Small Intestine

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UG lecture


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Small Intestine:

Small Intestine Dr Raghuveer Choudhary

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Overview - Nearly all chemical digestion and nutrient absorption occur in the small intestines.

Small Intestine :

Small Intestine Nearly all chemical digestion & nutrient absorption occurs here Longest part of digestive tract Circular folds of mucosa , villi, and microvilli – enhance surface area for absorption of nutrients

Three Parts of Small Intestine:

4 Three Parts of Small Intestine

Gross anatomy :

Gross anatomy Three regions: Duodenum - first 25 cm Jejunum – next 2.5 m Ileum – last 3.6 m ; ends at ileocecal junction – joins cecum of large intestine

The Small Intestine plays key role in digestion and absorption of nutrient, 90% of nutrient absorption occurs in the small intestine :

The Small Intestine plays key role in digestion and absorption of nutrient, 90% of nutrient absorption occurs in the small intestine The Duodenum The segment of small intestine closest to stomach 25 cm (10 in.) long “Mixing bowl” that receives: chyme from stomach digestive secretions from pancreas and liver

The Jejunum :

The Jejunum Is the middle segment of small intestine 2.5 meters (8.2 ft) long Is the location of most: chemical digestion nutrient absorption The Ileum The final segment of small intestine 3.5 meters (11.48 ft) long

Microscopic anatomy :

Microscopic anatomy Largest folds – circular folds – mucosa & submucosa – plicae circularis From duodenum to middle ileum – mixing & nutrient absorption Mucosa – projections – villi – largest in duodenum & get smaller – covered by absorptive cells and goblet cells Core of villus – arteriole, capillary, venule, lacteal

Wall of Small Intestine:

10 Wall of Small Intestine

Digestive System:

Digestive System Or folds Kerckring – well developed in duodenum and jejunum; inc absorptive area 3x Less in distal small int; inc Absorptive area another 10x

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- The surface area inside the small intestine is greatly increased by circular folds, villi, and microvilli. villi

Structure of the Villi in the Small Intestine:

13 Structure of the Villi in the Small Intestine On each epithelial cell on each villus ; inc absorptive capacity for another 20x

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Microvilli (brush border) brush border enzymes

Intestinal Villus:

15 Intestinal Villus

Small Intestine:

Small Intestine Each villus is a fold in the mucosa. Covered with columnar epithelial cells interspersed with goblet cells. Epithelial cells at the tips of villi are exfoliated and replaced by mitosis in crypt of Lieberkuhn. Lamina propria contain lymphocytes, capillaries, and central lacteal. Insert fig. 18.12

Intestinal Villi :

Intestinal Villi A series of fingerlike projections: in mucosa of small intestine Covered by simple columnar epithelium:covered with microvilli Intestinal glands have goblet cells between columnar epithelial cells Eject mucins onto intestinal surfaces Brush Border Enzymes are integral membrane proteins ,on surfaces of intestinal microvilli Break down materials in contact with brush border by trypsinogen

Digestive Anatomy: Histological Overview:

Digestive Anatomy: Histological Overview Figure 21-2e: ANATOMY SUMMARY: The Digestive System

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Epithelial cells cover the mucosa

Secretions of Small Intestine:

21 Secretions of Small Intestine peptidase – breaks down peptides into amino acids sucrase, maltase, lactase – break down disaccharides into monosaccharides lipase – breaks down fats into fatty acids and glycerol enterokinase – converts trypsinogen to trypsin somatostatin – hormone that inhibits acid secretion by stomach cholecystokinin – hormone that inhibits gastric glands, stimulates pancreas to release enzymes in pancreatic juice, stimulates gallbladder to release bile secretin – stimulates pancreas to release bicarbonate ions in pancreatic juice

Absorption in Small Intestine:

Duodenum and jejunum: Carbohydrates, amino acids, lipids, iron, and Ca 2+ . Ileum: Bile salts, vitamin B 12 , electrolytes, and H 2 0. Absorption in Small Intestine

Digestive Secretions: (7 L / Day From Tissues into Lumen):

Digestive Secretions: (  7 L / Day From Tissues into Lumen ) Figure 21-5: Daily mass balance in the digestive system Salivary glands Pancreas Water Enzymes Mucus Ions: H + , K + , Na + HCO 3 - , Cl - Mass Balance (H 2 O)

