Digestion, Absorption
and Transport
Objectives for Chapter 3
• List the organs and accessory organs of the
digestive tract.
• Discuss the main roles of each of the organs
of the digestive tract.
• Identify how other body systems assist the
digestive tract.
• Name a digestive disorder that can occur for
each of the following: mouth, esophagus,
stomach, and intestines.
What Is Digestion?
Digestion: a multi-step process of breaking
down foods into absorbable components
using mechanical and chemical means in
the gastrointestinal (GI) tract
• Gastrointestinal tract consists of:
– Mouth
– Esophagus
– Stomach
– Small and large intestines
– Other organs
What is Digestion?
• Anatomy of the GI
– mouth  esophagus  stomach 
small intestine  large intestine
• GI tract is about 23 feet long with
extensive surface area for nutrient
absorption.
– Cells lining GI tract are replaced every 3–5
days.
What Is Digestion?
• Main roles of the GI tract are to:
– Break down food into smallest components
– Absorb nutrients
– Prevent microorganism or other harmful
compounds in food from entering tissues of
the body
• GI tract is highly efficient: 92-97% of
nutrients from food are digested and
absorbed.
Digestion Is Mechanical and
Chemical
Mechanical digestion: chewing, grinding
food to aid swallowing
– Peristalsis: the forward, rhythmic muscular
contraction that moves food through GI tract
Chemical digestion: digestive juices and
enzymes break down food into absorbable
nutrients
Digestion
• The mouth
– Saliva released: contains water, electrolytes,
mucus and a few enzymes
• Softens, lubricates, dissolves food particles
– Bolus (food mass) moves into pharynx, is
swallowed, and enters the esophagus.
– Epiglottis closes off trachea during swallowing
to prevent food from lodging in the windpipe.
– involves mastication (chewing),
The epiglottis
prevents food
from entering
the trachea
when you
swallow.
Digestion
• The esophagus
– Esophagus leads food into the stomach
– Bolus is pushed down esophagus by peristalsis.
• Lower Esophageal Sphincter (LES): at bottom
of esophagus relaxes and allows food into
stomach
• LES then closes to prevent backflow of
hydrochloric acid (HCl) from stomach
– “Reflux” of stomach acid causes “heartburn” (irritation
of esophagus lining).
Peristalsis:
Muscles
around
organs of GI
tract constrict
in wavelike
manner to
move food
along.
Digestion
• The stomach
– Stomach stores, mixes and prepares food for
digestion
– grinds the bolus to a semi-liquid mass called chyme.
• Stomach produces powerful digestive
secretions:
– HCl: activates enzyme pepsin, enhances absorption
of minerals, breaks down connective tissue of meat
• Makes the stomach acidic with a pH of 1.5 – 1.7
• Mucus protects stomach lining from damage
– Digestive enzymes, intrinsic factor (for vitamin B12
absorption), stomach hormone gastrin
Digestion
The Stomach (cont’d)
• The pyloric sphincter regulates the flow of
partially digested food into the small intestine.
Allows about 1 tsp of chyme to enter the small
intestine every 30 seconds
• Gastrin: stimulates digestive activities and
secretion of HCl, increases gastric motility and
emptying
• Liquids, carbohydrates, low-fiber and low-calorie
foods exit stomach faster
• High fiber, fat and protein foods exit slower, keep
you feeling full longer
Digestion
• The small intestine
– Most digestion and absorption occur in the
small intestine
– receives digestive juices from the gallbladder
and the pancreas.
