The Digestive System PART A Digestive System Overview

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The Digestive System PART A

The Digestive System PART A

Digestive System: Overview n n n The alimentary canal or gastrointestinal (GI) tract digests

Digestive System: Overview n n n 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

Figure 23. 1

Figure 23. 1

Digestive Process n n The GI tract is a “disassembly” line ¨ Nutrients become

Digestive Process n n 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

Figure 23. 2

Figure 23. 2

Gastrointestinal Tract Activities n n n Ingestion – taking food into the digestive tract

Gastrointestinal Tract Activities n n n 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

Peristalsis and Segmentation Figure 23. 3

Peristalsis and Segmentation Figure 23. 3

Gastrointestinal Tract Activities n n n Chemical digestion – catabolic breakdown of food Absorption

Gastrointestinal Tract Activities n n n 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 n n External environment for the digestive process Regulation of digestion involves:

GI Tract n n 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 n n Mechano and chemoreceptors respond to: ¨ Stretch,

Receptors of the GI Tract n n Mechano and chemoreceptors respond to: ¨ Stretch, osmolarity, and p. H ¨ 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 n n Intrinsic controls ¨ Nerve plexuses near

Nervous Control of the GI Tract n n 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 ¨ CNS centers and extrinsic autonomic nerves

Nervous Control of the GI Tract Figure 23. 4

Nervous Control of the GI Tract Figure 23. 4

Peritoneum and Peritoneal Cavity n n Peritoneum – serous membrane of the abdominal cavity

Peritoneum and Peritoneal Cavity n n 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

Peritoneum and Peritoneal Cavity Figure 23. 5 a

Peritoneum and Peritoneal Cavity Figure 23. 5 a

Peritoneum and Peritoneal Cavity n n n Mesentery – double layer of peritoneum that

Peritoneum and Peritoneal Cavity n n n Mesentery – double layer of peritoneum that provides: ¨ Vascular and nerve supplies to the viscera ¨ Hold digestive organs in place and store fat Retroperitoneal organs – organs outside the peritoneum Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum

Peritoneum and Peritoneal Cavity Figure 23. 5 b

Peritoneum and Peritoneal Cavity Figure 23. 5 b

Blood Supply: Splanchnic Circulation n Arteries and the organs they serve include ¨ The

Blood Supply: Splanchnic Circulation n 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

Blood Supply: Splanchnic Circulation n Hepatic portal circulation: ¨ Collects nutrient-rich venous blood from

Blood Supply: Splanchnic Circulation n Hepatic portal circulation: ¨ Collects nutrient-rich venous blood from the digestive viscera ¨ Delivers this blood to the liver for metabolic processing and storage

Histology of the Alimentary Canal n n From esophagus to the anal canal the

Histology of the Alimentary Canal n n 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 a specific digestive function

Histology of the Alimentary Canal Figure 23. 6

Histology of the Alimentary Canal Figure 23. 6

Mucosa n n n Moist epithelial layer that lines the lumen of the alimentary

Mucosa n n n Moist epithelial layer that lines the lumen of the alimentary canal Three major functions: ¨ Secretion of mucus ¨ Absorption of end products of digestion ¨ Protection against infectious disease Consists of three layers: a lining epithelium, lamina propria, and muscularis mucosae

Mucosa: Epithelial Lining n n n Simple columnar epithelium and mucussecreting goblet cells Mucus

Mucosa: Epithelial Lining n n n Simple columnar epithelium and mucussecreting goblet cells Mucus secretions: ¨ Protect digestive organs from digesting themselves ¨ Ease food along the tract Stomach and small intestine mucosa contain: ¨ Enzyme-secreting cells ¨ Hormone-secreting cells (making them endocrine and digestive organs)

Mucosa: Lamina Propria and Muscularis Mucosae n n Lamina Propria ¨ Loose areolar and

Mucosa: Lamina Propria and Muscularis Mucosae n n Lamina Propria ¨ Loose areolar and reticular connective tissue ¨ Nourishes the epithelium and absorbs 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 n n n Submucosa – dense connective tissue containing elastic fibers,

Mucosa: Other Sublayers n n n 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 n n n Composed of two major intrinsic nerve plexuses: ¨

Enteric Nervous System n n n 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 n Oral or buccal cavity: ¨ Is bounded by lips, cheeks, palate, and

