Digestive System1

DIGESTIVE SYSTEM

Two Types of Systems  Incomplete digestive

system  One-way, saclike digestive cavity  Complete digestive system  A tube with an opening at each end

The Digestive Tract  The human digestive tract is a tube with

specialized regions and organs between the mouth and the anus.  Food is ingested, mechanically processed, and chemically digested to small molecules that are absorbed; indigestible remains are eliminated.  Parts of the digestive tract produce digestive enzymes.

Digestive system

Digestive System Tasks     

Break up, mix, and move food material Secrete enzymes into tube where digestion occurs Digest (breaks down) food particles into smaller molecules Absorb nutrients and fluids Eliminate wastes and residues

Major Components    

Mouth (oral cavity) Pharynx (throat) Esophagus Gut  Stomach  Small intestine  Large intestine  Rectum  Anus

Three Accessory Organs  The pancreas, liver, and gallbladder are

accessory organs of digestion; their secretions assist digestion.  Accessory organs are not part of the digestive tube but produce enzymes and other substances that assist digestion.  These three accessory organs send secretions to the duodenum via ducts.

Accessory Organs 

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Pancreas  Secretes digestive enzymes Liver  Secretes bile Gallbladder  Stores and concentrates bile Salivary glands  Secrete saliva

The Pancreas  The pancreas secretes: • enzymes that break down all major food molecules • sodium bicarbonate (NaHCO3) to neutralize acid in chyme. • Insulin which is a hormonal control over glucose metabolism.

The Liver  The liver produces bile, which is stored in

the gallbladder.  Bile emulsifies fats; it is a yellowish-green substance containing bilirubin from hemoglobin breakdown and bile salts derived from cholesterol.  The liver acts as gatekeeper to the blood and receives blood from the small intestine by way of the hepatic portal vein.

 The functions of the liver are many:      

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detoxifies blood, stores iron and vitamins, makes plasma proteins, stores glucose as glycogen, produces urea from amino acids, removes bilirubin after dismantling blood cells, and regulates blood cholesterol level when producing bile salts.

The Gallbladder  The gallbladder is a pear-shaped

muscular organ that stores bile until it is sent to the duodenum.  Water is reabsorbed in the gallbladder making the bile thick and mucus-like.  Bile enters the duodenum via the common bile duct. ---------------------------------------------------------- Gallstones are crystals of cholesterol.

Conditions for Digestion  For digestion to occur:    

the correct enzyme, optimum pH, optimum temperature, and the correct substrate must be present.

 Exact conditions can be determined during

laboratory experiments.  Most digestive enzymes, aside from pepsin, require a basic pH.

So how exactly does our digestive system work?

Human Digestive System  A complete system

with many

specialized organs  About 6.5 to 8 meters long if extended  Lined with mucus-secreting epithelium  Movement is one way, from mouth to anus

Adult mouth

The Teeth  Twenty deciduous (baby) teeth are

replaced by 32 adult teeth.  Each tooth has a crown and a root.  The crown has a layer of enamel, dentin, and an inner pulp with nerves and blood vessels that extend into the root.  The tongue mixes the chewed food with saliva and then forms the mixture into a mass called a bolus in preparation for swallowing.

Saliva  Produced by salivary glands at back of

mouth and under tongue  Saliva includes  Salivary amylase (enzyme)  Bicarbonate (buffer)  Mucins (bind food into bolus)  Water

Swallowing  Complex reflex  Tongue forces food into pharynx  Epiglottis and vocal cords close off

trachea; breathing temporarily ceases  Food moves into esophagus, then through esophageal sphincter into stomach

Swallowing

The Pharynx  The air passage and food passage cross

in the pharynx because the trachea is ventral to the esophagus.  Swallowing occurs in the pharynx and is a reflex action.  During swallowing, the air passage is usually blocked off by the soft palate and uvula, and the trachea moves under the epiglottis to cover the glottis opening to the windpipe.

The Esophagus  The esophagus is a muscular tube that

conducts food through the thoracic cavity and diaphragm into the stomach.  Peristalsis begins in the esophagus; this collapsed tube moves the bolus of food downward after swallowing occurs.  Heartburn is a burning pain when acidic stomach contents enter the esophagus.

 No chemical digestion occurs in the

esophagus.  The entrance of the esophagus to the stomach is marked by a constriction, called a sphincter; the sphincter must relax in order for food to enter the stomach.  The sphincter prevents food from backing up into the esophagus.

The Stomach  The stomach expands to store food.

