Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5^TH Edition, , Benjamine Cummings Publisher, 2001 Prepare from : V.A. Austin’s PowerPpoint Presentation (ISBN: 0-8053-5469-7), CD ROM: Pearson Education, Inc. , 2003.

Chapter 24

The Digestive System

Digestive System: Overview

• The alimentary canal or gastrointestinal (GI) tract digests and absorbs food

• Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine

• Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas

Digestive Process

• The GI tract is a “disassembly” line

• Nutrients become more available to the body in each step

• There are six essential activities:

• ingestion, propulsion, and mechanical digestion

• chemical digestion, absorption, and defecation

Essential Activities of Digestion

• Ingestion – taking food into the digestive tract

• Propulsion – swallowing and peristalsis

• Peristalsis – waves of contraction and relaxation of muscles in the organ walls

• Mechanical digestion – chewing, mixing, and churning food

• Chemical digestion – catabolic breakdown of food

• Absorption – movement of nutrients from the GI tract to the blood or lymph

• Defecation – elimination of indigestible solid wastes

GI Tract

• External environment for the digestive process

• Regulation of digestion involves:

• Mechanical and chemical stimuli – stretch receptors, osmolarity, and presence of substrate in the lumen

• Extrinsic control by CNS centers

• Intrinsic control by local centers

Receptors of the GI Tract

• Mechano- and chemoreceptors respond to:

• Stretch, osmolarity, and pH

• Presence of substrate, and end products of digestion

• They initiate reflexes that:

• Activate or inhibit digestive glands

• Mix lumen contents and move them along

Nervous Control of the GI Tract

• Intrinsic controls

• Nerve plexuses near the GI tract initiate short reflexes

• Short reflexes are mediated by local enteric plexuses (gut brain)

• Extrinsic controls

• Long reflexes arising within or outside the GI tract

• Involve CNS centers and extrinsic autonomic nerves

Digestive System Organs and Peritoneum

• Peritoneum – serous membrane of the abdominal cavity

• Visceral – covers external surface of most digestive organs

• Parietal – lines the body wall

• Peritoneal cavity

• Lubricates digestive organs

• Allows them to slide across one another

• Mesentery – double layer of peritoneum that provides:

• Vascular and nerve supplies to the viscera

• A means to hold digestive organs in place and store fat

• Retroperitoneal organs – organs outside the peritoneum

• Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum

Homeostatic Imbalance

• Peritonitis – inflammation of the peritoneum caused by a piercing wound, perforating ulcer, or burst appendix

• Often, infected membranes tend to stick together localizing the infection

• Generalized or widespread peritonitis is dangerous and often lethal

• Treatment includes removing infectious debris and massive doses of antibiotics

Blood Supply: Splanchnic Circulation

• Arteries and the organs they serve include

• The hepatic, splenic, and left gastric: spleen, liver, and stomach

• Inferior and superior mesenteric: small and large intestines

• Hepatic portal circulation:

• Collects nutrient-rich venous blood from the digestive viscera

• Delivers it to the liver for metabolic processing and storage

Histology of the Alimentary Canal

• From esophagus to the anal canal the walls of the GI tract have the same four tunics

• From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa

• Each tunic has a predominant tissue type and specific digestive function

Mucosa

• Moist epithelial layer that lines the lumen of the alimentary canal

• Its three major functions are:

• Secretion of mucus

• Absorption of the end products of digestion

• Protection against infectious disease

• Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae

Mucosa: Epithelial Lining

• Consists of simple columnar epithelium and mucus-secreting goblet cells

• The mucus secretions:

• Protect digestive organs from digesting themselves

• Ease food along the tract

• Stomach and small intestine mucosa contain:

• Enzyme-secreting cells and

• Hormone-secreting cells (making them endocrine and digestive organs)

