Transcript Document
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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