Digestive System
Chapter 22 – Day 1
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Digestive system
Respiratory System
♦ Brings O2 to the body
Cardiovascular System
♦ Brings O2 to the cells
In tissues O2 is used for cellular respiration, BUT…
O2 is only one of the ingredients – you also need
♦ Glucose
Digestive System
♦ Brings glucose to the body
♦ Other sources of nutrients
♦ And other building blocks to make cells
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Digestive system
Food is broken down & processed
Nutrients are absorbed
Waste products are eliminated
All of this happens in the digestive tract
A.K.A. alimentary canal or GI tract
The digestive tracts is a long, continuous muscular
tube starting at the esophagus
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Digestive system
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Fig. 21.11
Digestive system - Processes
In order for nutrients to reach cells the digestive
system carries out several processes
♦ Ingestion = food in the mouth
♦ Mechanical processing = physically breaking food into bits
♦ Digestion = enzymes and hormones
♦ Absorption = organic molecules enter interstitial fluid
♦ Excretion = elimination of unwanted materials
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Digestive system – Cell/Tissue layers
The digestive system has a distinct system of tissue
layers
There is an “open” surface inside the GI tract –
epithelial tissue covering on outside
4 layers – from inside to outside
♦ These are there ALL ALONG the GI tract
♦ Mucosa
• innermost layer
♦ Submucosa
♦ Muscularis externa
♦ Serosa
• outermost layer
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Fig. 22.3
Cell/Tissue layers - MUCOSA
 Digestive epithelium
 Connective tissue – lamina propria
 (smooth) muscle tissue – muscularis mucosa
 Type of epithelium varies along the GI tract
♦ Pharynx & esophagus handle large masses of food = stratified epithelium
♦ In the stomach – food becomes liquid & passes to intestines
♦ A great deal of absorption happens in the intestines
• Simple columnar epithelium
• Secrete mucous
• Highly folded epithelial surface (small intestine)
 Lamina propria – areolar connective tissue right below
basement membrane of epithelium
 Contains blood vessels, nerves, mucous glands (in the
intestines this is where you find the Peyer’s patches)
 Muscularis mucosa = smooth muscle – helps movement in the
GI tract
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Cell/Tissue layers - SUBMUCOSA
Dense connective tissue
Nerve network
♦ Submucosal plexus
Communication with the nervous system – helps
muscle movement
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Cell/Tissue layers – MUSCULARIS
EXTERNA
Smooth Muscle Tissue
Nerve network
♦ Mysenteric plexus
More forceful contractions for movement
2 layers of muscle
♦ Longitudinal
♦ Circular
Allows 2 types of movement
♦ Rocking motion
♦ Forward movement
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Cell/Tissue layers – SEROSA
Fibrous tissue on outside
Protective covering which helps in attachment to
abdominal wall
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Fig. 22.3
Other important general features
Membranes
♦ Perotineal cavity
• 2 membrane layers
♦ Encloses abdominal portion of digestive tract
♦ Serosa = visceral peritoneum (inner layer)
♦ Parietal peritoneum = lines abdominal wall (outer layer)
♦ Additional serous membrane connects serosa/visceral to
parietal peritoneum
♦ The space between the 2 layers if filled with fluid
• Peritoneal fluid
• Secreted by cells of the peritoneum
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Other important general features
Mesenteries
♦ Sheets of serous membrane allow blood vessels, nerves,
and lymph vessels to pass through & network
♦ Stabilize the position of the GI tract
• Prevents “flopping around”
♦ OMENTUS – associated with the stomach (Fig. 22-12)
• Lesser omentum & greater omentum
• Contains adipose tissue (fat around the belly)
♦ Mesentery proper
• Within small intestine, between loops
♦ Mesocolon
• Stabilizes large intestine
♦ Areas without mesentaries = pancreas & duodenum – which
are attached to the abdominal wall for stability
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Secretions
Within the digestive tract – secretions are evident
throughout
♦ Mucus
• To lubricate food & “walls”
♦ Digestive enzymes
• Break chemical bonds
♦ Electrolytes
• Buffers - change pH
• Cofactor for enzymes
Parts of digestive system & direction of food…
Oral cavity → Pharynx → Esophagus → Stomach →
→ Small Intestines → Large Intestines → Rectum → Anus
Explore important structures, secretions, absorption,
digestion process, movement of food
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Oral Cavity = Mouth
Consists of
♦ Cheek/walls
• Buccal walls (lined with membrane)
♦ Teeth
♦ Tongue
♦ Palate (roof of mouth)
Function of Oral Cavity
♦ Mastication = chewing
♦ Food is broken into smaller pieces
♦ Tough fibers are separated for access to enzymes
♦ Your teeth can apply up to 2 tons of pressure
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Oral Cavity = Mouth
During chewing secretions are produced
♦ Saliva (& mucus)
♦ Saliva = water + mucus + digestive enzymes + solutes
♦ Water – dissolves food as it is broken up
• Chemicals are then able to bind to taste buds = taste sensation
♦ Mucus – lubricates food
• makes it smaller & smoother for swallowing
♦ Digestive enzymes – break chemical bonds
• α – amylase: breaks starch (polysaccharide) into maltose
(disaccharide)
• Also have lysozymes – destroy bacteria (control population)
♦ Other solutes – help maintain pH (6.35-6.85)
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Oral Cavity = Mouth
 Saliva is secreted by SALIVARY GLANDS
 Mucus is secreted by BUCCAL GLANDS
♦ Located in the epithelium of the mouth
 Salivary glands: there are 3 types/locations
♦ Parotid:
• Pair
• Inferior & anterior to ears
• Empty into roof of mouth
