Digestive System 446t1i

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Anabolism ‡ Uses raw materials to synthesize essential compound

Catabolism ‡ Decomposes substances to provide energy cells need to function ‡ Catabolic Reactions- Require two essential ingredients: 1.oxygen 2.organic molecules broken down by intracellular enzymes: ‡ m carbohydrates, fats, and proteins

Components of the Digestive System

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Digestive Tract ‡ Gastrointestinal (GI) tract or alimentary canal ‡ Is a muscular tube ‡ Extends from oral cavity to anus ‡ es through: ² ² ² ² ² ²

pharynx esophagus stomach small intestine large intestine anus

· Functions of the Digestive System

1. Ingestion: ²

occurs when materials enter digestive tract via the mouth

2. Mechanical processing: ² ²

crushing and shearing makes materials easier to propel along digestive tract

3. Digestion: ² ² ²

is the chemical breakdown of food into small organic fragments for absorption by digestive epithelium

· Functions of the Digestive System 4. Secretion: ² ² ²

is the release of water, acids, enzymes, buffers, and salts by epithelium of digestive tract by glandular organs

5. Absorption: ² ² ²

movement of organic substrates, electrolytes, vitamins, and water across digestive epithelium into interstitial fluid of digestive tract

·. Excretion: ²

removal of waste products from body fluids

ining of the Digestive Tract ‡ Protects surrounding tissues against: ² corrosive effects of digestive acids and enzymes ² mechanical stresses, such as abrasion ² bacteria

Racteria ‡ Is ingested with food or resides in digestive tract ‡ Attacked by macrophages, and immune system cells ‡ In lamina propria (underlying layer of areolar tissue)

Peritoneal Cavity ‡ Is located within the abdominopelvic cavity ‡ ined with serous membrane consisting of: superficial mesothelium covering a layer of areolar tissue ‡ Peritoneal Fluid-is produced by serous membrane lining ‡ Provides essential lubrication

Mesenteries

Figure 24²2a, b

Mesenteries

Figure 24²2c, d

Mesenteries (1) ‡ Are double sheets of peritoneal membrane ‡ Suspend portions of digestive tract within peritoneal cavity by sheets of serous membrane: ² that connect parietal peritoneum ² with visceral peritoneum

Mesenteries (2) ‡ Areolar tissue between mesothelial surfaces: ² provides an access route to and from the digestive tract ² for age of blood vessels, nerves, and lymphatic vessels

‡ Stabilize positions of attached organs ‡ Prevent intestines from becoming entangled

üistological Organization of the Digestive Tract ‡ Major layers of the digestive tract: ² Mucosa- Is the inner lining of digestive tract Is a mucous membrane consisting of: epithelium, moistened by glandular secretions

² submucosa ² muscularis externa ² serosa

Structure of the Digestive Tract

Figure 24²3

The Digestive Epithelium ‡ Mucosal epithelium is simple or stratified: ² depending on location, function, and stresses

‡ Oral cavity, pharynx, and esophagus: ² mechanical stresses ² lined by stratified squamous epithelium

‡ Stomach, small intestine, and most of large intestine: ² absorption ² simple columnar epithelium with goblet cells

ining of Digestive Tract ‡ Folding increases surface area for absorption: 1. longitudinal folds, disappear as digestive tract fills 2. permanent transverse folds (plicae)

Muscularis Mucosae ‡ Narrow band of smooth muscle and elastic fibers in lamina propria ‡ Smooth muscle cells arranged in 2 concentric layers:inner layer encircles lumen (circular muscle) outer layer contains muscle cells parallel to tract (longitudinal layer)

The Submucosa ‡ Is a layer of dense irregular connective tissue ‡ Surrounds muscularis mucosae ‡ üas large blood vessels and lymphatic vessels ‡ May contain exocrine glands: ² secrete buffers and enzymes into digestive tract

Muscularis Externa Structure ‡ Is dominated by smooth muscle cells ‡ Are arranged in: ² inner circular layer ² outer longitudinal layer ² Function:

‡ Involved in: ² mechanical processing ² movement of materials along digestive tract

‡ Movements coordinated by enteric nervous system (ENS): ² sensory neurons ² interneurons ² motor neurons

The Movement of Digestive Materials ‡ Ry muscular layers of digestive tract: ² consist of visceral smooth muscle tissue

