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