14. digestive system

The Digestive System

Histology

Introduction

• The digestive system consists of a muscular digestive (tube) tract and accessory organs- digestive tract: oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine

- accessory organs: teeth, tongue, and glandular organs (salivary glands, lever, pancreas) which secrete into ducts

• Food enters the digestive tract, along the way secretions of the glandular organs assist in preparing organic and inorganic nutrients for absorption

Functions of the Digestive System

• Ingestion – occurs when foods and liquids enter the digestive tract via the mouth

• Mechanical processing – squashing with the tongue, tearing and crushing with the teeth- swirling, mixing, churning, and propulsive motions provide mechanical processing after swallowing

• Digestion – chemical and enzymatic breakdown of carbohydrates, lipids, and proteins- small organic molecules can be absorbed by the digestive epithelium

• Secretion – digestion involves the action of acids, enzymes, and buffers- produced by the lining but mostly by the accessory organs (pancreas)

• Absorption – movement of organic molecules, electrolytes, vitamins, and water across the digestive epithelium - into the interstitial fluid of the digestive tract

• Excretion – into the digestive tract, primarily by the accessory glands (especially the liver)

• Compaction – progressive dehydration of indigestible materials and organic wastes (feces)- defecation is the elimination of feces from the body

Lining of the Digestive Tract

• Plays a defensive role by protecting surrounding tissues against:

1. the corrosive effects of digestive acids and enzymes

2. mechanical stresses, such as abrasion

3. pathogens that are swallowed with food or that reside within the digestive tract

Components of the Digestive System Fig 25.1

Histological Organization of the Digestive Tract

Major layers of the digestive tract include the:• Mucosa • Submucosa• Muscularis externa• Serosa

The Mucosa

• A mucous membrane consisting of a layer of loose CT covered by an epithelium moistened by glandular secretions- plicae (folds) increase surface area for absorption

- lamina propria, underlying layer of areolar tissue (BVs, sensory nerve endings, lymphatic vessels, smooth muscle fibers

- muscularis mucosae, smooth muscle fibers arranged in 2 thin concentric layers:

- the circular layer, inner layer encircles the lumen and

- the longitudinal layer, outer layer whose muscle fibers lie parallel to the long axis of the tract

Histology of the Mucosa

Organ Epithelium

Mouth Nonkeratinized Stratified Squamous

Pharynx Nonkeratinized Stratified Squamous

Esophagus Nonkeratinized Stratified Squamous

Stomach Simple Columnar

Small Intestine Simple Columnar

Large Intestine Simple Columnar

Anus Nonkeratinized Stratified Squamous

Histology of the Mucosa

Organ Folds of the epithelium

Esophagus none

Stomach L: Rugae, S: gastric pits

Small Intestine L: Plicae circulares, Villi S: Crypts of Lieberkuhn, microvilli

Large Intestine L: Haustra S: Intestinal glands

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 25.2a

The Submucosa

• A layer of dense, irregular CT – surrounds the muscularis mucosae- contains large BVs and lymphatics

- some regions contain exocrine glands that secrete buffers and enzymes into the lumen

- in the outer margin the submucosal plexus, a network of nerve fibers and scattered neuron cell bodies innervates the mucosa

- include the sensory neurons, parasympathetic ganglia, and sympathetic postganglionic fibers

Histology of the Submucosa

Organ Specialized structures

Esophagus Submucosal mucous glands

Stomach None

Duodenum Brunner’s glands

Ileum Peyer’s Patches

Large Intestine None

The Muscularis Externa

• Dominated by smooth muscle fibers – surrounds the submucosa- fibers are arranged in circular (inner) and longitudinal (outer) layers

- essential in mechanical processing and in propulsion of materials along the digestive tract

- movements coordinated by the myenteric (Auerbach) plexus, a network of parasympathetic ganglia and sympathetic postganglionic fibers (sandwiched between the muscle layers)

- muscularis externa forms sphincters (valves) that help prevent materials from moving along the tract at the wrong time or direction

Histology of the Muscularis

Organ Smooth muscle layers

Esophagus 2, circular and longitudinal

Stomach 3, oblique, circular, and longitudinal

Small Intestine 2, circular and longitudinal

Large Intestine 2, circular and longitudinal

The Serosa

• A serous membrane – covers the muscularis externa along most regions of the digestive tract within the peritoneal cavity- no serosa surrounds the muscularis externa of the oral cavity, pharynx, esophagus, and rectum - instead adventitia, a fibrous sheath of collagen fibers attaches the digestive tract to adjacent structures

• Serosa or visceral peritoneum is continuous with the parietal peritoneum that lines the inner surfaces of the body wall

Histology of the Serosa

Organ Serosa

Esophagus Adventitia due to the fact that the esophagus is not in a cavity

Stomach Visceral Peritoneum

Small Intestine Visceral Peritoneum

Large Intestine Visceral Peritoneum

Anus Adventitia

Histology of the Digestive System

Basic Histological Layers1. Mucosa

a. Epitheliumb. Lamina Propriac. Muscularis Mucosae

2. Submucosaa. Submucosal plexus

“Plexus of Meissner”

3. Muscularisa. Myenteric plexus

“Plexus of Auerbach

4. Serosa

Histological Structure of the Digestive TractCopyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 25.2b

Muscularis Layers and the Movement of Digestive Materials

• Visceral smooth muscle tissue – digestive tract– a single smooth muscle cell: 5-10um in diameter and 30-200um in length

- surrounded by CT (no tendons or aponeuroses)

- contractile proteins not organized into sacromeres

- muscle cells are nonstriated, involuntary muscle

- contractions are as strong as skeletal or cardiac muscle

- muscle cells are arranged in sheets

- adjacent cells are electrically connected by gap junction

- 1 cell contracts spreads like a wave throughout the tissue

stimulus: activation of a motor neuron, local response to chemicals, hormones, O2 & CO2 levels, stretching, irritation

• Contractile filaments of smooth muscle cells are not rigidly organized- plasticity, tolerate extreme stretching, a stretched cell adapts to its new length and retains ability to contract on demand

- pacemaker cells in the muscularis mucosae and muscularis externa undergo spontaneous depolarization

- which trigger contractions leading to 2 types of movement: peristalsis and segmentation

Peristalsis• Muscularis externa via peristalsis propels materials from 1 region of the tract to another

• Waves of contractions move a bolus of food –

1. circular muscles contract behind the digestive contents

2. followed by longitudinal muscle contraction that shorten adjacent segments

3. wave of contraction in circular muscle layer forces bolus forward

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig 25.3a

Segmentation• Contractions in most areas of the small intestine and some regions of the large intestine churn and fragment the digestive materials

• The contents are mixed with intestinal secretions

• No net movement is produced


Figure 25.3

Peristalsis and Segmentation• May be triggered by pacesetter cells, hormones,

chemicals, and physical stimulation

• Peristaltic waves can also be initiated by afferent and efferent fibers- glossopharyngeal, vagus or pelvic nerves

