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Chapter 9.1

Introduction to the Digestive
Why Do We Eat Food???
? To obtain the raw materials for growth and maintenance from nutrients in food
? Nutrients provide energy (chemical energy) that fuels all biological processes and
physical activities such as growth and movement
What Do We Eat?
? Carbohydrates
? Proteins
? Lipids
? Water
? Vitamins and
What Do We Eat?
Vitamins and Minerals
? A vitamin is a compound that an organism needs as a nutrient only in small amounts
? Regulate cell functions, growth, and development
? Classified as fat soluble or water soluble
? Minerals are naturally occurring elements that the body uses to carry out metabolic
processes and to build or repair tissues
? Necessary for bone formation, nerve impulse transmission, and muscle contractions
? There are 4 main steps
? Ingestion – taking in of nutrients
? Digestion – physical and chemical breakdown of complex food molecules
? Absorption – transfer of digested nutrients into the bloodstream
? Egestion – the removal of waste food materials from the body
Human Digestive
? Often referred to as the
gastrointestinal tract (GI tract)
? Is approximately 7-9 meters long
depending on size and age of
the individual
Digestion in the Mouth
� Physical digestion
Food is broken down into smaller pieces by the teeth (mastication)
The front teeth are used to grab and cut food, back teeth are used for grinding and
crushing food
� Chemical Digestion
Saliva contains enzymes (chemicals that increase the rate of chemical reactions in
living things) which breaks down food into simpler particles.
� Amylase – breaks down starch into smaller disaccharides and monosaccharides
� Mucus – protective secretion that acts as a lubricant to aid swallowing
? At this point the food is now called
bolus and is pushed to the back of the
mouth where it is swallowed
? As the food is pushed into the pharynx
? The soft palate is raised to prevent
food from entering the nasal
? The larynx is raised against a flap of
soft tissue called the epiglottis,
preventing food from entering the
trachea and lungs
Going from the Mouth to the Stomach
? After swallowing, the bolus moves to the
stomach by way of the esophagus, a long
muscular tube
? When bolus enters the esophagus it activates the
smooth muscles to undergo rhythmic, wave-like
contractions called peristalsis
The Stomach
? Is a J-shaped, muscular organ where
physical and chemical digestion occurs
? The entry and exit inside the stomach is
controlled by circular muscles called
? The gastroesophageal sphincter is located
between the esophagus and the stomach
? When relaxed it allows food to enter the
stomach, when contracted it prevents food
from re-entering the esophagus
? The pyloric sphincter is located between the
stomach and small intestine
The Stomach
Has four layers each with a special structure and function
? The innermost layer is the mucosa and is extensively
folded and produces gastric juices (mixture of
digestive enzymes, acid and mucus)
? The next is the submucosa- made of connective tissue
containing networks of nerves and blood vessels
? The third layer is the muscularis and consists of smooth
muscles which contract frequently, churning the
mixing the food with gastric juices to produce a
mixture called chyme
? The outermost layer is the serosa and holds the
stomach in place and secretes a lubricating fluid
eliminating friction between organs
Chemical Digestion in the Stomach
? Digestion is controlled by enzymes, hormones and nerves
? When nerves detect the presence of food, the hormone gastrin is released
? Gastrin stimulates the release of gastric juice
? Gastric juice is mostly made of mucus but also contains acid and digestive
? The mucus coats and protects the lining of the stomach from acid and
digestive enzymes
Chemical Digestion in the Stomach
? The acid in gastric juice is very strong, ranges from pH of 2 – 3
? The acid kills many harmful microorganisms ingested with food
? The acid activates other digestive enzymes such as pepsinogen and converts it to
its active form, pepsin which breaks down proteins into amino acids
Stomach Disorders
Acid Reflux
? When the gastroesophageal sphincter does not close properly, acid can enter the
? Causes a burning sensation known as heartburn or acid reflux
Stomach Ulcers
? One species of bacteria is able to survive by secreting acid-neutralizing enzymes and
burrows into the mucosa preventing mucus from forming and protecting the stomach lining
? When the stomach lining is exposed to the acid an open sore, or ulcer, is formed
? Can bleed and be quite painful and can also be dangerous
Anatomy & Physiology of Digestive System
Physical / Mechanical Digestion
Chemical Digestion
Mouth (Buccal or Oral
Mastication- breakdown of food
by the action of teeth (food now
becomes bolus)
Salivary glands secretes amylase
(present in saliva) that
breakdowns carbohydrates.
Fats are partially broken down
Peristalsis (rhythmic, wave-like
Partial chemical breakdown of
proteins & fats
Churning Action (Propulsion and
Hormone called Gastrin stimulates
Gastric juice & Pepsin which
together bring about protein