Digestive System
Chapter 14
The Digestive System Notes Part 1
I. OVERVIEW OF THE DIGESTIVE SYSTEM
A. Functions of the Digestive System
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1. Ingestion - Consuming food
2. Digestion - Breakdown of ingested food into smaller molecule
3. Absorption - Passage of nutrients into the blood
4. Metabolism- Converting nutrients into raw materials for
Production of cellular energy (ATP)
5. Elimination - Removing indigestible wastes
B. Types of Digestion
1. Mechanical Digestion:
 Chewing, mixing, churning, cutting, tearing, or mashing of
food.
2. Chemical Digestion:
 Enzymes break down food molecules into their building blocks.
o Carbohydrates are broken down to simple sugars.
o Proteins are broken down to amino acids.
o Fats are broken down to fatty acids and alcohols
II. ORGANS OF THE DIGESTIVE SYSTEM
A. Two main types of digestive organs
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1. Organs along the Alimentary canal (Gastrointestinal tract): – continuous coiled hollow
tube over 30 ft. long
o Mouth, pharynx, esophagus, stomach, small and large intestine, rectum
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2. Accessory digestive organs: Organs that contribute secretions (fluids) & enzymes to aid
in digestion
o Salivary glands, liver, gall bladder, pancreas
B. ALIMENTARY CANAL ORGANS
1. ORAL CAVITY (Mouth, Teeth, & Salivary Glands)
a. Mouth
 Food enters the mouth where digestion begins
o MECHANICAL DIGESTION: Mastication (chewing) of food by teeth
o CHEMICAL DIGESTION: Saliva contains the enzyme amylase that breaks
down carbohydrates and starches in the mouth
 Bolus - masticated food mixed with saliva
b. Teeth
 Function: chew food
 Humans have 2 sets:
o 1. Deciduous (baby or milk) teeth
 20 teeth are fully formed by age two
o 2. Permanent teeth (adult) teeth
Digestive System
Chapter 14
 Replace deciduous teeth between the ages of 6 to 12
 A full set is 32 teeth, but some people do not have wisdom teeth
Classification of teeth
o Incisors – used for biting food
o Canines – used for tearing food
o Premolars - (bicuspids) – used for grinding food
o Molars – Used for crushing food
Parts of a tooth
o Crown – exposed part of tooth made of enamel
o Neck – connects crown to root
o Root – Periodontal membrane attached to the bone,
contains blood vessels and nerves
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c. Salivary Glands
 Produce saliva - mixture of mucus and serous fluids
 Helps to form a food bolus
 Contains salivary amylase to begin starch digestion
 Dissolves chemicals so they can be tasted
2. PHARYNX
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The passageway for air and food
Has 3 parts: (see diagram)
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Swallowing:
o Reflex that causes the epiglottis to move
blocking the trachea (windpipe)
o The esophagus opens opens to receive the
bolus
3. ESOPHAGUS
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"Food tube" links the pharynx to stomach
the diaphragm
Muscle layers contract to squeeze food
through the stomach
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Longitudinal inner layer
Circular outer layer
Peristalsis – alternating
waves of muscle contraction
4. STOMACH
a. Structure & Path of Food
 Thick-walled organ located on the left side of the abdominal
cavity
 Bolus (food) enters through the cardio esophageal sphincter
o Prevents the backup of stomach acid into
esophagus (acid reflux )
 Bolus (food) enters through the four regions of the
stomach:
o Cardiac region – near the heart
o Fundic region
o Pyloric region – main part of stomach
through
through to
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Chapter 14
o Pyloric region – funnel-shaped end
Chyme (mixture of bolus & gastric juices) exits through the pyloric
sphincter into the small intestine
 MECHANICAL DIGESTION: Muscle of stomach contract to churn and
mix food with gastric juices.
 Deep folds called rugae allow stomach to expand when filled with
food
b. Coverings of the stomach
 Peritoneum – membrane that covers the abdominal wall and organs
o Visceral peritoneum – covers the organ
o Parietal peritoneum – covers the cavity walls
 Mesentery holds the stomach & intestines in place in abdominopelvic cavity
c. Gastric Juices
 CHEMICAL DIGESTION: Lining of the stomach secretes gastric juices that break food.
