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Biology 221 Anatomy & Physiology II. TOPIC 8 Digestive System. Chapter 18 pp. 651-677. E. Lathrop-Davis / E. Gorski / S. Kabrhel. Digestive System Functions. Provide nutrients in usable form Remove unusable wastes. Digestive System Overview . Two main groups of organs:

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Topic 8 digestive system l.jpg

Biology 221

Anatomy & Physiology II

TOPIC 8Digestive System

Chapter 18

pp. 651-677

E. Lathrop-Davis / E. Gorski / S. Kabrhel

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Digestive System Functions

  • Provide nutrients in usable form

  • Remove unusable wastes

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Digestive System Overview

Two main groups of organs:

  • Alimentary canal (a.k.a. Gastrointestinal tract)

    • tube through which food passes

    • responsible for digestion and absorption of food

    • mouth, pharynx, esophagus, stomach, small intestines, large intestines

  • Accessory organs

    • organs, glands and structures which aid digestion but not part of GI tract itself

    • teeth, tongue, salivary glands, pancreas, liver, gall bladder

Fig. 24.1, p. 888

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Processes of Digestion

  • Ingestion – entrance of food and drink into mouth

  • Mechanical digestion – physical breakdown into smaller pieces

  • Propulsion – movement through gut

  • Chemical digestion – breakage of molecules into smaller compounds

  • Absorption – uptake of nutrients from lumen

  • Defecation – removal of indigestible material

Fig. 24.2, p. 889

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  • Serous membrane

  • parietal peritoneum – lines abdominal cavity

    • organs posterior to the parietal peritoneum are retroperitoneal

  • visceral peritoneum (serosa)– covers surfaces of most abdominal organs

    • Mesenteries – double layer of peritoneum extending from body wall to digestive organs

    • intraperitoneal organs are those in mesentaries

  • Peritoneal cavity – fluid-filled “space” between visceral and parietal peritoneum

  • Peritonitis – inflammation of the peritoneum

See also Fig. 24.5, p. 891

Fig,. 24.30, p. 929

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Splanchnic Circulation

Arteries serving the digestive organs:

  • Celiac Trunk – very short; gives rise to:

    • hepatic artery – liver; gall bladder; stomach; duodenum

    • left gastric artery – stomach; inferior esophagus

    • splenic artery – spleen; stomach; pancreas

  • Superior Mesenteric Artery – small intestines; most of large intestine; pancreas

  • Inferior Mesenteric Artery – large intestine

Fig. 20.22, p. 761

Fig. 20.22, p. 759

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Hepatic Circulation: Hepatic portal system

  • Veins draining digestive organs and carrying nutrient-rich blood to liver

    • gastric vein– drains stomach

    • superior mesenteric vein– drains small intestine

    • splenic vein– drains spleen

      • inferior mesenteric vein – drains large intestine

  • Venous blood from hepatic portal system mixes with arterial blood (hepatic artery) in liver

Fig. 20.27, p. 771

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Hepatic Circulation: Hepatic Veins

  • drain venous blood from liver into inferior vena cava

Fig. 20.27, p. 771

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  • Mucous membrane lining gut

  • Consists of:

    • epithelium – lining

    • lamina propria – areolar connective tissue layer

    • muscularis mucosae – smooth muscle

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Mucosa: Epithelium

  • Type varies depending on location

    • stratified squamous epithelium found in mouth, esophagus and anal canal

    • simple columnar epithelium found in stomach and intestines

  • Secretes mucus, digestive enzymes, hormones

  • Provides intact barrier to protect against entry of bacteria

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Mucosa: Lamina Propria

  • Layer of areolar connective tissue

  • Blood capillaries nourish epithelium, absorb and transport digested nutrients

  • Lymphatic capillaries provide drainage for interstitial fluid and transport fats to venous circulation

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Mucosa: Muscularis Mucosae

  • Smooth muscle used for local movement and to hold mucosa in folds (small intestine)

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  • Dense connective tissue superficial to mucosa

  • Highly vascularized

  • Many lymphatic vessels

  • Lymph nodules

    • MALT = mucosa-associated lymphatic tissue

    • especially in small (Peyer’s Patches) and large intestines

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Muscularis Externa (Muscularis)

  • Two layers in most organs (3 in stomach)

    • circular layer

    • longitudinal layer

  • Peristalsis moves material through gut

  • Segmentation helps mix material with digestive enzymes in small intestine

Fig. 24.3, p. 890

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  • Visceral peritoneum – simple squamous epithelium (mesothelium) with areolar CT