Mucosa: Lamina Propria and Muscularis Mucosae

• Lamina Propria

• Loose areolar and reticular connective tissue

• Nourish the epithelium and absorb nutrients

• Contains lymph nodes (part of MALT) important in defense against bacteria

• Muscularis mucosae– smooth muscle cells that produce local movements of mucosa

Mucosa: Other Sublayers

• Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves

• Muscularis externa – responsible for segmentation and peristalsis

• Serosa – the protective visceral peritoneum

• Replaced by the fibrous adventitia in the esophagus

• Retroperitoneal organs have both an adventitia and serosa

Enteric Nervous System

• Composed of two major intrinsic nerve plexuses

• Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa

• Myenteric nerve plexus:

• Major nerve supply that controls GI tract mobility

• Segmentation and peristalsis are largely automatic involving local reflex arcs

• Linked to the CNS via long autonomic reflex arc

Mouth

• Oral or buccal cavity:

• Is bounded by lips, cheeks, palate, and tongue

• Has the oral orifice as its anterior opening

• Is continuous with the oropharynx posteriorly

• To withstand abrasions:

• The mouth is lined with stratified squamous epithelium

• The gums, hard palate, and dorsum of the tongue are slightly keratinized

Lips and Cheeks

• Have a core of skeletal muscles

• Lips: orbicularis oris

• Cheeks: buccinators

• Vestibule – bounded by the lips and cheeks externally and teeth and gums internally

• Oral cavity proper– area that lies within the teeth and gums

• Labial frenulum – median fold that joins the internal aspect of each lip to the gum

Palate

• Hard palate – underlain by palatine bones and palatine processes of the maxillae

• Assists the tongue in chewing

• Slightly corrugated on either side of the raphe (midline ridge)

• Soft palate – mobile fold formed mostly of skeletal muscle

• Closes off the nasopharynx during swallowing

• Uvula projects downward from its free edge

• Palatoglossal and palatopharyngeal arches form the borders of the fauces

Tongue

• Occupies the floor of the mouth and fills the oral cavity when mouth is closed

• Functions include:

• Gripping and repositioning food during chewing

• Mixing food with saliva and forming the bolus

• Initiation of swallowing, and speech

• Intrinsic muscles change the shape of the tongue

• Extrinsic muscles alter the tongue’s position

• Lingual frenulum secures the tongue to the floor of the mouth

Homeostatic Imbalance

• Ankyloglossia – congenital situation where the lingual frenulum is extremely short

• Commonly referred to as being “tongue-tied”

• Corrected surgically by cutting the frenulum

Tongue

• Superior surface bears three types of papillae

• Filiform – give the tongue roughness and provide friction

• Fungiform – scattered widely over the tongue and give it a reddish hue

• Circumvallate – V-shaped row in back of tongue

• Sulcus terminalis – groove that separates the tongue into two areas:

• Anterior 2/3 residing in the oral cavity

• Posterior third residing in the oropharynx

Salivary Glands

• Produce and secrete saliva that:

• Cleanses the mouth

• Moistens and dissolves food chemicals

• Aids in bolus formation

• Contains enzymes that breakdown starch

• Three pairs of extrinsic glands – parotid, submandibular, and sublingual

• Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

• Parotid – lies anterior to the ear between the masseter muscle and skin

• Parotid duct – opens into the vestibule next to the second upper molar

• Submandibular – lies along the medial aspect of the mandibular body

• Its ducts open at the base of the lingual frenulum

• Sublingual – lies anterior to the submandibular gland under the tongue

• It opens via 10-12 ducts into the floor of the mouth

Homeostatic Imbalance

• Mumps – inflammation of the parotid glands caused by myxovirus

• Signs and symptoms – moderate fever and pain in swallowing acidic foods

• In adult males, mumps carries 25% risk that testes may become infected, leading to sterility

Saliva: Source and Composition

• Secreted from serous and mucous cells of salivary glands

• A 97-99.5% water, hypo-osmotic, slightly acidic solution containing

• Electrolytes – Na⁺, K⁺, Cl^–, PO₄^2–, HCO₃^–

• Digestive enzyme – salivary amylase

• Proteins – mucin, lysozyme, defensins, and IgA

• Metabolic wastes – urea and uric acid

Control of Salivation

• Intrinsic glands keep the mouth moist

• Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to:

