chapter 25 - Digestive system Flashcards
Digestive system
digestive tract (alimentary canal) and accessory organs (Fig 25.1) • digestive tract includes: oral cavity, pharynx, esophagus, stomach, small intestine and large intestine • accessory organs = teeth, tongue, salivary glands, liver and pancreas
Functions of digestive systems
• processing of food
o ingestion = entry of food and liquids into digestive tract via the oral cavity
o mechanical processing = physical breakdown of ingested substances
tongue (squishing), teeth (tearing, grinding), and propulsive motion of digestive tract (mixing, swirling and churning)
o digestion = chemical and enzymatic breakdown of macromolecules (carbohydrates, lipids, protein) into units that can be absorbed
o secretion = acids, enzymes and buffers released by lining of digestive tracts and accessory organs
o absorption = movement of organic molecules, electrolytes, vitamins and water across digestive epithelium and into interstitial fluid of digestive tract
o excretion = movement of waste products into digestive tract – primarily via liver
o compaction = progressive dehydration of indigestible materials and organic wastes forming feces
o defecation = elimination of feces from the body
defense against
o corrosive effects of digestive acids and enzymes
o mechanical stresses – e.g. abrasion
o pathogens
how many layers
• 4 major layers with variations along tract related to specific functions of each organ
• Mucosa
= inner lining
o mucous membrane
– loose (areolar) connective tissue covered by epithelium moistened by glandular secretion
epithelium = simple columnar or non-keratinized stratified squamous depending on location
lamina propria = underlying layer of areolar tissue
• contains blood vessels, sensory nerve ending, lymphatic fibers, smooth muscle fibers and scattered mucosa associated lymphatic tissue (MLAT = leukocytes)
o plicae
organization of mucosa in certain areas into transverse or longitudinal folds
increase surface area for absorption
o muscularis mucosae
two thin concentric layers of smooth muscle below the lamina propria
contraction changes shape of lumen and plicae
separates mucosa from submucosa
• Submucosa
dense irregular connective tissue
o contains large blood vessels, lymphatics, and in some areas exocrine glands
o submucosal (Meissner’s) plexus
nerve network (with neuron cell bodies) along outer margin of submucosa action affects muscularis mucosae and glandular secretions
• Muscularis Externa
= layer dominated by smooth muscle fibers
o smooth muscle = small spindle-shaped cells (5-10 µm diameter, 30-200 µm length)
non-striated – contractile proteins not organized into sarcomeres, involuntary muscle
arranged in sheets, electrically connected by gap junctions
smooth muscle can be activated by
- motor neurons
- local chemicals, hormones, carbon dioxide or oxygen levels
- physical factors – stretching, irritation
- pacemaker cells – undergo spontaneous depolarizations
o movements produced
peristalsis = waves of muscular contraction that move bolus (small mass of food) along length of digestive tract
• circular smooth muscle contracts behind bolus, then longitudinal forcing bolus forward
segmentation = mediated mainly by circular smooth muscle contractions that move and churn but do not produce net movement in any particular direction
o myenteric (Auerbach’s) plexus
plexus = nerve network
between inner circular and outer longitudinal smooth muscle layers
parasympathetic activation increases muscular tone and stimulates contraction
sympathetic output inhibits muscular activity resulting in relaxation
myenteric reflexes = short reflexes that do not involve the central nervous system
o sphincters
= thickened areas of circular smooth muscle that act as valves
located in specific areas to prevent materials moving in wrong direction or from one area to the next at inappropriate times
• Serosa
visceral peritoneum - serous membrane that covers most of the outer wall of the digestive tract
o mesenteries
fused, double-sheets of peritoneum – visceral with parietal peritoneum
areolar connective tissue between allows access route for passage of blood vessels, nerves and lymphatics to and from digestive tract
stabilize relative positions of attached organs and prevents entanglement of intestines during digestive movements or sudden changes in body position
greater omentum =
large pouch of mesentery extends from inferior margin of stomach between body wall and anterior surface of small intestine
• loose connective tissue between layers contains thick layer of adipose tissue (Fig 25.10)
• may serve as energy reserve and insulator reducing heat loss across anterior abdominal wall