• Afferent fibers synapse within the myenteric plexus – produce localized myenteric reflexes- short reflexes do not involve the CNS - enteric nervous system, neural network that coordinates short reflexes (as many neurons and NTs as the SC)- long reflexes, involve interneurons and motor neurons in the CNS control large-scale peristaltic waves

Mesenteries

• Most regions of the digestive tract are suspended by sheets of serous membrane- connect the parietal with the visceral peritoneum

• Mesenteries - fused, double sheets of peritoneal membrane - stabilize positions of attached organs

- prevent entanglement of intestines

- areolar CT between the mesothelial surfaces provide a route for BVs, nerves, and lymphatics, to and from the digestive tract

- during development, the digestive tract and accessory organs are suspended by dorsal and ventral mesenteries

• Lesser omentum - remnant of the embryonic ventral mesentery- persists only on ventral surface of the stomach, between the stomach and liver- and the falciform ligament, that lies between the liver and the anterior abdominal wall and diaphragm

• Greater omentum - dorsal mesentry becomes enlarged to form a pouch- loose CT within the mesentery contains a thick layer of adipose tissue, the lipids are an important energy reserve- provides insulation that reduces heat loss- contains numerous lymph nodes to help protect the body from pathogens that evaded the defenses of the digestive tract

• Mesentery proper – all but the first 25cm of the small intestine is suspended by this thick mesenterial sheet- provides stability but permits some independent movement

• Mesocolon – attached to the large intestine

• Transverse mesocolon – suspends the middle portion of the large intestine (transverse colon)

• Sigmoid mesocolon – suspends the sigmoid colon that leads to the rectum and anus

The Oral Cavity

• Digestive tract – path of food from the mouth to the anus

• The oral cavity includes:

- the tongue, salivary glands, teeth, mastication

• Functions include:1) analysis of material before swallowing

2) mechanical processing through the actions of the teeth, tongue, and palatal surfaces

3) lubrication by mixing with mucous and salivary secretions

4) limited digestion of carbohydrates by a salivary enzyme

The Oral Cavity

• Oral (buccal) cavity – lined by the oral mucosa- stratified squamous epithelium protects from abrasion

- mucosa of the cheeks (lateral walls) are formed by buccal fat pads and the buccinator muscles

- mucosa of the cheeks are continuous with the lips (labia)

- the vestibule, space between the cheeks, lips, and teeth

- gingivae, ridge of oral mucosa or the gums surrounds the base of each tooth

- roof is formed by the hard and soft palates

- hard separates oral from the nasal cavity; soft separates the oral from the nasopharynx (closes it off for swallowing)

- uvula helps prevent food from entering the pharynx prematurely

• Mylohyoid muscle – gives additional support

• Posterior soft palate supports the uvula and the 2 pairs of muscular pharyngeal arches- palatoglossal arches extend between the soft palate and the base of the tone and consist of a mucous membrane and an underlying palatoglossus muscle- palatopharyngeal arches extend from the soft palate to the side of the pharynx; consist of a mucous membrane and the underlying palatopharyngeus muscle

• Palatine tonsils – lie between the palatoglossal and palatopharyngeal arches

• Fauces – entrance to the oropharynx

Lips and Cheeks

• Both structures important in mastication and speech

• Lips (labia): orbicularis oris muscle within– Keratinized stratified squamous exterior is thin and

color of blood in dermis gives a red/pink color.– Labial frenula (mucous folds) extend from alveolar

processes of maxilla and mandible to the upper and lower lips, respectively.

– Many facial muscles act to move lips

• Cheeks: lateral walls of oral cavity– Buccinator muscle– Buccal fat pad

Palate and Palatine Tonsils

• Palate– Hard palate: anterior, supported by palatine

process of maxilla and palatine bone• Slightly corrugated on either side of midline raphe

– Soft palate: posterior, consists of skeletal muscle and connective tissue

• Closes off nasopharynx during swallowing• Uvula: projects from posterior of soft palate

• Palatine tonsils: lateral walls of fauces– Housed within palatoglossal and palatopharyngeal

arches

The Tongue

• Primary functions include:

1) mechanical processing by compression, abrasion, and distortion

2) manipulation to assist in chewing and preparing the material for swallowing

3) sensory analysis by touch, temperature, and taste receptors

4) secretion of mucins and an enzyme that aids in fat digestion

• Tongue divisions include:- an anterior body or oral portion

- posterior root or pharyngeal portion

- superior surface, dorsum of the body, contains numerous papillae

• Thickened epithelium covering each papilla provides additional friction- many have taste buds along the edges

• Secretions of small glands of the tongue’s epithelium extend into the lamina propria- contain water mucins and lingual lipase (triglycerides)

• Lingual frenulum (‘small bridle’) connects the body of the tongue to the mucosa of the oral floor

Tongue• Functions to moves food in mouth, sensory analysis by

taste buds, participates in speech and swallowing• Muscular organ with free anterior surface and attached

posterior surface. – Covered with moist stratified squamous epithelium

– Intrinsic muscles: change shape of the tongue– Extrinsic muscles: protrude or retract tongue, move

side to side• Innervated by the hypoglossal nerve

• Lingual frenulum attaches tongue inferiorly to floor of oral cavity

Tongue

• Terminal sulcus: groove that divides tongue into anterior 2/3 in oral cavity posterior 1/3 in oropharynx • Anterior region: in oral cavity; has papillae, some of which have taste buds• Posterior region: in oropharynx; no papillae; abundant lymphoid tissue (lingual tonsils)

Salivary Glands

• 3 pairs of salivary glands secrete into the oral cavity – parotid, sublingual, submandibular– each covered by a fibrous capsule- saliva is transported through a network of fine ducts to a single large drainage duct- the main duct penetrates the capsule and opens onto the surface of the oral mucosa

• Salivary glands produce 1.0 – 1.5 L saliva/day- 70% from the submandibular; 25% from the parotid; 5% from the sublingual- 99.4% water + ions, buffers, metabolites, enzymes- mucins, glycoproteins produce lubricating effects

• Parotid salivary glands - the largest ~20g- secretions are drained by a parotid (Stensen’s) duct

- salivary amylase breaks down complex carbohydrates

• Sublingual glands - covered by the mucous membrane of the floor of the mouth- numerous sublingual ducts (of Rivinus) open along either side of the lingual frenulum

• Submandibular gands – found in the floor of the mouth along the medial surfaces of the mandible inferior to the mylohyoid line- submandibular (Wharton’s) ducts open into the mouth on either side of the lingual frenulum, posterior to the teeth

Regulation of the Salivary Glands• Controlled by the ANS – receives both

parasympathetic and sympathetic innervation- an object placed within the mouth can stimulate receptors triggering a salivary reflex- innervation by CN VII, IX or X

