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B. Digestive System• 1. Functions of digestive tract

proper• a. Primary: digestion and

absorption of nutrients.• (1) Mechanical (physical)

digestion: breakdown of large masses into smaller particles.• (b) Main purpose:

increase the surface-area of the material for chemical digestion

• (c) Dilution of food materials by digestive juices.

• (d) Lubrication of food

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B. Digestive System• (2) Chemical digestion: chemical breakdown of

organic molecules.• (a) Digestive enzymes.• (b) Purpose: absorption of nutrients from

lumen.• 1. Except for lipids, large nutrients cannot

be taken in ex: monosaccharides, amino acids.

• (c) Further dilution by digestive juices.• (3) Absorption: of nutrients from gut lumen into

wall.• (a) Mechanisms of uptake by lumenal

epithelium: diffusion, active transport.

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B. Digestive System

• b. Secondary functions: • moving materials along digestive tract; • immune defense; • mutualistic intestinal microorganisms; • reabsorption of H2O; • elimination of wastes; etc.

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B. Digestive System

• (2) Wall structure of tubular organs.• (a) The naming system is similar to

that of blood vessels.• (b) The wall may have most or all

of the following layers, from lumenal surface outward: (4 major layers.)

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B. Digestive System• 1. Mucosa• a. mucus-producing• provides a mucus

coating to the lumenal surface for protection, lubrication, etc.;

• layer is glandular.• b. Mucosa composed of

following possible sub layers:• (1) Lumenal epithelium

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B. Digestive System

• (2) Lamina propria: below lumenal epithelium• loose-dense interwoven

connective tissue• may have glandular tubular

extensions of the lumenal epithelium

• no blood vessels larger than arterioles and venules

• cells (especially those of the immune system) abundant in this connective tissue

• Rarely absent.

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B. Digestive System

• (3) Muscularis mucosae• deepest sub layer of the

mucosa• absent in some• thin layer of smooth muscle

tissue; gentle contractions; expel gland contents; inc. contact area

• May extend up into the lamina propria.

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B. Digestive System• 2. Submucosa --absent in some organs• a. Just below the mucosa; no sub-

layers.• b. Mostly loose-dense interwoven

connective tissue• c. Contains blood small arteries and

small veins.• d. Cells; fibroblasts, immune-system

cells, etc., more sparse than in the lamina propria.

• e. Clumps of adipocytes, submucosal plexus small lymphatic vessels, etc.

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B. Digestive System

• 3. Muscularis Externa ("Muscularis")• a. Main smooth (usually)

muscle layer; contractions -and relaxations for peristalsis.

• b. Composed of two sub layers:

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B. Digestive System

• (1) Inner, circular layer• thicker of the two sub layers• smooth muscle fibers oriented

perpendicular to the main axis of the tube; encircle the tube

• contraction causes constriction, relaxation causes dilation.

• (2) Outer, longitudinal layer• thinner of the two sub layers• smooth muscle fibers oriented

along the main axis of the tube; • contraction causes shortening,

while relaxation causes lengthening of the tube.

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B. Digestive System

• c. Myenteric plexus; network of neural tissue (overall form: cylindrical net) between these two sub layers • coordinating

the contraction activities of the two sub layers

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B. Digestive System

• 4. Tunica Adventitia (aka, "serosa" --watery surface layer)• a. Outermost layer of the

wall. • b. Composed of dense-to-

loose connective tissue; contains largest blood vessels, lymphatic vessels and nerves of wall; may contain adipose tissue.)b

• c. Sometimes absent--especially when organ not surrounded by body cavity.

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B. Digestive System

• 2. Functions of accessory organs• a. Primary: production of digestive juices• b. Secondary functions: • immune defense; • production and release of hormones; • energy storage and release; • Fe storage and release; • production of plasma proteins; • intermediary metabolism; etc.

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Esophagus

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Esophagus

• (1) General features• (a) Esophagus carries

food from the pharynx to the stomach.

• (b) Lumenal epithelium subject to abrasion from food materials; stratified squamous epithelium.

• (c) Upper part involved in swallowing; muscularis externa is skeletal muscle.

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Esophagus • (2) Mucosa is highly folded • dilate to accommodate large

mass; longitudinal folds.• a) Epithelium of mucosa:

stratified squamous epithelium unkeratinized and thick.

• b) Mucosal mucus glands; tubular extensions into lamina propria from lumenal epithelium.

• c) Lamina propria composed of unusually cellular (via immune-defense cells) dense C.T.

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Esophagus

• d) Muscularis mucosae absent in upper 1/5 to 1/6 of organ• present as network of

smooth muscle fibers ; 1/5 to about 1/3 the way down the organ

• remainder, complete layer of smooth muscle tissue

• oriented longitudinally in wall.

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Esophagus

• (3) Submucosa: is continuous; extends up into folds• dense C.T• also present: loose C.T.,

small muscular arteries, small veins, small lymphatic vessels small nerves

• perhaps lymphatic nodules

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Esophagus • 4) Muscularis externa• a) Upper 1/4 : composed of

skeletal muscle tissue- one circular and one longitudinal layer.

• b) From 1/4 to 1/2 way down: transitions from skeletal to smooth tissue.• 1. Smooth muscle tissue first

appears as inner, circular layer within layer of skeletal muscle tissue.

• c) Lower half of organ: composed of smooth muscle tissue on inner circular and outer longitudinal layers.

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Esophagus • Esophagus-gastric junction (slide #29) (gaster: a root meaning stomach)• (1) At junction of esophagus with stomach• inner circular layer of the muscularis externa is thickened• the cardiac sphincter of the stomach• prevent the acidic contents of the stomach from moving up into the

esophagus• "heartburn."

• (2) The stratified squamous epithelium of the esophagus transitions abruptly to simple columnar epithelium.