Download The digestive system

THE
DIGESTIVE
SYSTEM
By: Erica Oates & Maschal Mohiuddin
What is the digestive system?
 The digestive system:
 ingests and breaks down food
 absorbs nutrients
 eliminates undigested material
21.1 Animals ingest their food in a
variety of ways
 Three dietary categories:
 Herbivores:
 Definition: an animal that eats only plants or algae
(mainly autotrophic organisms)
 Examples: cattle, gorillas, sea urchins, snails
 Carnivores:
 Definition: an animal that mainly eats other animals
 Examples: lions, hawks, spiders, whales
 Omnivores:
 Definition: an animal that ingests both plants and
animals
 Examples: humans, crows, cockroaches, raccoons
How do animals obtain and ingest
their food?
 Suspension feeders
 Extract food particles suspended in the surrounding air
 Examples: clams, oysters, humpback whales
 Substrate feeders
 Live in or on their food source and eat their way through it
 Example: caterpillars
 Fluid feeders
 Obtain their food by sucking nutrient-rich fluids from a living host
that is either a plant or an animal
 Examples: aphids, bloodsuckers
 Bulk feeders
 Ingest large pieces of food, may use utensils such as tentacles,
claws, teeth, pincers to tear off pieces of meat
 Example: grey herring
Quick Question:
 Blue whales, the largest animals ever to live,
feed on krill. Name their diet category and
type of feeding.
21.2 Overview: Food processing
occurs in four stages
 Ingestion
 The act of eating
 Digestion
 The breaking down of food into molecules small
enough for the body to absorb
 Mechanical
 Chemical
 Absorption
 The cells lining the digestive tract take up the
products of digestion (small molecules)
 Nutrients travel in the blood to body cells where
they are joined together to make macromolecules
of the cells or broken down further to provide
energy
 Elimination
 Undigested material passes out of the digestive
tract
Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
(enzymatic hydrolysis)
Nutrient
molecules
enter body
cells
Food
INGESTION
Undigested
material
DIGESTION
ABSORPTION
ELIMINATION
Quick Question:
 What are the two main digestive processes?
21.3 Digestion occurs in
specialized compartments
 Food vacuoles
 Simplest digestive compartments
 Food is engulfed by phagocytosis and the newly
formed food vacuole fuses with the lysosome,
containing hydrolytic enzymes
 Example: sponges
 Most animals have a more complex internal
compartment for digesting food
 Digestion occurs outside of cells so larger food can
be devoured than what would fit in a vacuole
 Gastrovascular cavity:
 Definition: a digestive compartment with a single
opening (mouth)
 Process:
 Gland cells lining the gastrovascular cavity secrete
digestive enzymes
 Digestive enzymes break down the soft tissues of
the prey
 Other cells engulf small food particles
 Food particles are broken down into food vacuoles
Tentacles
Mouth
Food
Epidermis
Mesoglea
Gastrodermis
Nutritive
muscular
cells
Flagella
Gland cells
Food vacuoles
Mesoglea
Gastrovascular
cavity
 Alimentary canal:
 Definition: a digestive tract with two openings, a
mouth and an anus
 Since food moves in one direction, specialized
regions of the tube can carry out digestion and
absorption of nutrients in sequence
Process
 Food entering the mouth usually passes into
a pharynx, or throat.
 Depending on the species, the esophagus
may channel food to a crop, gizzard, or
stomach.
 Crop: a pouch-like organ in which food is softened
and stored
 Stomachs and gizzards may also store food
temporarily, but they are more muscular and they
churn and grind the food.
 Intestine:
 Chemical digestion and nutrient absorption occur
here
 Anus:
 Undigested materials are expelled through here
Crop Gizzard
Intestine
Esophagus
Pharynx
Anus
Mouth
Typhlosole
Lumen of intestine
Earthworm
Foregut
Midgut
Esophagus
Hindgut
Rectum
Anus
Crop
Mouth
Grasshopper
Gastric ceca
Esophagus
Stomach
Gizzard
Intestine
Mouth
Crop
Anus
Bird
Quick Question:
 What is an advantage of an alimentary canal,
compared to a gastrovascular cavity?
21.4 The human digestive system consists
of an alimentary canal and accessory
glands
 Process:
 Food enters the mouth, is chewed in the oral
cavity, and pushed by the tongue into the pharynx
 Once food is swallowed, muscles propel it through
the alimentary canal by:
 Peristalsis - alternating waves of contraction and
relaxation of the smooth muscles lining the canal
 Enables us to process and digest food even while lying
down
 Sphincters
 Definition: muscular, ringlike valves that regulate
the passage of food into and out of the stomach
 The sphincter controlling the passage out of the
stomach works like a drawstring to close it off.