• Parasympathetic stimulation accelerates secretion by all of the salivary glands- produce large amounts of watery saliva

• Sympathetic activation results in secretion of a small volume of viscous saliva - high enzyme concentration reduced volume produces the sensation of dry mouth

The Teeth• Perform chewing, or mastication of food

- breaks down tough CTs and plant fibers- saturates material with salivary secretions and enzymes

• Dentine – mineralized matrix similar to bone- contains no living cells; is the bulk of each tooth- cytoplasmic processes extend into the dentine from cells in the central pulp cavity (spongy and highly vascularized)- root canal, receives the BVs and nerves for the pulp cavity: dental artery, vein, & nerve enter through the apical foramen

• Root of each tooth is anchored into the alveolus by collagen fibers of the periodontal ligament- extends from the dentine of the root to the alveolar bone - creates a strong articulation, the gomphosis

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 25.7a

• Enamel – covers the dentine of the crown- contains densely packed calcium phosphate crystal

• Epithelial cells of the gingival sulcus form tight attachments to the tooth

• Cementum covers the dentine of the root, providing protection and anchoring the periodontal ligament- similar to bone, less resistant to erosion than dentine


Figure 25.7d

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFig 25.7c

• Incisors – blade-shaped teeth useful for clipping or cutting

• Cuspids or canines – conical with a sharp ridgeline and pointed tip; useful for tearing or slashing

• Bicuspids or premolars – contain 1 or 2 roots, have flattened crowns with prominent ridges; used for crushing, mashing, and grinding

• Molars – very large flattened crowns with prominent ridges, typical have 3 or more roots; used for crushing and grinding


Figure 25.7b


Figure 25.7e

Mastication

• Muscles of mastication close the jaws and slide or rock the lower jaw from side to side

- food is forced back and forth between the vestibule and the rest of the oral cavity

- movement results in part from the masticatory muscles but also the buccal, labial, and lingual muscles

- material is shredded and moistened with salivary secretions

- the tongue compacts the debris into a small oval mass or bolus, that can be swallowed

The Pharynx

• Serves as a common passageway for food, liquids, and air – divisions include:nasopharynx, oropharynx, laryngopharynx

• Deep to the lamina propria of the mucosa is a dense layer of elastic fibers, bound to underlying skeletal muscles involved in swallowing- the pharyngeal constrictors (superior, middle, and inferior) push the bolus toward the esophagus- the palatopharyngeus and stylopharyngeus muscles elevate the larynx- the palatal muscles raise the soft palate and adjacent portions of the pharyngeal wall

The Swallowing Process

• Pharyngeal muscles cooperate with muscles of the oral cavity and esophagus to initiate the swallowing process or deglutition- complex process initiates voluntarily but proceeds involuntarily once initiated

- divided into buccal, pharyngeal, and esophageal phases

Begins with compression of the bolus against the hard palateSubsequent retraction of the tongue forces the bolus into the pharynx and in elevation of the soft palate by the palatal muscles, thereby isolating the nasopharynxBuccal phase – strictly voluntary; however in the oropharynx, involuntary reflexes are initiated and the bolus is moved toward the stomach

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 25.8a, b

Begins when the bolus comes in contact with the palatal arches, the posterior pharyngeal wall, or bothElevation of the larynx by the palatopharyngeus and stylopharyngeus muscles, and folding of the epiglottis direct the bolus past the closed glottisIn less than a second, the pharyngeal constrictor muscles propel the bolus into the esophagusDuring the time it takes for the bolus to travel through the pharynx and into the esophagus, the respiratory centers are inhibited and breathing ceases

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 25.8c, d

Starts at opening of upper esophageal sphincter

After passing through - bolus is pushed along the esophagus by peristaltic waves

Approach of the bolus triggers the opening of the weak lower esophageal sphincter (cardiac sphincter) and the bolus continues into the stomach


Fig 25.8

The Esophagus

• A hollow muscular tube – transports foods and liquids to the stomach- located posterior to the trachea

- enters the peritoneal cavity through the esophageal hiatus, before emptying into the stomach

- about 25 cm long and 2 cm in diameter

- begins at the level of the cricoid cartilage anterior to vertebra C6 and ends anterior to vertebra T7

• Receives blood from the esophageal arteries and branches of the1) thyrocervical trunk & external carotid arteries of the neck,2) bronchial arteries and esophageal arteries of the mediastinum, and the3) inferior phrenic artery and left gastric artery of the abdomen

• Venous blood from the esophageal capillaries collect - into the esophageal, inferior thyroid, azygos, & gastric veins

• Innervation is by the vagus and sympathetic trunks via the esophageal plexus

• Sphincter muscles are not well-defined, the - upper and lower esophageal sphincters (cardiac sphincter), are similar in function to other sphincters

Microscopic View of the Esophagus

Endoscopic View of the Esophagus

The Stomach

• Performs 3 major functions:

1) bulk storage of ingested food

2) mechanical breakdown of ingested food, and

3) chemical digestion of ingested food through disruption of chemical bonds by acids and enzymes

• Mixing of ingested substances with acids and enzymes secreted by stomach glands produces a viscous, strongly acidic, soupy mixture or chyme

Figure 25.12

The stomach – intraperitoneal, occupies the left hypochondriac, epigastric, and portions of the umbilical and left lumbar regions

Anatomy of the Stomach

• J-shaped stomach: - short lesser curvature (medial surface), long greater curvature (lateral surface)

- cardia, where esophagus contacts the medial surface

- cardiac orifice, esophageal lumen opens into this orifice

- fundus, contacts inferior & posterior surface of diaphargm

- body, area between the fundus and the curve of the J

- pylorus, the curve of the J, divided into the pyloric antrum and pyloric canal ; as mixing movements occur during digestion, the pylorus changes shape

- pyloric sphincter regulates release of chyme from the pyloric orifice into the duodenum


Figure 25.10a

c) the pyloric region, pyloric valve, & duodenum


Figure 25.10b, c

b) radiograph of the stomach & duodenum, after swallowing a barium solution

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin CummingsFigure 25.11a

• Stomach volume increases at mealtimes; decreases as chyme enters the small intestine

• Rugae – longitudinal folds permit expansion of the gastric lumen; during expansion, the epithelial lining, flattens out, and the rugae become less prominent; in a full stomach rugae almost disappear

Figure 25.11b

• Greater omentum forms a large pouch (hangs like an apron from the greater curvature – adipose tissue provides padding and insulation, protects anterior and lateral surfaces, an important energy reserve

• Lesser omentum a smaller pouch in the ventral mesentery between the lesser curvature and liver – provides an access route for BVs and other structures entering or leaving the liver

Blood Supply to the Stomach

Celiac artery–3 branches supply blood to the stomach• Left gastric artery supplies blood to the lesser

curvature and cardia

• Splenic artery supplies the fundus and the greater curvature through the left gastroepiploic artery

