Download Chapter 16 The Digestive System

Chapter 16
The Digestive System
Functions of the Digestive System
1. take in food
2. break down food
3. absorb digested molecules
4. provide nutrients
5. eliminate wastes
Two Divisions of the Digestive System
1. alimentary canal (digestive tract)
•
digests and absorbs
•
oral cavity to anus
2. accessory digestive organs
Organs of the Digestive System
Figure 14.1
Digestive Tract
 muscular tube approximately 9 M long
 open at both ends
 Wall has 4 layers with some variation
due to specific function.
Layers of Digestive Tract
 innermost: mucosa
 epithelial cells which line cavity
 attached to connective tissue
 surrounded by thin muscular layer
 protects tissues of canal
 carries out secretion and
absorption
Layers of Digestive Tract
 next: submucosa
 thick layer of loose connective tissue
housing blood vessels, lymph
vessels, and nerves
 nourishes surrounding layers of
tract
Layers of Digestive Tract
 next: muscular layer
 inner: circular smooth muscle
 outer: longitudinal smooth muscle
 propels food through digestive tract
 contains lots of nerves to control
movement and secretion
Layers of Digestive Tract
 outermost: serosa
 visceral peritoneum
 protects underlying tissue
 secretes serous fluid to keep tract
from sticking to other tissues of
abdominal cavity
Layers of Alimentary Canal Organs
Figure 14.3
Mouth (Oral Cavity) Anatomy
 teeth – 32 total in adults
 incisors, premolars,
molars, wisdom teeth
 tongue – thick, muscular
organ
 functions in digestion,
taste and speech
 attaches to posterior
part of mouth and by
frenulum
 covered by mucous
membrane
 houses taste buds
within papillae
 salty, sweet, sour,
bitter
Mouth (Oral Cavity) Anatomy
 tonsils
 lingual and palatine
 palate – roof of oral cavity
 hard = bone
 soft = muscle and
connective tissue
 separates oral and
nasal cavities
 uvula – fleshy
projection of the
soft palate
Salivary Glands
 produce saliva – mixture of mucous and serous fluids
 keeps mouth moist
 contains enzymes that aid in chemical digestion
 3 pairs in oral cavity
 parotid – located anterior to ears
 secrete serous and mucous fluids
 submandibular – base of jaw
 ducts open below tongue
 secrete more serous than mucous fluids
 sublingual – smallest, below tongue
 duct opens into floor of oral cavity
 secretes thick, stringy mucous fluid
Saliva
 produce 1 liter a day
 serous fluid: parotid and submandibular glands
 contains amylase, enzyme to break down starches
 lysozyme: reduces bacterial growth in mouth
 mucous: contains mucin for lubrication
 regulated by parasympathetic stimulation (involuntary
nervous system)
 increase in secretions with sight, smell, thought of
food
Processes of the Mouth
 mechanical digestion
 mastication (chewing) of food
 mixing of food with saliva by tongue
 chemical digestion
 salivary amylase breaks down starches
 allow for the sense of taste
 NO food absorption occurs in mouth.
Swallowing
tongue, soft palate, pharynx and esophagus
 phase 1: voluntary (mouth)
 Bolus (food mass) is forced into pharynx by tongue.
 phase 2: involuntary (pharynx)
 Pharynx connects mouth and esophagus.
 Receptors in pharynx sense food which triggers
swallowing reflexes.
 Epiglottis covers larynx.
 phase 3: involuntary (esophagus)
 Esophagus connects pharynx to stomach.
 Peristalsis moves food from esophagus to stomach.
 Lower esophageal sphincter helps prevent regurgitation
of food.
Stomach Anatomy
 located on the left side of the abdominal
cavity
 Food enters at the cardiac opening.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Stomach Functions
 acts as a storage tank for food
 site of food breakdown
 start of protein digestion
 very little absorption
 delivers chyme (processed food) to the small
intestine
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Regions of the stomach
Cardiac region – near the heart
Fundus – most superior part
Body
Pylorus – funnel-shaped terminal end
Food empties into the small intestine at the pyloric
sphincter.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Stomach Anatomy
Figure 14.4a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Muscles of the stomach
• different than rest of digestive tract
• 3 layers
1. outer longitudinal layer
2. middle circular layer
3. inner oblique layer
• produce churning action – mechanical
digestion
Specialized Mucosa of the Stomach
 gastric glands – secrete gastric juice
 3 types of cells
1. mucus cells – produce a sticky alkaline mucus
which protects stomach lining
2. chief cells – secrete protein-digesting enzymes
(pepsinogen which becomes pepsin)
3. parietal cells – secrete hydrochloric acid
 rugae – internal folds of the mucosa
 allows stomach to stretch when full
Other Stomach Secretions
• gastrin – hormone that increases stomach secretions
• intrinsic factor – aids in absorption of vitamin B12
• cholecystokinin – hormone that stimulates gastric
secretions
Production of gastric juices is regulated by hormones
and the nervous system.
Humans produce about 2 L per day.
How Stomach Digests Food
1. As food enters digestive tract, gastrin is produced
which causes production of gastric juices.
2. Food mixes with gastric juices and forms chyme
which passes to pyloric region.
3. As food empties, secretions from stomach wall are
inhibited by nervous system.
Stomach Movement
2 types resulting from stomach wall muscle
contractions
1. mixing – from weak contractions
mix ingested food with stomach secretions to form chyme
2. peristalsis – from stronger contractions
force chyme toward and through pyloric sphincter
Structure of the Stomach Mucosa
Figure 14.4b, c
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Small Intestine
 absorbs products of digestion
 receives secretions from pancreas and liver
 transports remaining residue to large intestine
 6 meters long
 ileocecal junction – joins ileum and large intestine
 Ileocecal sphincter and ileocecal valve keep materials
moving in right direction.
Subdivisions of the Small Intestine
Duodenum
 attached to the stomach
 curves around the head of the pancreas
 25 cm long
 Common bile duct from liver and pancreatic duct join
each other and empty into duodenum.
Jejunum
 2.5 meters; attaches anteriorly to the duodenum
Ileum
 3.5 meters, extends from jejunum to large intestine
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Subdivisions of the Small Intestine
Duodenum, jejunum and ileum have same
structure except:
 there is a gradual decrease in diameter
 decrease in thickness of the walls
 decrease in # of circular folds
 decrease in # of villi
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Chemical Digestion in the Small Intestine
3 modifications to increase surface area up to 600
times
1. circular folds – run perpendicular to long axis of tract
2. villi – tiny finger-like projections formed by mucosa
3. microvilli – extensions of villi found in absorptive
cells
Mucous Lining of Small Intestine
composed of simple columnar epithelium
1. absorptive cells
have microvilli
produce digestive enzymes
absorb digested food
2. goblet cells
produce protective mucus
3. granular cells
protect intestinal epithelium from bacteria
4. endocrine cells
produce regulatory hormones
Secretions of the Small Intestine
• lubricate and protect intestinal wall from acidic
chyme and digestive enzymes
• keep chyme in liquid form
• produced by mucosa, pancreas, liver
• peptidases – break proteins into amino acids
• disaccharidases – break down complex sugars
into monosaccharides
Absorption in the Small Intestine
major function of small intestine
most in duodenum and jejunum
 monosaccharides – absorbed through active
transport or facilitated diffusion by villi
 amino acids – absorbed through active transport by
villi and carried away in blood
 fatty acids – dissolve into cell membranes of villi and
carried away by lymphatic vessels
 water – absorbed through osmosis into villi
 electrolytes – absorbed by active transport by villi
Movement in the Small Intestine
• mixing and propulsion –
mechanical breakdown
• peristaltic contraction – causes
chyme to move along tract;
occur along entire digestive
tract
• segmental contraction – short
movements to mix contents
Large Intestine