‡ Smooth Muscle along digestive tract: ² has rhythmic cycles of activity ² controlled by pacesetter cells

‡ Cells undergo spontaneous depolarization: ² triggering wave of contraction through entire muscular sheet

Peristalsis |

Figure 24²4

Peristaltic Motion 1. Circular muscles contract behind bolus: ²

while circular muscles ahead of bolus relax

2. ongitudinal muscles ahead of bolus contract: ²

shortening adjacent segments

3. Wave of contraction in circular muscles: ²

forces bolus forward

Segmentation ‡ Cycles of contraction: ² Churn and fragment bolus ² mix contents with intestinal secretions

‡ Does not follow a set pattern: ² does not push materials in any 1 direction

The Regulation of Digestive Activities A





Figure 24²5

Neural Mechanisms ‡ Control: ² movement of materials along digestive tract ² secretory functions

‡ Motor neurons: ² control smooth muscle contraction and glandular secretion

‡ Short Reflexes control small segments of digestive tract

Digestive üormones ‡ At least 18 hormones that affect: ² most aspects of digestive function ² activities of other systems

‡ Are peptides ‡ Are produced by enteroendocrine cells in digestive tract ‡ Reach target organs after distribution in bloodstream

ocal Mechanisms ‡ Prostaglandins, histamine, and other chemicals ‡ Released into interstitial fluid ‡ Affect adjacent cells within small segment of digestive tract ‡ Coordinating response to changing conditions: ² m variations in local pü, chemical, or physical stimuli

‡ Affect only a portion of tract

The Oral Cavity

Figure 24²·

4 Functions of the Oral Cavity ‡ Sensory analysis: ² of material before swallowing

‡ Mechanical processing: ² through actions of teeth, tongue, and palatal surfaces

‡ ubrication: ² mixing with mucus and salivary gland secretions

‡ imited digestion: ² of carbohydrates and lipids

Oral Mucosa ‡ ining of oral cavity ‡ üas stratified squamous epithelium

The Mucosa Inferior to the Tongue ‡ Is thin and vascular enough to rapidly absorb lipid-soluble drugs ‡ Cheeks- Are ed by pads of fat and the buccinator muscles ‡ ips- labia - the mucosa of each cheek is continuous with that of the lips

]estibule ‡ Space between the cheeks (or lips) and the teeth Gingivae (Gums) ‡ Ridges of oral mucosa ‡ Surround base of each tooth on alveolar processes of maxillary bones and mandible ‡ The Oral Cavity ‡ Roof formed by hard and soft palates ‡ Tongue dominates the floor

Uvula ‡ A dangling process ‡ üelps prevent food from entering pharynx prematurely ‡ Is ed by posterior margin of soft palate Tonsils ‡ ies between palatoglossal and palatopharyngeal arches, on each side

The Tongue ‡ Manipulates materials inside mouth ‡ May bring foods into oral cavity 4 Functions of the Tongue 1. Manipulation: 2. Mechanical processing: ² ² ²

compression, abrasion, and distortion assists in chewing prepares material for swallowing

4 Functions of the Tongue 3. Sensory analysis: ²

touch, temperature, and taste receptors

4. Secretion: ² ²

‡

mucins enzyme lingual lipase

ingual Papillae has fine projections on superior surface (dorsum) of tongue which are covered in thick epithelium

Sublingual Glands ‡ Small glands extend into underlying lamina propria ‡ Secretions flush tongue·s epithelium ‡ Contain water, mucins, and enzyme lingual lipase - Enzyme, works over broad pü range (3.0²·.0) ‡ Starts lipid digestion immediately

Muscles of the Tongue ‡ Extrinsic tongue muscles-Perform all gross movements of the tongue ‡ Are large muscles ‡ Intrinsic tongue muscles- Change shape of the tongue ‡ Assist extrinsic muscles during precise movements, as in speech

The Salivary Glands

Figure 24²7

Salivary Glands ‡ 3 pairs secrete into oral cavity ‡ Each pair has distinctive cellular organization: ² and produces saliva with different properties

‡ Produce 1.0²1.5 liters of saliva each day: ² 70% by submandibular glands ² 25% by parotids ² 5% by sublingual glands

Saliva ‡ 99.4% water ‡ 0.·% includes: ² electrolytes (Na+, Cl³, and üCO3³) ² buffers ² glycoproteins (mucins)- Glycoproteins, responsible for lubricating action ² antibodies ² enzymes ² waste products

