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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

Biology 221

Anatomy & Physiology II

TOPIC 8Digestive System

Chapter 18

pp. 651-677

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

digestive system functions
Digestive System Functions
  • Provide nutrients in usable form
  • Remove unusable wastes
digestive system overview
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

processes of digestion
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

  • 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

splanchnic circulation
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

hepatic circulation hepatic portal system
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

hepatic circulation hepatic veins
Hepatic Circulation: Hepatic Veins
  • drain venous blood from liver into inferior vena cava

Fig. 20.27, p. 771

  • Mucous membrane lining gut
  • Consists of:
    • epithelium – lining
    • lamina propria – areolar connective tissue layer
    • muscularis mucosae – smooth muscle

mucosa epithelium
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

mucosa lamina propria
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

mucosa muscularis mucosae
Mucosa: Muscularis Mucosae
  • Smooth muscle used for local movement and to hold mucosa in folds (small intestine)

  • 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

muscularis externa muscularis
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

  • 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)
enteric nervous system
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)
central nervous system control
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
mouth oral cavity
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)
mouth palate
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

mouth arches
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

mouth tongue
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

mouth tongue22
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

salivary glands
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

  • 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
salivary proteins
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
control of salivation
Control of Salivation
  • Sympathetic division
    • stimulates production of mucin-rich saliva, or
    • inhibits salivation altogether at high levels
control of salivation27
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
  • 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


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

tooth structure
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

  • 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
  • Runs from laryngopharynx through mediastinum to stomach
  • All 4 layers present in wall
    • Mucosa – consists of stratified squamous epithelium
    • Submucosa – mucus-secreting esophageal glands

  • 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

structures associated with the esophagus
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
esophageal disorders
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
digestive processes in mouth pharynx and esophagus
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)
digestive processes in mouth pharynx and esophagus37
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

stomach gross anatomy
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

stomach histology
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

microscopic anatomy
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

gastric pit cells
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)
gastric pit cells42
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
digestive processes in stomach
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
digestive processes in stomach44
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
regulation of gastric secretion
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

stimulation of gastric secretion
Stimulation of Gastric Secretion
  • Cephalic Phase (cerebral)
  • Gastric Phase (stomach)
  • Intestinal Phase (duodenum)

Fig. 24.16, p. 910

inhibition of gastric secretion
Inhibition of Gastric Secretion
  • Cephalic Phase (cerebral)
  • Gastric Phase (stomach)
  • Intestinal Phase (duodenum)

Fig. 24.16, p. 910

gastric disorders
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
gastric disorders con t
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)
small intestine gross structure
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

small intestine duodenum
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

small intestine jejunum ileum
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

small intestine innervation
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

small intestine blood supply
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

small intestine overview of special anatomical features
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

small intestine plicae circularis
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

small intestine villi
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

small intestine microvilli
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

small intestine mucosa
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

small intestine submucosa
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

small intestine muscularis serosa
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
small intestine muscularis serosa62
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
small intestine digestive processes
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

small intestine lipid digestion
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

small intestine protein digestion
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

small intestine carbohydrate digestion
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

small intestine nucleic acid digestion
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

small intestine absorption of carbohydrates
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)
small intestine absorption of proteins and nucleic acids
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
small intestine absorption of lipids
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
small intestine absorption of vitamins
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)
small intestine absorption of electrolytes
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
small intestine movement
Small Intestine: Movement
  • Peristalsis moves chyme through intestine
  • Segmentation mixes chyme with intestinal juice

Fig. 24.3, p. 890

hormonal control of small intestine activity
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
nervous system control of small intestine activity
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
accessory glands liver gross anatomy
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

liver hepatic ducts
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

liver ligaments
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

liver blood supply
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

liver microscopic anatomy
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

liver microscopic anatomy con t
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

liver microscopic anatomy con t82
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

liver functions
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
liver functions bile
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)
control of bile production
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

liver disorders disease
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)
accessory glands gall bladder
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

control of bile release
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

disorders of the gall bladder
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
accessory glands pancreas
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

composition of pancreatic juice
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
pancreatic enzymes con t
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
control of pancreatic secretion
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

pancreas endocrine role insulin
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
pancreas endocrine role glucagon
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
disorders of the pancreas
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)
large intestine
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

large intestine subdivisions
Large Intestine: Subdivisions
  • Cecum
    • vermiform appendix
  • Colon
    • ascending
    • transverse
    • descending
    • sigmoid
  • Rectum

Fig. 24.29, p. 928

large intestine anal canal
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

large intestine histology
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

intestinal flora
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
digestion in large intestine
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
movements in large intestine
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
  • Parasympathetic reflex relaxation of smooth muscle (internal) sphincter
  • Voluntary relaxation of external sphincter (skeletal muscle)

Fig. 24.32, p. 931

large intestine disorders
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
large intestine disorders106
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