Fueling Body Activities: Digestion
Chapter 43
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Outline
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Types of Digestive Systems
– Vertebrate Digestive Systems
 The Mouth and Teeth
 Esophagus and Stomach
 The Small Intestine
 The Large Intestine
 Accessory Organs
Neural and Hormonal Regulation of Digestion
Food Energy and Energy Expenditure
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Types of Digestive Systems
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Single-celled organisms digest their food
intracellularly.
Other animals digest their food
extracellularly within a digestive cavity.
– digestive enzymes released into a cavity
 Specialization occurs when the
digestive tract, or alimentary canal, has
a separate mouth and anus.
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Types of Digestive Systems
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Ingested food may be stored or first
subjected to physical fragmentation.
– Chemical digestion occurs next.
 Hydrolysis reactions liberate subunit
molecules.
 Products pass through the epithelial
lining of the gut into the blood
(absorption).
 Waste products are excreted.
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Vertebrate Digestive Systems
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Consists of tubular gastrointestinal tract and
accessory digestive organs.
– mouth and pharynx
– esophagus - delivers food to stomach
– stomach - preliminary digestion
– small intestine - absorption
– large intestine - water absorption
– cloaca or rectum - waste storage
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Human Digestive System
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Vertebrate Digestive Systems
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Accessory digestive organs include:
– liver
 produces bile
– gallbladder
 stores and concentrates bile
– pancreas
 produces pancreatic juice
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Vertebrate Digestive Systems
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Tubular gastrointestinal tract has a
characteristic layered structure.
– mucosa - epithelium
– submucosa - connective tissue
– muscularis - double layer of smooth muscle
– serosa - connective tissue
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Gastrointestinal Tract Layers
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The Mouth and Teeth
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Vertebrate teeth
– Carnivorous mammals have pointed teeth
that lack flat grinding surfaces.
– Herbivores must pulverize cellulose of cell
walls of plant tissue before digestion.
 have large, flat teeth suited to grinding
– Humans are essentially carnivores in the
front, and herbivores in the back.
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Generalized Vertebrate Dentition
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The Mouth and Teeth
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Mouth
– The tongue mixes food with saliva.
 moistens and lubricates food
 secretions controlled by nervous system
 Taste-sensitive neurons in the mouth
send impulses to the brain, which
responds by stimulating the salivary
glands.
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The Mouth and Teeth
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When food is ready to be swallowed, the
tongue moves it to the back of the mouth.
– elevated by soft palate
– pressure against pharynx triggers an
automatic, involuntary reflex
 larynx contracted and raised
 glottis pushed against epiglottis
 keeps food out of respiratory tract
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Human Pharynx, Palate, and Larynx
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Esophagus and Stomach
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Structure and function of the esophagus
– Swallowing center stimulates successive
waves of contraction that moves food
along esophagus to stomach.
 controlled by ring of smooth muscle
(sphincter)
Structure and function of the stomach
– Surface is highly convoluted, enabling it to
fold when empty and expand as it fills with
food.
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Esophagus and Stomach
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Secretory systems
– Exocrine glands contain two cell types:
 parietal cells - secrete hydrochloric acid
 chief cells - secrete pepsinogen
Action of acid
– Human stomach produces about 2 liters of
HCl and other gastric juices everyday.
 helps denature food proteins
 chyme
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Stomach and Duodenum
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Esophagus and Stomach
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Ulcers
– Gastric ulcers are rare because epithelial
cells in the mucosa are protected by a layer
of alkaline mucus.
 Susceptibility increased when mucosal
barriers are weakened by Helicobacter
pylori.
– Chyme leaves the stomach through the
pyloric sphincter.
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The Small Intestine
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Digestion
– approximately 4.5 m long, and divided into
duodenum, jejunum and ileum
– epithelial wall covered with villi
 covered by microvilli
 greatly increase surface area
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Small Intestine
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Accessory Organs
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Secretions of the pancreas
– Pancreatic fluid is secreted into duodenum
through the pancreatic duct.
 host of enzymes: trypsin, chymotrypsin,
pancreatic amylase, and lipase
 Digest proteins into smaller
polypeptides, polysaccharides into
shorter sugar chains, and fat into free
fatty acids.