 Accessory Glands
 Salivary glands
 Pancreas
 Gall bladder
 Liver
Salivary
glands
Mouth
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestines
Large
intestines
Rectum
Anus
A schematic diagram of the
human digestive system
Quick Question:
 By what process does food move from the
pharynx to the stomach of an astronaut in the
weightless environment of a space station?
21.5 Digestion begins in the
oral cavity
 While you anticipate your food, your salivary
glands start delivering saliva through ducts to
the oral cavity, even before you take a bite.
 This is a response to the sight or smell of
food.
 The presence of food in your oral cavity
continues to stimulate salivation.
TEETH
Incisors
Canine
Premolar
Tongue
Salivary
Glands
Molars
Wisdom
Teeth
 Saliva:
 Contains several substances important in food
processing:
 Glycoprotein - protects the soft lining of the mouth
and lubricates food for easier swallowing
 Buffers – neutralize food acids, helping prevent
tooth decay
 Antibacterial agents – kill many of the bacteria that
enter the mouth with food
 Digestive enzyme amylase
 Mechanical and chemical digestion:
 Begin in the oral cavity
 Chewing makes it easier to swallow and exposes more
food surface to digestive enzymes
 Four kinds of teeth:
 Starting at the front:
 Two bladelike incisors – help with biting your food
 Canine teeth – help rip apart your food
 Two premolars and three molars – grind and crush your
food
 Tongue
 a muscular organ covered with taste buds
 manipulates food and helps shape it into a ball, called
a bolus
Quick Question:
 Chewing functions in what kind of digestion?
What does the amylase in saliva do?
21.6 After swallowing, peristalsis
moves food through the esophagus to
the stomach
 Our breathing and swallowing are usually carefully




coordinated
When you start to swallow the tongue pushes the
bolus of food into the pharynx, triggering the
swallowing reflex
The esophageal sphincter relaxes and allows the
bolus to enter the esophagus
At the same time, the larynx moves upward and tips
the epiglottis (a flap of cartilage and fibrous
connective tissue) down over the opening of the
larynx
In this position, the epiglottis prevents food from
passing into the trachea
Bolus
Bolus of
of food
food
Tongue
Tongue
Epiglottis
Epiglottis
up
up
Glottis
Glottis
down
down
and
and open
open
Pharynx
Pharynx
Epiglottis
Epiglottis
up
up
Glottis
Glottis
Larynx
Larynx
Esophageal
Esophageal
sphincter
sphincter
contracted
contracted
Epiglottis
Epiglottis
down
down
Esophagus
Esophagus
Glottis
Glottis up
up
and
and closed
closed
Trachea
Trachea
To
To lungs
lungs
Esophageal
Esophageal
sphincter
sphincter
relaxed
relaxed
Esophageal
Esophageal
sphincter
sphincter
contracted
contracted
Relaxed
Relaxed
muscles
muscles
To
To stomach
stomach
Contracted
Contracted
muscles
muscles
Relaxed
Relaxed
muscles
muscles
Stomach
Stomach
 The esophagus is a muscular tube that
conveys food boluses from the larynx to the
stomach.
 The structure of the esophagus fits its
function:
 A tough yet elastic connective tissues allow it to
stretch to accommodate a bolus
 Layers of circular and longitudinal smooth
muscles for peristalsis
 A stratified epithelial lining that replenishes cells
abraded off during swallowing
Quick Question:
 What prevents food from going down the
wrong tube?
21.7 The Heimlich maneuver
can save lives
 Procedure invented by Dr. Henry Heimlich in
the 1970s
 Allows people with little medical training to
step in and aid a choking victim
 Necessary when our swallowing mechanism
goes awry, like when someone eats too
quickly or fails to chew food thoroughly
 Choking happens when air is not allowed to
enter the trachea or pharynx because they
are blocked
Quick Question:
 If food is stuck in the pharynx, what effect
could it have on nearby structures?