Intestinal Enzymes:

Microvilli contain brush border enzymes that are not secreted into the lumen. Brush border enzymes remain attached to the cell membrane with their active sites exposed to the chyme. Absorption requires both brush border enzymes and pancreatic enzymes. Intestinal Enzymes

Intestinal Secretions:

Intestinal Secretions Watery intestinal juice 1.8 liters per day enter intestinal lumen Moistens chyme Assists in buffering acids Keeps digestive enzymes and products of digestion in solution Intestinal Movements Chyme arrives in duodenum Weak peristaltic contractions move it slowly toward jejunum

Small Intestine:

27 Small Intestine Secretes digestive enzymes Peptidases Amino- Di- Tri- Sucrases Maltase Lactase Saccharidases Di- Tri- Lipase Nucleases

Small Intestine:

28 Small Intestine Absorbs 80% ingested water Electrolytes Vitamins Minerals Carbonates Active/facilitated transport Monosaccharides Proteins Di-/tripeptides Amino acids Lipids Monoglycerides Fatty acids Micelles Chylomicrons

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brush border enzymes activates zymogens complete digestion of carbohydrates and proteins

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- 1-2 L of intestinal juice per day. - The duodenum endocrine cells secret cholecystokinin (CCK) and secretin. (Both are hormones.) Secretion by the small intestines

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Chemical Digestion and Absorption of Nutrients

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Chemical Digestion and Absorption of Nutrients Carbohydrates Proteins Lipids Nucleic Acids Vitamins Minerals Water

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Digestion and Absorption of Carbohydrate - Most digestible dietary carbohydrate is starch. The starch digestion begins in the mouth by salivary amylase. But fully digestion of starch occurs in the small intestines.

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- Starch is digested to oligosaccharides (3-8 glucose residues), disaccharide maltose , and glucose .

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starch oligosaccharides glucose pancreatic amylase Brush border enzymes Intestinal lumen Intestinal epithelial cells blood glucose glucose

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Glucose is absorbed by: - sodium-dependent glucose transporter (SGLT). - solvent drag

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Digestion and Absorption of Proteins

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- Proteins are digested by proteases and peptidases . - Protein digestion starts in the stomach.

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Protein digestion continues in the small intestine by pancreatic enzymes trypsin and chymotrypsin.

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Protein digestion is completed in the small intestine by brush border enzymes carboxypeptidase , aminopeptidase , and dipeptidase . Amino acid absorption is similar to that of monosaccharides, via several sodium-dependent amino acid cotransporters .

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proteins short peptides amino acids Gastric pepsin & pancreatic proteases Brush border enzymes Intestinal lumen Intestinal epithelial cells blood

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Digestion and Absorption of Lipids Lipids are digested by enzymes called lipases . Most fat digestion occurs in the small intestine via several steps.

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CHEMICAL DIGESTION OF CARBOHYDRATES STARCH SUCROSE LACTOSE mouth stomach small intestine salivary amylase (absorbed into blood of villus) MALTOSE pancreaticamylase brush border maltase glucose + glucose SUCROSE glucose + fructose (absorbed into blood of villus) brush border sucrase LACTOSE glucose + galactose (absorbed into blood of villus) brush border lactase

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CHEMICAL DIGESTION OF PROTEINS PROTEINS mouth stomach small intestine (absorbed into blood of villus) SMALLER PROTEINS, POLYPEPTIDES trypsin trypsinogen brush border peptidases peptides pepsin pepsinogen HCl enterokinase chymotrypsin chymotrypsinogen trypsin carboxypeptidase procarboxypeptidase trypsin amino acids from pancreas from intestinal glands

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CHEMICAL DIGESTION OF FATS FATS mouth stomach small intestine lingual lipase (absorbed into lymph of villus) EMULSIFIED FATS pancreatic lipase + cholesterol esterase CHYLOMICRONS gastric lipase from pancreas reassembled into bile FATTY ACIDS, GLYCEROLS, GLYCERIDES from gallbladder/liver minimal effects minimal effects

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REGULATION OF THE SMALL INTESTINE psychic stimuli stretch of stomach increased parasympathetic impulses via vagus nerve increased small intestinal motility (gastroileal reflex) and secretion + relaxation of ileocecal sphincter gastroileal reflex chemoreceptors in stomach increased gastrin secretion stretch of small intestine increased enteric nervous system activity GASTRIC PHASE