  • Adventitia – dense connective tissue covering without epithelium; found around esophagus

  • Retroperitoneal organs have both a serosa (of parietal peritoneum) and adventitia (on side abutting body wall)

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Enteric Nervous System

Intrinsic nerve plexuses:

  • Enteric neurons

    • neurons able to act independently of central nervous system

    • communicate with each other to control GI activity

  • Two main enteric plexuses:

    • submucosal nerve plexus – regulates glands in submucosa and smooth muscle of muscularis mucosae

    • myenteric nerve plexus – regulates activity of muscularis externa (with aide of submucosal nerve plexus)

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Central Nervous System Control

  • Enteric nerve plexuses linked to CNS by visceral afferent (sensory) fibers

  • Digestive system receives motor input from sympathetic and parasympathetic divisions of autonomic nervous system

    • parasympathetic outflow generally increases activity

    • sympathetic outflow generally decreases activity

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Mouth (Oral Cavity)

  • Oral oriface is anterior opening

  • Mouth is continuous with oropharynx

  • Lips and cheeks keep food in oral cavity

  • Three layers of tissue:

    • mucosa (stratified squamous epithelium)

    • submucosa

    • muscularis externa (skeletal muscle)

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Mouth: Palate

  • Hard palate

    • palatine process of maxilla

    • palatine bones

  • Soft palate

    • muscle only

    • prevents food from entering nasopharynx during swallowing

    • uvula (part that hangs down in middle)

Fig. 24.7, p. 895

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Mouth: Arches

  • Palatoglossal arch – anchors soft palate to tongue

  • Palatopharyngeal arch anchors soft palate to wall of oropharynx

  • Fauces – area between arches

    • palatine tonsils – located in fauces

Fig. 24.7, p. 895

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Mouth: Tongue

  • Lingual tonsil – sits at base of tongue; protects against invasion by bacteria

  • Taste buds

    • contain receptors for taste

    • found in some papillae

Fig. 24.8, p. 896

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Mouth: Tongue

  • Tongue forms bolus

    • ball of food

    • makes food easier to swallow

    • keeps food between teeth

  • Muscles – served by nerve XII

    • intrinsic muscles – within tongue (not attached to bone); allow tongue to change shape for swallowing and speech

    • extrinsic muscles – attach to bone or soft palate; alter tongue position (protrusion, retraction, side-to-side)

Fig. 24.7, p. 895

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Salivary Glands

  • Produce saliva

  • Two groups of salivary glands

    • intrinsic glands (buccal glands) – within oral cavity

    • extrinsic glands – 3 pairs (see A&P I Unit VI for innervation)

      • parotid glands (connected to oral cavity by parotid duct; mumps is a viral infection of the parotid glands)

      • sublingual glands

      • submandibular glands

Fig. 24.9, p. 897

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  • Mucus cells produce mucus (less common)

  • Watery saliva produced by serous cells; composition:

    • 97-99.5% water

    • slightly acidic (pH ~ 6.8)

    • electrolytes (ions such as Na+, K+, Cl-, PO4=, HCO3-)

    • metabolic wastes (urea, uric acid)

    • proteins

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Salivary Proteins

  • Mucin – glycoprotein portion of mucus that lubricates oral cavity)

  • Lysozyme – antibacterial

  • IgA– antibodies that prevent antigens from attaching to mucus membrane

  • Defensins – secreted by neutrophils; act as local antibiotic and chemotatic agent when mucosa is damaged

  • Salivary amylase – hydrolyzes starch

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Control of Salivation

  • Sympathetic division

    • stimulates production of mucin-rich saliva, or

    • inhibits salivation altogether at high levels

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Control of Salivation

  • Parasympathetic division of ANS stimulates activity

    • chemoreceptors (excited most by acidic substances) and baroreceptors (excited by mechanical stimuli) send messages to salivatory nuclei in pons and medulla

    • parasympathetic motor output results in salivation

    • psychological control – response to visual, olfactory stimuli, even thoughts of food

    • salivary nuclei are stimulated by irritation to lower GI tract

  • Parasympathetic nerves

    • facial – to submandibular, sublingual

    • glossopharyngeal – to parotids

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  • Lie in alveoli of mandible and maxilla (see A&P I axial skeleton lab)

  • Primary dentition = deciduous teeth (20 milk or baby teeth)

    • roots are absorbed as permanent teeth grow in, causes baby teeth to fall out

Fig. 24.10, p. 899

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Permanent dentition = adult teeth (32)

  • 8 Incisors (central and lateral)

  • 4 Canines (eyeteeth)

