestion are absorbed in the blood or lymph
• Food must enter mucosal cells and then move into blood or lymph capillaries
6. Defecation
• Elimination of indigestible substances from the GI tract in the form of feces

 Food ingestion and breakdown
• Food is placed into the mouth

• Essentially, no food absorption occurs in the mouth

 Food propulsion—swallowing and peristalsis
• Pharynx and esophagus have no digestive function
• Serve as passageways to the stomach
• Pharynx functions in swallowing (deglutition

 Food propulsion—swallowing and peristalsis
1. Buccal phase

2.Pharyngeal-esophageal phase

• Food breakdown
• Gastric juice is regulated by neural and hormonal factors
• Presence of food or rising pH causes the release of the hormone
gastrin
• Gastrin causes stomach glands to produce:
• Protein-digesting enzymes
• Mucus
• Hydrochloric acid

• Hydrochloric acid makes the stomach contents very acidic
• Acidic pH

• Food breakdown
• Protein-digestion enzymes
• Pepsin—an active protein-digesting enzyme
• Rennin—works on digesting milk protein in infants; not produced in adults
• Alcohol and aspirin are virtually the only items absorbed in the
stomach

Propulsion: waves of peristalsis occur from the fundus to the pylorus,
forcing food past the pyloric sphincter
2. Grinding: the pylorus meters out chyme into the small intestine (3 mil lilit ers
at a time)
3. Retropulsion: peristaltic waves close the pyloric sphincter, forcing
contents back into the stomach; the stomach empties in 4–6 hours

• Chyme breakdown and absorption

 Intestinal enzymes and enzyme-rich pancreatic juice help to
complete digestion of all nutrient groups

Alkaline content neutralizes acidic chyme and provides the proper
environment for the pancreatic enzymes to operate

• Release of pancreatic juice from the pancreas into the duodenum is
stimulated by:
• Vagus nerves
• Local hormones that travel via the blood to influence the release of pancreatic
juice (and bile)

 Hormones (secretin and CCK) also target the liver and gallbladder to
release bile

Water is absorbed along the length of the small intestine
• End products of digestion

• Substances are transported to the liver by the hepatic portal vein or
lymph

Nutrient breakdown and absorption
• No digestive enzymes are produced
• Resident bacteria (normal flora or microbiota) digest remaining
nutrients

Water, vitamins, ions, and remaining water are absorbed
• Remaining materials are eliminated via feces

Propulsion of food residue and defecation
• Sluggish peristalsis begins when food residue arrives
• Haustral contractions are the movements occurring most frequently in
the large intestine
• Mass movements are slow, powerful movements that occur three to
four times per day

Presence of feces in the rectum causes a defecation reflex

Microbiota (normal flora)
• Includes all bacteria, viruses, parasites and fungi living in or on the
human body
• Outnumber the cells of the human body
• Microbiome
• Intersection of the human body and its health, microbiota, all of their
genes, environment

Energy value of food is measured in kilocalories (kcal) or
Calories (C)

• Nutrient—substance used by the body for growth,
maintenance, and repair
• Major nutrients
• Carbohydrates
• Lipids
• Proteins
• Water
• Minor nutrients
• Vitamins
• Minerals

 Healthy Eating Pyramid
• Issued in 1992
• Six major food groups arranged horizontally
• MyPlate
• Issued in 2011 by the USDA
• Five food groups are arranged by a round plate

Vitamins
• Most vitamins function as coenzymes
• Found mainly in fruits and vegetables
• Certain vitamins (A, C, E) appear to be anticancer

Minerals
• Mainly important for enzyme activity
• Foods richest in minerals: vegetables, legumes, milk, and some meats

Metabolism
• Metabolism is all of the chemical reactions necessary to
maintain life
• Catabolism—substances are broken down to simpler substances;
energy is released and captured to make adenosine triphosphate (AT
P)
• Anabolism—larger molecules are built from smaller ones

Carbohydrate Metabolism

Carbohydrates are the body’s preferred source to produce
cellular energy (ATP)
• Glucose (blood sugar)
• Major breakdown product of carbohydrate digestion
• Fuel used to make ATP

 Cellular respiration
• As glucose is oxidized, carbon dioxide, water, and ATP are formed