21.8 The stomach stores food and
breaks it down with acid and enzymes
 Some chemical digestion occurs in the stomach
 The stomach secretes gastric juice which is made up of
mucus, enzymes and strong acid
 The interior surface of the stomach wall is highly folded and
dotted with pits leading into tubular gastric glands
 Gastric glands have three types of cells that secrete
different components of the gastric juice
 Mucous cells- secrete mucus which lubricates and protects the
cells lining the stomach
 Parietal cells- secrete hydrogen ions and chloride ions which
combine in the cavity of the stomach to form hydrochloric acid
 Chief cells- secrete pepsinogen, an inactive form of the enzyme
pepsin
How pepsinogen, hydrochloric
acid and pepsin interact:
 Pepsinogen and hydrochloric acid are
secreted into the cavity of the stomach
 The hydrochloric acid converts pepsinogen to
pepsin
 Pepsin then activates more pepsinogen
starting a chain reaction
 Pepsin begins the chemical digestion of
proteins
Esophagus
Cardiac orifice
Stomach
5 µm
Pyloric sphincter
Interior surface of stomach
Small
intestine
Folds of
epithelial
tissue
Epithelium
Pepsinogen
Gastric gland
Pepsin
(active enzyme)
HCl
Pepsinogen and HCl
are secreted into the
lumen of the stomach.
HCl converts
pepsinogen to pepsin.
Pepsin then activates
more pepsinogen,
starting a chain
reaction. Pepsin
begins the chemical
digestion of proteins.
Mucus cells
Chief cells
Parietal cells
Chief cell
Parietal cell
What prevents gastric juice from
digesting away the stomach lining?
 Secreting pepsin in the inactive form of
pepsinogen helps protect the cells of the
gastric glands
 Mucus helps protect the stomach lining from
both pepsin and acid
Other protections for the
stomach:
 Our gastric glands do not secrete acidic gastric
juice constantly; their activity is regulated by a
combination of nerve signals and hormones
 Once you have food in your stomach, substances
in the food stimulate cells in the stomach wall to
release the hormone gastrin into the circulatory
system
 Gastrin circulates in the blood stream, returning
to the stomach wall
 When it arrives there, it stimulates additional
secretion of gastric juice
 Negative-feedback mechanism:
 Inhibits the secretion of gastric juice when the
stomach contents become too acidic
 The acid inhibits the release of gastrin
 As a result of mixing and enzyme action,
nutrient-rich acidic broth known as chyme
develops
 The sphincter between the stomach and the
small intestine helps regulate the passage of
chyme from the stomach into the small
intenstine
Quick Question:
 If you add pepsinogen to a test tube
containing protein dissolved in distilled
water, not much protein will be digested.
What inorganic chemical could you add to the
tube to accelerate protein digestion? What
effect will it have?
21.9 Digestive ailments include
acid reflux and gastric ulcers
 A stomach full of digestive juice laced with strong acid:
 Breaks apart the cells in our food
 Kills bacteria
 Begins the digestion of proteins
 These chemicals can be harmful:
 Acid reflux –
 back flow of chyme into the lower end of the esophagus,
 causes the feeling called heartburn
 GERD
 Gastroesophageal reflux disease
 Harms the lining of the esophagus
 Treated with:
 Lifestyle changes
 Medications
 Surgery – strengthens the lower esophageal sphincter
Can all this acid also cause
problems in the stomach?
 A gel-like coat of mucus usually protects the
stomach wall from the corrosive effect of
digestive juice
 Gastric ulcers, or open sores, can develop in the
stomach wall
 Respond to a combination of antibiotics and bismuth
 Helicobacter pylori – a spiral shaped bacterium,
which is the primary culprit of gastric ulcers
 Defense by white blood cells causes gastritis
Quick Question:
 In contrast to most microbes, the species that
causes ulcers thrives in an environment with a
very low ________.
21.10 The small intestine is the major
organ of chemical digestion and nutrient
absorption
 The chemical digestion of large molecules
occurs in the small intestine.
 The nutrients that result from this digestion
are absorbed into the blood from the small
intestine.