  • 8 Bicuspids = premolars

  • Molars

    • 4 first molars

    • 4 second molars

    • 4 third molars

      • wisdom teeth

      • may become impacted as grow in

Fig. 24.10, p. 899

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Tooth Structure

  • Crown - covered by enamel (hardest substance in body); underlain with dentin

  • Neck

  • Root

    • cementum – calcified connective tissue covering dentin of root; attaches root to periodontal ligament which anchors tooth to alveolus

    • no enamel (dentin is under cementum)

    • pulp cavity – houses blood vessels and nerves that enter/leave via apical foramen in the root canal

Fig. 24.11, p. 900

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  • Only oropharynx and laryngopharynx are involved in digestion (nasopharynx is only respiratory)

  • Lined with nonkeratinized stratified squamous epithelium

  • Mucus-producing glands in submucosa produce mucus that lubricates food

  • Skeletal muscle responds to somatic reflexes to move food quickly past laryngopharynx

  • No serosa or adventitia

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  • Runs from laryngopharynx through mediastinum to stomach

  • All 4 layers present in wall

    • Mucosa – consists of stratified squamous epithelium

    • Submucosa – mucus-secreting esophageal glands

Esophagus33 l.jpg

  • Muscularis – changes type as goes down

    • top 1/3 – skeletal muscle

    • middle 1/3 – mix

    • bottom 1/3 – smooth muscle

  • Adventitia – dense connective tissue covering

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Structures Associated with the Esophagus

  • Upper esophageal sphincter – controls movement of material from pharynx into esophagus

  • Esophageal hiatus – opening in diaphragm that allows esophagus to pass from thoracic cavity into abdominal cavity

  • Gastroesophageal (cardiac) sphincter

    • thickening of smooth muscle of inferior esophagus

    • aided by diaphragm to close bottom of esophagus

    • helps prevent reflux of acidic gastric juice

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Esophageal Disorders

  • heartburn – failure of lower esophageal sphincter to close completely allowing acidic gastric juice into esophagus

  • hiatus hernia – protrusion of the superior portion of stomach above diaphragm

  • esophageal ulcer – erosion of wall due to chronic reflux of stomach acid

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Digestive Processes in Mouth, Pharynx and Esophagus

  • Ingestion

  • Mechanical Digestion

    • mastication by teeth (with aid of tongue)

    • formation of bolus

  • Chemical digestion by salivary amylase produced by salivary glands

    • breaks starch and glycogen into smaller fragments (including maltose [disaccharide] if left long enough)

    • continues activity until reaches acid stomach

  • Absorption– essentially none (except some drugs, e.g., nitroglycerine)

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Digestive Processes in Mouth, Pharynx and Esophagus

  • Movement – deglutition (swallowing)

    • moves food from oral cavity to stomach

    • voluntary in oral cavity (buccal phase)

    • reflexive in pharynx

    • involuntary peristalsis where smooth muscle is found

Fig. 24.13, p.904

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Stomach: Gross Anatomy

  • Cardiac region (cardia)

  • Fundus - temporary storage area

  • Body

    • greater curvature

    • lesser curvature

  • Pyloric region– distal portion

    • Pyloric sphincter– controls movement of chyme into small intestine

Fig. 24.14, p. 905

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Stomach Histology

  • Mucosa

    • simple columnar epithelium

    • muscularis mucosae throws mucosa into folds called rugae

  • Submucosa– connective tissue

  • Muscularis – 3 layers create mixing waves in addition to peristalsis

    • longitudinal layer

    • circular layer

    • oblique layer

  • Serosa - covers stomach

Fig. 24.14, p. 905

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Microscopic Anatomy

  • Surface composed mainly of goblet cells (secrete mucus)

  • Gastric pits

    • tight junctions between epithelial cells prevent acidic gastric juice from reaching underlying layers

    • contain gastric glands which secrete gastric juice

      • mucus neck cells

      • parietal (oxyntic) cells

      • chief (zymogenic) cells

      • enteroendocrine cells

Fig. 24.15, p. 906

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Gastric Pit Cells

  • Mucous neck cells secrete bicarbonate-rich mucus

  • Parietal (oxyntic) cells secrete:

    • HCl (buffered by bicarbonate rich mucus)

    • intrinsic factor (essential to absorption of Vit. B12 by small intestine)

  • Chief (zymogenic) cells secrete:

    • pepsinogen (inactive form of the protease pepsin for protein hydrolysis)

    • minor amounts of lipases (lipid hydrolysis)