• Events of three main metabolic pathways of cellular
respiration
1. Glycolysis

2.Citric acid cycle (Krebs cycle

3. Electron transport chain

Fat Metabolism (1 of 2)
• Fats
• Insulate the body
• Protect organs
• Build some cell structures (membranes and myelin sheaths)
• Provide reserve energy
• Excess dietary fat is stored in subcutaneous tissue and other
fat depots

• When carbohydrates are in limited supply, more fats are
oxidized to produce ATP

 Excessive fat breakdown causes blood to become acidic (acidosis or
ketoacidosis

• Proteins form the bulk of cell structure and most functional
molecules
• Proteins are carefully conserved by body cells
• Amino acids are actively taken up from blood by body cells

Amino acids are oxidized to form ATP mainly when other fuel
sources are not available
• Ammonia, released as amino acids are catabolized, is
detoxified by liver cells that combine it with carbon dioxide to
form urea

The Central Role of the Liver in Metabolism
(1 of 7)
• Liver is the body’s key metabolic organ
• Roles in digestion
• Manufactures bile
• Detoxifies drugs and alcohol
• Degrades hormones
• Produces cholesterol, blood proteins (albumin and clotting proteins)
• Plays a central role in metabolism
• Liver can regenerate if part of it is damaged or remove

To maintain homeostasis of blood glucose levels, the liver
performs:
• Glycogenesis—“glycogen formation”
• Glucose molecules are converted to glycogen and stored in the liver
• Glycogenolysis—“glycogen splitting”
• Glucose is released from the liver after conversion from glycogen
• Gluconeogenesis—“formation of new sugar”
• Glucose is produced from fats and proteins

 Fats and fatty acids are picked up by the live

• Blood proteins made by the liver are assembled from amino
acids
• Albumin is the most abundant protein in blood
• Clotting proteins
• Liver cells detoxify ammonia
• Ammonia is combined with carbon dioxide to form urea, which is
flushed from the body in urine

 Cholesterol metabolism and transport
• Cholesterol is not used to make ATP

Most cholesterol (85%) is produced in the liver; only 15% is from the
diet. They are transported by lipoproteins (lipid-protein complexes) known
as LDLs and HDLs

• Cholesterol metabolism and transport
• Low-density lipoproteins (LDLs) transport cholesterol to body cells
• Rated “bad lipoproteins” since they can lead to atherosclerosis
• High-density lipoproteins (HDLs) transport cholesterol from body cells
to the live

Body Energy Balance (1 of 9)
• Energy intake = Total energy output
(heat + work + energy storage)

Metabolic rate and body heat production
• Nutrients yield different amounts of energy
• Energy value is measured in kilocalories (kcal)
• Carbohydrates and proteins yield 4 kcal/gram
• Fats yield 9 kcal/gram

Basic metabolic rate (BMR)—amount of heat produced by
the body per unit of time at rest
• Average BMR is about 60 to 72 kcal/hour for an average 70-
kgr(154-lb) adult

Factors that influence BMR
• Surface area—a small body usually has a higher BMR
• Gender—males tend to have higher BMRs
• Age—children and adolescents have higher BMRs
• The amount of thyroxine produced is the most important control
factor

Total metabolic rate (TMR)—total amount of kilocalories the
body must consume to fuel ongoing activities
• TMR increases dramatically with an increase in muscle activity
• TMR must equal calories consumed to maintain homeostasis
and maintain a constant weight

Body temperature regulation
• When foods are oxidized, more than 60% of energy escapes as heat,
warming the body
• The body has a narrow range of homeostatic temperature
• Must remain between 35.6°C and 37.8°C
• (96°F and 100°F)
•

Developmental Aspects of the
Digestive System and Metabolism
The alimentary canal is a continuous, hollow tube present by
the fifth week of development
• Digestive glands bud from the mucosa of the alimentary tube
• The developing fetus receives all nutrients through the
placenta
• In newborns, feeding must be frequent, peristalsis is inefficient,
and vomiting is common

Newborn reflexes
• Rooting reflex helps the infant find the nipple
• Sucking reflex helps the infant hold on to the nipple and swallow
• Teething begins around age 6 months

• • Functions of the mouth
• Mastication (chewing) of food
• Tongue mixes masticated food with saliva
• Tongue initiates swallowing
• Taste buds on the tongue allow for taste

Functions of the mouth
• Mastication (chewing) of food
• Tongue mixes masticated food with saliva
• Tongue initiates swallowing
• Taste buds on the tongue allow for taste