CHAPTER 48
LECTURE
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The Digestive System
Chapter 48
Types of Digestive Systems
• Heterotrophs are divided into three groups
based on their food sources
1. Herbivores are animals that eat plants
exclusively
2. Carnivores are animals that eat other
animals
3. Omnivores are animals that eat both plants
and other animals
3
Types of Digestive Systems
• Single-celled organisms and
sponges digest their food
intracellularly
• Other multicellular animals
digest their food extracellularly
– Within a digestive cavity
• Cnidarians and flatworms have
a gastrovascular cavity
– Only one opening, and no
specialized regions
4
Types of Digestive Systems
• Specialization occurs when the digestive
tract has a separate mouth and anus
– Nematodes have the most primitive digestive
tract
• Tubular gut lined by an epithelial membrane
– More complex animals have a digestive tract
specialized in different regions
5
Types of Digestive Systems
6
Types of Digestive Systems
• Ingested food may be stored or first
subjected to physical fragmentation
• Chemical digestion occurs next
– Hydrolysis reactions liberate the subunit
molecules
• Products pass through gut’s epithelial
lining into the blood (absorption)
• Wastes are excreted from the anus
7
Vertebrate Digestive Systems
• Consists of a tubular gastrointestinal tract and
accessory organs
• Mouth and pharynx – entry
• Esophagus – delivers food to stomach
• Stomach – preliminary digestion
• Small intestine – digestion and absorption
• Large intestine – absorption of water and
minerals
• Cloaca or rectum – expel waste
8
9
Vertebrate Digestive Systems
• Accessory organs
– Liver
• Produces bile
– Gallbladder
• Stores and concentrates bile
– Pancreas
• Produces pancreatic juice
• Digestive enzymes and bicarbonate buffer
10
Vertebrate Digestive Systems
• Gastrointestinal tract is layered
– Mucosa – innermost
• Epithelium that lines the interior, or lumen, of the
tract
– Submucosa
• Connective tissue
– Muscularis
• Circular and longitudinal smooth muscle layers
– Serosa – outermost
• Epithelium covering external surface of tract
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12
Mouth and Teeth
• Many vertebrates have teeth
used for chewing or
mastication
• Birds
– Lack teeth
– Break up food in a twochambered stomach
– Gizzard – muscular chamber
that uses ingested pebbles to
pulverize food
13
• Carnivores – pointed teeth that lack flat grinding
surfaces
• Herbivores – large flat teeth suited for grinding
cellulose cell walls of plant tissues
• Humans have carnivore-like teeth in the front
and herbivore-like teeth in the back
14
Mouth and Teeth
• Inside the mouth, the tongue mixes food
with saliva
– Moistens and lubricates the food
– Contains salivary amylase, which initiates the
breakdown of starch
– Salivation is controlled by the nervous system
• Tasting, smelling, and even thinking or talking
about food stimulate increased salivation
15
Mouth and Teeth
• Swallowing
– Starts as voluntary action
• Continued under involuntary control
– When food is ready to be swallowed, the
tongue moves it to the back of the mouth
– Soft palate seals off nasal cavity
– Elevation of the larynx (voice box) pushes the
glottis against the epiglottis
• Keeps food out of respiratory tract
16
Mouth and Teeth
17
The Esophagus
• Muscular tube connecting the esophagus
to the stomach
• Actively moves a bolus through peristalsis
• Swallowing center in brain stimulates
successive one-directional waves of
contraction
• Sphincter opens to allow food to enter
stomach
– Humans lack a true sphincter here
18
19
The Stomach
• Saclike portion of tract
• Convoluted surface allows expansion
• Contains 3rd layer of smooth muscles for mixing food
with gastric juice
• 3 kinds of secretory cells
– Mucus-secreting cells
– Parietal cells
• Secrete HCl and intrinsic factor (for vitamin B12
absorption)
– Chief cells
• Secrete pepsinogen (inactive form of pepsin)
20
The Stomach
21
The Stomach (Cont.)