• Common hepatic artery supplies blood to the lesser and greater curvatures of the pylorus - through the right gastric artery, the right gastroepiploic artery and the gastroduodenal artery

- gastric and gastroepiploic veins drain blood from the stomach into the hepatic portal vein

Musculature of the Stomach

• Muscularis mucosae and muscularis externa contain extra layers of smooth muscle- extra layers strength the stomach wall

- perform the mixing and churning activites essential to chyme formation

• Muscularis mucosae contains an additional outer, circular layer of muscle fibers

• The muscularis externa has an extra inner, oblique layer of smooth muscle

Histology of the Stomach

• Simple columnar epithelium lines all regions of the stomach – a secretory sheet- produces a carpet of mucus that covers the luminal surfaces and protects the epithelium against the acids and enzymes in the gastric lumen

• Gastric pits (shallow depressions) open onto the gastric surface- mucous cells at the base, or neck, actively divide to replace superficial cells shed continuously into the chyme

- continual replacement of epithelial cells provides an additional defense against the gastric contents ( when stomach acid and enzymes penetrate the mucous layers)

Gastric Secretory Cells

• In the stomach fundus and body, each gastric pit communicates with several gastric glands- simple branched tubular glands dominated by 3 types of secretory cells: parietal cells, chief cells, and enteroendocrine cells (scattered between the parietal and chief cells)

- parietal and chief cells work together to secrete about 1500 ml of gastric juice per day

Parietal Cells

• Or oxyntic cells - secrete intrinsic factor and hydrochloric acid (HCl)- common along proximal portions of each gastric gland

- intrinsic factor facilitates absorption of vitamin B12 (necessary for normal erythropoiesis) across the intestinal lining

- HCl lowers pH of the gastric juice, kills microorganisms, breaks down cell walls and CTs in food, and activates secretions of the chief cells

Chief Cells

• Or zymogen cells – secretes pepsinogen - most abundant near the base of a gastric gland

- pepsinogen converted by HCl to pepsin, an active proteolytic enzyme

- in newborn infants, also produce rennin and gastric lipase (enzymes important for digestion of milk)

- rennin coagulates milk proteins, and gastric lipase initiates digestion of milk fats

Enteroendocrine Cells

• Produce at least 7 different secretions- G cells, enteroendocrine cells most abundant in gastric pits of the pyloric region secrete the hormone gastrin

- gastrin, released when food enters the stomach, stimulates the secretory activity of both parietal and chief cells; also promotes smooth muscle activity in the stomach wall (enhances mixing and churning activity)


Figure 25.13b, e, f

e) parietal and mucous neck cells – outer portion of a gastric gland

f) chief and parietal cells – deepest portions of a gastric gland

Low power view of the Stomach

Low and High power view of the Stomach Mucosa

Regulation of the Stomach

• Production of acid and enzymes by the gastric mucosa can be directly controlled by the CNS- and indirectly regulated by local hormones

• Parasympathetic innervation via the vagus nerve- sight or thought of food triggers motor output in the vagus nerve postganglionic fiber stimulation of parietal cells, chief cells, and mucous cells causes an increase in acids, enzymes, and mucus production arrival of food stimulates stretch receptors in the stomach wall and chemoreceptors in the mucosa reflexive contractions occur in the muscularis layers of the stomach wall and gastrin is released by enteroendocrine cells

• Sympathetic innervation via branches of the celiac plexus

- activation leads to the inhibition of gastric activity

- in addition, the small intestine release two hormones that inhibit gastric secretion

- secretin and cholecystokinin stimulate secretion by both the pancreas and liver; the depression of gastric activity is a secondary, but complementary effect

Chemical Digestion in the StomachChemical Digestion in the Stomach

• 1.1. pepsinogen pepsin (pH 1 - 3)pepsinogen pepsin (pH 1 - 3)

proteins peptidesproteins peptides

• 2. gastric lipase2. gastric lipase

• 3. rennin (infant only)3. rennin (infant only)

HCl

Mode of Hydrochloric Acid Mode of Hydrochloric Acid SecretionSecretion

• Pepsid ACPepsid AC

• ZantacZantac

Three Phases of Stomach ControlThree Phases of Stomach Control

• Cephalic PhaseCephalic Phase• Gastric PhaseGastric Phase• Intestinal PhaseIntestinal Phase

Stomach Regulation-First PhaseStomach Regulation-First Phase• 1. cephalic phase1. cephalic phase

a. psychic stimulia. psychic stimuli

b. vagus nerveb. vagus nerve

c. increased motility and secretionc. increased motility and secretion

Stomach Regulation-Cephalic Phase

Cephalic phase

thought and anticipation of food

parasympathetic output via the vagus nerve (X)

stimulation of stomach’s enteric nervous system

PSYCHIC STIMULI

sight, taste, smell of food sound of food preparation

increased gastric secretion + increased gastric motility

Stomach Regulation-Second PhaseStomach Regulation-Second Phase• 2. gastric phase2. gastric phase

a. stretch receptors and a. stretch receptors and

chemoreceptorschemoreceptors

b. local parasympathetic responseb. local parasympathetic response

c. gastrinc. gastrin

Stomach Regulation-Gastric Phase

increased stretch of stomach wall

direct stimulation of stomach’s enteric nervous system

increased gastric secretion + increased gastric motility

food enters the stomach

input to brainstem

parasympathetic output via the vagus nerve (X)

increased gastrin secretion

increased pH stimulates chemoreceptors

Positive Feedback Control of Positive Feedback Control of Gastric SecretionGastric Secretion

Negative Feedback of the Gastric Phase

CONTROLLED CONDITION

RECEPTORChemoreceptors and stretch receptors

increased pH and stretch of stomach wall, and generate nerve impulses that pass to

the control centers

CONTROL CENTER

Enteric nervous system and medullary neurons generate parasympathetic impulses that pass to the effectors

EFFECTORSParietal cells of the gastric mucosa

secrete HCl and the muscularis contracts more vigorously (increased frequency and

strength of mixing waves)

RETURN TO HOMEOSTASISIn response, there is increased acidity in stomach chyme and the mixing waves

begin emptying the stomach. An empty stomach is a return to homeostasis.