4 Functions of Saliva 1. ubricating the mouth 2. Moistening and lubricating materials in the mouth 3. Dissolving chemicals that: ² ²

stimulate taste buds provide sensory information

4. Initiate digestion of: ² ²

complex carbohydrates by enzyme salivary amylase (ptyalin or alpha-amylase) lipids by enzyme lingual lipase

Teeth

Figure 24²8

The Teeth ‡ Tongue movements food across occlusal surfaces of teeth ‡ Chew (masticate) food ‡ Tooth Structure: ‡ Dentin: ² a mineralized matrix similar to that of bone ² does not contain cells ‡ Pulp cavity: ² receives blood vessels and nerves through the root canal ‡ Root of each tooth sits in a bony socket (alveolus) ‡ A layer of cementum covers dentin of the root: ² providing protection and anchoring periodontal ligament

The Crown ‡ ‡ ‡ ‡

Exposed portion of tooth Projects beyond soft tissue of gingiva Dentin covered by layer of enamel Gingival Sulcus is a shallow groove surrounding the neck of each tooth

‡

Dental Arches

Contain 4 types of teeth: 1. Incisors- Rlade-shaped teeth ocated at front of mouth Used for clipping or cutting, üave a single root 2. Cuspids (canines)-Conical Sharp ridgeline, Pointed tip Used for tearing or slashing, üave a single root 3.Ricuspids (premolars)-Flattened crowns Prominent ridges, Used to crush, mash, and grind, üave 1 or 2 roots 4. Molars-]ery large, flat crowns ‡ With prominent ridges, Used for crushing and grinding, üave 3 or more roots

Primary and Secondary Dentitions

Figure 24²9

Dental Succession ‡ During embryonic development, 2 sets of teeth form:

Primary dentition or deciduous teeth‡ Also called primary teeth, milk teeth, or baby teeth ‡ 20 temporary teeth of primary dentition ‡ 5 on each side of upper and lower jaws: ² 2 incisors,1 cuspid, 2 deciduous molars

Secondary dentition or permanent dentition Replaces deciduous teeth, 32 permanent teeth ‡ 8 on each side, upper and lower: ² 2 incisors, 1 cuspid, 5 molars

Mastication ‡ Also called chewing ‡ Food is forced from oral cavity to vestibule and back: ² crossing and recrossing occlusal surfaces

‡ Mastication muscles close the jaws ‡ Slide or rock lower jaw from side to side ‡ Chewing involves mandibular: ² elevation and depression ² protraction and retraction ² medial and lateral movement

The Pharynx ‡ A common ageway for solid food, liquids, and air ‡ Nasopharynx ‡ Oropharynx ‡ aryngopharynx ‡ Food es through oropharynx and laryngopharynx to esophagus

The Esophagus

Figure 24²10

The Esophagus ‡ A hollow muscular tube ‡ About 25 cm long and 2 cm wide ‡ Conveys solid food and liquids to the stomach ‡ Regins posterior to cricoid cartilage ‡ Is innervated by fibers from the esophageal plexus

5 Characteristics of the Esophageal Wall ‡

Wall of esophagus has 3 layers: ² ² ²

mucosal submucosal muscularis

1. Mucosa contains: ²

nonkeratinized and stratified squamous epithelium

2. Mucosa and submucosa: ²

form large folds that extend the length of the esophagus

3. Muscularis mucosae: ²

consists of irregular layer of smooth muscle

5 Characteristics of the Esophageal Wall 4. Submucosa contains esophageal glands: ² ²

which produce mucous secretion reduces friction between bolus and esophageal lining

5. Muscularis externa: ²

has usual inner circular and outer longitudinal layers

The Swallowing Process

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4 Functions of the Stomach 1. Storage of ingested food 2. Mechanical breakdown of ingested food 3. Disruption of chemical bonds in food material: ²

by acids and enzymes

4. Production of intrinsic factor: ²

glycoprotein required for absorption of vitamin R12 in small intestine

The Stomach

Figure 24²12a

The Stomach

Figure 24²12b

Anatomy of the Stomach ‡ The stomach is shaped like an expanded J: ² short lesser curvature forms medial surface ² long greater curvature forms lateral surface

‡ Anterior and posterior surfaces are smoothly rounded ‡ Shape and size vary: ² from individual to individual ² from 1 meal to the next