22
The Stomach
• Low pH in the stomach helps denature food proteins
– Activates pepsin and keeps it functioning
• No significant digestion of carbohydrates or fats occurs
• Absorption of some water (aspirin and alcohol)
• Mixture of partially digested food and gastric juice is
called chyme
• Peptic ulcer – commonly caused by bacteria
• Leaves the stomach through the pyloric sphincter to
enter the small intestine
23
The Small Intestine
• About 4.5 m long – small diameter
• Consists of duodenum, jejunum, and ileum
• Receives
– Chyme from stomach
– Digestive enzymes and bicarbonate from
pancreas
– Bile from liver and gallbladder
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• Epithelial wall is covered with villi
– Villi are covered by microvilli
– Greatly increase surface area
• Microvilli participate in digestion and absorption
– Brush border enzymes
• Many adults lack the enzyme lactase
– Have lactose intolerance
25
Accessory Organs
• Pancreas
– Pancreatic fluid is secreted into the duodenum
through the pancreatic duct
– Enzymes
• Trypsin and chymotrypsin – proteins into smaller
polypeptides
• Pancreatic amylase – polysaccharides into shorter
sugars
• Lipase – fats into free fatty acids and
monoglycerides
– Bicarbonate neutralizes acidic chyme
26
– Exocrine and endocrine gland
Accessory Organs
• Liver
– Body’s largest internal organ
– Secretes bile
• Bile pigments (waste products) and bile salts (for
emulsification of fats)
• Gallbladder
– Stores and concentrates bile
– Arrival of fatty food in the duodenum triggers a neural
and endocrine reflex that stimulates the gallbladder to
contract, causing bile to be transported through the
common bile duct and injected into the duodenum
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28
Absorption
• Amino acids and monosaccharides are
transported through epithelial cells to blood
– Blood carries these products to the liver via the
hepatic portal vein
• Fatty acids and monoglycerides diffuse into
epithelial cells
– Reassembled into triglycerides and then chylomicrons
– Enter the lymphatic system and later join the
circulatory system
• Almost all fluid reabsorbed in small intestine
29
Absorption
30
The Large Intestine (colon)
• Much shorter than small intestine, but has larger
diameter
• Small intestine empties directly into the large
intestine at a junction where two vestigial
structures, cecum and appendix, remain
• No digestion occurs
• Function to reabsorb water, remaining
electrolytes, and vitamin K
• Prepare waste for expulsion
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32
The Large Intestine
• Many bacteria live and reproduce within
the large intestine
• Feces compacted and passed to rectum
• Feces exit anus
– Smooth muscle sphincter (involuntary)
– Striated muscle sphincter (voluntary)
33
Variations in Digestive Systems
• Digestive tracts of some animals contain
bacteria and protists that convert cellulose
into substances the host can absorb
– Minor in humans
– Essential to some animals
• Herbivores have longer digestive tracts
– Greater time for digestion of cellulose
– Modifications to enhance digestion of plant
material
34
35
• Ruminants have a fourchambered stomach
– Rumen, reticulum,
omasum
– True stomach –
abomasum
– Rumen has cellulosedegrading microbes
– Contents can be
regurgitated and
rechewed
• Rumination
– Evolved only once
36
Variations in Digestive Systems
• Foregut
fermentation
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Cow
Horse
Rat
Human
Baboon
Langur
– Convergent
evolution
– Modified lysozyme to
take on new role of
digesting bacteria in
stomach
– Same 5 amino acids
changed
37
Variations in Digestive Systems
• Rodents, horses, deer, and rabbits digest
cellulose in the cecum
– Regurgitation of contents is not possible
• However, some such animals practice
coprophagy
– Eat their feces to absorb nutrients on the
second passage of food
– Cannot remain healthy if prevented from
eating feces
38
Variations in Digestive Systems
• All mammals rely on intestinal bacteria to
synthesize vitamin K, which is required for
blood clotting
• Birds, which lack these bacteria, must
consume the required quantities of vitamin
K in their diet
39
Regulation of the Digestive Tract
• Gastrointestinal activities are coordinated
by the nervous and endocrine systems
• Nervous system stimulates salivary and
gastric secretions in response to sight,
smell, and consumption of food
• In the stomach, proteins stimulate the
release of gastrin
– Triggers the secretion of HCl and pepsinogen
from the gastric glands
40
Regulation of the Digestive Tract
• Enterogastrones or duodenal hormones
– Inhibit stomach contractions and prevent
additional chyme from entering duodenum
– Cholecystokinin (CCK), secretin, and gastric
inhibitory peptide (GIP)
• All inhibit gastric motility and secretions
– CCK also stimulates gallbladder contraction
and pancreatic enzyme secretion
– Secretin also stimulates the secretion of
pancreatic bicarbonate
41
42
Accessory Organ Function
• Liver
– Chemically modifies the substances absorbed
from the digestive tract before they reach the
rest of the body
– Ingested alcohol and other drugs are taken
into liver cells and metabolized
– Removes toxins, pesticides, and carcinogens,
converting them to less toxic forms
– Regulates levels of steroid hormones
– Produces most proteins found in plasma
43
Accessory Organ Function
• Regulation of blood glucose
– After a carbohydrate-rich meal
• Insulin stimulates removal of excess blood glucose
by liver and skeletal muscles (glycogen)
– When blood glucose levels decrease
• Glycogenolysis – glucagon stimulates liver to
break down glycogen to release glucose into blood
• Gluconeogenesis – liver converts other molecules
into glucose if fasting continues
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46
Food Energy
• Ingestion of food serves two primary
functions
1. Source of energy
2. Source of raw material
• Basal metabolic rate (BMR)
– Minimal amount of energy consumed under
defined resting conditions
• Continued ingestion of excess food energy
results primarily in accumulation of fat
47
Regulation of Food Intake
• Control mechanism links food intake to
energy balance
– Leptin – peptide hormone
•
•
•
•
•
Key to appetite control
Produced by adipose tissue
Leptin receptor located in hypothalamus
Reduced leptin signals brain to intake food
Research on leptin in humans ongoing
48
Regulation of Food Intake
• Other hormones involved in the control of
feeding and energy include
– Insulin, GIP, and CCK, which signal satiety
– Ghrelin which stimulates food intake
– Efferent control of feeding
• Neuropeptide Y (NPY) induces feeding activity
• Melanocyte-stimulating hormone (α-MSH) which
suppresses it
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Essential Nutrients
• Animal cannot manufacture these for itself but are
necessary for health and so must be obtained in the diet
• Vitamins
– Humans, apes, monkeys, and guinea pigs have lost
the ability to synthesize ascorbic acid (vitamin C)
• Amino acids – humans require 9
• Long-chain unsaturated fatty acids
– Vertebrates can synthesize cholesterol, a key
component of steroid hormones, but some
carnivorous insects cannot
• Minerals
51