Food entering stomach disrupts homeostasis by causing an increase in

gastric juice pH AND stretch (distention) of stomach wall

Stomach Regulation-Third PhaseStomach Regulation-Third Phase• 3. intestinal phase3. intestinal phase

a. stretch receptors and a. stretch receptors and chemoreceptorschemoreceptors b. enterogastric reflexb. enterogastric reflex c. hormonesc. hormones (1) gastrin (+)(1) gastrin (+) (2) cholecystokinin (CCK) (-)(2) cholecystokinin (CCK) (-) (3) secretin (-)(3) secretin (-) (4) gastric inhibitory peptide (4) gastric inhibitory peptide (GIP) (-)(GIP) (-)

Stomach Regulation-Intestinal Phase

increased stretch of duodenal wall

direct stimulation of duodenum’s enteric

nervous system

chyme enters the duodenum

decreased stomach activity

cholecystokininsecretin

increased enteric endocrine cell activity

secretion ofenterogastric

reflex

input to brainstem

increased sympathetic output to stomach

increased stomach activity

enteric gastrindecreased parasympathetic

output from the vagus nerve (X) to stomach

NET EFFECTgastric inhibition

inhibits

inhibits inhibits

Gastric Emptying

distention of stomach

STIMULATION OF GASTRIC EMPTYING

increased gastrin secretion increased vagal activity

contraction of gastroesophageal sphincterrelaxation of pyloric sphincter

increased rate of mixing wavesincreased gastric secretion

increased rate of emptying

partially digested proteinsalcoholcaffeine

distention of duodenum

INHIBITION OF GASTRIC EMPTYING

enterogastric reflex

decreased rate of emptying

partially digested proteins, fatty acids,

glucose in duodenum

secretion of cholecystokinin and

secretin

contraction of pyloric sphincterdecreased rate of mixing waves

decreased gastric secretion

Stomach AbsorptionStomach Absorption

• Accomplishments of digestion to this point in the GI tractAccomplishments of digestion to this point in the GI tract• starch maltose by salivary amylase (action stops in stomach)• proteins partially digested proteins (action of pepsin)• lipids partially digested fats (action of lingual and gastric lipase)• creation of chyme from food, drink, saliva, and gastric juice

• Stomach Absorption

• 1. water1. water• 2. electrolytes2. electrolytes• 3. certain drugs (aspirin)3. certain drugs (aspirin)• 4. alcohol4. alcohol

The Small Intestine• Primary role in the digestion and absorption of

nutrients – about 90%- averages 6 m in length (range 5 -8.3m) with a diameter ranging from 4 cm at the stomach to 2.5 cm at the junction with the large intestine

- stabilized by mesenteries attached to the dorsal body wall

- movement during digestion is restricted by the stomach, large intestine, abdominal wall, and pelvic girdle

- plicae circulares, transverse folds in the intestinal lining are permanent and do not disappear as intestine fills; roughly 800 plicae increases surface area for absorption

• 3 subdivisions – the duodenum, jejunum, ileum

The Small Intestine


Fig 25.14

The Duodenum

• Shortest and widest segment - about 25 cm long – a mixing bowl that receives chyme from the stomach and digestive enzymes from the pancreas and liver- almost all digestive enzymes enter from the pancreas- connected to the pylorus of the stomach- interconnection guarded by the pyloric sphincter- from its start at the pyloric sphincter, curves in a C that encloses the pancreas- contains numerous mucous glands and the duodenal submucosal (Brunner’s) glands - bile duct and pancreatic duct come together at a muscular chamber, the duodenal ampulla or hepatopancreatic ampulla

The Jejunum

• About 2.5 m long – where the bulk of chemical digestion and nutrient absorption occurs

- duodenojejunal flexure, marks the boundary between the duodenum and the jejunum (the small intestine re-enters the peritoneal cavity to become intraperitoneal)

- supported by a sheet of mesentery

- plicae and villi are prominent over the proximal half

- small, isolated, individual lymphoid nodules are present in the lamina propria

The Ileum

• Last segment and the longest, averages 3.5 m - ends at the ileocecal valve, a sphincter which controls flow of materials from the ileum into the cecum of the large intestine

- plicae and villi diminish in size and number

- lymphoid nodules become more numerous and fuse together to form large masses called aggregated lymphoid nodules, or Peyer’s patches

- Peyer’s patches are most abundant near the entrance to the large intestine, which normally contains large numbers of potentially harmful bacteria

Support of the Small Intestine

• Duodenum has no supporting mesentery

• Jejunum and ileum – supported by an extensive, fan-shaped mesentery, the mesentery proper- BVs, lymphatics, nerves pass through the mesentery CT

- BVs involved: intestinal arteries, branches of the superior mesenteric artery and superior mesenteric vein

- parasympathetic innervation provided by the vagus nerve

- sympathetic innervation involves postganglionic fibers from the superior mesenteric ganglion

Histology of the Small Intestine

• Intestinal villi – fingerlike projections of the mucosa- each villus is covered by a simple columnar epithelium- the apical epithelial surfaces are carpeted with microvilli (‘brush border’)- epithelium also contain plicae circularis, each plica supports a forest of villi; each villus is covered by epithelial cells whose exposed surfaces contain microvilli- arrangement increases total area for absorption to more than 200 m2

- at the base of the villi are the entrances to the intestinal crypts or crypts of Lieberkuhn where stem cell division continually renew epithelial cells- crypts also contain enteroendocrine cells that produce several horomones, including cholecystokinin and secretin

• Lamina propria of each villus contains an extensive network of capillaries - absorb and carry nutrients to the hepatic portal circulation

• Also contain a terminal lymphatic called a lacteal – transports materials that cannot enter the capillaries such as large lipid-protein complexes


Fig 25.15

Histology of the Intestinal Wall


Fig 25.15d,e


Figure 25.16Regions of the Small Intestine

Scanning view of the Small Intestine demonstrating Plicae Circularis

High power view of the duodenal Mucosa

High Power View of Villi

X-sectional view of the duodenum

Gross view and low-power view

of the ileum

High-power view of the Ileum demonstrating Peyer’s patches

Regulation of the Small Intestine

• As absorption occurs, weak peristaltic contractions slowly move materials along the small intestine- movements are controlled primarily by neural reflexes involving the submucosal and myenteric plexuses

- parasympathetic (vagal) stimulation increases sensitivity of these reflexes, accelerates peristaltic contractions and segmentation movements

- the ileocecal valve allows passage of material into the large intestine

- hormonal and CNS controls regulate the secretory output of intestinal juice

- sympathetic stimulation inhibits secretion

Intestinal Juice and Brush Border Intestinal Juice and Brush Border EnzymesEnzymes

• MaltaseMaltase

• LactaseLactase

• PeptidasesPeptidases

• DextrinasesDextrinases

• NucleosidasesNucleosidases

• PhosphatasesPhosphatases

Small Intestine-Mechanical Small Intestine-Mechanical DigestionDigestion

• 1. segmentation1. segmentation

• 2. peristalsis2. peristalsis

Review of Chemical Digestion of Carbohydrates

STARCH SUCROSE LACTOSE

mouth

stomach

small intestine

salivary amylase

(absorbed into blood of villus)

MALTOSE

pancreaticamylase

brush border maltase

glucose + glucose

SUCROSE

glucose + fructose(absorbed into blood of villus)

brush border sucrase

LACTOSE

glucose + galactose(absorbed into blood of villus)

brush border lactase

Review the Chemical Digestion of ProteinsReview the Chemical Digestion of Proteins