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Gastric Pit Cells

  • Enteroendocrine cells – release hormones and hormone-like products into the lamina propria where they are picked up by blood and carried to other digestive organs

    • gastrin – generally stimulatory

    • histamine – stimulates H+ secretion

    • somatostatin – generally inhibitory

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Digestive Processes in Stomach

  • Mechanical digestion

    • mixing waves help break food into smaller particles

  • Chemical digestion – produces chyme (pH ~ 2)

    • acid (HCl) secreted by parietal cells breaks some bonds and activates pepsinogen into pepsin

    • pepsin –

      • produced as pepsinogen by chief cells

      • hydrolyses proteins

    • rennin – protease secreted in children that acts on milk proteins

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Digestive Processes in Stomach

  • Movement

    • mixing waves mix food with acid and enzymes

    • peristalsis moves material through stomach and into small intestine

  • Absorption – limited to lipid soluble substances

    • alcohol

    • aspirin

    • some other drugs

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Regulation of Gastric Secretion

Controlled by nervous system and hormones

  • Hormonal control

    • gastrin stimulates secretion

    • somatostatin, gastric inhibitory protein (GIP), and cholecystokinin inhibit secretion

  • Neural control:

    • autonomic control (CNS)

      • parasympathetic division

        • Vagus (X) nerve

      • sympathetic division - thoracic spinal nerves

    • local enteric nerve reflexes

      • distension of stomach stimulates activity

      • distension of duodenum inhibits activity

See Fig. 24.16, p. 910

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Stimulation of Gastric Secretion

  • Cephalic Phase (cerebral)

  • Gastric Phase (stomach)

  • Intestinal Phase (duodenum)

Fig. 24.16, p. 910

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Inhibition of Gastric Secretion

  • Cephalic Phase (cerebral)

  • Gastric Phase (stomach)

  • Intestinal Phase (duodenum)

Fig. 24.16, p. 910

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Gastric Disorders

  • Gastritis –inflammation of underlying layers of wall

  • Gastric ulcers –erosions of stomach wall

    • Helicobacter infections associated with ~90% of all ulcers (uncertain as to whether it is causitive agent)

    • non-infectious ulcers associated with persistent inflammation

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Gastric Disorders (con’t)

  • Emesis = vomiting

    • usually caused by

      • extreme stretching of stomach or small intestine, or

      • presence of irritants in stomach (e.g., bacterial toxins, excessive alcohol, spicy foods, certain drugs)

    • emetic center in medulla initiates impulses to

      • contract abdominal muscles (increases intra-abdominal pressure)

      • relax cardiac sphincter

      • raise soft palate (closes off nasal passages)

    • excessive vomiting results in dehydration and metabolic alkalosis (increased blood pH)

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Small Intestine: Gross Structure

  • Diameter ~ 2.5 cm

  • Length ~ 2-4 m (8-13’) (in cadaver, 6-7 m [20-21’] because muscle is not contracted)

  • Small intestine designed for secretion (especially proximal end) and absorption

    • site of most chemical digestion

    • site of most absorption

  • pH 7-8

  • Three areas:

    • duodenum (25 cm)

    • jejunum

    • ileum

Fig. 24.21, p. 916

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Small Intestine: Duodenum

  • Receives chyme from stomach

  • Hepatopancreatic ampulla

    • union of common bile duct and pancreatic duct

    • opens via major duodenal papilla

    • hepatopancreatic sphincter (sphincter of Oddi) controls entry of fluid from ampulla

  • Duodenal (Brunner’s) glands – secrete alkaline mucus

Fig. 24.20, p. 915

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Small Intestine: Jejunum & Ileum

  • Jejunum

    • extends from duodenum to ileum

  • Ileum

    • extends from jejunum to large intestine

    • ileocecal valve controls movement of material into large intestine

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Small Intestine: Innervation

  • Parasympathetic impulses supplied by Vagus nerve stimulates activity

  • Sympathetic impulses supplied by thoracic splanchnic nerves inhibit activity

  • Enteric nervesact locally

Fig. 14.4, p. 517

Fig. 14.5, p. 519

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Small Intestine: Blood Supply

  • Arteries:

    • common hepatic artery serves duodenum

    • superior mesenteric artery serves most of small intestine

  • Veins:

    • superior mesenteric veindrains entire small intestine

Fig. 20.22, p. 761

Fig. 20.27, p. 771

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Small Intestine: Overview ofSpecial Anatomical Features

  • Plicae circularis – circular folds

  • Villi – fingerlike projections of intestinal wall

  • Microvilli – projections of cell membranes

See Fig. 24.21, p. 916

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Small Intestine: Plicae Circularis

  • Circular folds

  • Deep, permanent folds of mucosa and submucosa

  • Force chyme to spiral through lumen

    • mixes chyme with intestinal juice

    • slows movement

      Why is this helpful?