 Gastric juices contain pepsin, HCl, and mucus
o Pepsin breaks down proteins into amino acids
o Hydrochloric acid (HCl) is a very strong acid (pH 1 or
2) that activate enzymes and kills bacteria
o Mucus protects the lining of the stomach from the
acidic environment
o Replaced every few days
o If the mucus layer is broken down by
increased acid, an ulcer may form
o Peptic Ulcer: Open sores on the lining of the
digestive tract that may bleed and be very painful
 Caused by bacteria (Helicobacter pylori), regular
use of pain relievers or other medications
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The Digestive System Notes Part 2
II. ORGANS OF THE DIGESTIVE SYSTEM (continued)
B. ALIMENTARY CANAL ORGANS
Review: 1. Oral Cavity (mouth, teeth, salivary glands) > 2. Pharynx > 3. Esophagus > 4. Stomach
5. SMALL INTESTINE
a. Overview
 Muscular tube extending form the pyloric sphincter to the ileocecal valve
 Up to 22 feet long
 Functions:
 Main digestive organ – most digestion occurs
in the small intestine
 Carbohydrate and protein digestion is
completed
 Fat digestion begins
 Main site of nutrient absorption into the
blood
b. Structure
 Small Intestine has 3 parts:
o duodenum
o First 25 cm, attached to the stomach
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Chapter 14
o Receives secretions from the liver and pancreas to aid in digestion
Jejunum
o Next 3 feet
o Contains folds and villi for absorption
o Ileum
o Last 6-7 feet
o Extends from jejunum to large intestine
Walls of the small intestine
Has 3 features that increase the surface area for maximum absorption of nutrients
o 1. Circular folds – permanent transverse folds in the mucosa and submucosa
o 2. Villi – fingerlike projections of the mucosa
o Goblet cells in villi produce mucus to lubricate chyme
o Villi contain absorptive cells and blood capillaries to absorb nutrients
o 3. Microvilli – microscopic extensions from the plasma membrane
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c.
CHEMICAL DIGESTION in the Small Intestine
 Digestion Is completed in the small intestine
 Accessory organs aid in digestion by secreting
enzymes into the small intestine
o Bile from the gall bladder
 Breaks down lipids
o Pancreatic juices from the pancreas
 Sodium bicarbonate neutralizes the
acid
d. Absorption in the Small Intestine
 When food has been broken down to its nutrients
& waste products, the nutrients must be absorbed by the body
 Most nutrient absorption occurs in the small intestine.
 Most substances are absorbed by active transport through cell membranes
 Lipids are absorbed by diffusion
 After absorption, nutrients are transported to the cells through the bloodstream
6. LARGE INTESTINE
 Once digestion & absorption of nutrients is complete, the chyme moves into the large intestine by peristalsis
 Also known as the colon
 Larger in diameter, but shorter than the small intestine
a. Functions of the Large Intestine
 Absorption of water
 Does not participate in digestion of food
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Chapter 14
 Eliminates indigestible food from the body as feces
 Goblet cells produce mucus to act as a lubricant
b. Structure
 Ascending Colon – includes the cecum
(saclike structure) with a small projection
called the appendix
 Transverse Colon
 Descending Colon
 Sigmoid Colon
c. Food Breakdown & Absorption in the Large
Intestine
 Bacteria breaks down indigestible materials
o Source of odor
o Produce some vitamin K and B
o Release gases
 Water vitamins K and vitamin B are
absorbed
 Remaining materials are eliminated as feces
7. RECTUM
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Feces moves from the sigmoid colon of the large intestine to the rectum by peristalsis
Feces Is stored in the rectum until the involuntary internal
anal sphincter is triggered to open
Defecation or expulsion of feces occurs when the voluntary
external anal sphincter relaxes, and the feces passes out of
the body
 Happens daily
 Triggered by presence of feces in
rectum
C.
ACCESSORY DIGESTIVE ORGANS
1. PANCREAS
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Produces a wide spectrum of digestive enzymes that
break down all types of food
 Enzymes are secreted into the duodenum of the small intestine
a. Secretions of the Pancreas
1. Pancreatic Amylase: Helps complete digestion of starch
2. Trypsin, etc: Carry out about half of all protein digestion
3. Lipase: Responsible for fat digestion
4. Nucleases: Digest nucleic acids
5. Sodium Bicarbonate: Neutralize acidic
chyme
 Enzymes are transported to the small intestine
through the common bile duct (shared with the
liver)
2. LIVER
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Largest organ in the body
Has 2 main lobes
o Larger right lobe
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o Smaller left lobe
The liver also contributes to digestion by producing bile
a. Bile
 Bile is a greenish substance that emulsifies fat
 Stored in gall bladder
 Composed of bile salts & pigments bilirubin from hemoglobin breakdown), cholesterol, phospholipids,
& electrolytes
 Helps to digest fatty foods
b. Role of the Liver in Metabolism
 Removes drugs and alcohol from blood
 Degrades hormones
 Produce cholesterol, blood proteins (albumin and clotting
proteins)
 Plays a central role in metabolism
 Stores products of digestion as glycogen to be released as needed
3. GALL BLADDER
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Sac found in hollow fossa of liver
Stores bile produced by the liver
Bile is introduced into the duodenum in the presence of fatty
food
Gallstones can cause blockages
III. Control of Digestive Activity
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Mostly controlled by reflexes via the parasympathetic division of
nervous system
Chemical and mechanical receptors are in organ walls that trigger
reflexes
Stimuli include:
o Stretch of the organ
o pH of the contents
o Presence of breakdown products
Reflex include:
o Activation or inhibition of glandular secretions
o Smooth muscle activity
Processes of the Digestive System (See diagram)