Mouth n 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

Mouth n To withstand abrasions: ¨ The mouth is lined with stratified squamous epithelium

Mouth n To withstand abrasions: ¨ The mouth is lined with stratified squamous epithelium ¨ The gums, hard palate, and dorsum of the tongue are slightly keratinized

Anatomy of the Oral Cavity: Mouth Figure 23. 7 a

Anatomy of the Oral Cavity: Mouth Figure 23. 7 a

Lips and Cheeks n n Have a core of skeletal muscles ¨ Lips: orbicularis

Lips and Cheeks n n 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

Oral Cavity and Pharynx: Anterior View Figure 23. 7 b

Oral Cavity and Pharynx: Anterior View Figure 23. 7 b

Palate n Hard palate – underlain by palatine bones and palatine processes of the

Palate n 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)

Palate n n Soft palate – mobile fold formed mostly of skeletal muscle ¨

Palate n n 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 n n Occupies the floor of the mouth and fills the oral cavity

Tongue n n 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

Tongue n n n Intrinsic muscles change the shape of the tongue Extrinsic muscles

Tongue n n n 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

Tongue n Superior surface bears three types of papillae ¨ Filiform – give the

Tongue n 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

Tongue n Sulcus terminalis – groove that separates the tongue into two areas: ¨

Tongue n Sulcus terminalis – groove that separates the tongue into two areas: ¨ Anterior 2/3 residing in the oral cavity ¨ Posterior third residing in the oropharynx

Tongue Figure 23. 8

Tongue Figure 23. 8

Salivary Glands n Produce and secrete saliva that: ¨ Cleanses the mouth ¨ Moistens

Salivary Glands n Produce and secrete saliva that: ¨ Cleanses the mouth ¨ Moistens and dissolves food chemicals ¨ Aids in bolus formation ¨ Contains enzymes that break down starch

Salivary Glands n n Three pairs of extrinsic glands – parotid, submandibular, and sublingual

Salivary Glands n n Three pairs of extrinsic glands – parotid, submandibular, and sublingual Intrinsic salivary glands (buccal glands) – scattered throughout the oral mucosa

Salivary Glands n n n Parotid – lies anterior to the ear between the

Salivary Glands n n n Parotid – lies anterior to the ear between the masseter muscle and skin ¨ Parotid duct opens into the vestibule next to 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

Salivary Glands Figure 23. 9 a

Salivary Glands Figure 23. 9 a

Saliva: Source and Composition n n Secreted from serous and mucous cells of salivary

Saliva: Source and Composition n n Secreted from serous and mucous cells of salivary glands 97 -99. 5% water, hypo-osmotic, slightly acidic solution containing ¨ Electrolytes – Na+, K+, Cl–, PO 42–, HCO 3– ¨ Digestive enzyme – salivary amylase ¨ Proteins – mucin, lysozyme, defensins, and Ig. A ¨ Metabolic wastes – urea and uric acid

Control of Salivation n Intrinsic glands keep the mouth moist Extrinsic salivary glands secrete

Control of Salivation n Intrinsic glands keep the mouth moist Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to: ¨ Ingested food which stimulates chemoreceptors and mechanoreceptors ¨ The thought of food Strong sympathetic stimulation inhibits salivation and results in dry mouth

Teeth n n n Primary and permanent dentitions have formed by age 21 Primary

Teeth n n n 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 ¨ Usually 32 permanent teeth

Deciduous Teeth Figure 23. 10. 1

Deciduous Teeth Figure 23. 10. 1

Permanent Teeth Figure 23. 10. 2

Permanent Teeth Figure 23. 10. 2

Classification of Teeth n n Teeth are classified according to their shape and function

Classification of Teeth n n Teeth are classified according to their shape and function ¨ Incisors – chisel-shaped teeth for cutting or nipping ¨ Canines – fanglike teeth that tear or pierce ¨ Premolars (bicuspids) and molars – have broad crowns with rounded tips; best suited for grinding or crushing During chewing, upper and lower molars lock together generating crushing force

Dental Formula: Permanent Teeth n A shorthand way of indicating the number and relative

Dental Formula: Permanent Teeth n A shorthand way of indicating the number and relative position of teeth ¨ Written as ratio of upper to lower teeth for the mouth

Dental Formula: Permanent Teeth ¨Deciduous: 2 I (incisors), 1 C (canine), 2 M (molars)