‡ Stomach typically extends between levels of vertebrae T7 and 3

4 Regions of the Stomach ‡ Cardia, Fundus, Rody, Pylorus ‡ Muscularis mucosae and muscularis externa: ² contain extra layers of smooth muscle cells ² in addition to circular and longitudinal layers

The Stomach ining

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Figure 24²13

Gastric Pits ‡ Are shallow depressions that open onto the gastric surface ‡ Mucous cells: ² at base, or neck, of each gastric pit ² actively divide, replacing superficial cells

Chyme- Mixture of secretions and food in the stomach ‡ In fundus and body of stomach: ² extend deep into underlying lamina propria

‡ Each gastric pit communicates with several gastric glands ² ‡ Secretory cells in gastric glands: ² parietal cells ² chief cells

The Secretion of üydrochloric Acid

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Figure 24²14

Pyloric Glands ‡ Pyloric Glands in the pylorus: ² produce mucous secretion

G Cells produce gastrin ‡ D Cells in pyloric glands ‡ Release somatostatin: ² hormone that inhibits release of gastrin

The Phases of Gastric Secretion

Table 24²1

Digestion in the Stomach ‡ Stomach performs preliminary digestion of proteins by pepsin: ² some digestion of carbohydrates (by salivary amylase) ² lipids (by lingual lipase)

‡ Stomach contents: ² become more fluid ² pü approaches 2.0 ² pepsin activity increases ² protein disassembly begins Although digestion occurs in the stomach, nutrients are not absorbed there

Associated Glandular Organs ‡ Stomach:

‡ Pancreas:

² gastric juices ² stomach acids ² pepsin

² digestive enzymes ² buffers

‡ iver: ² Rile

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Segments of the Intestine

Figure 24²1·

The Small Intestine The Small Intestine ² ‡ Plays key role in digestion and absorption of nutrients ‡ 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 bowlµ that receives: ² chyme from stomach ² digestive secretions from pancreas and liver

The Jejunum ‡ Is the middle segment of small intestine ‡ 2.5 meters (8.2 ft) long ‡ Is the location of most: ² chemical digestion ² nutrient absorption

The Ileum ‡ The final segment of small intestine ‡ 3.5 meters (11.48 ft) long

The Intestinal Wall ' & Figure 24²17

Intestinal ]illi ‡ A series of fingerlike projections: ² 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 Rrush Rorder Enzymes are integral membrane proteins ,on surfaces of intestinal microvilli ‡ Rreak down materials in with brush border by trypsinogen

Enteroendocrine Cells ‡ In intestinal glands ‡ Produce intestinal hormones: ² Gastrin, Cholecystokinin, Secretin

‡ Rrunners glands-submucosal glands of duodenum ‡ Produce copious mucus: ² when chyme arrives from stomach

Functions of the Duodenum ‡ To receive chyme from stomach ‡ To neutralize acids before they can damage the absorptive surfaces of the small intestine ‡ Duodenum has few plicae ‡ Small villi

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

Peristaltic Contractions ‡ Myenteric reflexes,Not under CNS Parasympathetic stimulation: ² accelerates local peristalsis and segmentation

‡ Gastroenteric Reflex Stimulates motility and secretion: ² along entire small intestine

‡ Gastroileal Reflex triggers relaxation of ileocecal valve: ‡ Allows materials to : ² from small intestine into large intestine

The Pancreas ‡ A compound tubuloalveolar gland



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Figure 24²18

Common Rile Duct from the liver and gallbladder meets pancreatic duct near duodenum obules of the Pancreas ‡ Are separated by connective tissue partitions (septa) ‡ Contain blood vessels and tributaries of pancreatic ducts ‡ In each lobule: ² ducts branch repeatedly ² end in blind pockets (pancreatic acini)

‡ Pancreatic Islets -Endocrine tissues of pancreas ‡ Scattered (1% of pancreatic cells)

Functions of the Pancreas 1. Endocrine cells: ² ²

of pancreatic islets secrete insulin and glucagon into bloodstream

2. Exocrine cells: ² ²

‡ ‡ ‡

acinar cells epithelial cells of duct system

Pancreatic Secretions - 1000 ml (1 qt) pancreatic juice per day Controlled by hormones from duodenum Contain pancreatic enzymes