• Ingested food which stimulates chemoreceptors and pressoreceptors

• The thought of food

• Strong sympathetic stimulation inhibits salivation and results in dry mouth

Teeth

• Primary and permanent dentitions have formed by age 21

• Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months

• Permanent – enlarge and develop causing the root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years

• All but the third molars have erupted by the end of adolescence

• There are usually 32 permanent teeth

Classification of Teeth

• Teeth are classified according to their shape and function

• Incisors – chisel-shaped teeth adapted for cutting or nipping

• Canines – conical or fanglike teeth that tear or pierce

• Premolars (bicuspids) and molars – have broad crowns with rounded tips and are best suited for grinding or crushing

• During chewing, upper and lower molars lock together generating crushing force

Dental Formula

• A shorthand way of indicating the number and relative position of teeth

• Written as ratio of upper to lower teeth for the mouth

• Primary: 2I (incisors), 1C (canine), 2M (molars)

• Permanent: 2I, 1C, 2PM (premolars), 3M

Tooth Structure

• Two main regions – crown and the root

• Crown – exposed part of the tooth above the gingiva (gum)

• Enamel – acelluar, brittle material composed of calcium salts and hydroxyapatite crystals is the hardest substance in the body

• Encapsules the crown of the tooth

• Root – portion of the tooth embedded in the jawbone

• Neck – constriction where the crown and root come together

• Cementum – calcified connective tissue

• Covers the root

• Attaches it to the periodontal ligament

• Periodontal ligament

• Anchors the tooth in the alveolus of the jaw

• Forms the fibrous joint called a gomaphosis

• Gingival sulcus – depression where the gingival borders the tooth

• Dentin – bonelike material deep to the enamel cap that forms the bulk of the tooth

• Pulp cavity – cavity surrounded by dentin that contains pulp

• Pulp – connective tissue, blood vessels, and nerves

• Root canal – portion of the pulp cavity that extends into the root

• Apical foramen – proximal opening to the root canal

• Odontoblasts – secrete and maintain dentin throughout life

Homeostatic Imbalance

• Root canal therapy – blows to the teeth can cause swelling and consequently pinch off the blood supply to the tooth

• The nerve dies and may become infected with bacteria

• Then the cavity is sterilized and filled with an inert material

• The tooth is then capped

Tooth and Gum Disease

• Dental caries – gradual demineralization of enamel and dentin by bacterial action

• Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth

• Acid produced by the bacteria in the plaque dissolves calcium salts

• Without these salts, organic matter is digested by proteolytic enzymes

• Daily flossing and brushing help prevent caries by removing forming plaque

Tooth and Gum Disease: Periodontitis

• Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartar

• Accumulation of calculus:

• Disrupts the seal between the gingivae and the teeth

• Puts the gums at risk for infection

• Periodontitis – serious gum disease resulting from an immune response

• Attack of the immune system against intruders:

• Also carves pockets around the teeth and

• Dissolves bone away

Pharynx

• From the mouth, the oro- and laryngopharynx allow passage of:

• Food and fluids to the esophagus

• Air to the trachea

• Lined with stratified squamous epithelium and mucus glands

• Has two skeletal muscle layers

• Inner longitudinal

• Outer pharyngeal constrictors

Esophagus

• Muscular tube going from the laryngopharynx to the stomach

• Travels through the mediastinum and pierces the diaphragm

• Joins the stomach at the cardiac orifice

Homeostatic Imbalance

• Heartburn (gastroesophageal reflux disease or GERD) – burning, radiating substernal pain caused by acidic gastric juice regurgitated into the esophagus

• Caused by excess eating or drinking, and conditions that force abdominal contents superiorly (e.g., extreme obesity, pregnancy, and running)

• Hiatus hernia – structural abnormality in which the superior part of the stomach protrudes slightly above the diaphragm

• Prolonged episodes can lead to esophagitis, ulcers, and cancer

Esophageal Characteristics

• Esophageal mucosa – nonkeratinized stratified squamous epithelium

• The empty esophagus is folded longitudinally and flattens when food is present

• Glands secrete mucus as a bolus moves through the esophagus

• Muscularis changes from skeletal (superiorly) to smooth muscle (inferiorly)