Review the Chemical Digestion of LipidsReview the Chemical Digestion of Lipids

Regulation of small intestinal Regulation of small intestinal secretion and motilitysecretion and motility

• 1. local reflexes1. local reflexes

• 2. parasympathetic reflexes 2. parasympathetic reflexes

(vagus nerve)(vagus nerve)

• 3. gastrin3. gastrin

Regulation of the Small Intestine

psychic stimuli stretch of stomach

increased parasympathetic impulses via vagus nerve

increased small intestinal motility secretion

+ relaxation of ileocecal sphincter

gastroileal reflex

chemoreceptors in stomach

increased gastrin secretion

stretch of small intestine

increased enteric nervous system activity

GASTRIC PHASE

Small Intestine AbsorptionSmall Intestine Absorption

• 1. monosaccharides1. monosaccharides• 2. 2. amino acidsamino acids hepatic portal bloodhepatic portal blood liver inferior vena cava general circulation

• 3. fats 3. fats triglyceridestriglycerides chylomicronschylomicrons lymph lacteals intestinal trunk thoracic duct general circulation

• 4. water4. water

blood capillary

lactealwith

chylomicrons

monosaccharidesamino acids

superior mesenteric vein

hepatic portal vein

thoracic duct

inferiorvena cava

lymphatic vessel

Water absorptionWater absorption

• GI tract fluids/24 hoursGI tract fluids/24 hours

• saliva = 1 L small intestine =saliva = 1 L small intestine =• ingested liquids = 2L 8 Lingested liquids = 2L 8 L• gastric juice = 2 L large intestine =gastric juice = 2 L large intestine =• bile = 1L 0.9 Lbile = 1L 0.9 L• pancreatic juice = 2 Lpancreatic juice = 2 L• intestinal juice = 1Lintestinal juice = 1L• total = 9 Ltotal = 9 L 0.1 L0.1 L

Ingested or secreted into GI tract Absorbed into blood

Excretedin feces

The Large Intestine

• Aka the large bowel, shaped like a horse-shoe - begins at the end of the ileum, ends at the anus- average length of ~1.5 m and a width of ~7.5 cm

• Divided into 3 parts:

1) Cecum – first portion, appears as a pouch

2) Colon - largest portion

3) Rectum – last 15 cm and the end of the digestive tract

• Major functions of the large intestine:1) reabsorption of water and electrolytes, and compaction of intestinal contents into feces2) absorption of important vitamins produced by bacterial action3) storage of fecal material before defecation

• Blood supply: - receives blood from tributaries of the superior mesenteric and inferior mesenteric arteries- venous blood is collected by the superior mesenteric and inferior mesenteric veins


Figure 25.17

The Cecum• The ileum attaches to the medial surface of the

cecum – opens into the cecum at the ileal papilla

• Muscles encircling the opening form the ileocecal valve – regulates passage of materials

• Cecum collects and stores arriving materials – begins the process of compaction

• Vermiform appendix – ~9 cm is attached to the posteromedial surface of the cecum- the mesoappendix (band of mesentery) connects the appendix to the ileum and cecum

- mucosa and submucosa dominated by lymphoid nodules, primarily functions as a lymphoid system organ (tonsils)

The Colon

• Has a larger diameter and a thinner wall than the small intestine1) Wall of the colon forms a series of pouches or haustra (sing., haustrum) – permits distension and elongation; creases between the haustra extend into the mucosal lining2) Taeniae coli – 3 longitudinal ribbons of the smooth muscles3) Omental (fatty) appendices – teardrop-shaped sacs of fat of the serosa

Regions of the Colon

• Colon is subdivided into 4 regions: the ascending colon, transverse colon, descending colon, and sigmoid colon

• The ascending colon – begins at the superior border of the cecum

- ascends to the inferior surface of the liver

- at the right colic flexure, or hepatic flexure colon turns to the left; marks the end of the ascending colon and beginning of the transverse colon

• The transverse colon – curves anteriorly at the hepatic flexure- crosses the abdomen from right to left

- initial segment is intraperitoneal, supported by the transverse mesocolon

- the left side passes inferior to the greater curvature of the stomach and becomes secondarily retroperitoneal; the gastrocolic ligament attaches it to the stomach

- near the spleen, at the left colic flexure or splenic flexure makes a right-angle bend and proceeds caudally

• The descending colon proceeds inferiorly along the left side of the abdomen- it is secondarily retroperitoneal and so it is firmly attached to the abdominal wall

- at the iliac fossa, descending colon enters sigmoid colon

• The sigmoid colon – S-shaped segment- only ~15 cm begins at the sigmoid flexure and ends at the rectum

- it curves posterior to the urinary bladder and is suspended by the sigmoid mesocolon

• The sigmoid colon discharges fecal waste into the rectum- the last portion, the anal canal contains small longitudinal folds called the anal columns - anal canal ends at the anus, or anal orifice - epidermis close to the anus becomes keratinized and identical to the surface of the skin- veins in the lamina propria and submucosa of the anal canal can become distended, producing hemorrhoids- circular muscle layer of the muscularis externa forms the internal anal sphincter – not under voluntary control- external anal sphincter, a ring of skeletal muscle encircles the distal portion of the anal canal – under voluntary control

Histology of the Large Intestine

• Characteristics that distinguish the large intestine from the small intestine:- wall is relatively thin, diameter of the colon is ~3 times larger than the small intestine

- lacks villi

- goblet cells are much more abundant

- distinctive intestinal crypts with deeper glands dominated by goblet cells; secretion occurs as local stimuli trigger reflexes involving the local nerve plexuses

- large lymphoid nodules are scattered throughout the lamina propria and extend into the submucosa

- longitudinal layer of the muscularis externa reduced to the muscular bands of the taeniae coli

High power view of the colon demonstrating intestinal glands

Large Intestine

Regulation of the Large Intestine

• Movement of ingested materials: - from cecum to transverse colon occurs slowly thorugh peristaltic activity and haustral churning, - allows fecal material to be converted into a sludgy paste- powerful peristaltic contractions (mass movements) from the transverse colon occurs a few times per day- stimulus: distension of the stomach and duodenum- commands are relayed over the intestinal nerve plexuses- contractions force fecal materials into the rectum causes conscious urge to defecate leads to internal sphincter relaxation (defecation reflex)- fecal material moves into the anal canal- defecation occurs by voluntary relaxation of external anal sphincter

Chemical digestion in the Chemical digestion in the large intestinelarge intestine

• 1. bacteria fermentation1. bacteria fermentation

• 2. bacteria secrete vitamin K and some 2. bacteria secrete vitamin K and some

B complex vitaminsB complex vitamins

Large Intestine AbsorptionLarge Intestine Absorption

• 1. simple molecules and vitamins1. simple molecules and vitamins• 2. most remaining water (~900 ml/day) 2. most remaining water (~900 ml/day)