Pancreatic Enzymes ‡ Pancreatic alpha-amylase: ² a carbohydrase ² breaks down starches ² similar to salivary amylase

‡ Pancreatic lipase: ² breaks down complex lipids ² releases products (m fatty acids) that are easily absorbed

‡ Nucleases: ² break down nucleic acids

‡ Proteolytic enzymes: ² break certain proteins apart ² proteases break large protein complexes ² peptidases break small peptides into amino acids

Proteolytic Enzymes ‡ 70% of all pancreatic enzyme production ‡ Secreted as inactive proenzymes ‡ Activated after reaching small intestine ‡ Trypsin- An active protease ‡ Enterokinase in duodenum: ² converts trypsinogen to trypsin

The iver ‡ Is the largest visceral organ (1.5 kg) ‡ ies in right hypochondriac and epigastric regions ‡ Extends to left hypochondriac and umbilical regions ‡ Performs essential metabolic and synthetic functions

The Anatomy of the iver r



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The Porta üepatis ‡ ´Doorway to the liverµ ‡ Region where blood vessels and other structures converge on liver ‡ ]essels reach liver by traveling within connective tissue of lesser omentum

üepatic Rlood Supply ‡ 1/3 of blood supply: ² arterial blood from hepatic artery proper

‡ 2/3 venous blood from hepatic portal vein, originating at: ² ² ² ²

esophagus stomach small intestine most of large intestine

‡ üepatocytes are liver cells ‡ Adjust circulating levels of nutrients: ² through selective absorption and secretion

‡ Rlood eaving the iver returns to systemic circuit ‡ ]ia hepatic veins: ² which open into inferior vena cava

iver üistology

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üepatocytes ‡ In a liver lobule form a series of irregular plates arranged like wheel spokes ‡ Rlood enters liver sinusoids: ² from small branches of hepatic portal vein ² from hepatic artery proper

‡ As blood flows through sinusoids: ² hepatocytes absorb solutes from plasma ² and secrete materials such as plasma proteins

The Rile Duct System ‡ iver secretes bile fluid: ² into a network of narrow channels (bile canaliculi) ² between opposing membranes of adjacent liver cells

‡ The Right and eft üepatic Ducts Collect bile from all bile ducts of liver lobes ‡ Unite to form common hepatic duct which leaves the liver

3 Functions of the iver 1. Metabolic regulation 2. üematological regulation 3. Rile production

3 Functions of the iver Metabolic Regulation ‡ The liver regulates: 1. 2. 3. 4. 5.

composition of circulating blood nutrient metabolism waste product removal nutrient storage drug inactivation

Composition of Circulating Rlood ‡ All blood leaving absorptive surfaces of digestive tract: ² enters hepatic portal system ² flows into the liver

‡ iver cells extract nutrients or toxins from blood: ² before it reaches systemic circulation through hepatic veins

‡ iver removes and stores excess nutrients: ² corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities

Metabolic Activities of the iver ‡ ‡ ‡ ‡ ‡ ‡ ‡

Carbohydrate metabolism ipid metabolism Amino acid metabolism Waste product removal ]itamin storage Mineral storage Drug inactivation

The iver and üematological Regulation (1 of 2) ‡ argest blood reservoir in body ‡ Receives 25% of cardiac output ‡ Performs · hematological regulation functions

The iver and üematological Regulation (2 of 2) 1. Phagocytosis and antigen presentation 2. Synthesis of plasma proteins 3. Removal of circulating hormones 4. Removal of antibodies 5. Removal or storage of toxins ·. Synthesis and secretion of bile

ipid Digestion and Absorption ‡ Dietary lipids are not water soluble ‡ Mechanical processing in stomach creates large drops containing lipids ‡ Pancreatic lipase is not lipid soluble: ² interacts only at surface of lipid droplet

Functions of Rile ‡ Rile salts break droplets apart (emulsification): ² increases surface area exposed to enzymatic attack ² creates tiny emulsion droplets coated with bile salts

The Gallbladder and Rile Ducts

Figure 24²21

The Gallbladder ‡ Is a pear-shaped, muscular sac ‡ Stores and concentrates bile prior to excretion into small intestine ‡ Is located in the fossa on the posterior surface of the liver·s right lobe ‡ Cystic duct extends from gallbladder ‡ Union with common hepatic duct forms common bile duct ‡ Stores bile ‡ Releases bile into duodenum: ² only under stimulation of hormone cholecystokinin (CCK)