Digestive Processes in the Mouth

• Food is ingested

• Mechanical digestion begins (chewing)

• Propulsion is initiated by swallowing

• Salivary amylase begins chemical breakdown of starch

• The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach

Deglutition (Swallowing)

• Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and 22 separate muscle groups

• Buccal phase – bolus is forced into the oropharynx

• Pharyngeal-esophageal phase – controlled by the medulla and lower pons

• All routes except into the digestive tract are sealed off

• Peristalsis moves food through the pharynx to the esophagus

Stomach

• Chemical breakdown of proteins begins and food is converted to chyme

• Cardiac region – surrounds the cardiac orifice

• Fundus – dome-shaped region beneath the diaphragm

• Body – midportion of the stomach

• Pyloric region – made up of the antrum and canal which terminates at the pylorus

• The pylorus is continuous with the duodenum through the pyloric sphincter

• Greater curvature – entire extent of the convex lateral surface

• Lesser curvature – concave medial surface

• Lesser omentum – runs from the liver to the lesser curvature

• Greater omentum – drapes inferiorly from the greater curvature to the small intestine

• Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system

• Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal system)

Microscopic Anatomy of the Stomach

• Muscularis – has an additional oblique layer that

• Allows the stomach to churn, mix and pummel food physically

• Breaks down food into smaller fragments

• Epithelial lining is composed of:

• Goblet cells that produce a coat of alkaline mucus

• Gastric pits containing gastric glands that secrete:

• Gastric juice

• Mucus

• Gastrin

Glands of the Stomach Fundus and Body

• Gastric glands of the fundus and body have a variety of secretory cells

• Mucous neck cells – secrete acid mucus

• Parietal (oxyntic) cells – secrete HCl and intrinsic factor

• Chief (zymogenic) cells – produce pepsinogen

• Pepsinogen is activated to pepsin by:

• HCl in the stomach

• Pepsin itself by a positive feedback mechanism

• Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria

Stomach Lining

• The stomach is exposed to the harshest conditions in the digestive tract

• To keep from digesting itself, the stomach has a mucosal barrier with:

• A thick coat of bicarbonate-rich mucus on the stomach wall

• Epithelial cells that are joined by tight junctions

• Gastric glands that have cells impermeable to HCl

• Damaged epithelial cells are quickly replaced

Digestion in the Stomach

• The stomach:

• Holds ingested food

• Degrades it both physically and chemically

• Delivers chyme to the small intestine

• Enzymatically digests proteins with pepsin

• Secretes intrinsic factor required for absorption of vitamin B₁₂

Regulation of Gastric Secretion

• Neural and hormonal mechanisms regulate the release of gastric juice

• Stimulatory and inhibitory events occur in three phases

• Cephalic (reflex) phase: prior to food entry

• Gastric phase: once food enters the stomach

• Intestinal phase: as partially digested food enters the duodenum

Cephalic Phase

• Excitatory events include:

• Sight or thought of food

• Stimulation of taste or smell receptors

• Inhibitory events include:

• Loss of appetite or depression

• Decrease in stimulation of the parasympathetic division

Gastric Phase

• Excitatory events include:

• Stomach distension

• Activation of stretch receptors (neural activation)

• Activation of chemoreceptors by peptides, caffeine, and rising pH

• Release of gastrin to the blood

• Inhibitory events include:

• A pH lower than 2

• Emotional upset which overrides the parasympathetic division

Intestinal Phase

• Excitatory phase – low pH and partially digested food enters the duodenum

• Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum

• Initiate inhibition of local reflexes and vagal nuclei

• Closes the pyloric sphincter

• Releases enterogastrones that inhibit gastric secretion

Regulation and Mechanism of HCl Secretion

• HCl secretion is stimulated by ACh, histamine, and gastrin

• All work through second messenger systems

• Release of hydrochloric acid:

• Is low if only one ligand binds to parietal cells

• Is prolific if all three ligands bind to parietal cells

• Antihistamines and cimetidine block H₂ receptors and decrease HCl release

Response of the Stomach to Filling

• Stomach pressure remains constant until about 1L of food is ingested

• Relative unchanging pressure results from reflex-mediated relaxation and plasticity

• Reflex-mediated events include:

• Receptive relaxation – as food travels in the esophagus, stomach muscles relax

• Adaptive relaxation – the stomach dilates in response to gastric filling

• Plasticity – intrinsic ability of smooth muscle to exhibit the stress-relaxation response

Gastric Contractile Activity

• Peristaltic waves move toward the pylorus at the rate of 3 per minute

• This basic electrical rhythm (BER) is initiated by pacemaker cells (cells of Cajal)

• Most vigorous peristalsis and mixing occurs near the pylorus

• Chyme is either:

• Delivered in small amounts to the duodenum or

• Forced backward into the stomach for further mixing

Regulation of Gastric Emptying

• Gastric emptying is regulated by:

• The neural enterogastric reflex

• Hormonal (enterogastrone) mechanisms

• These mechanisms inhibit gastric secretion and duodenal filling

• Carbohydrate-rich chyme moves through the duodenum quickly

• Fat-laden chyme is digested more slowly causing food to remain in the stomach longer

Homeostatic Imbalance

• Vomiting (emesis) – the stomach empties via a different route (oral)

• Causes include extreme stretching, irritants such as bacterial toxins, excessive alcohol, spicy foods, and certain drugs

• The emetic center of the medulla initiates a number of motor responses

• Diaphragm and abdominal wall muscle contract

• Cardiac sphincter relaxes and soft palate closes off the nasal passages

• Excessive vomiting can cause dehydration and upset electrolyte and pH balance

Small Intestine: Gross Anatomy

• Runs from pyloric sphincter to the ileocecal valve

• Has three subdivisions: duodenum, jejunum, and ileum

• The bile duct and main pancreatic duct:

• Join the duodenum at the hepatopancreatic ampulla

• Are controlled by the sphincter of Oddi

• The jejunum extends from the duodenum to the ileum

• The ileum joins the large intestine at the ileocecal valve

Microscopic Anatomy of the Small Intestine

• Structural modifications of the small intestine wall increase surface area

• Plicae circulares: deep circular folds of the mucosa and submucosa

• Villi: fingerlike extensions of the mucosa

• Microvilli: tiny projections of absorptive mucosal cells’ plasma membranes

Small Intestine: Histology of the Wall

• The epithelium of the mucosa is made up of:

• Absorptive cells and goblet cells

• Interspersed T cells (intraepithelial lymphocytes), and

• Enteroendocrine cells

• Intestinal crypts cells secrete intestinal juice

• Peyer’s patches are found in the submucosa

• Brunner’s glands in the duodenum secrete alkaline mucus

Intestinal Juice

• Secreted by intestine glands in response to distension or irritation of the mucosa

• It is slightly alkaline and isotonic with blood plasma

• Is largely water, enzyme-poor, but contains mucus

Liver

• The largest gland in the body

• Superficially has four lobes – right, left, caudate, and quadrate

• The falciform ligament:

• Separates the right and left lobes anteriorly

• Suspends the liver from the diaphragm and anterior abdominal wall

• The ligamentum teres:

• Is a remnant of the fetal umbilical vein

• Runs along the free edge of the falciform ligament

Liver: Associated Structures

• The lesser omentum anchors the liver to the stomach

• The hepatic blood vessels enter the liver at the porta hepatis

• The gallbladder rests in a recess on the inferior surface of the right lobe

• Bile leaves the liver via

• Bile ducts which fuse into the common hepatic duct

• The common hepatic duct fuses with the cystic duct

• These two ducts form the bile duct

Microscopic Anatomy of the Liver

• Hexagonal-shaped liver lobules are the structural and functional units of the liver

• Composed of hepatocyte (liver cell) plates radiating outward from a central vein