Dental Formula: Permanent Teeth ¨Deciduous: 2 I (incisors), 1 C (canine), 2 M (molars) = 20 ¨Permanent: 2 I, 1 C, 2 PM (premolars), 3 M = 32 2 I 2 I 1 C 1 C 2 PM 3 M 3 M X 2 (32 teeth)

Tooth Structure n n Two main regions – crown and the root Crown –

Tooth Structure n n Two main regions – crown and the root Crown – exposed part of the tooth above the gingiva Enamel – acellular, brittle material composed of calcium salts and hydroxyapatite crystals; the hardest substance in the body ¨ Encapsules the crown of the tooth Root – portion of the tooth embedded in the jawbone

Tooth Structure n n Neck – constriction where the crown and root come together

Tooth Structure n n Neck – constriction where the crown and root come together Cementum – calcified connective tissue ¨ Covers the root ¨ Attaches it to the periodontal ligament

Tooth Structure n n Periodontal ligament ¨ Anchors the tooth in the alveolus of

Tooth Structure n n Periodontal ligament ¨ Anchors the tooth in the alveolus of the jaw ¨ Forms the fibrous joint called a gomphosis Gingival sulcus – depression where the gingiva borders the tooth

Tooth Structure n n n Dentin – bonelike material deep to the enamel cap

Tooth Structure n n n 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

Tooth Structure n n n Root canal – portion of the pulp cavity that

Tooth Structure n n n 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

Tooth Structure Figure 23. 11

Tooth Structure Figure 23. 11

Tooth and Gum Disease n Dental caries – gradual demineralization of enamel and dentin

Tooth and Gum Disease n 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 n n Gingivitis – as plaque accumulates, it calcifies

Tooth and Gum Disease: Periodontitis n n 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 Immune system attacks intruders as well as body tissues, carving pockets around the teeth and dissolving bone

The Digestive System PART B

The Digestive System PART B

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

Pharynx n n n 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 n n n Muscular tube going from the laryngopharynx to the stomach Travels

Esophagus n n n Muscular tube going from the laryngopharynx to the stomach Travels through the mediastinum and pierces the diaphragm Joins the stomach at the cardiac orifice

Esophagus Figure 23. 12

Esophagus Figure 23. 12

Esophageal Characteristics n n Esophageal mucosa – nonkeratinized stratified squamous epithelium The empty esophagus

Esophageal Characteristics n n 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 (superior third) to smooth muscle (inferior third)

Digestive Processes in the Mouth n n n Food is ingested Mechanical digestion begins

Digestive Processes in the Mouth n n n 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) n n Coordinated activity of the tongue, soft palate, pharynx, esophagus, and

Deglutition (Swallowing) n n Coordinated activity of the tongue, soft palate, pharynx, esophagus, and 22 separate muscle groups Buccal phase – bolus is forced into the oropharynx

Deglutition (Swallowing) n n Pharyngeal-esophageal phase – controlled by the medulla and lower pons

Deglutition (Swallowing) n n 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

Bolus of food Tongue Uvula Pharynx Epiglottis Bolus Epiglottis Glottis Esophagus Trachea (a) Upper

Bolus of food Tongue Uvula Pharynx Epiglottis Bolus Epiglottis Glottis Esophagus Trachea (a) Upper esophageal sphincter contracted Bolus (b) Upper esophageal sphincter relaxed Relaxed muscles Bolus of food Longitudinal muscles contract, shortening passageway ahead of bolus Gastroesophageal sphincter closed (d) (c) Upper esophageal sphincter contracted Circular muscles contract, constricting passageway and pushing bolus down Relaxed muscles Stomach (e) Figure 23. 13

Stomach n n n Chemical breakdown of proteins begins and food is converted to

Stomach n n n 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

Stomach n n Greater curvature – entire extent of the convex lateral surface Lesser

Stomach n n 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

Stomach n n Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous

Stomach n n Nerve supply – sympathetic and parasympathetic fibers of the autonomic nervous system Blood supply – celiac trunk, and corresponding veins (part of the hepatic portal system)

Figure 23. 14 a

Figure 23. 14 a

Microscopic Anatomy of the Stomach n Muscularis – has an additional oblique layer that:

Microscopic Anatomy of the Stomach n Muscularis – has an additional oblique layer that: ¨ Allows the stomach to churn, mix, and pummel food physically ¨ Breaks down food into smaller fragments