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Monosaccharides Amino acids ABSORPTION IN THE SMALL INTESTINE hepatic portal blood  liver  inferior vena cava  general circulation Fats = chylomicrons lymph  lacteals  intestinal trunk  thoracic duct  general circulation Water blood capillary lacteal with chylomicrons monosaccharides amino acids superior mesenteric vein hepatic portal vein thoracic duct inferior vena cava lymphatic vessel GI TRACT FLUIDS PER 24 HOURS ingested or secreted into tract absorbed into blood or lymph Saliva = 1 L Ingested liquids = 2 L Gastric juice = 2 L Bile = 1 L Pancreatic juice = 2 L Intestinal juice = 1L Total = 9 L Small intestine = 8 L Large intestine = 0.9 L excreted in feces 100 – 150 ml

Digestion and Absorption of Carbohydrates:

Digestion and Absorption of Carbohydrates Salivary amylase: Begins starch digestion. Pancreatic amylase: Digests starch to oligosaccharides. Oligosaccharides hydrolyzed by brush border enzymes. Glucose is transported by secondary active transport with Na + into the capillaries. Insert fig. 18.32

Digestion and Absorption of Protein:

Digestion begins in the stomach when pepsin digests proteins to form polypeptides. In the duodenum and jejunum: Endopeptidases cleave peptide bonds in the interior of the polypeptide: Trypsin. Chymotrypsin. Elastase. Exopeptidases cleave peptide bonds from the ends of the polypeptide: Carboxypeptidase. Aminopeptidase. Digestion and Absorption of Protein

Digestion and Absorption of Protein (continued):

Digestion and Absorption of Protein (continued) Free amino acids absorbed by cotransport with Na + . Dipeptides and tripeptides transported by secondary active transport using a H + gradient to transport them into the cytoplasm. Hydrolyzed into free amino acids and then secreted into the blood. Insert fig. 18.33

Digestion and Absorption of Lipids:

Arrival of lipids in the duodenum serves as a stimulus for secretion of bile. Emulsification: Bile salt micelles are secreted into duodenum to break up fat droplets. Pancreatic lipase and colipase hydrolyze triglycerides to free fatty acids and monglycerides. Colipase coats the emulsification droplets and anchors the lipase enzyme to them. Form micelles and move to brush border. Digestion and Absorption of Lipids

Digestion and Absorption of Lipids (continued):

Free fatty acids, monoglycerides, and lysolecithin leave micelles and enter into epithelial cells. Resynthesize triglycerides and phospholipids within cell. Combine with a protein to form chylomicrons. Secreted into central lacteals. Digestion and Absorption of Lipids (continued)

Transport of Lipids:

In blood, lipoprotein lipase hydrolyzes triglycerides to free fatty acids and glycerol for use in cells. Remnants containing cholesterol are taken to the liver. Form VLDLs which take triglycerides to cells. Once triglycerides are removed, VLDLs are converted to LDLs. LDLs transport cholesterol to organs and blood vessels. HDLs transport excess cholesterol back to liver. Transport of Lipids

Absorption of Fat:

Absorption of Fat Insert fig. 18.36

Intestinal Contractions and Motility:

Intestinal Contractions and Motility 2 major types of contractions occur in the small intestine: Peristalsis: Slow movement. Pressure at the pyloric end of small intestine is greater than at the distal end. Segmentation: Major contractile activity of the small intestine. Contraction of circular smooth muscle. Mix chyme. Insert fig. 18.14

Contractions of Intestinal Smooth Muscles:

Contractions of Intestinal Smooth Muscles Occur automatically in response to endogenous pacemaker activity. Rhythm of contractions is paced by graded depolarizations called slow waves. Slow waves produced by interstitial cells of Cajal. Slow waves spread from 1 smooth muscle cell to another through nexuses. Insert fig. 18.15

Contractions of Intestinal Smooth Muscles:

When slow waves above threshold, it triggers APs by opening of VG Ca 2+ channels. Inward flow of Ca 2+ : Produces the upward depolarization phase. Stimulates contraction of smooth muscle. Repolarization: VG K + channels open. Slow waves decrease in amplitude as they are conducted. May stimulate contraction in proportion to the magnitude of depolarization. Contractions of Intestinal Smooth Muscles