• Portal triads are found at each of the six corners of each liver lobule

• Portal triads consist of a bile duct and

• Hepatic artery – supplies oxygen-rich blood to the liver

• Hepatic portal vein – carries venous blood with nutrients from digestive viscera

• Liver sinusoids – enlarged, leaky capillaries located between hepatic plates

• Kupffer cells – hepatic macrophages found in liver sinusoids

• Hepatocytes’ functions include:

• Production of bile

• Processing bloodborne nutrients

• Storage of fat-soluble vitamins

• Detoxification

• Secreted bile flows between hepatocytes toward the bile ducts in the portal triads

Homeostatic Imbalance

• Hepatitis – inflammation of the liver often due to viral infection

• Viruses causing hepatitis are catalogued has HVA through HVF

• HVA and HVE are transmitted enterically and cause self-limiting infections

• Hepatitis B is transmitted via blood transfusions, contaminated needles, and sexual contact, and increases the risk of liver cancer

• Hepatitis C produces chronic liver infection

• Nonviral hepatitis is caused by drug toxicity and wild mushroom poisoning

• Cirrhosis – diffuse and progressive chronic inflammation of the liver

• Typically results from chronic alcoholism or severe chronic hepatitis

• The liver becomes fatty and fibrous and its activity is depressed

• Scar tissue obstructs blood flow in the hepatic portal system causing portal hypertension

Composition of Bile

• A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes

• Bile salts are cholesterol derivatives that:

• Emulsify fat

• Facilitate fat and cholesterol absorption

• Help solubilize cholesterol

• Enterohepatic circulation recycles bile salts

• The chief bile pigment is bilirubin, a waste product of heme

The Gallbladder

• Thin-walled, green muscular sac on the ventral surface of the liver

• Stores and concentrates bile by absorbing its water and ions

• Releases bile via the cystic duct which flows into the bile duct

Regulation of Bile Release

• Acidic, fatty chyme causes the duodenum to release:

• Cholecystokinin (CCK) and secretin into the bloodstream

• Bile salts and secretin transported in blood stimulate the liver to produce bile

• Vagal stimulation causes weak contractions of the gallbladder

• Cholecystokinin causes:

• The gallbladder to contract

• The hepatopancreatic sphincter to relax

• As a result, bile enters the duodenum

Homeostatic Imbalance

• Gallstones – crystallization of cholesterol which can obstruct the flow of bile

• Current treatments include: dissolving the crystals with drugs, pulverizing them with ultrasound, vaporizing them with lasers, and surgical removal of the gallbladder

• Obstructive jaundice – yellowish skin caused by bile pigments deposited in the skin

• Due to blocked bile ducts

Pancreas

• Location

• Lies deep to the greater curvature of the stomach

• The head is encircled by the duodenum and the tail abuts the spleen

• Exocrine function

• Secretes pancreatic juice which breaks down all categories of foodstuff

• Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes

• The pancreas also has an endocrine function – release of insulin and glucagon

Composition and Function of Pancreatic Juice

• Water solution of enzymes and electrolytes (primarily HCO₃)

• Neutralizes acid chyme

• Provides optimal environment for pancreatic enzymes

• Enzymes are released in inactive form and activated in the duodenum

• Examples include

• Trypsinogen is activated to trypsin

• Procarboxypeptidase is activated to carboxypeptidase

• Active enzymes secreted

• Amylase, lipases, and nucleases

• These enzymes require ions or bile for optimal activity

Regulation of Pancreatic Secretion

• Secretin and CCK are released when fatty or acidic chyme enters the duodenum

• CCK and secretin enter the bloodstream

• Upon reaching the pancreas:

• CCK induces the secretion of enzyme-rich pancreatic juice

• Secretin causes secretion of bicarbonate-rich pancreatic juice

• Vagal stimulation also causes release of pancreatic juice

Digestion in the Small Intestine

• As chyme enters the duodenum

• Carbohydrates and proteins are only partially digested

• No fat digestion has taken place

• Digestion continues in the small intestine

• Chyme is released slowly into the duodenum

• Because it is hypertonic and has low pH, mixing is required for proper digestion