• Secretes digestive enzymes
♦ Sublingual
• Under tongue
• Empties near base of tongue
• Secretes mucus
♦ Submandibular
• Back of tongue
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Oral Cavity = Mouth
Chewing and lubrication converts food particles into a
slimy ball = BOLUS (mass of food)
Bolus is then swallowed
The process of swallowing – begins DEGLUTITION
♦ Bolus leaves mouth to enter esophagus and then travels to
stomach
♦ Forces epiglottis to close larynx
♦ Triggers deglutition center in medulla
• To start involuntary muscle movement
♦ Bolus tracks from
• Oral cavity → pharynx = buccal phase
• Pharynx → esophagus = pharyngeal movement/phase
• Esophagus → stomach = esophageal movement/phase
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Oral Cavity: Food Movement
Movement through the pharynx is fast → slides
through oropharynx → laryngopharynx
Esophagus
♦ 10 inch long tube
• Contains 2 special muscular areas = sphincters (openings)
• UPPER ESOPHAGEAL SPHINCTER:
◦
◦
◦
◦
◦
guards entrance to esophagus
Contracted – constricts & closes opening
For food to pass muscle must be relaxed
Relaxes at the time of swallowing
Food slides into the esophagus from pharynx
• CARDIAC SPHINCTER
◦ At junction of esophagus and stomach
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Oral Cavity: Food Movement
In the esophagus – the bolus makes its way to the
stomach, more mucus is secreted to keep up
lubrication
The bolus moves with the help of SMOOTH muscle
activity
There are 2 types of movement in digestive system
Forward Direction & Side – to – Side Movement (next)
For muscle contraction to take place:
♦ Submucosal plexus & submyenteric plexus must be
activated by the autonomic nervous system
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Movement of Bolus
Forward movement
♦ Waves of contraction in muscularis externa
♦ Along length of tube
♦ Process of peristalsis
♦ Circular muscles contract behind bolus
♦ Longitudinal muscles contract at cardiac sphincter
♦ Wave of relaxation opens entrance to stomach
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Movement of Food (bolus)
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Fig. 22.4
Movement of Bolus
Forward movement
♦ Waves of contraction in muscularis externa
♦ Along length of tube
♦ Process of peristalsis
♦ Circular muscles contract behind bolus
♦ Longitudinal muscles contract at cardiac sphincter
♦ Wave of relaxation opens entrance to stomach
Side to side movement
♦ No set direction
♦ Helps to mix bolus with mucus for more lubrication
♦ Contraction in muscles → segmentation
♦ Mostly in large & small intestine – helps to fragment bolus
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Stomach Anatomy
Shape
Sphincters
♦ Cardiac
♦ Pyloric
Folds = rugae
♦ Deep muscular folds
Mucosa
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Fig. 22.12
Stomach Anatomy
Mucosa
♦ Gastric pits with gastric glands
♦ Secretory cells
4 types of secretory cells:
Cell
Chief cells
→
Parietal cells
→
Mucus cells
→
Enteroendocrine cells →
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Secretion
Pepsinogen
HCl (acid)
Mucus
Gastrin (hormone)
Stomach Processes
 What happens to food when it enters the stomach?
 Digestion & Secretion – almost no absorption
 3 phases of secretion in the stomach
1. Cephalic
♦ Begins at the sight of food
♦ Gastrin is secreted
♦ Stimulates HCl & pepsinogen
 Food enters the stomach
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Phases of Gastric Secretion in Stomach
2. Gastric Phase
Secretion
 Mucus is secreted to protect stomach lining
 More gastrin, more pepsinogen
 Acidic environment – pH drops (pepsinogen →
pepsin at low pH)
 Secretions stop when pH reaches 2.0
Digestion
 Proteins in food →pepsin →amino acids
 Milk proteins →gastric lipase → amino acids & renin
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Phases of Gastric Secretion in Stomach
2. Gastric Phase
Mixing
 Rugae become stretched – stomach is distended
 Muscular contractions mix food for several hours
 Food becomes watery mixture
♦ Chyme (acidic)
 After several hours of mixing waves of contractions
(peristalsis) reach the lower end/base of the stomach
– near the pyloric sphincter
 Sphincter opens & closes with each wave
 Squirts chyme into the duodenum
 The Duodenum secretes enteric gastrin
♦ starts next phase
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Phases of Gastric Secretion in Stomach
2. Gastric Phase
General info/reminders
 After 2-6 hours, the stomach is emptied
 Some macromolecules move faster through the
stomach:
♦ Carbohydrates
♦ Proteins
♦ Then fats
 Remember NO absorption in the stomach except for
EtOH, H2O, aspirin (alcohol is absorbed fast – gets
to brain fast)
 On to next phase = intestinal phase
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Phases of Gastric Secretion in Stomach
3. Intestinal Phase
Food moves to intestine = gastric emptying
 Small intestine secretes 2 hormones:
 Cholecystokinin (CCK)
♦ Is released when proteins & fat are in the chyme
♦ Inhibits gastric secretions
♦ Triggers pancreas secretion
 Secretin
♦ Released when pH in duodenum drops below 4.5
♦ Stimulates bicarbonate release from pancreas
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• Deactivates pepsin
• Inhibits stomach secretions
• Stimulates bile secretion from liver
Swallowing
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Fig. 22.11
Alvioli – Capillary Interface
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Fig. 22.4
ld
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Mechanics of Respiration
Ventilation
♦ = mechanical process
♦ involves the diaphragm and skeletal muscles (intercostal
muscles)
Breathing consists of 2 phases:
♦ Inspiration
• air is taken into the lungs
♦ Expiration
• Air passes out of the lungs
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Alvioli – Capillary Interface
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Fig. 21.11