lesser omentum
= between stomach, proximal duodenum and liver
peritonitis = inflammation of peritoneal membrane that interferes with normal functioning of affected organs
• ascites = accumulation of fluid in peritoneal lining creates characteristic abdominal swelling
o areas not covered by serosa
adventitia - dense network of collagen fibers that attaches pharynx, esophagus and rectum to adjacent structures
Oral (Buccal) Cavity
• allows:
o analysis of material before swallowing
o mechanical processing of food
o lubrication by mixing mucous and salivary secretions
o limited digestion of carbohydrates
• oral cavity is lined by
oral mucosa
non-keratinized stratified squamous epithelium
o provides protection from abrasion
• lateral walls
cheeks – mucosa, buccal fat pads and buccinator muscles
• anterior border
labia (lips) and gingiva = ridge of oral mucosa
o vestibule = space between cheeks, lips and teeth
• roof
hard and soft palates
o hard palate = mucosa over maxilla and palatine bones
separates oral cavity from nasal cavity
o soft palate = mucosa, palatoglossus and palatopharyngeal muscles
separates oral cavity from nasopharynx
closes off nasopharynx while swallowing
uvula
dangles from posterior margin and help prevent food from entering nasopharynx
palatine tonsils
– lie between the pharyngeal arches
• floor
tongue and support from mylohyoid muscle
o tongue
contains two groups of skeletal muscles – intrinsic and extrinsic
• extrinsic muscles - perform all gross movements of tongue
• intrinsic muscles – alter shape of tongue and aid extrinsic muscles
• both controlled by hypoglossal nerve (CN XII)
tongue aid in
aids in mechanical processing (compression) of food
manipulation of food to assist in chewing and to prepare in swallowing
provides sensory analysis of food by touch, temperature and taste receptors
secretion of mucins and lingual lipase - enzyme that aids in fat digestion
anterior body, posterior root
dorsum
surface and contain fine projections called papillae
• provide additional friction that aid in movement of materials by tongue
• contain taste buds
lingual frenulum
thin fold of mucous membrane that connects inferior surface of tongue to mucosa of oral floor
• prevents excessive movements of tongue
• ankyloglossia
condition (“tongue-tied”) that arises when frenulum too restrictive
• Salivary glands
= accessory organs that empty saliva into oral cavity (Fig 25.5)
o saliva lubricates mouth, moistens food, dissolves chemicals that stimulate taste buds, helps control populations of oral bacteria
o parotid salivary gland
largest – produces 25% of saliva
extends from inferior surface of zygomatic arch over the superficial surface of masseter muscle
serous cells produce thick secretion containing salivary amylase – begins digestion of complex carbohydrates
parotid ducts empty into vestibule at level of second upper molar
o sublingual salivary glands
produces 5% of saliva
mucous cells covered by mucous membrane on floor of mouth
ducts empty on either side of lingual frenulum
o submandibular salivary glands
produces 75% of saliva
serous and mucous cells located in glands located along medial surface of mandible in floor of mouth
ducts empty of either side of lingual frenulum posterior to teeth
o secretions controlled by
autonomic nervous system – parasympathetic nerves increase and sympathetic nerves decrease secretions
o mump virus
– preferentially targets salivary glands and usually occurs between age 5–9
infection of post adolescent males can affect testes causing sterility
infection of pancreas may produce temporary or permanent diabetes
mumps vaccine has almost eliminated disease in Canada and US
• Teeth
accessory organs (Fig 25.7) o aid in mechanical breakdown of food through mastication (chewing)
o dentine (dentin)
= mineralized matrix (similar to bone) that makes up bulk of tooth – does not contain living cells
o pulp cavity
– contains living cells with cytoplasmic extensions the extend into the dentin
spongy and highly vascular
o root
– portion below the gingiva (gums)
contains root canal with apical foramen that allows dental artery, vein and nerve to service pulp cavity
periodontal ligament = dense regular fibrous connective tissue that anchor root into the alveolus (part of the alveolar margin of the maxilla and mandible) = gomphosis (joint)
cement (cementum) covers dentin of root – provide protection and anchoring for periodontal ligament
o neck
boundary between root and crown
o crown
visible portion of tooth that projects above the gingiva (gums)
enamel = densely packed crystals of calcium phosphate that cover the crown
