Chapter 23Digestive System

Lecture 11

Part 3: Large Intestine and Defecation

Marieb’s HumanAnatomy and

PhysiologyNinth Edition

Marieb Hoehn

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Large Intestine

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001 *

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Histology of the Large Intestine

Walls of large intestine are much thinner than the small intestine, however, the lumen is larger

Note lack of villi and presence of numerous goblet cells (mucus)

No enzymes produced; any digestion is from previously introduced enzymes or bacteria

*

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Functions of Large Intestine• little or no digestive function

• absorbs water, bile salts, and electrolytes

• secretes mucus (lubrication, binding, protection, pH)

• conversion of bilirubin (uro- and stercobilinogen)

• houses intestinal flora (~800 species of bacteria) and absorbs vitamins liberated by bacterial action (K, B5, and Biotin); produces intestinal gas (flatus)

• forms and stores feces

• carries out defecation

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The Rectum, Anal Canal, and Anus

Temporary storage of fecal material in rectum triggers the urge to defecate

Internal anal sphincter is usually contracted but relaxes in response to distension. External sphincter must be tensed reflexively to retain feces

(Keratinzed strat. squamous epithelium)

Procto- = anus or rectum

Rectal valves

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Movements of Large Intestine

• slower and less frequent than those of small intestine

• mixing movements (haustral churning every 30 min)

• mass movements - usually follow meals (stimulated by distension of stomach and duodenum)

- gastrocolic reflex- duodenocolic reflex

- peristaltic wave from transverse colon through rest of large intestine

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Parasympathetic Defecation Reflex

Note that this reflex opens the internal sphincter and closes the external sphincter

Need voluntary relaxation of the external sphincter for defecation

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Feces• water (75%), solids (25%)

• electrolytes

• mucus

• bacteria (30% of solids) and sloughed epithelial cells

• bile pigments altered by bacteria provide color (mainly urobilins and stercobilins)

• odor produced by bacterial compounds (indoles and skatoles, phenols, H2S, ammonia)

• indigestible materials

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Review

• Liver– Two major lobes– Blood supply and drainage– Functional unit is the lobule

• Plates of hepatocytes

• Portal triads

• Blood/bile flow in lobules

– Three major categories of function• Metabolic regulation

• Hematological regulation

• Bile production

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Review

• Gall Bladder (GB)– Major function – storage and concentration of bile

– Stimulated to contract by CCK

– Path of bile from GB• Cystic duct

• Common bile duct

• Hepatopancreatic sphincter (Oddi) – CCK relaxes

• Duodenal ampulla

– Bile• Water, bile salts, bile pigments, electrolytes, cholesterol

• Function is emulsification of fats for digestion

• Absorption of fats

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Review

• Small Intestine– Three major parts

• Duodenum – mixing chamber; mucus• Jejunum – digestion• Ileum – connects to cecum of large intestine

– Blood supply and drainage– Suspended from the posterior abdominal wall

by the mesentery– Surface area greatly increased by

• Plicae• Villi• Microvilli

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Review

• Small Intestine (cont’d)– Secretions– Control of secretion and absorption– Absorption

• Protein, CHO, electrolytes – facilitated diffusion

• Fats via chylomicrons and lacteals -> circulation

– Movements• Local via myenteric plexuses

• Long distance via stomach filling– Gastroenteric reflex

– Gastroileal reflex

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Review

• Large Intestine– Three major parts

• Cecum (veriform appendix)

• Colon (ascending, transverse, descending)

• Rectum

– Blood supply and drainage– Haustra (pouches)– Longitudinal muscle of external muscularis

reduced to long strips (taeniae coli)– Wall is thinner than small intestine, less

muscle, no villi

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Review

• Large Intestine (cont’d) – Rectum

• Temporary storage of feces – urge to defecate• Internal sphincter contracted until distended• External sphincter is voluntary

– Major Functions• Absorption of water, bile salts, and electrolytes• Absorbs vitamins produced by bacteria (K, biotin, B5)• Converts bilirubin

– Movements slower and less frequent than small intestine

• Mass movements• Stimulated by gastric and duodenal distension