• Feces consists of:Feces consists of:1. water (about 100 ml/day)1. water (about 100 ml/day)2. undigested foodstuffs (plant fibers = cellulose)2. undigested foodstuffs (plant fibers = cellulose)3. bacteria3. bacteria4. products of bacterial decomposition4. products of bacterial decomposition5. sloughed epithelial cells5. sloughed epithelial cells

Defecation Reflex in the AdultDefecation Reflex in the Adult• 1. distention of the rectum 1. distention of the rectum stimulates stretch receptorsstimulates stretch receptors• 2. sacral parasympathetic area output, 2. sacral parasympathetic area output, causing:causing:

a. contraction of the descending colon, a. contraction of the descending colon, sigmoid colon, and rectum; andsigmoid colon, and rectum; and

b. reflex relaxation of the b. reflex relaxation of the internal anal sphincterinternal anal sphincter• 3. voluntary relaxation of the 3. voluntary relaxation of the external anal sphincterexternal anal sphincter

(in the infant, this is also reflexive)(in the infant, this is also reflexive)• 4. expulsion of feces4. expulsion of feces

Mechanical Digestion in the Large Mechanical Digestion in the Large IntestineIntestine

• 1. haustral churning1. haustral churning• 2. mass peristalsis 2. mass peristalsis

(gastrocolic reflex)(gastrocolic reflex)• 3. peristalsis3. peristalsis

Ano-Rectal Junction

Inflammation of the Colon – IBD is of unknown origin - thought to be an autoimmune disorder as an individual develops an immune reaction to their own intestinal tract (affects ~1 million in the US)

- 2 major forms: ulcerative colitis (develop ulcers of the colon) and Crohn’s disease (distal segment of the ileum)

Fig 25.24

Accessory Glandular Digestive Organs

• Include the salivary glands, the liver, the gallbladder, and the pancreas- glandular organs produce and store enzymes and buffers essential to normal digestive function

- in addition salivary glands, liver, and pancreas have exocrine functions

The Liver

• The largest visceral organ and one of the most versatile – weighs ~ 1.5 kg- lies within the right hypochondriac and epigastric regions

• Provides essential metabolic and synthetic actions that fall into 3 basic categories: metabolic regulation, hematological regulation, bile production

• Metabolic regulation - all blood leaving the digestive tract enters the hepatic portal system - circulating levels of carbohydrates, lipids, and amino acids are regulated by the liver

- hepatocytes extract absorbed nutrients or toxins from the blood before it enters the hepatic veins

- hepatocytes monitor circulating levels of metabolites and adjust them as necessary

- excess nutrients are removed and stored, deficiencies are corrected by mobilizing stored reserves or performing appropriate synthetic activities

- circulating toxins and metabolic waste are removed for subsequent inactivation, storage, or excretion

- fat-soluble vitamins (A,D,K,E) are absorbed and stored

• Hematological regulation – the liver is the largest blood reservoir (receives ~25% of the cardiac output)- as blood passes through the liver sinusoids:1) phagocytic cells in the liver remove old or damaged RBCs, cellular debris, and pathogens from circulation2) hepatocytes synthesize plasma proteins that contribute to the osmotic concentration of the blood, transports nutrients, establishes clotting and complement systems

• Synthesis and secretion of bile - by the liver - bile is stored in the gallbladder and excreted into the lumen of the duodenum- consists mostly of water + minor amounts of ions, bilirubin (pigment derived from hemoglobin), and bile salts- water and ions assist in dilution and buffering of acids in chyme; bile salts associate with lipids in order to facilitate their breakdown into fatty acids suitable for absorption

Anatomy of the Liver

• The largest intraperitoneal organ- anterior surface, a ventral mesentery, the falciform ligament, marks division between left and right lobes

- thickening in the inferior margin is the round ligament, or ligamentum teres, a fibrous band that marks the path of the degenerated fetal umbilical vein

- coronary ligament suspends the liver from the inferior surface of the diaphragm

- the superior, anterior, and posterior surfaces are referred to as the diaphragmatic surfaces

- inferior surface is referred to as the visceral surface

- inferior to the small caudate lobe is the quadrate lobe

Figure 25.20

Blood Supply to the Liver

• Afferent BVs travel within the CT of the lesser omentum and converge at the porta hepatis (‘doorway to the liver’)

• Hepatic artery proper and the hepatic portal vein deliver blood to the liver- blood returns to the systemic circuit through the hepatic veins that open into the inferior vena cava

- the arterial supply provides oxygenated blood and the hepatic portal vein supplies nutrients and other chemicals absorbed from the liver


Figure 25.20

The classical description of the 4 lobes was based on the superficial topography of the liver

New terminology subdivides the lobes into segments based on the major subdivisions of the hepatic artery, portal vein, and hepatic ducts

Fig 25.20

Liver Lobules

• Each lobe is divided by CT into ~100,000 liver lobules – basic functional units of the liver- hepatocytes form a series of plates (like spokes around a wheel), each plate is only 1 cell thick

- exposed heptocyte surfaces are covered with short microvilli

- sinusoids between adjacent plates empty into the central vein

- sinusoidal lining includes a large number of Kupffer (stellate reticuloendothelial) cells, part of the monocyte-macrophage system

- Kupffer cells engulf pathogens, cell debris, damaged blood cells and also any heavy metals (tin and mercury)

• Blood enters liver sinusoids from small branches- 6 portal areas, or hepatic triads, one at each of the 6 corners of the lobule; contains 3 structures:

1) a branch of the hepatic portal vein

2) a branch of the hepatic artery proper

3) a small branch of the bile duct

- as blood flows through the sinusoids, hepatocytes absorb and secrete materials into the bloodstream

- blood then leaves the sinusoids and enters the central vein of the lobule

- central veins merge to form the hepatic veins that empty into the inferior vena cava

Kupffer cell deals with gut endotoxins & other bad stuff

CELL TYPES & ARRANGEMENT OF LIVER

Microvilli

Tight junctions Bile canaliculus

Hepatocytes in plates

SPACE OF DISSE has ECM materials, but no distinct basal lamina

Fenestrated Endothelial cells

SINUSOID

Reticular fiber STELLATE CELL

makes collagen fibrils & ECM materials

Liver Histology


Figure 25.21

Bile Secretion and Transport

• Bile is secreted into a network of narrow channels called bile canaliculi- canaliculi eventually connect with fine bile ductules that carry bile to a bile duct in the nearest portal area

• Right and left hepatic ducts collect bile from all of the bile ducts of the liver lobes- ducts unite to form the common hepatic duct that leaves the liver

- bile within the common hepatic duct may either 1) flow into the common bile duct that empties into the duodenum 2) enter the cystic duct that leads to the gallbladder

BileBile

• 1. is a detergent1. is a detergent• 2. emulsification of fats2. emulsification of fats

