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Biology
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2. 38–2 The Process of Digestion
Photo Credit: © Fred Hossler/Visuals Unlimited
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What are the organs of the digestive system?
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The digestive system includes: mouth (pharynx) esophagus stomach small intestine large intestine
Other structures add secretions to the digestive system and aid in digestion. (ie. salivary glands, pancreas, and liver)
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The Digestive System
Mouth
Pharynx
Salivary glands
Esophagus
Liver
Stomach
The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Because the pancreas and most of the gallbladder are behind other organs, their locations are indicated by dotted lines.
Pancreas
Gallbladder
Large intestine
Small intestine
Rectum
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The Mouth
What is the function of the digestive system?
To convert foods into simpler molecules - these can be absorbed/used by the cells of the body
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The Mouth
The Mouth
Chewing begins mechanical digestion - physical breakdown of food into smaller pieces Teeth: cut, tear, and crush food into small pieces
Salivary glands: secrete saliva - which moistens food and makes it easier to chew
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The Mouth
Saliva: - helps ease the passage of food - begins the process of chemical digestion - contains amylase - enzyme that breaks down sugars
Saliva also contains lysozyme - an enzyme that fights infection
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The Esophagus
The Esophagus
Food tube which connects mouth to the stomach
Food is moved along by contractions called peristalsis - these contractions squeeze food into the stomach
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The Esophagus
Esophagus
Bolus
Peristalsis
Muscles contracted
Stomach
Muscles in the walls of the esophagus contract in waves. Each wave pushes the chewed clump of food, or bolus, in front of it. Eventually, the bolus is pushed into the stomach.
The cardiac sphincter: closes the esophagus after food has passed into the stomach.
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The Stomach
The Stomach
Continues mechanical and chemical digestion
Alternating contractions of three smooth muscle layers churn food
Food can stay in stomach 2-6 hours!
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The Stomach
Chemical Digestion
The stomach lining has millions of gastric glands that release substances into the stomach.
Some glands produce mucus, which lubricates and protects the stomach wall.
Other glands produce hydrochloric acid, which makes the stomach contents very acidic.
Other glands produce pepsin, an enzyme that digests protein.
Pepsin and hydrochloric acid begin protein digestion
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The Stomach
Digestive enzymes break down foods and make nutrients available to the body.
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The Stomach
Mechanical Digestion 
The stomach churns to produce a mixture known as chyme
Chyme flows into the small intestine through the pyloric valve between the stomach and small intestine.
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The Small Intestine
The Small Intestine
Chyme is pushed into the duodenum
- the first of three parts of the small intestine - where most digestive enzymes enter the intestine
Important Note: Most chemical digestion and absorption of food occurs in the small intestine
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The Small Intestine
Accessory Structures of Digestion
Liver
Bile duct
Pancreas
Gallbladder
Pancreatic duct
Duodenum
Accessory structures, including the liver and pancreas, add secretions to the digestive system. The pancreas secretes enzymes that help break down carbohydrates, proteins, and fats.
To rest of small intestine
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The Small Intestine
Accessory Structures of Digestion
Pancreas:
produces enzymes that break down all foods
produces sodium bicarbonate - a base that neutralizes stomach acid
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The Small Intestine
Liver:
Produces bile.
Bile dissolves droplets of fat - this helps enzymes to break down fat
Bile is stored in the gallbladder
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19. Absorption in the Small Intestine
The small intestine is adapted for the absorption of nutrients.
Villi - cover the inner surface of the small intestine - provide enormous surface area for absorption
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20. Absorption in the Small Intestine
Villus
Small intestine
Circular folds
Epithelial cells
Villi
Capillaries
Lymph vessel
The lining of the small intestine consists of folds that are covered with tiny projections called villi. Within each villus there is a network of blood capillaries and lymph vessels that absorb and carry away nutrients. 
Vein
Artery
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The Large Intestine
The Large Intestine
- also known as the colon
- removes water from the chyme.
-water is absorbed quickly - leaving undigested materials behind (feces)
Concentrated waste material passes through the rectum and is eliminated from the body
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22. Digestive System Disorders
Stomach acids sometimes damage the lining - producing a hole in the stomach wall - known as a peptic ulcer - most ulcers are caused by a bacteria( H. pylori )
Other digestive disorders include diarrhea and constipation
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38–2
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38–2
Food is moved through the esophagus into the stomach by
air pressure.
muscle contractions.
gravity.
swallowing.
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38–2
A gland that has both endocrine and exocrine functions is the
liver.
spleen.
pancreas.
gallbladder.
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38–2
The enzyme in saliva that begins the digestion of starch is
amylase.
pepsin.
lysozyme.
peptidase.
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38–2
Stomach muscles contract to churn and mix stomach fluids and food, producing a mixture known as
chyme.
amylase.
bile.
acid.
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38–2
Absorption of vitamins, minerals, and digested food molecules takes place in the
stomach.
small intestine.
large intestine.
duodenum.
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29. END OF SECTION