Without CCK ‡ üepatopancreatic sphincter remains closed ‡ Rile exiting liver in common hepatic duct cannot flow through common bile duct into duodenum ‡ Rile enters cystic duct: ² is stored in gallbladder

CCK ‡ Is released whenever chyme enters duodenum ‡ Relaxes hepatopancreatic sphincter ‡ Stimulates contractions in gallbladder: ² pushes bile into small intestine

‡ Amount secreted depends on lipid content of chyme

The Gallbladder and Rile Modification ‡ Full gallbladder contains 40²70 ml bile ‡ Rile composition gradually changes in gallbladder: ² water is absorbed ² bile salts and solutes become concentrated

Gallstones ‡ Are crystals of insoluble minerals and salts ‡ Form if bile is too concentrated ‡ Small stones may be flushed through bile duct and excreted

Coordination of Secretion and Absorption ‡ Neural and hormonal mechanisms coordinate activities of digestive glands ‡ Regulatory mechanisms center around duodenum: ² where acids are neutralized and enzymes added

Activities of Major Digestive Tract üormones

Figure 24²22

Intestinal üormones ‡ Intestinal tract secretes peptide hormones with multiple effects: ² in several regions of digestive tract ² in accessory glandular organs

üormones of Duodenal Enteroendocrine Cells ‡ Coordinate digestive functions: ² secretin ² cholecystokinin (CCK) ² gastric inhibitory peptide (GIP) -when fats and carbohydrates enter small intestine ² vasoactive intestinal peptide (]IP) - Inhibits acid production in stomach ² Gastrin- Stimulates acids and enzyme production ² Enterocrinin- when chyme enters small intestine,Stimulates mucin production

Intestinal Absorption ‡ It takes about 5 hours for materials to : ² from duodenum ² to end of ileum

‡ Movements of the mucosa increases absorptive effectiveness: ² stir and mix intestinal contents ² constantly change environment around epithelial cells

The arge Intestine r "

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Figure 24²23

Functions of the arge 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 arge Intestine: 1. Cecum: ²

the pouchlike first portion -Stores materials and begins compaction

2. Colon: the largest portion, üas a larger diameter and thinner wall than small intestine ‡ The wall of the colon: ²

forms a series of pouches (haustra) which permit expansion and elongation of colon

3. Rectum: the last 15 cm of digestive tract

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: ² mesoappendix connects appendix to ileum and cecum

4 regions :the Ascending Colon ‡ Regins at superior border of cecum ‡ Ascends along right lateral and posterior wall of peritoneal cavity: ² to inferior surface of the liver

The Transverse Colon- Curves anteriorly from right colic flexure, ² Crosses abdomen from right to left ‡ Is ed by transverse mesocolon ‡ Is separated from anterior abdominal wall by greater omentum

The Descending Colon ‡ Proceeds inferiorly along left side: ² 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 ‡ ies posterior to urinary bladder ‡ Is suspended from sigmoid mesocolon ‡ Empties into rectum

Rlood Supply of the arge Intestine ‡ Receives blood from tributaries of: ² superior mesenteric and inferior mesenteric arteries

‡ ]enous blood is collected from: ² superior mesenteric and inferior mesenteric veins

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 is exit of the anal canal ‡ üas keratinized epidermis like skin

Anal Sphincters ‡ Internal anal sphincter: ² circular muscle layer of muscularis externa ² has smooth muscle cells, not under voluntary control

‡ External anal sphincter: ² encircles distal portion of anal canal ² a ring of skeletal muscle fibers, under voluntary control

Mucosa and Glands of the Colon | + '

Figure 24²24

Glands of the arge 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 arge Intestine‡ ess than 10% of nutrient absorption occurs in large intestine ‡ Prepares fecal material for ejection from the body

Absorption in the arge Intestine ‡ Reabsorption of water ‡ Reabsorption of bile salts: ² in the cecum ² transported in blood to liver

‡ Absorption of vitamins produced by bacteriaorganic molecules ‡ Important as cofactors or coenzymes in metabolism ‡ Normal bacteria in colon make 3 vitamins that supplement diet ‡ Absorption of organic wastes

3 ]itamins Produced in the arge Intestine 1. ]itamin K: ² ²

a fat-soluble vitamin required by liver for synthesizing 4 clotting factors, including prothrombin