 A length of more than 6 meters
Sources of digestive enzymes
and bile:
 Two large organs, the pancreas and the liver,
contribute to digestion in the small intestine
 Pancreas
 Produces pancreatic juice, a mixture of digestive
enzymes and an alkaline solution rich in bicarbonate
 The bicarbonate acts as a buffer to neutralize the acidity
of chyme as it enters the small intestine
 Liver
 Produces bile, which contains bile salts that emulsify fats,
making them more susceptible to attack by digestive
enzymes
 Gallbladder
 Stores bile until it is needed in the small intestine
 Intestinal wall itself produces digestive
enzymes, some of which are secreted into the
lumen (cavity) of the small intestine while
others are bound to the surface of epithelial
cells
 Duodenum
 First 25 cm of the small intestine
 Where chyme squirted from the stomach mixes
with :
 Bile from the gallbladder
 Pancreatic juice from the pancreas
 Digestive enzymes from gland cells in the intestinal
wall
Liver
Bile
Gallbladder
Stomach
Acid chyme
Intestinal
juice
Pancreas
Duodenum of
small intestine
Digestion in the small
intestine:
 All four types of large molecules are digested
in the small intestine
 Carbohydrates
 Digestion that began in the oral cavity is completed
in the small intestine
 Pancreatic amylase hydrolyzes starch into maltose
 Maltase then splits maltose into glucose
 Proteins
 Digestion originally begins in the stomach
 Pancreas and duodenum secrete hydrolytic
enzymes that completely dismantle the
polypeptides into amino acids
 Enzymes trypsin and chymotrypsin break the
polypeptides into smaller p0lypeptides
 Two other enzymes, aminopeptidase and
carboxypeptidase, split off one amino acid at a
time
 Dipeptidase hydrolyzes the fragments, only two
or three amino acids long
 Nucleic acids
 Nucleases from the pancreas split DNA and RNA into
their component nucleotides
 Fats
 Remain undigested until they reach the duodenum
 Hydrolysis is different because fats are insoluble in
water, which results in a special process:
 Bile salts and bile cause fat globules to be physically
broken up into smaller fat droplets, called emulsification
 Lipase – a pancreatic enzyme that breaks fat molecules
down into fatty acids and glycerol
 Main function of the rest of the small intestine is
the absorption of nutrients and water
Absorption in the small
intestine:
 Since the small intestine has a huge surface area, it is well-
suited for absorbing nutrients
 The surface area results from several kinds of folds and
projections, which are found around the inner wall of the
intestine, such as large circular folds with small, finger-like
projections called villi
 Each of the epithelial cells lining a villus has many tiny surface
projections called microvilli
 Microvilli extend into the lumen of the intestine and greatly
increase the surface area across which nutrients are absorbed
 Mechanisms of absorption:
 Simple diffusion
 Other nutrients are pumped against concentration gradients into
the epithelial cells
Vein carrying blood
to hepatic portal
vessel
Nutrient
absorption
Microvilli
(brush border)
Blood
capillaries
Epithelial
cells
Muscle layers
Epithelial cells
Large
circular
folds
Villi
Lacteal
Villi
Intestinal wall
Lymph
vessel
Quick Question:
 Amylase is to ________ as _________ is to
DNA.
21.11 One of the liver’s many functions
is processing nutrient-laden blood from
the intestines
 Location – between the intestines and the
heart
 Capillaries from the small and large intestines
converge into veins that lead into the hepatic
portal vein
 This large vessel transports nutrients absorbed by
the intestines directly to the liver
 Functions of the liver:
 Removes excess glucose from the blood and




converts it to glycogen, which is stored in liver
cells
Regulates body metabolism
Converts many of the nutrients it receives into
new substances
Modifies and detoxifies substances absorbed by
the digestive tract before the blood carries these
materials to the heart for distribution to the rest
of the body
Produces bile, which aids in the digestion of fats
and also processes nitrogen waste from the
breakdown of proteins for disposal in urine
Quick Question:
 What two functions of the liver relate to the
hepatic portal vein?
21.12 The large intestine reclaims
water and compacts the feces
 The large intestine, or colon, is about 1.5
meters long and 5 cm in diameter
 Joins the small intestine at a T-shaped junction,
where a sphincter controls the passage of
unabsorbed food material out of the small
intestine
 Cecum - one arm of the T, a blind pouch which
absorbs water and salts from undigested food
 Appendix – a small, fingerlike extension of the
cecum that contains a mass of white blood cells
that make a minor contribution to immunity
Functions of the large
intestine:
 One major function is to absorb water from the
alimentary canal
 As water is absorbed, the remains of the digested food
become more solid as they move along the colon by
peristalsis
 Feces - waste products that consist mainly of indigestible plant
fibers and prokaryotes that normally live in the colon
 Stored in the final portion of the colon, the rectum, until they
can be eliminated
 If the lining of the colon is irritated, the colon is less effective in
reclaiming water and diarrhea may result
 Constipation occurs when peristalsis moves the feces along to
slowly and they become to compacted
 Results from a diet that does not include enough plant fiber
Quick Question:
 Explain why treatment with antibiotics for an
extended period may cause a vitamin K
deficiency.
21.13 Evolutionary adaptations of
vertebrate digestive systems often relate
to diet
 Large expandable stomachs are common
adaptations in carnivores
 The length of an animal’s digestive tract is
often correlated with diet
 Herbivores and omnivores have longer alimentary
canals, relative to their body size, than carnivores
 Herbivores also have cellulose-digesting microbes
housed in special chambers
 Ruminants such as cows process food with
the aid of microbes in four stomach chambers
Small intestine
Stomach
Small
intestine
Cecum
Colon
(large
intestine)
Carnivore
Herbivore
Quick Question:
 What are two advantages of a longer
alimentary canal in herbivores?