Download Introducing Digestion

Introducing Digestion
Digestion: the process by which the food we eat is broken down into nutrients that can be absorbed
into the blood and carried to the cells of the body
Digestive system: The collective unit of organs responsible for digestion
4 main steps:
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Ingestion: taking in nutrients
Digestion: physical and chemical breakdown of complex food molecules
Absorption: the transfer of digested nutrients from the digestive system to the bloodstream
Egestion: the removal of waste food materials from the body
Specialized digestive systems:
1. Simple digestive systems: animals like flatworms or jellyfish have a digestive sac with a single
opening into a gastrovascular cavity. Serves as an entry for and an exit for waste. Food is taken
in and digested by chemicals released into the cavity and the nutrients are absorbed by cells
that line the cavity, which further diffuse to other cells.
2. Complete Digestive system: Animals such as earthworms have a digestive tract with two
openings (one for food intake and one for elimination). The tract is organized into specialized
regions (mouth, pharynx, esophagus, crop, gizzard, intestine) that enable the breakdown and
absorption of food as food moves along the tract in one direction.
Human Digestive System:
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Complete digestive system
The digestive tract in humans, unlike earth worms is much longer than our body
Referred to as the gastrointestinal tract (GI tract)
7-9m in length
The digestive system relies on other organ systems; muscle and bone( to catch and ingest),
circulatory (transport of oxygen and nutrients), the nervous system(regulate actions of digestive
organs) and the endocrine system (delivery of hormones and enzymes like insulin for proper
digestion)
Pharynx
Digestion in the mouth
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Digestion begins in the mouth
Food is broken down into smaller pieces by the teeth ( physical digestion)
o Teeth in the front of your mouth (incisor and canines), are specialized for cutting an
grabbing your food.
o Molars ( back broad and flat teeth), are specialized for grinding and crushing food
o The type of teeth an animal has dictates the food it eats
Type of tooth
Incisor
Canine
Premolar
Molar*
Function
Bite or cut
food
Tear or shred
food
Grind food
Crush food
Number in Adult
Mouth
8
4
8
12 (8 molars and 4 wisdom
teeth)
Saliva
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The presence of food in our mouth triggers the salivary glands. Which secrete a watery
substance called saliva.
Saliva contains enzymes, chemicals that increase the rate of chemical reactions in living things.
o Enzymes in saliva =Amylase: breaks down starch into simpler disaccharides (chemical
digestion)
Saliva dissolves food particles, making it possible to taste food and forms it into a ball (bolus)
that can be swallowed
Saliva contains mucus, a protective secretion that acts as a lubricant and aids in swallowing
On average we produce 0.75 to 1.5 L of saliva per day.
Swallowing
Once the food has been chewed and mixed with saliva, the tongue pushes it to the back of the mouth
where it is swallowed
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Food is pushed into the pharynx (throat), the soft palate is raised to prevent food from entering
the nasal passage
The larynx is raised against the epiglottis, a flap of cartilage at the root of the tongue, which is
depressed during swallowing to cover the opening of the windpipe, this covers the trachea to
prevent food from entering the lungs
Food moves from the pharynx to the stomach via the esophagus, a long muscular tube.
Food moves down the esophagus in rhythmic, involuntary, wavelike contractions of the smooth
muscles of the gastrointestinal tract. This action is known as peristalsis.
Peristalsis is responsible for the downward movement and it takes 8 seconds for food to move
down the esophagus. Food would still move downward even if you were to stand on your head!
The Stomach
esophagus
serosa
esophagus
Gastroesophageal
sphincter
circular
Pyloric sphincter
esophagus
oblique
submucosa
Mucosa (gastric
folds)
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J-shaped muscular organ that continues mechanical and chemical digestion
Can store up to 2L of food
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Sphincters: control the movement into and out of the stomach
o Gastroesophageal: prevents stomach acid from entering when contracted
o Pyloric: a ring of tightly closed muscle that allows chyme to enter intestines
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4 layers:
o Mucosa: innermost and extensively folded layer. Secretes gastric juices and mucus.
Epithelial cells divide rapidly to heal any damage from the gastric juices (lining replaced
every 3 days)
o Submucosa: a layer of connective tissue that contains the nerves and blood vessels
o Muscularis: consists of 3 layers of smooth muscle (longitudinal, circular, oblique)
 Muscles contract frequently, churning and mixing food (physical digestion)
o Serosa: smooth outer layer, holds the stomach in place and secretes a lubricating fluid
that eliminates friction with other organs
Muscularis
longitudinal
Chemical Digestion
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Gastric juice breaks down the stomach contents into a semi-solid called Chyme (a mixture of
gastric juices and food)
Gastric Juices:
o about 2L per day is secreted
o the secretion known as gastric juice contains a variety of substances that aid in the
digestion of food
o composed of acid, mucus and enzymes
Contents of gastric juice are:
o Hydrochloric acid - loosens tough fibrous material kills bacteria that enters stomach with
food (pH of 2-3)
o Pepsin – an enzyme that breaks down proteins into amino acids and polypeptide
o Lipase – an enzyme that breaks down triglycerides into fatty acids and glycerol
Process:
o Nerves in the lining of the stomach detect when food enters the stomach and stimulates
the secretion of gastric juice and stimulates muscular contractions
o Nerves in the submucosa detect the presence of food and release the hormone gastrin
 Gastrin: a hormone that releases hydrochloric acid and pepsinogen into the
stomach
 Pepsinogen when mixed with stomach acid is converted to its active form
pepsin
Complications
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Acid reflux: occurs if the gastroesophageal sphincter dose not close completely and acid from
the stomach enters the esophagus
o Causes a burning sensation in the lower throat that we call heart burn
o Caused by an overfilled stomach, and excess acid production due to smoking
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Stomach ulcers: a lesion or sore on the epithelium of the stomach. Can bleed and can be painful.
o Occurs when the stomach lining is exposed to stomach acid
o Caused by the bacteria Heliobacter pylori burrowing into the mucosa
o Cured with antibiotics