• Required substances needed are supplied by the liver

• Virtually all nutrient absorption takes place in the small intestine

Motility of the Small Intestine

• The most common motion of the small intestine is segmentation

• It is initiated by intrinsic pacemaker cells (Cajal cells)

• Moves contents steadily toward the ileocecal valve

• After nutrients have been absorbed:

• Peristalsis begins with each wave starting distal to the previous

• Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine

Control of Motility

• Local enteric neurons of the GI tract coordinate intestinal motility

• Cholinergic neurons cause:

• Contraction and shortening of the circular muscle layer

• Shortening of longitudinal muscle

• Distension of the intestine

• Other impulses relax the circular muscle

• The gastroileal reflex and gastrin:

• Relax the ileocecal sphincter

• Allow chyme to pass into the large intestine

Large Intestine

• Has three unique features:

• Teniae coli – three bands of longitudinal smooth muscle in its muscularis

• Haustra – pocketlike sacs caused by the tone of the teniae coli

• Epiploic appendages – fat-filled pouches of visceral peritoneum

• Is subdivided into the cecum, appendix, colon, rectum, and anal canal

• The saclike cecum:

• Lies below the ileocecal valve in the right iliac fossa

• Contains a wormlike vermiform appendix

Homeostatic Imbalance

• Appendicitis – inflammation of the appendix resulting from blockage that traps infectious bacteria in its lumen

• If the appendix ruptures, feces containing bacteria spray over the abdominal contents causing peritonitis

• Treatment is surgical removal of the appendix

Colon

• Has distinct regions: ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon

• The transverse and sigmoid portions are anchored via mesenteries called mesocolons

• The sigmoid colon joins the rectum

• The anal canal, the last segment of the large intestine, opens to the exterior at the anus

Valves and Sphincters of the Rectum and Anus

• Three valves of the rectum stop feces from being passed with gas

• The anus has two sphincters:

• Internal anal sphincter composed of smooth muscle

• External anal sphincter composed of skeletal muscle

• These sphincters are closed except during defecation

Large Intestine: Microscopic Anatomy

• Colon mucosa is simple columnar epithelium except in the anal canal

• Has numerous deep crypts lined with goblet cells

• Anal canal mucosa is stratified squamous epithelium

• Anal sinuses exude mucus and compress feces

• Superficial venous plexuses are associated with the anal canal

• Inflammation of these veins results in itchy varicosities called hemorrhoids

Bacterial Flora

• The bacterial flora of the large intestine consist of:

• Bacteria surviving the small intestine that enter the cecum and

• Those entering via the anus

• These bacteria:

• Colonize the colon

• Ferment indigestible carbohydrates

• Release irritating acids and gases (flatus)

• Synthesize B complex vitamins and vitamin K

Functions of the Large Intestine

• Other than digestion of enteric bacteria, no further digestion takes place

• Vitamins, water, and electrolytes are reclaimed

• Its major function is propulsion of fecal material toward the anus

• Though essential for comfort, the colon is not essential for life

Motility of the Large Intestine

• Haustral contractions

• Slow segmenting movements that move the contents of the colon

• Haustra sequentially contract as they are stimulated by distension

• Presence of food in the stomach:

• Activates the gastrocolic reflex

• Initiates peristalsis that forces contents toward the rectum

Homeostatic Imbalance

• Diverticulosis – small herniation (diverticula) of the mucosa of the colon walls caused by lack of bulk in the colon

• Most common in the sigmoid colon in people over 70

• Diverticulitis – inflamed diverticula that can be life threatening if the diverticula rupture

Defecation

• Distension of rectal walls caused by feces

• Stimulates contraction of the rectal walls

• Relaxes the internal anal sphincter

• Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs

Homeostatic Imbalance

• Diarrhea – watery stool resulting from any condition that rushes food residue through the large intestine too quickly

• This causes insufficient time for water absorption

• Prolonged diarrhea may result in dehydration and electrolyte imbalance

• Constipation – hard stool that is difficult to pass resulting from residues staying in the intestine too long