Cells and Electrical Events in the Muscularis:

Cells and Electrical Events in the Muscularis Insert fig. 18.16

Peristaltic Contractions:

Peristaltic Contractions Myenteric reflexes,Not under CNS Parasympathetic stimulation: accelerates local peristalsis and segmentation Gastroenteric Reflex Stimulates motility and secretion: along entire small intestine Gastroileal Reflex triggers relaxation of ileocecal valve: Allows materials to pass: from small intestine into large intestine

Mechanical Processes Motility of the Small Intestine:

66 Mechanical Processes Motility of the Small Intestine Segmentation is the most common motion of the small intestine, which is the contracting and relaxing of smooth muscle. Pacemaker cells in the smooth muscle initiate segmentation, although the duodenum depolarizes more frequently (12 –14 contractions per minute) then the ileum (8 – 9 contractions per minute).

Mechanical Processes Motility of the Small Intestine:

67 Mechanical Processes Motility of the Small Intestine This allows ample time for complete digestion and absorption as contents move towards the ileocecal valve. Long and short reflexes and hormones alter the intensity of segmentation. Parasympathetic activity enhances and sympathetic activity decreases segmentation.


68 Peristalsis Peristalsis occurs after most nutrients have been absorbed and is regulated on the basis of which neurons are stimulated. Peristaltic waves sweep slowly along the duodenum to sweep out debris, bacteria, and meal remnants. This is called the migrating mobility complex and it’s function is to keep bacteria from settling into the small intestine. The enteric neurons of the GI tract coordinate the mobility patterns. Impulses sent proximally by the cholinergic neurons cause contraction and shortening of the muscle layer.


69 Peristalsis Impulses sent distally to certain interneurons cause shortening of the longitudinal muscle layer and distension of the intestine, in response to Ach releasing neurons. Other impulses sent distally by activated VIP or NO releasing enteric neurons that cause relaxation of the circular muscle. As a result, as the proximal area constricts and forces chyme along the tract, the lumen of the intestine enlarges to receive it, where it moves toward the ileocecal sphincter.


70 Peristalsis Most of the time, the ileosphincter valve is constricted and closed. Two mechanisms; one neural, the other hormonal cause it relax and allow food residues to enter the cecum. Enhanced activity in the stomach initiates the gastroileal reflex that enhances force of segmentation in the ileum. Gastrin released by the stomach increases motility of the ileum and relaxes the ileocecal sphincter. Once the chyme has passed through, it exerts backpressure that closes the valve’s flaps and prevents regurgitation.

Segmentation movements of the small intestine:

71 Segmentation movements of the small intestine


72 Peristalsis

Small Intestinal Phase:

Small Intestinal Phase Digestion and Absorption Enormous surface area Large blood flow (up to 25% of cardiac output after meals) Extensive ENS controlling motility autonomously Segmental contractions Peristaltic contractions

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Peristalsis Segmentation Contractions A ring of muscle contraction appears on the oral side moves toward the anus, propelling the contents of the lumen in that direction; as the ring moves, the muscle on the other side of the distended area relaxes, facilitating smooth passage of the bolus. Segmentation contractions are a common type of mixing motility seen especially in the small intestine - segmental rings of contraction chop and mix the ingesta. Alternating contraction and relaxation of the longitudinal muscle in the wall of the gut also provides effective mixing of its contents.

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Small intestine Mixing contractions propulsive contractions

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Mixing contractions segmentation contractions

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Propulsive movements segmentation contractions peristalsis Peristaltic rush

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peristalsis 1. Occur in any part of the small intestine 2. Move analward at a slow velocity 3. Are weak and die out after traveling only 3 to 5 centimeters 4. The net movement of chyme in the small intestine is very slow

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peristalsis Distention of the stomach elicit peristalsis of small intestine through myenteric plexus, which is called gastroenteric reflex the entry of chyme into the duodenum

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Peristaltic rush Powerful and rapid peristalsis Travel long distances to sweep the contents of the intestine into the colon and thereby relieve the small intestine irritative chyme and excessive distention Is produced by intense irritation of the intestinal mucosa, infectious diarrhea