2. Riotin: ² ²

a water-soluble vitamin important in glucose metabolism

3. Pantothenic acid: ² ²

a water-soluble vitamin required in manufacture of steroid hormones and some neurotransmitters

Organic Wastes ‡ Racteria convert bilirubin to urobilinogens and stercobilinogens: ² urobilinogens absorbed into bloodstream are excreted in urine ² urobilinogens and stercobilinogens in colon convert to urobilins and stercobilins by exposure to oxygen

‡ Racteria break down peptides in feces and generate: ² ammonia: ‡ as soluble ammonium ions

² indole and skatole: ‡ nitrogen compounds responsible for odor of feces

² hydrogen sulfide: ‡ gas that produces ´rotten eggµ odor

The Defecation Reflex !



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Figure 24²25

Movements of the arge Intestine ‡ Gastroileal and gastroenteric reflexes: ² 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 arge 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 2 positive loops ² both loops triggered by stretch receptors in rectum

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 ‡ A typical meal contains: ² Carbohydrates, proteins ² ipids, water ² Electrolytes, vitamins Digestive system handles each nutrient differently: ² large organic molecules: ‡ must be digested before absorption can occur

² water, electrolytes, and vitamins: ‡ can be absorbed without processing ‡ may require special transport

Summary: Chemical Events in Digestion

Figure 24²2·

Processing Nutrients ‡ The digestive system: ² breaks down physical structure of food ² disassembles component molecules

‡ Molecules released into bloodstream are: ² absorbed by cells

‡ Rroken down to provide energy for ATP synthesis: ² used to synthesize carbohydrates, proteins, and lipids

Digestive Enzymes ‡ Are secreted by: ² ² ² ²

salivary glands tongue stomach pancreas

‡ Rreak molecular bonds in large organic molecules: ² carbohydrates, proteins, lipids, and nucleic acids ² in a process called hydrolysis

Digestive Enzymes ‡ Are divided into classes by targets: ² carbohydrases: ‡ break bonds between simple sugars

² proteases: ‡ break bonds between amino acids

² lipases: ‡ separate fatty acids from glycerides

‡ Rrush border enzymes break nucleotides into: ² sugars ² phosphates ² nitrogenous bases

Complex Carbohydrate Digestion ‡ Proceeds in 2 steps: 1. carbohydrases (from salivary glands and pancreas)-Function at pü ·.7²7.5: ² salivary amylase - From parotid and submandibular salivary glands, Rreaks down starches (complex carbohydrates),Produces: ² disaccharides (2 simple sugars) ² trisaccharides (3 simple sugars) ² pancreatic alpha-amylase2. brush border enzymes- Fragment disaccharides and trisaccharides into monosaccharides (simple sugars):maltase splits maltose into 2 glucose

Absorption of Monosaccharides ‡ Intestinal epithelium absorbs monosaccharides: ² by facilitated diffusion and cotransport ² via a carrier protein

Facilitated Diffusion vs. Cotransport ‡ Facilitated diffusion: ² moves only 1 molecule or ion through cell membrane, does not require ATP ² will not occur against opposing concentration gradient

‡ Cotransport: ² ² ² ²

moves more than 1 molecule or ion at the same time transported materials move in same direction may require ATP to preserve cell homeostasis can occur against opposing concentration gradient

ipid Digestion ‡ Involves: ² lingual lipase from glands of tongue-Regins triglycerides breakdown in mouth

‡ Continues for limited time within stomach ‡ Digests 20% of lipids before chyme enters duodenum ‡ Rile salts improve chemical digestion: ² by emulsifying lipid drops into tiny droplets ² providing better access for pancreatic lipase ² pancreatic lipase from pancreas-Rreak off 2 fatty acids, leaving monoglycerides ,Are water-soluble enzymes, Attack only exposed surfaces of lipid drops

Triglycerides ‡ Are the most important and abundant dietary lipids ‡ Consist of 3 fatty acids attached to 1 molecule glycerol

ipid Absorption ‡ Intestinal cells synthesize new triglycerides from monoglycerides and fatty acids ‡ Triglycerides and other absorbed molecules are coated with proteins: ² creating chylomicrons