Fig. 24.21, p. 916

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Small Intestine: Villi

  • Finger-like projections of mucosa (over 1 mm tall)

  • Each villus contains:

    • blood capillary bed

    • lacteal

    • smooth muscle - allows villus to shorten

      • increases contact between villus and “soup” in lumen

      • “milks” lacteal

See also Fig. 24.22, p. 917;

Fig. 24.21, p. 916

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Small Intestine: Microvilli

  • Extensions of cell membrane

  • Called brush border

  • Functions:

    • secrete brush border enzymes

    • increase surface area for absorption

Fig. 24.21, p. 916

See also Fig. 24.22, p. 917

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Small Intestine: Mucosa

  • Renewed every 3-6 days

  • Simple columnar epithelium

    • goblet cells – secrete mucus

    • absorptive cells – absorb nutrients

      • bound by tight junctions

      • microvilli

  • Lamina propria

    • blood vessels

    • lacteals

  • Intestinal crypts (crypts of Lieberkuhn) – between villi

    • most cells secrete intestinal juice

    • Paneth cells secrete lysozyme (antibacterial)

See Fig. 24.21, p. 916

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Small Intestine: Submucosa

  • Peyer’s patches – lymphatic tissue for protection against disease

  • Duodenal (Brunner’s) glands

    • secrete alkaline mucus rich in bicarbonate to raise pH of chyme from <3 to >7

    • only in duodenum

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Small Intestine: Muscularis & Serosa

  • Muscularis – two layers of smooth muscle create two kinds of movement

    • peristalsis moves chyme through intestine

    • segmentation mixes chyme with intestinal juice

      • moves between segments a few cm at a time

      • intrinsic control in longitudinal muscle (intrinsic pacemaker cells)

    • intensity altered by nervous system and hormones

      • parasympathetic impulses increase strength of contraction

      • sympathetic impulses decrease it

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Small Intestine: Muscularis & Serosa

  • Serosa (visceral peritoneum) – outer covering

    • Mesenteries

      • visceral peritoneum

      • attach small intestine to posterior body wall

    • Intraperitoneal organs - surrounded and supported by mesenteries

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Small Intestine: Digestive Processes

  • Mechanical digestion – bile salts secreted by liver (stored in and released from gall bladder) emulsify fat globules (make them into smaller droplets) to increase surface area lipases have available to work on

  • Chemical digestion – hydrolysis of macromolecules

    • lipid digestion

    • protein digestion

    • carbohydrate digestion

    • nucleic acid digestion

See Fig. 24.33, p. 933

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Small Intestine: Lipid Digestion

  • Pancreatic lipase

  • Most common lipids are neutral fats (triglycerides)

    • glycerol + 1 fatty acid = monoglyceride

    • glycerol + 2 fatty acids = diglyceride

    • glycerol + 3 fatty acids = triglyceride

  • Triglycerides cleaved into glycerol and 3 fatty acids or monoglycerides and 2 fatty acids

Fig. 2.14, p. 48

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Small Intestine: Protein Digestion

  • Pancreatic and intestinal proteases break proteins into amino acids

  • Pancreatic proteases: trypsin, chymotrypsin and carboxypolypeptidase

    • secreted as inactive precoursers (trypsinogen, chymotrypsinogen, and procarboxypolypeptidase, respectively) to protect intestinal mucosa from being digested

    • cleave large proteins into small peptides

  • Intestinal proteases

    • include aminopeptidase, carboxypeptidase, dipeptidase

    • cleave small peptides into amino acids

See Fig. 2.17, p. 52

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Small Intestine: Carbohydrate Digestion

  • Starches – cleaved into short chains (oligosaccharides) and maltose (disaccharide) by pancreatic amylase secreted by pancreas

  • Disaccharides hydrolyzed by intestinal (brush border) enzymes:

    • maltase – cleaves maltose

    • lactase – cleaves lactose

    • sucrase – cleaves sucrose

Fig. 2.13, p. 46

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Small Intestine: Nucleic Acid Digestion

  • Pancreatic nucleases – cleave nucleic acids into nucleotides

  • Nucleosidases and phosphatases – cleave nucleotides into sugars, phosphates, bases

Fig. 2.22, p. 58

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Small Intestine: Absorption of Carbohydrates