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Ileocecal valve and ileocecal sphincter cecum ileum The lips of the ileocecal valve protrude into the lumen of the cecum to prevent the backflow of fecal contents from the colon into the small intestine Sphincter remains constricted and slows the emptying of ideal contents into the cecum

Activities of Major Digestive Tract Hormones:

Activities of Major Digestive Tract Hormones Figure 24–22

Intestinal Absorption :

Intestinal Absorption It takes about 5 hours for materials to pass: from duodenum to end of ileum Movements of the mucosa increases absorptive effectiveness: stir and mix intestinal contents constantly change environment around epithelial cells

Hormones of Duodenal Enteroendocrine Cells:

Hormones of Duodenal Enteroendocrine Cells Coordinate digestive functions: secretin cholecystokinin ( CCK ) gastric inhibitory peptide ( GIP ) -when fats and carbohydrates enter small intestine vasoactive intestinal peptide ( VIP ) - Inhibits acid production in stomach Gastrin - Stimulates acids and enzyme production Enterocrinin - when chyme enters small intestine,Stimulates mucin production

Large Intestine:

87 Large Intestine Extends from ileocecal valve to anus Regions Cecum – Appendix Colon Ascending Transverse Descending Rectum Anal canal

Large Intestine:

88 Large Intestine Histology No villi No permanent circular folds Smooth muscle Taeniae coli Haustra Epiploic appendages Otherwise like rest of Gl tract

Functions of the Large Intestine:

Functions of the Large Intestine Reabsorption of water Compaction of intestinal contents into feces Absorption of important vitamins produced by bacteria Storage of fecal material prior to defecation Also called large bowel Is about 1.5 meters long and 7.5 cm wide 3 Parts of the Large Intestine : Cecum : the pouchlike first portion -Stores materials and begins compaction Colon : the largest portion, Has a larger diameter and thinner wall than small intestine The wall of the colon: forms a series of pouches ( haustra ) which permit expansion and elongation of colon 3. Rectum : the last 15 cm of digestive tract

Colonic Phase:

Colonic Phase Function Final reabsorption of water and ions (proximal colon) 2 L enters  200 ml leaves Propulsion Elimination of remaining waste products (rectum)

Large Intestine:

Large Intestine The large intestine is wider than the small intestine and begins beyond the ileocecal sphincter and ends at the anus. The large intestine consists of the cecum ; the ascending colon, transverse colon, descending colon, and sigmoid colon; the rectum; and the anal canal.

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The longitudinal layer of smooth muscle is arranged in three discrete strips called teniae coli. Contractions of this discontinuous muscle layer cause the wall of the large intestine to form bulges known as haustra .

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colon is lined with transporting epithelial cells called colonocytes , which absorb fluid and transport electrolytes but do not express digestive enzymes.

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The main functions of the large intestine are completion of fluid absorption and the storage and elimination of fecal waste.

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Most water absorption occurs in the right colon (the cecum , the ascending colon, and the first half of the transverse colon). Prior to defecation, fecal waste is stored in the left colon (i.e., the distal half of the transverse colon, the descending colon, and the sigmoid colon)

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By the time fecal material reaches the rectum, it consists of a small volume of K+-rich fluid containing undigested plant fibers, bacteria, and inorganic material.

The Appendix:

The Appendix Also called vermiform appendix Is a slender, hollow appendage (about 9 cm long) Is dominated by lymphoid nodules (a lymphoid organ) Is attached to posteromedial surface of cecum: mesoappendix connects appendix to ileum and cecum

4 regions :the Ascending Colon :

4 regions :the Ascending Colon Begins at superior border of cecum Ascends along right lateral and posterior wall of peritoneal cavity: to inferior surface of the liver The Transverse Colon- Curves anteriorly from right colic flexure, Crosses abdomen from right to left Is supported by transverse mesocolon Is separated from anterior abdominal wall by greater omentum

The Descending Colon :

The Descending Colon Proceeds inferiorly along left side: to the iliac fossa (inner surface of left ilium) Is retroperitoneal, firmly attached to abdominal wall The Sigmoid Colon Is an S-shaped segment, about 15 cm long Starts at sigmoid flexure Lies posterior to urinary bladder Is suspended from sigmoid mesocolon Empties into rectum

Blood Supply of the Large Intestine:

Blood Supply of the Large Intestine Receives blood from tributaries of: superior mesenteric and inferior mesenteric arteries Venous blood is collected from: superior mesenteric and inferior mesenteric veins