Microscopic Anatomy of the Stomach n n n Epithelial lining is composed of: ¨

Microscopic Anatomy of the Stomach n n n Epithelial lining is composed of: ¨ Goblet cells that produce a coat of alkaline mucus n The mucous surface layer traps a bicarbonate-rich fluid beneath it Gastric pits contain goblet cells Gastric glands secrete gastric juice ¨ Mucus and hormones

Microscopic Anatomy of the Stomach Figure 23. 15 a

Microscopic Anatomy of the Stomach Figure 23. 15 a

Microscopic Anatomy of the Stomach Figure 23. 15 b

Microscopic Anatomy of the Stomach Figure 23. 15 b

Microscopic Anatomy of the Stomach Figure 23. 15 c

Microscopic Anatomy of the Stomach Figure 23. 15 c

Glands of the Stomach Fundus and Body n n n Gastric glands of the

Glands of the Stomach Fundus and Body n n n Gastric glands of the cardia and pylorus ¨ Secrete mucus Gastric glands of the pyloric antrum ¨ Secrete hormones Gastric glands of the fundus and body have a variety of secretory cells ¨ Mucous neck cells – secrete acid mucus ¨ Parietal cells – secrete HCl and intrinsic factor

Glands of the Stomach Fundus and Body ¨ Chief cells – produce pepsinogen n

Glands of the Stomach Fundus and Body ¨ Chief cells – produce pepsinogen n Pepsinogen is activated to pepsin by: ¨HCl in the stomach ¨Pepsin itself via a positive feedback mechanism ¨ Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria

Stomach Lining n n n The stomach is exposed to the harshest conditions in

Stomach Lining n n n 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 n Gastric glands that have cells impermeable to HCl Damaged epithelial cells are quickly replaced

Digestion in the Stomach n The stomach: ¨ Holds ingested food ¨ Degrades this

Digestion in the Stomach n The stomach: ¨ Holds ingested food ¨ Degrades this food both physically and chemically ¨ Delivers chyme to the small intestine ¨ Enzymatically digests proteins with pepsin ¨ Secretes intrinsic factor required for absorption of vitamin B 12

Regulation of Gastric Secretion n n Neural and hormonal mechanisms regulate the release of

Regulation of Gastric Secretion n n 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 n n Excitatory events include: ¨ Sight or thought of food ¨

Cephalic Phase n n Excitatory events include: ¨ Sight or thought of food ¨ Stimulation of taste or smell receptors ¨ Relayed to hypothalamus, medulla oblongata (vagus nucleus) n Stimulates stomach glands Inhibitory events include: ¨ Loss of appetite or depression ¨ Decrease in stimulation of the parasympathetic division

Gastric Phase n Excitatory events include: ¨ Stomach distension n Activation of stretch receptors

Gastric Phase n Excitatory events include: ¨ Stomach distension n Activation of stretch receptors (neural activation) ¨ Activation of chemoreceptors by peptides, caffeine, and rising p. H ¨ Release of gastrin to the blood n Increase release of HCl

Gastric Phase n Inhibitory events include: ¨ A p. H lower than 2 ¨

Gastric Phase n Inhibitory events include: ¨ A p. H lower than 2 ¨ Emotional upset that overrides the parasympathetic division

Regulation and Mechanism of HCl Secretion n HCl secretion is stimulated by ACh, histamine,

Regulation and Mechanism of HCl Secretion n HCl secretion is stimulated by ACh, histamine, and gastrin through second-messenger systems Release of hydrochloric acid: ¨ Is low if only one ligand binds to parietal cells ¨ Is high if all three ligands bind to parietal cells Antihistamines block H 2 receptors and decrease HCl release

Regulation and Mechanism of HCl Secretion Figure 23. 17

Regulation and Mechanism of HCl Secretion Figure 23. 17

Intestinal Phase – 2 components: n n Excitatory phase – low p. H; partially

Intestinal Phase – 2 components: n n Excitatory phase – low p. H; partially digested food enters the duodenum and encourages gastric gland activity through intestinal gastrin Inhibitory phase (enterogastric reflex)– distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum ¨ Initiates inhibition of local reflexes and vagal nuclei and stimulates sympathetic fibers ¨ Closes the pyloric sphincter ¨ Releases enterogastrones that inhibit gastric secretion