  • Absorption moves nutrients from lumen into cells, thence into interstitial fluid to blood or lymph

  • Carbohydrates – absorbed as monosaccharides by:

    • cotransport with Na+ (based on setting up Na+ gradient using active transport; glucose and galactose)

    • facilitated transport (fructose)

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Small Intestine: Absorption of Proteins and Nucleic Acids

  • Proteins – absorbed as amino acids

    • cotransport with Na+ (based on setting up Na+ gradient using active transport)

    • proteins rarely taken up intact (absorbed peptides may cause food allergies)

  • Nucleic acids – actively absorbed as components: sugar (ribose/deoxyribose), phosphate, nitrogen bases

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Small Intestine: Absorption of Lipids

  • Lipids

    • combine with bile salts to form micelles

    • absorbed passively through lipid bilayer as glycerol and fatty acids or monoglycerides

    • combine with proteins within cell to form chylomicrons which are then released into interstitial fluid

      • chylomicrons enter lymph through lacteals (lymphatic capillaries) in villi and are transported to subclavian veins

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Small Intestine: Absorption of Vitamins

  • Fat-soluble vitamins (DAKE) incorporated into micelles and absorbed in same manner as fats (passively through lipid bilayer)

  • Water-soluble vitamins (C, B complex) mostly absorbed by diffusion

    • exception is B12, which must bind to intrinsic factor produced in stomach to be actively absorbed in ileum (recognition of B12-intrinsic factor complex by receptors in plasma membrane of cells triggers active receptor-mediated endocytosis)

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Small Intestine: Absorption of Electrolytes

  • Most actively absorbed throughout small intestine

    • absorption based on how much is in food

    • Na+/K+ pump plays role (Na+ into/ K+ out)

    • K+passively absorbed based on gradient created by pump

  • Iron (Fe) and calcium (Ca) only absorbed in duodenum

    • depends on needs of body

    • iron actively transported into cells where it becomes bound to ferritin

    • calcium absorption regulated by vitamin D which serves as cofactor in Ca transport

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Small Intestine: Movement

  • Peristalsis moves chyme through intestine

  • Segmentation mixes chyme with intestinal juice

Fig. 24.3, p. 890

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Hormonal Control of Small Intestine Activity

Gastrin – secreted by stomach

  • stimulates contraction of intestinal smooth muscle

  • stimulates relaxation of ileocecal valve

    Vasoactive intestinal peptide (VIP) – from duodenum; acts on duodenum

  • stimulates secretion of bicarbonate-rich intestinal juice

    Somatostatin – from stomach and duodenum

  • inhibits blood flow and absorption from small intestine

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Nervous System Control of Small Intestine Activity

  • Sympathetic impulses decrease activity

  • Gastroileal reflex – initiated by increased activity in stomach

    • long reflex involving brain and parasympathetic innervation

    • parasympathetic impulses increase activity

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Accessory Glands: Liver Gross Anatomy

  • Largest gland in body, approximately 1.4 kg

  • Upper right hypochondriac and epigastric regions

  • 4 primary lobes: right, left, caudate, quadrate

  • Covered by serosa except for uppermost region just under diaphragm

Fig. 24.1, p. 888

See Fig. 24.23, p. 919

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Liver: Hepatic Ducts

  • Right hepatic duct – serves right lobe

  • Left hepatic duct – serves other lobes

  • Common hepatic duct

    • formed from union of right and left hepatic ducts

    • joins with cystic duct of gall bladder to form common bile duct,which joins with pancreatic duct to form hepato-pancreatic ampulla

Fig. 24.20, p. 915

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Liver: Ligaments

  • Falciform ligament

    • piece of mesentery that separates right and left lobes

    • suspends liver from diaphragm and anterior abdominal wall

  • Round ligament (= ligamentum teres) – remnant of umbilical vein

  • Ligamentum venosum

    • remnant of ductus venosus

See Fig. 24.23, p. 919

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Liver: Blood supply

  • Hepatic artery – arterial blood

  • Hepatic portal vein – receives nutrient-rich venous blood from stomach, intestines, pancreas, spleen (see lab for vessels)

  • Hepatic vein – drains venous blood into inferior vena cava

Fig. 20.27, p. 771

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Liver: Microscopic Anatomy

  • Designed to filter and process nutrient-rich blood

  • Composed of lobules with portal triad at each corner of hexagonal structure

    • branch of hepatic artery (HA)

    • branch of hepatic portal vein (HPV)

    • bile duct (BD)

Fig. 24.24, p. 921

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Liver: Microscopic Anatomy (con’t)