– long, narrow, coiled, has three segments:
• Duodenum (10 inches)
• Jejunum (8 feet)
• Ileum ((12 feet)
– Interior surface area tremendously increased
by villi, microvilli, circular folds
Surface Area in the Small
Intestine
Digestion
– Peristalsis moves chyme through intestine
– Segmentation mixes chyme with chemical
secretions
– Pendular movement enhances nutrient
absorption
• Chyme moves at rate of 1 cm/minute
• Total contact time 3–10 hours, depending
on amount and type of food
Digestion
• The large intestine
– Large intestine absorbs water and some nutrients
– Ileocecal sphincter: prevents backflow of fecal
matter into ileum
• Most of nutrients in chyme have been absorbed
when it reaches large intestine
• Large intestine has three sections: cecum,
colon, rectum
– About 5 feet long, 2.5 inches in diameter
– Absorbs water and electrolytes
– No digestive enzymes; chemical digestion done by
bacteria
Digestion & Absorption
• Intestinal matter passes through colon in
12-70 hours depending on age, health,
diet, fiber intake
– Bacteria in colon produce vitamin K and biotin
and break down fiber and undigested
carbohydrates
• Stool stored in rectum
Secretions of Digestion
• Enzymes – protein molecules which facilitates a
chemical reaction
• Saliva – made by salivary glands
• Gastric Juice – made by the stomach, very
acidic, HCL
• Pancreatic Juice/intestinal Enzymes – Contains
bicarbonate to neutralize acidic gastric juices as
it enters the small intestine
• Bile – secreted by the liver, stored in the gall
bladder, emulsify fat. Only acts on fat
Accessory Organs
Liver: largest gland in body
• Produces bile needed for fat digestion
• Metabolism of carbohydrates, fats and protein
• Stores nutrients: vitamins A, D, B12, E, copper,
iron, glycogen (glucose storage form)
• Detoxifies alcohol
Gallbladder: concentrates and stores bile
– Released into GI tract when fat is ingested
Accessory Organs
Pancreas:
– Produces hormones: insulin and glucagon,
regulate blood glucose
– Sodium bicarbonate neutralizes acidic
chyme, protects enzymes from inactivation
by acid
– Digestive enzymes:
• Amylase: digests carbohydrate
• Lipase: digests fats
• Trypsin, chymotrypsin, and carboxypeptidase:
digest protein
Other Body Systems
• Nervous system stimulates your appetite.
– Hormone ghrelin signals brain to eat when
stomach is empty.
• Circulatory system transports nutrients,
oxygen, waste products through your
blood.
• Lymphatic system distributes fat and fatsoluble nutrients through your lymph.
• Excretory system eliminates wastes from
circulatory system via the urine.
The
Circulatory and
Lymphatic
Systems
The Excretory System
Common Digestive Disorders
Disorders of the mouth and esophagus:
• Gingivitis and periodontal disease – gum
swelling, bleeding and oral pain
• Swallowing problems:
– Choking
– Food becomes lodged in the trachea.
– The larynx cannot make sounds.
– The Heimlich maneuver may need to be used.
Common Digestive Disorders
• Esophageal problems
– Heartburn (acid reflux) may be caused by
weak LES (lower esophageal sphincter)
•
•
•
•
•
•
Certain foods,
smoking,
drinking alcohol,
being overweight or obese,
tight-fitting clothes,
reclining after eating
Common Digestive Disorders
Stomach Disorders:
– Gastroenteritis – cause by bacteria or virus
“stomach flu”
– Peptic ulcers - a sore in the lining of the
stomach (gastric ulcer)
Gallbladder Disease:
– Gallstones – cause by high cholesterol bile,
may lead to obstruction of the bile duct
Common Digestive Disorders
Disorders of the Small Intestine:
– Nutrient malabsorption disorders
• Celiac disease: inability to digest gluten protein
– Duodenal ulcers - a sore in the lining of the
duodenum of the small intestine (duodenal
ulcer).
– Intestinal enzyme deficiencies
– Short bowel syndrome
Common Digestive Disorders
Disorders of the Large Intestine:
– Constipation may be due to insufficient fiber
and water intake, inactivity, stress, illness.
– Diarrhea is treated by fluid and electrolyte
replacement.
– Irritable bowel syndrome (IBS): alternating
patterns of diarrhea, constipation, abdominal
pain
– Colon cancer often begins with polyps.
• Curable when detected early and treated
Videos
• Digestion 1
• Digestion 2 – Enzymes