Release of Gastric Juice: Stimulatory Events Figure 23. 16. 1

Release of Gastric Juice: Stimulatory Events Figure 23. 16. 1

Release of Gastric Juice: Inhibitory Events Figure 23. 16. 2

Release of Gastric Juice: Inhibitory Events Figure 23. 16. 2

Response of the Stomach to Filling n n Stomach pressure remains constant until about

Response of the Stomach to Filling n n Stomach pressure remains constant until about 1 L of food is ingested Relative unchanging pressure results from reflex-mediated relaxation and plasticity

Response of the Stomach to Filling n n Reflex-mediated events include: ¨ Receptive relaxation

Response of the Stomach to Filling n n 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 n n n Peristaltic waves move toward the pylorus at the

Gastric Contractile Activity n n n Peristaltic waves move toward the pylorus at the rate of 3 per minute This basic electrical rhythm (BER) is set by pacemaker cells (cells of Cajal) on the longitudinal layer ¨ They set the rate of contraction Neural and hormonal stimulus ¨ Initiate the stimulus ¨ Regulate the strength of the stimulus

Gastric Contractile Activity n n Most vigorous peristalsis and mixing occurs near the pylorus

Gastric Contractile Activity n n 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

Gastric Contractile Activity Figure 23. 18

Gastric Contractile Activity Figure 23. 18

Regulation of Gastric Emptying n n Gastric emptying is regulated by: ¨ The neural

Regulation of Gastric Emptying n n Gastric emptying is regulated by: ¨ The neural enterogastric reflex ¨ Hormonal (enterogastrone) mechanisms These mechanisms inhibit gastric secretion and duodenal filling

Regulation of Gastric Emptying n n Carbohydrate-rich chyme quickly moves through the duodenum Fat-laden

Regulation of Gastric Emptying n n Carbohydrate-rich chyme quickly moves through the duodenum Fat-laden chyme is digested more slowly causing food to remain in the stomach longer

Regulation of Gastric Emptying Figure 23. 19

Regulation of Gastric Emptying Figure 23. 19

Small Intestine: Gross Anatomy n n n Runs from pyloric sphincter to the ileocecal

Small Intestine: Gross Anatomy n n n Runs from pyloric sphincter to the ileocecal valve Has three subdivisions: duodenum, jejunum, and ileum Mesentery proper attach the small intestine to the body wall

Small Intestine: Gross Anatomy n n n The bile duct and main pancreatic duct:

Small Intestine: Gross Anatomy n n n 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

Small Intestine: Microscopic Anatomy n Structural modifications of the small intestine wall increase surface

Small Intestine: Microscopic Anatomy n 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

Duodenum and Related Organs Figure 23. 20

Duodenum and Related Organs Figure 23. 20

Small Intestine: Microscopic Anatomy Figure 23. 21

Small Intestine: Microscopic Anatomy Figure 23. 21

Small Intestine: Histology of the Wall n The epithelium of the mucosa is made

Small Intestine: Histology of the Wall n The epithelium of the mucosa is made up of: ¨ Absorptive cells and goblet cells ¨ Enteroendocrine cells ¨ Interspersed T cells called intraepithelial lymphocytes (IELs) n IELs immediately release cytokines upon encountering Ag. Do not need “priming. ”

Small Intestine: Histology of the Wall n n n Cells of intestinal crypts secrete

Small Intestine: Histology of the Wall n n n Cells of intestinal crypts secrete intestinal juice Peyer’s patches are found in the submucosa of the ileum Brunner’s glands in the duodenum secrete alkaline mucus

Intestinal Juice n n n Secreted by intestinal glands in response to distension or

Intestinal Juice n n n Secreted by intestinal glands in response to distension or irritation of the mucosa Slightly alkaline and isotonic with blood plasma Largely water, enzyme-poor, but contains mucus

The ______ in the small intestine have a similar structure and function to the

The ______ in the small intestine have a similar structure and function to the ______ in the stomach. n n a. b. c. d. gastric pits… intestinal glands… gastric pits goblet cells… enteroendocrine cells brush border… rugae

Liver n n n The largest gland in the body Superficially has four lobes

Liver n n n 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

Liver n The ligamentum teres (round ligament): ¨ Is a remnant of the fetal

Liver n The ligamentum teres (round ligament): ¨ Is a remnant of the fetal umbilical vein ¨ Runs along the free edge of the falciform ligament

