notes: the digestive system (unit 8, part 1). digestion… digestion is the process of mechanically...
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NOTES: The Digestive
System (UNIT 8, part 1)
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DIGESTION…
Digestion is the process of
mechanically and chemically
breaking down foods and
absorbing the breakdown products.
The digestive system consists of an
alimentary canal and several accessory
organs.
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Alimentary canal includes:
● mouth
● pharynx
● esophagus
● stomach
● small intestine
● large intestine
● rectum
● anus
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Accessory organs are:
● salivary glands
● liver
● gallbladder
● pancreas
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GENERAL CHARACTERISTICS OF THE ALIMENTARY CANAL
*The alimentary canal is a muscular tube that passes through the body’s ventral cavity. Different regions carry out different functions, but certain structural characteristics are similar throughout its length.
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Structure of the alimentary wall:
4 DISTINCT LAYERS:• MUCOSA (or mucous membrane) -surface epithelium, underlying connective
tissue, and a small amount of smooth muscle
-in some regions, folds and tiny projections increase the surface area (for absorption)
• SUBMUCOSA -loose connective tissue, glands, blood
vessels, lymphatic vessels, and nerves
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4 layers of alimentary wall…● MUSCULAR LAYER-consists of two coats of smooth muscle tissue
and some nerves; produces movement of the tube
● SEROSA (or serous layer)-outer covering of the tube (also called the
visceral peritoneum)-protects underlying tissue-cells here secrete serous fluid, which moistens
and lubricates tube’s outer surface
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Movements of the tube:
*motor functions include:
MIXING MOVEMENTS
and
PROPELLING MOVMENTS
(include a wavelike motion
called PERISTALSIS)
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MOUTH (Oral Cavity)*receives food and
begins digestion
Tongue:
-rough projections (PAPILLAE)
on surface of tongue provide friction to handle food
-papillae also contain TASTE BUDS
-covering the root of the tongue
are masses of lymphatic tissue:
LINGUAL TONSILS
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MOUTH (Oral Cavity)
Palate:
-forms the roof of the oral cavity
-hard palate (anterior) and soft palate (posterior)
-soft palate extends posteriorly
and downward as a projection
called the UVULA
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MOUTH (Oral Cavity) Teeth:
-primary teeth (“baby teeth”) are replaced by the secondary teeth (permanent teeth)
-teeth are used to mechanically
break food into smaller pieces
(this increases the surface area
of food allowing digestive
enzymes to react more
effectively with the food
molecules)
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SALIVARY GLANDS
*salivary glands secrete saliva, which moistens food, helps bind food particles, begins chemical digestion of carbohydrates, makes taste possible, and helps cleanse the mouth.
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SALIVARY GLANDS
Serous cells: secrete a watery fluid that contains the digestive enzyme AMYLASE (starts to break down starch and glycogen into smaller disaccharides)
Mucous cells: secrete a thick fluid called mucus, which binds food particles and lubricates during swallowing
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SALIVARY GLANDSThere are 3 pairs of salivary glands:
1) Parotid glands (largest); contains only serous cells
2) Submandibular glands; contains both serous and mucous cells
3) Sublingual glands (underneath the tongue; secretions are mostly mucus…thick and stringy!)
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PHARYNX and ESOPHAGUS (important passageways!)
PHARYNX: connects the nasal and oral cavities with the larynx and esophagus
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ESOPHAGUSESOPHAGUS: a food passageway
from the pharynx to the stomach; passes through the diaphragm
-at distal end, circular smooth muscle fibers thicken and close the entrance to the stomach (prevents regurgitation);
-if some of the acidic stomach contents are regurgitated (reflux) into the esophagus, may cause HEARTBURN
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STOMACH*the stomach receives food, mixes it with
gastric juice, initiates protein digestion, carries on a limited amount of absorption, and moves food into the small intestine.
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STOMACH Structure / Parts:-J-shaped, pouch-like organ that
sits inferior to diaphragm in upper left of abdominal cavity
-has a capacity of about 1 L-thick folds in the inner lining disappear when
the stomach wall stretches-separated from the small intestine by a
powerful circular muscle: PYLORIC SPHINCTER
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STOMACH Gastric Secretions - produced by
GASTRIC GLANDS, which have 3 types of cells:● mucous cells: secrete mucus
● chief cells: secrete digestive enzymes
● parietal cells: secrete hydrochloric acid (HCl)
*together, these secretions form
GASTRIC JUICE
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STOMACH PEPSIN: most important digestive enzyme in
gastric juice
-secreted by chief cells in an inactive form: PEPSINOGEN
-when it comes into
contact with
hydrochloric acid,
it becomes the
active PEPSIN
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STOMACH
*a layer of thick mucus is produced by cells in the stomach’s inner lining…this protective coating prevents the stomach from digesting itself.
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STOMACH Mixing and emptying actions:
- CHYME = a mixture
of food particles and
gastric juice produced by
mixing
-chyme is emptied into the small intestine where chemical digestion continues
-the rate of emptying depends on the fluidity of chyme and the type of food