‡ Intestinal cells secrete chylomicrons into interstitial fluid by exocytosis

Protein Digestion ‡ Is complex and time-consuming: ² mechanical processing in oral cavity (mastication) and chemical processing in stomach acid (üCl) allows proteolytic enzymes to attack proteins ² pepsin: ‡ proteolytic enzyme ‡ works at pü 1.5²2.0 ‡ breaks peptide bonds within polypeptide chain

² when chyme enters duodenum: ‡ enterokinase from small intestine triggers conversion of trypsinogen to trypsin ‡ pü is adjusted to 7²8 ‡ pancreatic proteases begin working

Absorption of Amino Acids ‡ Dipeptidases: ² enzymes on epithelial surfaces of small intestine ² break short peptide chains into individual amino acids

‡ After diffusing to basal surface of cell: ² amino acids are released into interstitial fluid ² by facilitated diffusion and cotransport

Digestive Secretion and Absorption |

' Figure 24²27

Absorption of Ions and ]itamins

Table 24²4

Ion Absorption ‡ Osmosis does not distinguish among solutes: ² determined only by total concentration of solutes

‡ To maintain homeostasis: ² concentrations of specific ions must be regulated

‡ Sodium ion absorption: ² rate increased by aldosterone (steroid hormone from adrenal cortex)

‡ Calcium ion absorption: ² involves active transport at epithelial surface ² rate increased by parathyroid hormone (PTü) and calcitriol

Ion Absorption ‡ Potassium ion concentration increases: ² as other solutes move out of lumen

‡ Other ions diffuse into epithelial cells along concentration gradient ‡ Cation absorption (magnesium, iron): ² involves specific carrier proteins ² cell must use ATP to transport ions to interstitial fluid

‡ Anions (chloride, iodide, bicarbonate, and nitrate): ² are absorbed by diffusion or carrier mediated transport

‡ Phosphate and sulfate ions: ² enter epithelial cells by active transport

]itamins ‡ Are organic compounds required in very small quantities ‡ Are divided in 2 major groups: ² fat-soluble vitamins A, D, E, and K

- Their structure allows them to dissolve in lipids ² 9 water-soluble vitamins: ² all R vitamins (common in milk and meats) ² vitamin C (found in citrus)

‡ All but 1 of water-soluble vitamins easily diffuse across digestive epithelium ‡ ]itamin R12- Cannot be absorbed by intestinal mucosa in normal amounts: ² unless bound to intrinsic factor (glycoprotein secreted by parietal cells of stomach

5 Effects of Aging on the Digestive System 1. Division of epithelial stem cells declines: ²

digestive epithelium becomes more susceptible to damage by abrasion, acids, or enzymes 2. Smooth muscle tone and general motility decreases:peristaltic contractions become weaker

3. Cumulative damage from toxins (alcohol, other chemicals) absorbed by digestive tract and transported to liver for processing

5 Effects of Aging on the Digestive System 4. Rates of colon cancer and stomach cancer rise with age: ²

oral and pharyngeal cancers common among elderly smokers

5. Decline in olfactory and gustatory sensitivities: ²

lead to dietary changes that affect entire body

The Digestive System and Other Systems

Figure 24²28

SUMMARY (1 of 4) ‡ Digestive system: ² digestive tract ² accessory organs

‡ Digestive system functions: ² ² ² ² ²

ingestion mechanical processing digestion secretion absorption

SUMMARY (2 of 4) ‡ Oral cavity ‡ Ruccal cavity: ² oral mucosa

‡ Tongue: ² intrinsic tongue muscles ² extrinsic tongue muscles

‡ ‡ ‡ ‡ ‡

Salivary glands Teeth, Pharynx Esophagus Stomach: ² cephalic phase ² gastric phase ² intestinal phase

SUMMARY (3 of 4)

‡ Small intestine:

² gastroenteric reflex ² gastroileal reflex

‡ Pancreas ‡ iver: ² bile

‡ Gallbladder ‡ Intestinal hormones: ² ² ² ² ² ²

secretin cholecystokinin (CCK) gastric inhibitory peptide (GIP) vasoactive intestinal peptide (]IP) gastrin enterocrinin

SUMMARY (4 of 4) ‡ arge intestine: ² cecum ² colon ² rectum

Processing and absorption of nutrients ‡ Carbohydrate digestion and absorption ‡ ipid digestion and absorption ‡ Protein digestion and absorption ‡ Water absorption ‡ Ion absorption ‡ ]itamin absorption