Liver: Associated Structures n n The lesser omentum anchors the liver to the stomach

Liver: Associated Structures n n The lesser omentum anchors the liver to the stomach The gallbladder rests in a recess on the inferior surface of the right lobe

Liver: Associated Structures n Bile leaves the liver via: ¨ Bile ducts, which fuse

Liver: Associated Structures n Bile leaves the liver via: ¨ Bile ducts, which fuse into the common hepatic duct ¨ The common hepatic duct, which fuses with the cystic duct n These two ducts form the bile duct

Gallbladder and Associated Ducts Figure 23. 20

Gallbladder and Associated Ducts Figure 23. 20

Liver: Microscopic Anatomy n Hexagonal-shaped liver lobules are the structural and functional units of

Liver: Microscopic Anatomy n 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 Figure 23. 24 c

Liver: Microscopic Anatomy n Portal triads consist of a bile duct and ¨ Hepatic

Liver: Microscopic Anatomy n 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 Figure 23. 24 d

Liver: Microscopic Anatomy n n Liver sinusoids – enlarged, leaky capillaries located between hepatic

Liver: Microscopic Anatomy n n Liver sinusoids – enlarged, leaky capillaries located between hepatic plates Kupffer cells – hepatic macrophages found in liver sinusoids

Liver: Microscopic Anatomy n n Hepatocytes’ functions include: ¨ Production of bile ¨ Processing

Liver: Microscopic Anatomy n n 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

Microscopic Anatomy of the Liver Figure 23. 24 c, d

Microscopic Anatomy of the Liver Figure 23. 24 c, d

Composition of Bile n n A yellow-green, alkaline solution containing bile salts, bile pigments,

Composition of Bile n n 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 n n n Thin-walled, green muscular sac on the ventral surface of

The Gallbladder n n n 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 n n n Acidic, fatty chyme causes the duodenum to

Regulation of Bile Release n n n 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

Regulation of Bile Release n n Cholecystokinin causes: ¨ The gallbladder to contract ¨

Regulation of Bile Release n n Cholecystokinin causes: ¨ The gallbladder to contract ¨ The hepatopancreatic sphincter to relax ¨ Secretion of pancreatic juice As a result, bile and pancreatic juice enters the duodenum

Regulation of Bile Release 4 Vagal stimulation causes weak contractions of gallbladder 3 Bile

Regulation of Bile Release 4 Vagal stimulation causes weak contractions of gallbladder 3 Bile salts and secretin transported via bloodstream stimulate liver to produce bile more rapidly 5 Cholecystokinin (via bloodstream) causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum 1 Acidic, fatty chyme entering duodenum causes release of cholecystokinin and secretin from duodenal wall enteroendocrine cells 2 Cholecystokinin and secretin enter the bloodstream 6 Bile salts reabsorbed into blood Figure 23. 25

The Digestive System PART C

The Digestive System PART C

Pancreas n Location ¨ Lies deep to the greater curvature of the stomach ¨

Pancreas n Location ¨ Lies deep to the greater curvature of the stomach ¨ The head is encircled by the duodenum and the tail abuts the spleen

Pancreas n n Exocrine function ¨ Secretes pancreatic juice which breaks down all categories

Pancreas n n 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

Acinus of the Pancreas Figure 23. 26 a

Acinus of the Pancreas Figure 23. 26 a

Composition and Function of Pancreatic Juice n n Water solution of enzymes and electrolytes

Composition and Function of Pancreatic Juice n n Water solution of enzymes and electrolytes (primarily HCO 3–) ¨ Neutralizes acid chyme ¨ Provides optimal environment for pancreatic enzymes Pancreatic proteases are released in inactive form and activated in the duodenum ¨ Trypsinogen is activated to trypsin

Composition and Function of Pancreatic Juice n n Trypsin activates: ¨ Procarboxypeptidase to carboxypeptidase

Composition and Function of Pancreatic Juice n n Trypsin activates: ¨ Procarboxypeptidase to carboxypeptidase ¨ Chymotrypsinogen to chymotrypsin Active enzymes secreted ¨ Amylase, lipases, and nucleases ¨ These enzymes require ions or bile for optimal activity

Pancreatic Activation Figure 23. 27

Pancreatic Activation Figure 23. 27

Regulation of Pancreatic Secretion n n Secretin and CCK are released when fatty or