  • Sinusoids – specialized capillaries in which venous and arterial blood mix

    • hepatocytes (liver cells) just inside walls of sinusoid perform functions of liver

    • Kupffer cells (macrophages) found along wall – remove debris, bacteria, worn out RBCs

Fig. 24.24, p. 921

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Liver: Microscopic Anatomy (con’t)

  • Central vein drains lobule

    • join to form hepatic veins

  • Bile canaliculi = channels between hepatocytes

    • join to form bile ducts

    • bile flow is counter to blood flow

Fig. 24.24, p. 921

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Liver Functions

  • Process blood-borne nutrients

  • Store glucose (as glycogen)

  • Store fat-soluble vitamins

  • Store iron (Fe)

  • Detoxify poisons

  • Produce plasma proteins (see Topic 1)

  • Cleanse blood of debris, including bacteria and worn out RBCs

  • Produce bile

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Liver Functions: Bile

  • Consists of bile salts, bile pigments, cholesterol, neutral fats, phospholipids, electrolytes in water

  • Aid digestion of fat

    • emulsify (break up) fat globules into droplets

    • form micelles (ferry fats to mucosal wall)

  • Conserved by enterohepatic circulation (some is reabsorbed by ileum and returned to liver via hepatic portal system)

  • Main bile pigment is bilirubin

    • formed from breakdown of hemoglobin

    • metabolized by bacteria in large intestine (becomes brown pigment)

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Control of Bile Production

  • Stimulated by bile salts returning via hepatic portal blood

  • Stimulated by secretin (hormone secreted by small intestine in response to fats in chyme)

Fig. 24.25, p. 923

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Liver Disorders/Disease

  • Hepatitis – inflammation of liver, often caused by viral infection

    • transmitted enterically (HVA) or through blood (HVB, HVC, HVD)

    • blood-borne viruses are linked to chronic hepatitis and cirrhosis

  • Cirrhosis – chronic disease characterized by growth of scar tissue

  • Jaundice – yellowing of skin due to build up of bilirubin from liver disease or excessive destruction of RBCs (e.g., neonatal jaundice)

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Accessory Glands: Gall Bladder

  • Lies in depression on ventral surface of liver

  • Thin-walled, muscular sac (holds about 50 ml)

  • Stores and concentrates bile

  • Releases bile via cystic duct

  • Histology

    • mucosa – cells contain microvilli for reabsorption of water

    • submucosa – dense CT

    • muscularis – contracts to expel bile

    • serosa – over ventral portion only

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Control of Bile Release

  • Bile produced by liver backs up into gall bladder when hepatopancreatic sphincter is closed

  • Gall bladder releases bile into cystic duct when stimulated by cholecystokinin (secreted by duodenum) and/or parasympathetic impulses

  • Release inhibited by somatostatin produced by stomach and duodenum

Fig. 24.25, p. 923

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Disorders of the Gall Bladder

  • Gallstones (biliary calculi) – result from crystallization of cholesterol due to excess of cholesterol or too little bile salts

  • Obstructive jaundice – yellowish coloration of skin due to build up of bile pigments caused by blockage of bile ducts

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Accessory Glands: Pancreas

  • Mostly retroperitoneal, head encircled by duodenum, tail abuts spleen

  • Acinar cells (acini)

    • secrete pancreatic juice rich in enzymes, which are stored in zymogen granules until release

    • pancreatic juice excreted through pancreatic duct

  • Islets of Langerhans – endocrine cells

    • secrete insulin, glucagon, somatostatin

See Fig. 24.20, p. 915

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Composition of Pancreatic Juice

  • Watery, rich in bicarbonate (HCO3-)

    • bicarbonate makes it alkaline and neutralizes acidity of chyme

  • Digestive enzymes – see Small Intestine: Digestion

    • proteasesreleased as zymogens (inactive precursors)

      • trypsin – released as trypsinogen (activated by enterokinase enzyme in brush border cells)

      • carboxypeptidase & chymotrypsin – activated from precursors by trypsin

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Pancreatic Enzymes (con’t)

  • Digestive enzymes

    • amylase – hydrolyzes starch and glycogen (animal “starch”) into short carbohydrate chains and maltose

    • lipases – hydrolyze neutral fats into fatty acids and glycerol (or mono- and diglycerides)

    • nucleases – hydrolyze nucleic acids into nucleotides

    • nucleosidases – hydrolyze nucleotides into ribose, nitrogen bases and phosphate

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Control of Pancreatic Secretion