The Rectum:

The Rectum Forms last 15 cm of digestive tract Is an expandable organ for temporary storage of feces Movement of fecal material into rectum triggers urge to defecate The Anal Canal- Is the last portion of the rectum Contains small longitudinal folds called anal columns Anus or anal orifice i s exit of the anal canal Has keratinized epidermis like skin

Anal Sphincters:

Anal Sphincters Internal anal sphincter : circular muscle layer of muscularis externa has smooth muscle cells, not under voluntary control External anal sphincter : encircles distal portion of anal canal a ring of skeletal muscle fibers, under voluntary control

Mucosa and Glands of the Colon:

Mucosa and Glands of the Colon Figure 24–24 Colon has a Lack of villi Abundance of goblet cells Presence distinctive intestinal glands

Glands of the Large Intestine:

Glands of the Large Intestine Are deeper than glands of small intestine Are dominated by goblet cells Mucosa does not produce enzymes Provides lubrication for fecal material Physiology of the Large Intestine- Less than 10% of nutrient absorption occurs in large intestine Prepares fecal material for ejection from the body

Absorption in the Large Intestine:

Absorption in the Large Intestine Reabsorption of water Reabsorption of bile salts : in the cecum transported in blood to liver Absorption of vitamins produced by bacteria- organic molecules Important as cofactors or coenzymes in metabolism Normal bacteria in colon make 3 vitamins that supplement diet Absorption of organic wastes

3 Vitamins Produced in the Large Intestine :

3 Vitamins Produced in the Large Intestine Vitamin K: a fat-soluble vitamin required by liver for synthesizing 4 clotting factors, including prothrombin Biotin : a water-soluble vitamin important in glucose metabolism Pantothenic acid : a water-soluble vitamin required in manufacture of steroid hormones and some neurotransmitters

Organic Wastes :

Organic Wastes Bacteria convert bilirubin to urobilinogens and stercobilinogens : urobilinogens absorbed into bloodstream are excreted in urine urobilinogens and stercobilinogens in colon convert to urobilins and stercobilins by exposure to oxygen Bacteria break down peptides in feces and generate: ammonia: as soluble ammonium ions indole and skatole : nitrogen compounds responsible for odor of feces hydrogen sulfide : gas that produces “rotten egg” odor

The Defecation Reflex:

The Defecation Reflex Figure 24–25 Organic Wastes Bacteria feed on indigestible carbohydrates (complex polysaccharides): produce flatus , or intestinal gas, in large intestine

Movements of the Large Intestine :

Movements of the Large Intestine Gastroileal and gastroenteric reflexes: move materials into cecum while you eat Movement from cecum to transverse colon is very slow: allowing hours for water absorption Peristaltic waves move material along length of colon Segmentation movements ( haustral churning ) mix contents of adjacent haustra

Movements of the Large Intestine :

Movements of the Large Intestine Movement from transverse colon through rest of large intestine results from powerful peristaltic contractions ( mass movements ) Stimulus is distension of stomach and duodenum; relayed over intestinal nerve plexuses Distension of the rectal wall triggers defecation reflex : 2 positive feedback loops both loops triggered by stretch receptors in rectum

Elimination of Feces :

Elimination of Feces Requires relaxation of internal and external anal sphincters Reflexes open internal sphincter, close external sphincter Opening external sphincter requires conscious effort

Essential Nutrients:

Essential Nutrients A typical meal contains: Carbohydrates, proteins Lipids, water Electrolytes, vitamins Digestive system handles each nutrient differently: large organic molecules: must be digested before absorption can occur water, electrolytes, and vitamins: can be absorbed without processing may require special transport

Feces Formation and Defecation:

118 Feces Formation and Defecation Chyme dehydrated to form feces Feces composition Water Inorganic salts Epithelial cells Bacteria Byproducts of digestion Defecation Peristalsis pushes feces into rectum Rectal walls stretch Control Parasympathetic Voluntary

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DIARRHEA is defined as an increase in stool fluid volume of more than 200 mL within 24 hours In general terms, diarrhea may result from the delivery of more fluid to the colon than the colon can absorb, or it may result if feces move too rapidly through the colon to allow the colon to adequately absorb fluid. The general causes of diarrhea are:

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