Regulation of Pancreatic Secretion n n 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 bicarbonaterich pancreatic juice Vagal stimulation also causes release of pancreatic juice

Regulation of Pancreatic Secretion During cephalic and gastric phases, stimulation by vagal nerve fibers

Regulation of Pancreatic Secretion During cephalic and gastric phases, stimulation by vagal nerve fibers causes release of pancreatic juice and weak contractions of the gallbladder. 1 Acidic chyme entering duodenum causes the enteroendocrine cells of the duodenal wall to release secretin, whereas fatty, protein-rich chyme induces release of cholecystokinin. 2 Cholecystokinin and secretin enter bloodstream. 3 Upon reaching the pancreas, cholecystokinin induces the secretion of enzyme-rich pancreatic juice; secretin causes copious secretion of bicarbonate-rich pancreatic juice. Figure 23. 28

Digestion in the Small Intestine n As chyme enters the duodenum: ¨ Carbohydrates and

Digestion in the Small Intestine n As chyme enters the duodenum: ¨ Carbohydrates and proteins are only partially digested ¨ Almost no fat digestion has taken place

Digestion in the Small Intestine n Digestion continues in the small intestine ¨ Chyme

Digestion in the Small Intestine n Digestion continues in the small intestine ¨ Chyme is released slowly into the duodenum ¨ Because it is hypertonic and has low p. H, 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 in the Small Intestine n The most common motion of the small intestine

Motility in the Small Intestine n The most common motion of the small intestine is segmentation and not peristalsis ¨ It is initiated by intrinsic pacemaker cells (Cajal cells) ¨ Moves contents steadily toward the ileocecal valve

Motility in the Small Intestine n After nutrients have been absorbed: ¨ Peristalsis begins

Motility in the Small Intestine n After nutrients have been absorbed: ¨ Peristalsis begins in response to the motilin hormone with each wave starting distal to the previous ¨ Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine

Control of Motility n n Local enteric neurons of the GI tract coordinate intestinal

Control of Motility n n Local enteric neurons of the GI tract coordinate intestinal motility Cholinergic neurons cause: ¨ Contraction and shortening of the circular muscle layer proximally ¨ Shortening of longitudinal muscle distally ¨ Distension of the intestine

Control of Motility n n n The gastroileal reflex initiated by the increased activity

Control of Motility n n n The gastroileal reflex initiated by the increased activity of the stomach ¨ Increases segmentation in the ileum Gastrin released by the stomach ¨ Relax the ileocecal sphincter ¨ Increases the motility of the ileum Allow chyme to pass into the large intestine

Large Intestine n Has three unique features: ¨ Teniae coli – three bands of

Large Intestine n 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

Large Intestine n n Is subdivided into the cecum, appendix, colon, rectum, and anal

Large Intestine n n 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

Large Intestine Figure 23. 29 a

Large Intestine Figure 23. 29 a

Colon n n Has distinct regions: ascending colon, hepatic flexure, transverse colon, splenic flexure,

Colon n n 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 n n n Three valves of

Valves and Sphincters of the Rectum and Anus n n n 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

Mesenteries of Digestive Organs Figure 23. 30 b

Mesenteries of Digestive Organs Figure 23. 30 b

Mesenteries of Digestive Organs Figure 23. 30 c

Mesenteries of Digestive Organs Figure 23. 30 c

Mesenteries of Digestive Organs Figure 23. 30 d

Mesenteries of Digestive Organs Figure 23. 30 d

Large Intestine: Microscopic Anatomy n n n Colon mucosa is simple columnar epithelium except

Large Intestine: Microscopic Anatomy n n n Colon mucosa is simple columnar epithelium except in the anal canal There are no villi, no plicae, and no cells that secrete digestive enzymes Has numerous deep crypts lined with goblet cells

Large Intestine: Microscopic Anatomy n n Anal canal mucosa is stratified squamous epithelium Anal

Large Intestine: Microscopic Anatomy n n 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

Structure of the Anal Canal Figure 23. 29 b

Structure of the Anal Canal Figure 23. 29 b

Bacterial Flora n n The bacterial flora of the large intestine consist of: ¨

Bacterial Flora n n 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 n n Other than digestion of enteric bacteria, no

Functions of the Large Intestine n n 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 n n Haustral contractions ¨ Slow segmenting movements that