larger in diameter, but shorter than the small intestine
1.5 meters
extends from ileocecal valve to anus
major functions:
 absorption of water, electrolytes, vitamins K and B
 elimination of indigestible food (feces)
 production of mucus for lubrication
 No digestion of food occurs here.
 Bacteria produce vitamin K and B.
Divisions of the Large Intestine
 cecum – saclike first part of the large intestine
 appendix
 accumulation of lymphatic tissue
 hangs from the cecum
 colon
 ascending
 transverse
 descending
 S-shaped sigmoid (in pelvis)
 rectum
 anus
 external body opening
 contains 2 sphincters
Large Intestine
Figure 14.8
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
Structures of the Large Intestine
 3 bands of longitudinal muscle
 arranged in teniae coli
 create haustra (pocket like sacs in wall)
 large numbers of goblet cells
Absorption in the Large Intestine
 no digestive enzymes
 absorption of water and electrolytes and vitamins K and B
Propulsion in the Large Intestine
 mass movements
 slow, powerful contractions
 move contents toward anus
 occur two to three times per day
 cover more distance than peristaltic contractions
 Presence of feces in the rectum causes a defecation
reflex.
 Internal anal sphincter is relaxed.
 Defecation occurs with relaxation of the voluntary
(external) anal sphincter.
Pancreas
 located behind the stomach
 produces digestive enzymes that break down all categories
of food
 trypsin – protein digestion
 pancreatic amylase – starch digestion
 lipidase – fat digestion
 secreted into the duodenum
 Alkaline fluid introduced with enzymes neutralizes acidic
chyme.
Liver
 located on the right side of the body under the
diaphragm
 consists of four lobes
 capable of regeneration
 connected to the gall bladder via the common hepatic
duct




metabolizes carbohydrates, lipids, proteins
stores glycogen, vitamins A,D and B12, iron and blood
filters blood, removes toxins and damaged blood cells
secretes bile
Bile
 yellowish, green liquid
 includes water, bile salts, bile pigments, cholesterol
and electrolytes
 Bile salts
 emulsify fats.
 aid in absorption of fatty acids, cholesterol and some
vitamins
Gall Bladder
 sac found beneath liver
 stores bile from the liver
 releases bile into the duodenum in the presence of fatty
food
 release controlled by sphincter muscles
 Crystallized bile causes gallstones.
Processes of the Digestive System
Figure 14.11