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Pancreas
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Pancreas also functions as endocrine gland,
secreting hormones to control blood glucose.
– produced in islets of Langerhans
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Accessory Organs
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Liver and gallbladder
– Liver is largest internal organ of the body.
 Main secretion is bile, a fluid mixture of
bile pigments and bile salts delivered
into the duodenum during digestion.
 Bile pigments are waste products.
 Bile salts act as detergents.
 emulsification of fat
 stored in gallbladder
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The Small Intestine
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Absorption
– Glucose and amino acids enter the
bloodstream via the hepatic portal vein.
– Fat enters the lymphatic system.
– Approximately 9 liters of fluid passes
through the small intestine daily.
 Only about 50 g of solids and 100 ml of
liquid leave the body as feces.
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The Large Intestine
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Small intestine empties directly into the large
intestine at a junction where two vestigial
structures, cecum and appendix, remain.
– no digestion takes place, and only about
4% of absorption occurs there
 undigested material, primarily bacterial
fragments and cellulose, compacted and
stored
 compacted feces driven by peristaltic
contractions pass into rectum
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Variations in Vertebrate Digestive Systems
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Digestive tracts of some animals contain
bacteria and protists that convert cellulose
into substances the host can digest.
Ruminants have large, divided stomachs.
– rumen and reticulum
– omassum and abomasum
 rumination
– Rodents and lagomorphs practice
coprophagy.
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Four-Chambered Ruminant Stomach
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Variations in Vertebrate Digestive Systems
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All mammals rely on intestinal bacteria to
synthesize vitamin K.
– necessary for blood clotting
 prolonged treatment with antibiotics
greatly reduces bacterial populations in
the body
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Neural and Hormonal Regulation of Digestion
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Gastrointestinal activities are coordinated by
the nervous system and endocrine system.
– Stomach secretions are regulated by food
and gastrin.
– The passage of chyme into the duodenum
inhibits stomach contractions.
– Duodenum secretes other hormones that
inhibit stomach emptying and promote bile
release and bicarbonate secretion.
 enterogastrones
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Hormonal Control of Gastrointestinal Tract
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Accessory Organs
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Liver regulatory functions
– Liver chemically modifies substances
absorbed in the gastrointestinal tract
before they reach the rest of the body.
 also removes toxins and poisons, and
converts them into less toxic forms
– Liver regulates many compounds such as
steroid hormones, and produces most
proteins found in blood plasma.
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Accessory Organs
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Regulation of blood glucose concentration
– After a carbohydrate-rich meal, the liver and
skeletal muscles remove excess glucose
from blood and store it as glycogen.
 stimulated by insulin
 When glucose levels decrease, the
liver secretes glucose in the blood.
 breakdown of glycogen
 gluconeogenesis - process of
converting other molecules into
glucose
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Actions of Insulin and Glucagon
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Food Energy and Energy Expenditure
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Ingestion of food serves two primary
functions:
– provides source of energy
– provides raw materials that cannot be
manufactured by the organism
 Basal metabolic rate (BMR) refers to the
minimum rate of energy consumption
under defined resting conditions.
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Food Energy and Energy Expenditure
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If the amount of food energy taken in is
greater than the energy consumed per day,
the excess energy will be stored in glycogen
and fat.
– As glycogen reserves are limited,
ingestion of excess food energy results
primarily in the accumulation of fat.
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Food Energy and Energy Expenditure
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Regulation of food intake
– Recent human studies show activity of ob
gene and blood concentrations of leptin
(satiety factor) are higher in obese people
than in lean people.
 Leptin produced by obese people
appears to be normal.
 Most cases of human obesity may
result from reduced sensitivity to
action of leptin in the brain.
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Essential Nutrients
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Essential nutrients are substances an animal
cannot manufacture for itself but which are
necessary for health must be obtained in the
diet.
– essential amino acids
– unsaturated fatty acids
Essential minerals
– trace elements
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Summary
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Types of Digestive Systems
– Vertebrate Digestive Systems
 The Mouth and Teeth
 Esophagus and Stomach
 The Small Intestine
 The Large Intestine
 Accessory Organs
Neural and Hormonal Regulation of Digestion
Food Energy and Energy Expenditure
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