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Chapter 47
The Digestive
System
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
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Types of Digestive Systems
• Single-celled organisms and
sponges digest their food
intracellularly
• Other multicellular animals
digest their food extracellularly
– Within a digestive cavity
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Food
Wastes
Mouth
Tentacle
Body stalk
Gastrovascular
cavity
• Cnidarians and flatworms have
a gastrovascular cavity
– Only one opening, and no
specialized regions
3
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, pseudocoelomate
– More complex animals have a digestive tract specialized in
different regions
4
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
5
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
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Vertebrate Digestive Systems
• Accessory organs
– Liver
• Produces bile
– Gallbladder
• Stores and concentrates bile
– Pancreas
• Produces pancreatic juice
• Digestive enzymes and bicarbonate buffer
7
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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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
9
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Incisors
Premolars
Canines
Molars
Herbivore
Carnivore
Omnivore
Horse
Lion
Human
• 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
10
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
11
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
12
The Esophagus
• Muscular tube connecting the mouth to the stomach
• Actively moves a bolus through peristalsis
13
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)
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The Stomach
• Starts to break down proteins
• No significant digestion of carbohydrates or fats occurs
• Absorption of some water (aspirin and alcohol)
15
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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Accessory Organs
•
Pancreas
– Pancreatic fluid is secreted into the
duodenum through the pancreatic duct
– Enzymes to break down proteins, carbs
and fats
– Bicarbonate neutralizes acidic chyme
from stomach
– Exocrine and endocrine gland
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Accessory Organs
• Liver
– Body’s largest internal organ
– Secretes bile
• emulsifies fats
• Gallbladder
– Stores and concentrates bile
– Arrival of fatty food in the
duodenum triggers the
gallbladder to contract, causing
bile to be transported through
the common bile duct and
injected into the duodenum
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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
– Enter the lymphatic system and later join the
circulatory system
• Almost all fluid reabsorbed in small intestine
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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 and remaining electrolytes
• Prepare waste for expulsion
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The Large Intestine
• Many bacteria live and reproduce within
the large intestine. Provide humans with
Vitamin K
• Feces compacted and passed to rectum
• Feces exit anus
– Smooth muscle sphincter (involuntary)
– Striated muscle sphincter (voluntary)
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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
22
• Ruminants have a fourchambered stomach
– Contents can be
regurgitated and
rechewed
• Rumination
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reproduction or display.
Small intestine
Rumen
Esophagus
Reticulum
– Evolved only once
Abomasum Omasum
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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
http://www.petcaregt.com/images/Coprophagy-in-Rabbits.jpg
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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
25
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)
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Food Energy
• Ingestion of food serves two primary
functions
1. Source of energy
2. Source of raw material (to make things the
body needs)
• 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
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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
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