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Digestive System

Chapter 22 – Day 1

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Digestive systemRespiratory 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 systemFood is broken down & processedNutrients are absorbedWaste products are eliminated

All of this happens in the digestive tractA.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

Fig. 21.11

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Digestive system - ProcessesIn 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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Fig. 22.3

Digestive system – Cell/Tissue layersThe digestive system has a distinct system of tissue

layersThere is an “open” surface inside the GI tract –

epithelial tissue covering on outside4 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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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 - SUBMUCOSADense connective tissueNerve network

♦Submucosal plexus

Communication with the nervous system – helps muscle movement

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Cell/Tissue layers – MUSCULARIS EXTERNA

Smooth Muscle TissueNerve network

♦Mysenteric plexus

More forceful contractions for movement2 layers of muscle

♦Longitudinal♦Circular

Allows 2 types of movement♦Rocking motion♦Forward movement

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Cell/Tissue layers – SEROSAFibrous tissue on outsideProtective covering which helps in attachment to

abdominal wall

Fig. 22.3

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Other important general featuresMembranes

♦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 featuresMesenteries

♦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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SecretionsWithin the digestive tract – secretions are evident

throughout♦Mucus

•To lubricate food & “walls”

♦Digestive enzymes•Break chemical bonds

♦Electrolytes•Buffers pH

•Cofactor for enzymes

Parts of digestive system & direction of food…Explore important structures, secretions, absorption,

digestion process, movement of food

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Alvioli – Capillary Interface

Fig. 21.11

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ld

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Movement of BolusForward 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)

Fig. 22.4

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Movement of BolusForward 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 AnatomyShapeSphincters

♦Cardiac♦Pyloric

Folds = rugae♦Deep muscular folds

Mucosa

Fig. 22.12

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Stomach AnatomyMucosa

♦Gastric pits with gastric glands♦Secretory cells

4 types of secretory cells:

Cell SecretionChief cells → PepsinogenParietal cells → HCl (acid)Mucus cells → MucusEnteroendocrine cells → Gastrin (hormone)

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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 Stomach2. 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 Stomach2. Intestinal PhaseMixing 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 Stomach2. Gastric PhaseMixing 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