• May result from lack of fiber in the diet

Food Poisoning: Salmonella

• Salmonella is spread by:

• Contaminated eggs and egg products

• Infected food handlers with feces-contaminated hands

• Salmonella can cause:

• Bacteremia 4 to 7 days after infection

• Endocarditis, thrombi, bone infections, arthritis, and meningitis

• Diagnosis is by positive stool samples

• Salmonellosis is treated symptomatically

Chemical Digestion: Carbohydrates

• Absorption: via cotransport with Na⁺, and facilitated diffusion

• Enter the capillary bed in the villi

• Transported to the liver via the hepatic portal vein

• Enzymes used: salivary amylase, pancreatic amylase, and brush border enzymes

Chemical Digestion: Proteins

• Absorption: similar to carbohydrates

• Enzymes used: pepsin in the stomach

• Enzymes acting in the small intestine

• Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase

• Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases

Chemical Digestion: Fats

• Absorption: Diffusion into intestinal cells where they

• Combine with proteins and extrude chylomicrons

• Enter lacteals and are transported to systemic circulation via lymph

• Glycerol and short chain fatty acids

• are absorbed into the capillary blood in villi

• transported via the hepatic portal vein

• Enzymes/chemicals used: bile salts and pancreatic lipase

Fatty Acid Absorption

• Fatty acids and monoglycerides enter intestinal cells via diffusion

• They are combined with proteins within the cells

• Resulting chylomicrons are extruded

• They enter lacteals and are transported to the circulation via lymph

Chemical Digestion: Nucleic Acids

• Absorption: active transport via membrane carriers

• Absorbed in villi and transported to liver via hepatic portal vein

• Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines

Electrolyte Absorption

• Most ions are actively absorbed along the length of small intestine

• Na⁺ is coupled with absorption of glucose and amino acids

• Ionic iron is transported into mucosal cells where it binds to ferritin

• Anions passively follow the electrical potential established by Na⁺

• K⁺ diffuses across the intestinal mucosa in response to osmotic gradients

• Ca²⁺ absorption:

• Is related to blood levels of ionic calcium

• Is regulated by Vitamin D and parathyroid hormone (PTH)

Water Absorption

• 95% of water is absorbed in the small intestines by osmosis

• Water moves in both directions across intestinal mucosa

• Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells

• Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients

Malabsorption of Nutrients

• Results from anything that interferes with delivery of bile or pancreatic juice

• Factors that damage the intestinal mucosa (e.g., bacterial infection)

• Gluten enteropathy (adult celiac disease) – gluten damages the intestinal villi and reduces the length of microvilli

• Treated by eliminating gluten from the diet (all grains but rice and corn)

Embryonic Development of the Digestive System

• 3^rd week – endoderm has folded and foregut and hindgut have formed

• The midgut is open and continuous with the yolk sac

• Mouth and anal openings are nearly formed

• 8^th week – accessory organs are budding from endoderm

Homeostatic Imbalance

• Cleft palate – palatine bones, palatine process of the maxillae (or both) fail to fuse

• Tracheoesophageal fistula – opening between the esophagus and trachea

• Cystic fibrosis – impairs pancreatic activity

Developmental Aspects

• During fetal life, nutrition is via the placenta, but the GI tract is stimulated toward maturity by amniotic fluid swallowed in utero

• At birth, feeding is an infant’s most important function and is enhanced by

• Rooting reflex (helps infant find the nipple) and sucking reflex (aid in swallowing)

• Digestive system has few problems until the onset of old age

• During old age the GI tract activity declines, absorption is less efficient, and peristalsis is slowed

Cancer

• Stomach and colon cancers rarely have early signs or symptoms

• Metastasized colon cancers frequently cause secondary liver cancer

• Prevention is by regular dental and medical examinations

• Colon cancer is the 2^nd largest cause of cancer deaths in males (lung cancer is 1^st)

• Forms from benign mucosal tumors called polyps whose formation increases with age

• Regular colon examination should be done for all those over 50