The Large Intestine

The Large Intestine

• Terminal portion of the GI tract

• 1.5 m long and 6.5 cm in dm

• Extends from the ileum to the anus

• Attached to the posterior abdominal wall by its mesocolon

• Completion of absorption

• Production of certain vitamins

• Formation and expulsion of feces

Functions of Large intestine

• Haustral churning, peristalsis, and mass peristalsis drive the contents of the colon into the rectum

• Bacteria in the large intestine convert proteins to amino acids, break down amino acids & produce some B vitamins and vitamin K

• Absorbing some water, ions, and vitamins

• Forming feces

• Defecating (emptying the rectum)

Large intestine Regions

•Cecum

•Colon

• Rectum

•Anal canal

Anatomy of large Intestine

• Ileocecal sphincter

– Opening from the ileum into the large intestine

– Guarded by a fold of mucous membrane


   Cecum

– Hanging inferior to the ileocecal valve is the cecum

– Small pouch about 6 cm long

• Appendix or vermiform appendix

– Twisted, coiled tube

– Attached to the cecum

• Mesoappendix

– The mesentery of the appendix

– Attaches the appendix to the inferior part of the mesentery of the ileum

Colon

• Colon is divided into:

– Ascending (retroperitoneal)

– Transverse

– Descending (retroperitoneal)

– Sigmoid portion

Rectum

• The last 20 cm of the GI tract

• Lies anterior to the sacrum & coccyx

• The terminal 2–3 cm - anal canal

• The mucous membrane of the anal canal is arranged in longitudinal folds called anal columns - contain a network of arteries and veins

Anus

• The opening of the anal canal to the exterior, called the anus

• Anus - guarded by:

– Internal anal sphincter of smooth muscle (involuntary)

– External anal sphincter of skeletal muscle (voluntary)

Histology of Large Intestine

• Typical four layers found in the rest of the GI tract: mucosa, submucosa, muscularis, and serosa

Histology of Large Intestine - Mucosa

• Consists of simple columnar epithelium

• Lamina propria (areolar connective tissue)

• Muscularis mucosae (smooth muscle)

• The epithelium contains: absorptive and goblet cellslocated in long, straight, tubular intestinal glands (crypts of Lieberk├╝hn)

• Solitary lymphatic nodules are present

• No circular folds or villi (Small intestine contains)

• Microvilli are present on the absorptive cells

Sub mucosa

• Consists of areolar connective tissue

Muscularis

• Consists of:

– External layer of longitudinal smooth muscle

– Internal layer of circular smooth muscle

Serosa

– Part of the visceral peritoneum

– Small pouches of visceral peritoneum filled with fat are attached to teniae coli and are called omental (fatty) appendices

• Teniae coli

– Portions of the longitudinal muscles are thickened and forms three conspicuous bands

– Run most of the length of the large intestine

• Haustra

– Tonic contractions of the bands gather the colon into a series of pouches

– give the colon a puckered appearance

Mechanical Digestion in large Intestine

• The passage of chyme from the ileum into the cecum is regulated by the action of the ileocecal sphincter

• Immediately after a meal, a gastroileal reflex intensifies peristalsis in the ileum

• Forces any chyme into the cecum

• Gastrin also relaxes the sphincter

• Movements of the colon begins

Haustral churning: contraction and relaxation of haustral segments

Peristalsis: rhythmic contraction

– A final type of movement is mass peristalsis - begins at middle of the transverse colon

• Drives the contents of the colon into the rectum

Chemical Digestion in large Intestine

• Final stage of digestion occurs in the colon

• Mucus is secreted by the glands of the large intestine, but no enzymes are secreted

• Chyme is prepared for elimination by the action of bacteria

• Bacteria ferment any remaining carbohydrates and release hydrogen, carbon dioxide, and methane gases

• Bacteria also convert any remaining proteins to amino acids

• Break down the AA : Indole, skatole, hydrogen sulfide, and fatty acids

• Some of the indole and skatole is eliminated in the feces

• Rest is absorbed and transported to the liver

• Converted to less toxic compounds and excreted in the urine

• Bacteria also decompose bilirubin to simpler pigments

• Stercobilin (Pigment) gives feces their colour

Absorption and Feces Formation in Large intestine

• Chyme remains in the large intestine for 3–10 hours

• Become solid or semisolid because of water absorption – feces

• Chemically consist of:

– Water

– Inorganic salts

– Sloughed-off epithelial cells from the mucosa of GIT

– Bacteria & products of bacterial decomposition

– Unabsorbed digested materials

– Indigestible parts of food

• Large intestine also absorbs water, ions including sodium and chloride, and some vitamins

Defecation Reflex


Phages of Digestion

Digestive activities occur in 3 overlapping phases:

Phase 1: Cephalic phase

Phase 2: Gastric phase

Phase 3: Intestinal phase

Cephalic Phase of Digestion

• The smell, sight, thought, or initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem

• The brain stem then activates the facial (VII), glossopharyngeal (IX), and vagus (X) nerves

• The facial and glossopharyngeal nerves stimulate the salivary glands to secrete saliva

• The vagus nerves stimulate the gastric glands to secrete gastric juice

• The purpose of the cephalic phase of digestion is to prepare the mouth and stomach for food that is about to be eaten

Gastric Phase of Digestion

• Begins once food reaches the stomach

1) Neural Regulation

• Nerve impulses cause waves of peristalsis and continue to stimulate the flow of gastric juice from gastric glands

• The peristaltic waves mix the food with gastric juice

• Waves become strong enough - gastric emptying

• The pH of the stomach chyme & distension of the stomach decreases - gastric juice secretion suppresssed

2) Hormonal Regulation

• Stimulates gastric glands to secrete large amounts of gastric juice

• Strengthens the contraction of the LES to prevent reflux of acid chyme into the esophagus

• Increases motility of the stomach

• Relaxes the pyloric sphincter - promotes gastric emptying.

• Gastrin secretion is inhibited (pH drop below 2.0)

Intestinal Phase of Digestion

• Begins once food enters the small intestine

• Intestinal phase have inhibitory effects - slow the exit of chime from the stomach

• This prevents the duodenum from being overloaded

• Promote the continued digestion of foods that have reached the small intestine

• These activities of the intestinal phase of digestion are regulated by neural and hormonal mechanisms

Intestinal Phase of Digestion – Neural Regulation

• Distension of the duodenum by the presence of chyme causes the enterogastric reflex

• Stretch receptors in the duodenal wall send nerve impulses to the medulla oblongata

• Inhibit parasympathetic stimulation and stimulate the sympathetic nerves to the stomach

• As a result, gastric motility is inhibited & decreased gastric emptying

Intestinal Phase of Digestion – Hormonal Regulation

Cholecystokinin (CCK)

• Stimulates secretion of pancreatic juice

• Slows gastric emptying

• Produces satiety by acting on the hypothalamus in the brain

• Promotes normal growth and maintenance of the pancreas

Secretin

• Stimulates the flow of pancreatic juice that is rich in bicarbonate

• Buffer the acidic chyme that enters the duodenum

• Inhibits secretion of gastric juice

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