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Lecture 18The Digestive System
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Food for Energy and Growth
Food provides animals with energy and essential nutrients for growth
A healthy diet contains more carbohydrates than fatsIt also contains a lot of proteins
The pyramid of nutrition
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Who Is Overweight?
The body mass index is used to determine who’s overweight
BMI =body weight in kg
(height in m)2 (height in in)2
=(body weight in lbs) X 703
Obesity
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Many vertebrates are unable to manufacture one or more of the 20 amino acids needed to make proteins Humans are unable to synthesize 8 amino acids These essential amino acids must be obtained through food
In addition, all vertebrates cannot synthesize certain polyunsaturated fats
Trace elements Minerals required in small amounts Iodine, cobalt, zinc, molybdenum, manganese
Vitamins Essential organic substances required in small amounts Humans require at least 13 different vitamins
Vitamin C (ascorbic acid) if not in diet, the disease scurvy will develop
Essential Substances for Growth
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Evolution of the Digestive System
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Digestive System Overview
Consists of tubular gastrointestinal tract and accessory digestive organs Mouth and pharynx Esophagus – Delivers food to
the stomach Stomach – Some preliminary
digestion Small intestine – Digestion and
absorption Large intestine – Water and
mineral absorption Rectum – Waste excretion
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General Structure of Digestive Organs
The gastrointestinal tract has a characteristic layered structure
MucosaEpithelium
SubmucosaConnective tissue
MuscularisSmooth muscles
SerosaConnective tissue
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Breaking Down Food in the Mouth
Many vertebrates have teeth that are used for chewing (mastication)
Carnivores have pointed teeth adapted for cutting and shearing
Herbivores have large flat teeth well suited for grinding plant cellulose
Omnivores have carnivorous teeth in front and herbivorous teeth in the back
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The Structure of a Tooth
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Tooth and Gum Disease: Periodontitis
Dental caries – gradual demineralization of enamel and dentin by bacterial action Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth Acid produced by the bacteria in the plaque dissolves calcium salts Without these salts, organic matter is digested by proteolytic enzymes Daily flossing and brushing help prevent caries by removing forming plaque
Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartar
Accumulation of calculus: Disrupts the seal between the gingivae and the teeth Puts the gums at risk for infection
Periodontitis – serious gum disease resulting from an immune response Immune system attacks intruders as well as body tissues, carving pockets
around the teeth and dissolving bone
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Tongue
Superior surface bears three types of papillae Filiform – give the tongue roughness and provide friction Fungiform – scattered widely over the tongue and give it a reddish hue Circumvallate – V-shaped row in back of tongue
Sulcus terminalis – groove that separates the tongue into two areas: Anterior 2/3 residing in the oral cavity Posterior third residing in the oropharynx
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In The Mouth
The tongue mixes food with a solution called saliva Saliva moistens and
lubricates food and contains amylase which initiates breakdown of starch into maltose
The secretions of the salivary glands are controlled by the nervous system A continuous secretion of
about 0.5 milliliters per minute keeps the mouth moist
The presence of food in the mouth triggers an increased rate of secretion
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Prior to swallowing, the tongue moves food to the back of the mouth
The soft palate elevates, pushing against back wall of pharynx This stimulates neurons to send impulses to the swallowing center in
the brain Muscles contract and raise the larynx The glottis is pushed against the epiglottis which keeps food out of the
respiratory tract, and into the esophagus
Swallowing
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The esophagus is a muscular tube that connects the pharynx to the stomach
The swallowing center stimulates successive waves of contraction
Peristalsis moves food along the esophagus to the stomach
The Esophagus and Stomach
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Movement of food from esophagus into stomach is controlled by a ring of smooth muscle, the sphincter Humans lack a true
sphincter and thus, can regurgitate
The stomach is a saclike portion of the tract with a convoluted surface enabling expansion It contains an extra layer
of smooth muscles for mixing food with gastric juices
Two kinds of secretory cells
Parietal cells – Secrete hydrochloric acid
Chief cells – Secrete pepsinogen
The human stomach produces about 2 liters of HCl and other gastric secretions every day This gastric juice has a pH of ~ 2
It kills most bacteria ingested with food and also denatures food proteins
The mixture of partially digested food and gastric juice is termed chyme
Chyme leaves the stomach to the small intestine through the pyloric sphincter
The stomach and gastric glands
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The hormone gastrin regulates the synthesis of HCl
Overproduction of gastric acid can occasionally eat a hole through the stomach wall These gastric ulcers are rare
Susceptibility increases when mucosal barriers are weakened by Helicobacter pylori infection
Over 90% of gastrointestinal ulcers are duodenal ulcers Caused by too much chyme in the small intestine
Ulcers
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The small intestine is the body’s true digestive vat It breaks down large
molecules into smaller ones which are then absorbed into the bloodstream
The small intestine is ~ 6 m long The first 25 cm (~ 4%)
constitute the duodenum
The duodenum is the actual site of digestion The pancreas secretes
digestive enzymes into it The liver secretes bile salts
into it, to make fats easier to digest
The Small Intestine
The ileum is the rest of the small intestine (~ 96%) It is devoted to absorption
The lining is covered with finger-like projections called villi
Each cell covering a villus has cytoplasmic projections called microvilli which increase the absorptive surface
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Anatomy of the Small Intestine
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The Large Intestine
The large intestine, or colon is only 1 meter long But has a larger diameter than the small intestine
No digestion and little absorption take place in the large intestine Its primary function is to act as a refuse dump by collecting and compacting
solid wastes Feces pass through the rectum as a result of peristalsis and leave the body
through the anus
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Accessory Digestive Organs
Pancreas Functions as both an exocrine & endocrine
gland Exocrine: Cell clusters called acini secrete
Trypsin and chymotrypsin which digest proteins
Amylase which digests starch Lipase which digests fats Bicarbonate which neutralizes HCl in
chyme Endocrine: Cell clusters called Islets of
Langerhans secrete Insulin and glucagon which regulate sugar
levels in blood
Liver Largest internal organ of the body Its main exocrine secretion is bile
Aids in the digestion of fats in the duodenum
Chemically modifies substances absorbed in the gastrointestinal tract
Converts poisons into less toxic forms Produces most of the proteins found in plasma
Gall bladder Stores and concentrates bile Delivers it to the duodenum
via the bile duct
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Microscopic Anatomy of the Liver
Liver sinusoids – enlarged, leaky capillaries located between hepatic plates
Kupffer cells – hepatic macrophages found in liver sinusoids
Hepatocytes’ functions include: Production of bile Processing
bloodborne nutrients
Storage of fat-soluble vitamins
Detoxification
Secreted bile flows between hepatocytes toward the bile ducts in the portal triads
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Homeostasis
Homeostasis is the dynamic constancy of the internal environment Conditions fluctuate continuously within narrow limits
Homeostasis is essential for life
Play Hormones & Gastric Secretion
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Blood glucose levels are monitored by the Islets of Langerhans in the pancreas
Regulating Blood Glucose
When levels are high, insulin is released
When levels are low, glucagon is released