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THE DIGESTIVE SYSTEM
(pp. 978-984)
I. THE DIGESTIVE PROCESS
A. Steps in Digestion
1. _Ingest__ the food.
2. Break food down into molecules small enough to pass through _cell membranes____
3. _Absorb____ nutrient molecules.
4. Eliminate _undigestible food______
B. Digestive Process
There are two types of digestion used in the digestive process:
 Mechanical digestion - _Physical_ breakdown of large food particles into smaller particles. Does not involve
_chemical______ change. Always occurs first to increase _surface area__ for chemical digestion.
 Chemical digestion – Breakdown of complex molecules into simpler molecules by breaking _chemical bonds___.
Chemical digestion requires the work of _enzymes_____
C. Hunger
The _hypothalamus___ monitors _glucose___ concentration in the blood. As glucose _decreases__, the hypothalamus
stimulates the _medulla oblongata____ to send a message via a _motor neuron___ to contract the _smooth___ muscles of
the stomach, creating the sensation of hunger.
D. Organization of the Digestive System
 Alimentary Canal – Continuous and enclosed tube that extends from _mouth__ to _anus____.
 Accessory Organs – Help with digestion, but food _never passes through them___
o _Liver_______
o _Gall Bladder_
o _Pancreas___
II. DIGESTION
A. Oral Cavity
 Description - Includes the _mouth, teeth, tongue, and salivary glands________
 Function
o Mechanical digestion of all food particles begins through the action of the _teeth_ and _tongue___. Creates a
wad of food known as a _bolus_____.
 Incisors – Adapted for _biting___.
 Canines – Adapted for _ripping & tearing_.
 Molars – Adapted for _chewing___.
o Chemical Digestion - _Saliva________ moistens the food which enhances the taste and makes it easier to
swallow. In addition, it contains the enzyme, _amylase_______ which begins chemical digestion of
__starch__, a _polysaccharide____.
B. Esophagus
 Description – Soft, muscular tube
 Function - No _digestion___ occurs in the esophagus. _Smooth____ muscle contractions called
_peristalsis_________ move the bolus along. When the bolus reaches the end of the esophagus, a circular valve-like
structure called the _cardiac sphincter____ relaxes, and the bolus enters the stomach.
C. Stomach
 Description – Thick-walled, _muscular__ sac with deep folds that flatten out as the stomach fills. Produces a thick layer
of _mucus____ to protect it from its acidic contents.
 Function - Stomach serves as a “holding tank” so that digestion can take place gradually.
o Mechanical digestion of all nutrients continues due to churning action of the three layers of _smooth_____
muscle tissue. Produces a thick, liquid mush called _chyme___.
o Chemical digestion – Results from gastric juices secreted by stomach
 HCl (_hydrochloric _____ Acid) Gives the stomach a low pH which destroys any _pathogens______
present and activates the enzyme, pepsin.
 Pepsin – Pepsin begins the chemical digestion of _proteins___________.
D. Small Intestine
 Description - Small in _diameter____, but very long. Inner lining is folded into _villi____ to increase _surface
area_____ for absorption. Divided into 3 sections.
 Function
o Duodenum – The accessory organs to digestion are all involved as the chyme moves into the duodenum.
 Liver – Produces bile, a substance that _mechanically___ digests _lipids______.
 Gall Bladder – Stores & concentrates bile until it is secreted into duodenum
 Pancreas – Produces _digestive enzymes_____ to chemically digest all remaining nutrients. Also
secretes _sodium bicarbonate_________, a strong _base____ that neutralizes the acidic
_chyme____ as it is released a little at a time through the _pyloric sphincter__________________
from the stomach into the duodenum.
o Jejunum & Ileum - Site of _nutrient___ absorption. Absorbed into _capillaries____ and transported first to the
liver for _toxin___ removal and storage of extra glucose as _glycogen_________.
E. Large Intestine
 Description - Shorter than small intestine, but larger in _diameter______.
o Contains huge population of _mutualistic_______ bacteria that help with digestion and produce _vitamins___.
o Site of a dead-end tube with no apparent function called the _appendix_____.
 Function –Site of _water___ re-absorption.
 Disorders of the Large Intestine
o Diarrhea - Results when contents pass through too _quickly_________
o Constipation – Results when contents pass through too _slowly____
F. Rectum
 Description - Last portion of the alimentary canal.
 Function - Storage of _undigestible ______ material, including _cellulose___. _Peristalsis________ of large intestine
forces waste into rectum for storage until elimination via the _anus___ and relaxation of the anal _sphincter_____.