  • Secretin

    • released from small intestine in response to acidic chyme entering duodenum

    • stimulates acini to produce juice rich in bicarbonate

  • Cholecystokinin

    • released from duodenum in response to fatty or protein-rich chyme

    • stimulates acini to secrete juice rich in enzymes

  • Vagus nerve – stimulates secretion during cephalic and gastric phases of digestion

Fig. 24.28, p. 925

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Pancreas’ Endocrine Role: Insulin

  • Secreted when blood glucose increases

  • Lowers blood sugar by

    • stimulating uptake by body cells (except liver, kidney and brain)

    • stimulates glycogen formation in liver and skeletal muscle

    • inhibits gluconeogenesis (conversion of fats and protein to glucose) in liver

    • stimulates carbohydrate metabolism in most cells

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Pancreas’ Endocrine Role: Glucagon

  • Secreted in response to low blood glucose

  • Increases blood sugar by:

    • Promotes breakdown of glycogen by liver (glycogenolysis)

    • Stimulates synthesis of glucose from lactic acid and noncarbohydrate sources (gluconeogenesis) by liver

    • Stimulates release of glucose into blood by liver

    • Inhibits uptake and use of carbohydrates by skeletal muscle

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Disorders of the Pancreas

Pancreatitis – inflammation of the pancreas

  • may be caused by excessive fat in blood

  • activation of enzymes within pancreas (pancreas digests itself)

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Large Intestine

  • Located primarily in abdominal cavity, distal end is in pelvic cavity

  • Larger in diameter, but shorter (~1.5 m) than small intestine

  • Modifications:

    • teniae coli – reduction of longitudinal layer of muscularis

    • haustra – pocket-like sacs formed by motor tone of teniae coli

    • epiploic appendages – small, fat-filled pouches of visceral peritoneum

Fig. 24.29, p. 928

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Large Intestine: Subdivisions

  • Cecum

    • vermiform appendix

  • Colon

    • ascending

    • transverse

    • descending

    • sigmoid

  • Rectum

Fig. 24.29, p. 928

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Large Intestine: Anal Canal

  • Arranged as anal columns (long folds of anal mucosa)

    • composed of stratified squamous epithelium

    • anal sinuses secrete mucus when compressed by feces

  • Sphincters control defecation

    • internal anal sphincter – smooth muscle

    • external anal sphincter – skeletal muscle

Fig. 24.29, p. 928

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Large Intestine: Histology

  • Mucosa – thicker than in small intestine

    • crypts contain numerous goblet cells

    • simple columnar epi. with lots of goblet cells

    • stratified squamous in anal canal

  • Submucosa – thinner than in small intestine

    • less lymphatic tissue

  • Muscularis – longitudinal layer is modified as teniae coli

  • Serosa – covers all but region in pelvic cavity

See Fig. 24.31, p. 930

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Intestinal Flora

  • Resident bacteria dominated by Escherichia coli (E. coli)

  • Ferment some indigestible carbohydrates

    • results in mixture of irritating acids and gases

  • Synthesize B vitamins and vit. K

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Digestion in Large Intestine

  • No additional breakdown of molecules except by bacteria

  • Reabsorption of water and electrolytes (very important to water and electrolyte balance)

  • Absorption of vitamins produced by bacteria

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Movements in Large Intestine

Formation of feces

  • Haustral churning

    • slow process in which distention of hastrum stimulates contraction which moves food into next haustrum

    • mixes food residue and aids water reabsorption

  • Mass peristalsis

    • long, slow movements along length of large intestine force food toward rectum

    • stimulated by gastrocolic reflexes based on stretching of stomach

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  • Parasympathetic reflex relaxation of smooth muscle (internal) sphincter

  • Voluntary relaxation of external sphincter (skeletal muscle)

Fig. 24.32, p. 931

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Large Intestine: Disorders

  • Appendicitis – inflammation of the appendix, usually caused by bacterial infection

  • Diarrhea

    • watery stools due to shortened residence time

    • caused by irritants, bacterial or viral disease

    • loss of water and electrolytes can lead to dehydration and electrolyte imbalances

  • Constipation

    • hard stools due to increased time for water reabsorption

    • can also lead to electrolyte and pH imbalances

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Large Intestine: Disorders

  • Hemorrhoids – inflammation of the superficial anal veins

  • Colitis – inflammation of the colon

  • Diverticulosis

    • formation of small herniations in mucosa of large intestine

    • common in elderly, especially those whose diets are low in bulk (fiber from fruits and vegetables provides bulk)

  • Diverticulitis – inflammation of diverticula

  • Crohn’s disease – chronic inflammation; usually in ileum or large intestine