Food in the stomach is churned, mixing the food with gastric juices containing hydrochloric acid and pepsin for the digestion of protein to peptides.  Alcohol, but not food, is absorbed here.  In 2–6 hours, the soupy chyme leaves the stomach.  Ulcers are usually caused by a bacterial infection. 

Structure of the Stomach    

J-shaped organ lies below the diaphragm Sphincters at both ends Outer serosa covers smooth muscle layers Inner layer of glandular epithelium faces lumen

sphincters muscle

mucosa

serosa

Stomach Secretions  Secreted into lumen (gastric fluid)

Hydrochloric acid (HCl)  Mucus (protective)  Pepsinogen (inactive form of a proteindigesting enzyme) 

 Stomach cells also secrete the hormone

gastrin into the bloodstream

Small Intestine  Three regions   

Duodenum Jejunum Ilium

 Receives chyme

from stomach  Receives secretions from liver, gallbladder, and pancreas

The Small Intestine  The small intestine, averaging about 6

meters in length ( ~20 feet), is small in diameter.  The first 25 cm is the duodenum that receives bile from the gallbladder and pancreatic juice containing pancreatic lipase and trypsin for digestion of protein to peptides, as well as lipase for digestion of fat to glycerol and fatty acids.  Pancreatic juice contains NaHCO3 (sodium bicarb.) that is basic and neutralizes the acidic chyme.

 Enzymes that finish the process of

digestion are produced by the intestinal wall.  Walls of the small intestine have finger-like projections called villi where nutrient molecules are absorbed into the cardiovascular and lymphatic systems.  Villi have microvilli that increase the surface area available for absorption.

Walls of Small Intestine 

Projections into the intestinal lumen increase the surface area available for absorption

one villus

Anatomy of the small intestine

Absorption of Nutrients  Passage of molecules into internal

environment  Occurs mainly in jejunum and ileum of

small intestine  Segmentation mixes the lumen contents

against wall and enhances absorption

Junction of the small intestine and the large intestine

Large Intestine (Colon)  Concentrates and stores feces  Sodium ions are actively

transported out of lumen and water follows  Lining secretes mucus and bicarbonate

The Large Intestine  The large intestine consists of the cecum,

colon, rectum and anal canal.  The large intestine does not produce digestive enzymes but does absorb water, salts, and some vitamins.  The colon includes the ascending colon, the transverse colon, the descending colon, and the sigmoid colon.

 The appendix is an extension of the

cecum.  Indigestible material is stored in the rectum until the anus allows defecation.  Anaerobic bacteria in the feces break down indigestible material and produce some vitamins.

Intestinal Problems a.k.a. (“ A pain in the butt”)

 Polyps are small growths arising from the

epithelial lining that may be benign or cancerous.  Diarrhea and constipation are two common complaints of the large intestine. 

Causes of diarrhea include infection of the lower tract and nervous stimulation, both moving feces more rapidly than normal, but also causing dehydration if prolonged.

 Water and fiber in the diet can prevent

constipation where the feces become too dry and hard.  Hemorrhoids are enlarged and inflamed blood vessels at the anus; this condition is associated with chronic constipation.  Regular elimination reduces the time the colon wall is exposed to cancer-promoting agents in the feces and may help prevent cancer.

Dietary Concerns

Obesity 

Excess of fat in adipose tissues

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Levels have been rising since the 1980s

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1 in 4 Americans is now obese

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Possible reasons for the rise  

Increased portion sizes Increased intake of refined carbohydrates (may also be implicated in rising rates of diabetes)

Food Pyramid added fats and simple sugars

milk, yogurt, cheese group

fruit group

bread, cereal, rice, pasta group

legume, nut, poultry, fish, meat group

vegetable group

Dietary Essentials  Vitamins

Essential organic substances  Minerals  Essential inorganic substances 

Vitamins Fat soluble  Excess accumulates in tissue  Vitamins A, D, E, K

Fat insoluble  B vitamins  Pantothenic acid  Folate  Biotin  Vitamin C

Major Minerals Calcium Chloride Copper Fluorine Iodine Iron

Magnesium Phosphorus Potassium Sodium Sulfur Zinc

Body-Mass Index  An indicator of obesity-related health risk  BMI =

Weight (lbs) X 700 ----------------------------Height (inches)2

 BMI greater than or equal to 27 indicates

health risk

Maintaining Weight  Caloric input must equal caloric use  Calories burned depends upon

Activity level  Age  Height and build 