Motility of the Large Intestine n n 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 n Initiates peristalsis ( mass movements) that forces contents toward the rectum

Defecation n n Distension of rectal walls caused by feces: ¨ Stimulates contraction of

Defecation n n Distension of rectal walls caused by feces: ¨ Stimulates contraction of the rectal walls ¨ Relaxes the internal anal sphincter ¨ Contracts external anal sphincter Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs

Defecation Figure 23. 32

Defecation Figure 23. 32

Chemical Digestion: Carbohydrates n n Absorption: via cotransport with Na+, and facilitated diffusion ¨

Chemical Digestion: Carbohydrates n n 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 n n n Absorption: similar to carbohydrates Enzymes used: pepsin in

Chemical Digestion: Proteins n n n 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

Figure 23. 34

Figure 23. 34

Chemical Digestion: Fats n n Happens mainly in the small intestine Two phases: Emulsification

Chemical Digestion: Fats n n Happens mainly in the small intestine Two phases: Emulsification ¨ By bile salts ¨ Emulsion droplets Chemical digestion ¨ Pancreatic lipase n Free fatty acids n Monoglycerides

Chemical Digestion: Fats Figure 23. 35

Chemical Digestion: Fats Figure 23. 35

Lipids Absorption n Glycerol and short chain fatty acids are: ¨ Absorbed into the

Lipids Absorption n Glycerol and short chain fatty acids are: ¨ Absorbed into the capillary blood in villi ¨ Transported via the hepatic portal vein

Lipids Absorption n n n Micelles Fatty acids and monoglycerides enter intestinal cells via

Lipids Absorption n n n Micelles Fatty acids and monoglycerides enter intestinal cells via diffusion They are resynthesized into triglycerides Triglycerides are combined with proteins within the cells Resulting chylomicrons are extruded They enter lacteals and are transported to the circulation via lymph

Fatty Acid Absorption Fatty acids and monoglycerides associated with micelles in lumen of intestine

Fatty Acid Absorption Fatty acids and monoglycerides associated with micelles in lumen of intestine Lumen of intestine 1 Fatty acids and monoglycerides resulting from fat digestion leave micelles and enter epithelial cell by diffusion. Absorptive epithelial cell cytoplasm ER Golgi apparatus 2 Fatty acids are used to synthesize triglycerides in smooth endoplasmic reticulum. 3 Fatty globules are combined with proteins to form chylomicrons (within Golgi apparatus). 4 Vesicles containing chylomicrons migrate to the basal membrane, are extruded from the epithelial cell, and enter a lacteal (lymphatic capillary). Lacteal Chylomicron 5 Lymph in the lacteal transports chylomicrons away from intestine. Figure 23. 36

Chemical Digestion: Nucleic Acids n n n Absorption: active transport via membrane carriers Absorbed

Chemical Digestion: Nucleic Acids n n n 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 n n Most ions are actively absorbed along the length of small

Electrolyte Absorption n n 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+

Electrolyte Absorption n n K+ diffuses across the intestinal mucosa in response to osmotic

Electrolyte Absorption n n K+ diffuses across the intestinal mucosa in response to osmotic gradients Ca 2+ absorption: ¨ Is related to blood levels of ionic calcium ¨ Is regulated by vitamin D and parathyroid hormone (PTH)

Water Absorption n n 95% of water is absorbed in the small intestines by

Water Absorption n n 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 n n n Results from anything that interferes with delivery of

Malabsorption of Nutrients n n n 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 n n 3 rd week – endoderm has

Embryonic Development of the Digestive System n n 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

Embryonic Development of the Digestive System Figure 23. 37

Embryonic Development of the Digestive System Figure 23. 37

Developmental Aspects n n During fetal life, nutrition is via the placenta, but the

Developmental Aspects n n During fetal life, nutrition is via the placenta, but the GI tract is stimulated toward maturity by amniotic fluid swallowed in uterus At birth, feeding is an infant’s most important function and is enhanced by ¨ Rooting reflex (helps infant find the nipple) and sucking reflex (aids in swallowing)

Developmental Aspects n n Digestive system has few problems until the onset of old

Developmental Aspects n n 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 n n n Stomach and colon cancers rarely have early signs or symptoms

Cancer n n n 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

Cancer n n n Colon cancer is the 2 nd largest cause of cancer

Cancer n n n 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