• Produced continuously at slow rate• Secretion increased in response to:

vagus nerve – psychic and gastric phases

secretin – from the duodenum during intestinal phase

Physiology of the LiverPhysiology of the Liver

1. carbohydrate metabolism 4. detoxification1. carbohydrate metabolism 4. detoxification a. glycogenesis 5. synthesis and a. glycogenesis 5. synthesis and

excretion of bile excretion of bile b. glycogenolysis 6. storageb. glycogenolysis 6. storage c. gluconeogenesis 7. phagocytosis of c. gluconeogenesis 7. phagocytosis of

RBCs RBCs2. lipid metabolism 8. activation of vitamin D 2. lipid metabolism 8. activation of vitamin D 3. protein metabolism3. protein metabolism a. deamination (-NH2)a. deamination (-NH2) b. urea formationb. urea formation c. plasma protein productionc. plasma protein production

GallbladderGallbladder

• 1. anatomy1. anatomy• 2. rugae2. rugae• 3. cystic duct3. cystic duct• 4. stores/concentrates bile4. stores/concentrates bile• 5. sphincter of Oddi5. sphincter of Oddi• 6. CCK6. CCK

Biliary TractBiliary Tract• common hepatic duct +• cystic duct = • common bile duct +• main pancreatic duct = • ampulla of Vater• sphincter of Oddi

The Gallbladder• Hollow pear-shaped muscular sac - stores and

concentrates bile before- its excretion into the small intestine

- located in a fossa in the visceral surface of the right lobe

• Divided into 3 regions: fundus, body, and neck- the cystic duct leads from the gallbladder toward the porta hepatis, where the common hepatic duct and cystic duct unite to create the common bile duct

- at the duodenum, a muscular hepatopancreatic sphincter (sphincter of Oddi) surrounds the common bile duct, contraction seals off the passageway, prevents bile from entering the small intestine

The Gallbladder and Associated Bile Ducts


Figure 25.22

• The gallbladder has 2 major functions – bile storage and bile modification- when the hepatopancreatic sphincter is closed, bile enters the cystic duct - in the interim bile enters the cystic duct for storage- at capacity contains 40-70 ml of bile- composition gradually changes: water is absorbed, bile salts and other components become more concentrated- bile ejection occurs under stimulation of the hormone cholecystokinin (CCK)- CCK released into the bloodstream at the duodenum, when chyme arrives with large amounts of lipid and partially digested proteins- CCK causes relaxation of the hepatopancreatic sphincter and contraction of the gallbladder

Regulation of Bile SecretionRegulation of Bile Secretion

acid chyme in duodenum

REGULATION OF BILE SECRETION

enteroendocrine cells stimulated

cholecystokinin secretion

gallbladder contraction

relaxation of sphincter of Oddi

release of bile into duodenum

The Pancreas

• Lies posterior to the stomach, extends laterally from the duodenum toward the spleen- about 15 cm long, about 80 g (3 oz) and divided into 3 regions: broad head, slender body, short-blunted tail- thin transparent CT capsule wraps the pancreas

• Primarily an exocrine organ – produces digestive enzymes and buffers- large pancreatic duct (duct of Wirsung) delivers secretions to the duodenal ampulla- small accessory pancreatic duct (duct of Santorini) empties into the duodenum at the lesser duodenal papilla

PancreasPancreas• 1. gross anatomy1. gross anatomy• 2. main pancreatic duct2. main pancreatic duct• 3. hepatopancreatic ampulla3. hepatopancreatic ampulla• 4. accessory pancreatic duct4. accessory pancreatic duct• 5. 99% exocrine5. 99% exocrine• 6. 1% endocrine6. 1% endocrine

Pancreatic JuicePancreatic Juice• 1. sodium bicarbonate (NaHCO3)1. sodium bicarbonate (NaHCO3)

• 2. pancreatic amylase2. pancreatic amylase

• 3. pancreatic lipase and cholesterol esterase3. pancreatic lipase and cholesterol esterase

• 4. nucleases -- DNAse and RNAse4. nucleases -- DNAse and RNAse

• 5. protein-digesting enzymes5. protein-digesting enzymes

a. trypsinogen (inactive)a. trypsinogen (inactive)

b. chymotrypsinogen (inactive)b. chymotrypsinogen (inactive)

c. procarboxypeptidase (inactive)c. procarboxypeptidase (inactive)

Pancreatic Regulation-Neural Pancreatic Regulation-Neural Control and Endocrine ControlControl and Endocrine Control

• 1. vagus nerve1. vagus nerve

• 2. CCK = enzymes2. CCK = enzymes

• 3. secretin = NaHCO33. secretin = NaHCO3

Pancreatic Regulation

acid chyme in duodenum

ENDOCRINE CONTROL

enteroendocrine cells stimulated

increased secretion of bicarbonate ions

increased secretin increased cholecystokinin

increased secretion of enzymes

NEURAL CONTROL

psychic stimuli stretch of stomach

increased parasympathetic impulses via vagus nerve

increased pancreatic secretion

• Arterial blood reaches the pancreas through branches of the splenic, superior mesenteric and common hepatic arteries.

Figure 25.23

• Major branches include the pancreatic arteries and the pancreaticoduodenal arteries ( superior and inferior)• Splenic vein and its branches drain the pancreas

Histology of the Pancreas

• Partitions of CT divide pancreatic tissue into lobules - BVs and tributaries of the pancreatic ducts are found within these CT septa

• Pancreas is a compound tubuloacinar gland- within each lobule, ducts branch repeatedly before ending in pancreatic acini- a pancreatic acinus is lined by simple cuboidal epithelium, secretes a mixture of water, ions, and digestive enzymes (pancreatic juice) into the duodenum- pancreatic ducts secrete buffers (1° sodium bicarbonate) important in neutralizing the acid in chyme and stabilizing pH- pancreatic islets are scattered between the acini, account for only ~1% of the cellular population


Figure 25.23

Pancreatic enzymes classified according to their intended targets• Lipases digest lipids

• Carbodhydrases such as pancreatic amylase digest sugars and starches

• Nucleases attack nucleic acids

• Proteolytic enzymes break proteins apart such as proteinases (large protein complexes) and peptidases (small peptide chains)

Regulation of Pancreatic Secretion

• Occurs primarily in response to hormonal instructions from the duodenum- the hormone secretin is released when acidic chyme arrives in the small intestine

- secretin triggers the production of watery pancreatic juice containing buffers especially sodium bicarbonate

- a duodenal hormone, cholecystokinin, stimulates the production and secretion of pancreatic enzymes

Aging and the Digestive System

• Rate of epithelial stem cell division declines

• Smooth muscle tone decreases

• The effects of cumulative damage become apparent

• Cancer rates increase

• Changes in other systems have direct of indirect effects on the digestive system

14. digestive system

Documents

pearson benjamin

high power

external anal

common hepatic

common bile

lymphoid nodules

power view

cystic duct