• hardest biologically manufactured substance that requires adequate amounts of calcium, phosphates and vitamin D during childhood for proper formation
o types of teeth (anterior to posterior):
incisors
incisors – shovel-shaped, single root teeth located in front – allow nipping, cutting
cuspids (canines)
– conical with sharp ridge and point and single root – allow tearing and slashing
bicuspids (premolars)
– flattened crowns with prominent ridges, one or two roots – allow grinding, crushing and mashing
molars
– very large flattened crowns with prominent ridges, three or more roots – allow crushing and grinding
o dental succession
o dental succession – two sets of teeth
primary dentition = 1st set of temporary teeth
• deciduous teeth = 20 primary, milk or baby teeth
adult dentition = 2nd set
• permanent teeth = 32 – 3rd molar (wisdom tooth) erupts after the age of 17 (if at all)
• bolus
shredded food moistened with saliva and formed into small oval mass that can be swallowed
Pharynx
common passageway for food, liquids and air
• extends between the internal nares and larynx (entry to trachea)
• nasopharynx
= superior portion connected to posterior portion of nasal cavity and separated from oral cavity by soft palate
o lined by pseudostratified ciliated columnar epithelium containing numerous mucous cells
o pharyngeal tonsil located on posterior wall
o lateral walls contain opening of auditory tubes
• oropharynx
between soft palate and base of the tongue at the level of the hyoid bone
o lined by non-keratinized stratified squamous epithelium
• laryngopharynx
area between hyoid bone and epiglottis
o lined by non-keratinized stratified squamous epithelium
o epiglottis = shoehorn-shaped elastic cartilage (covered with mucous membrane) that projects superior to the glottis (opening of the larynx and entry to trachea) (Fig 24.6)
• during swallowing larynx is elevated and epiglottis folds back over glottis preventing entry of liquids or solid food into the respiratory passages
Esophagus
- hollow muscular tube approximately 25 cm (1ft) long and 2 cm (0.75 in) in diameter\
- that transports food and liquids to the stomach
- located posterior to trachea in the thorax
• esophageal hiatus
opening in the diaphragm through which esophagus passes to enter peritoneal cavity
• histological modifications of the esophageal wall
o mucosa
esophagus
non-keratinized stratified squamous epithelium that resists abrasion from food bolus
o submucosa contains
scattered esophageal glands = simple, branched tubular mucous producing glands
esophagus : outer layer
o contains adventitia (fibrous outer layer) instead of serosa
• swallowing process divided into 3 parts
buccal phase, pharyngeal phase and esophageal phase
o buccal phase
voluntarily controlled
extrinsic muscles of tongue compression of food bolus against hard palate and retraction of tongue forces bolus into pharynx
o pharyngeal phase
< 1second - involuntary reflexes initiated once bolus enters oropharynx
palatal muscle raise the soft palate and adjacent portions of the pharyngeal wall preventing entry of food into the nasopharynx
elevation of larynx (palatopharyngeus and stylopharyngeus muscles) and folding of epiglottis direct bolus past closed glottis
pharyngeal constrictors (superior, middle and inferior) that push bolus toward esophagus
o esophageal phase
– involuntary reflexes
allow opening of upper esophageal (or cardiac) sphincter
once food bolus passes through upper esophageal sphincter – sphincter closes and food is propelled toward stomach by peristaltic waves (contractions of esophageal smooth muscle)
followed by opening of lower esophageal sphincter and entry into the stomach
• achalasia
condition where shallowed bolus descends relatively slowly along esophagus due to abnormally weak peristaltic waves and does not trigger opening of lower esophageal sphincter
o food materials accumulate at the base of the esophagus and repeated secondary peristaltic waves cause discomfort
o treatment = weakening of lower esophageal sphincter by cutting circular smooth muscle at base or using a balloon catheter and expanding balloon until muscle layers tear
• acid reflux
brief, limited reflux of stomach contents into lower part of esophagus after meals producing burning sensation
o increased incidence with abdominal obesity, pregnancy and reclining after large meals
• esophagitis
inflammation of esophagus from frequent or prolonged acid reflux
o few defences against attack by acids and digestive enzymes resulting in inflammation, esophageal erosion and intense discomfort
o GERD = gastroesophageal reflux disease – elevating head of bed and medication reduce symptoms
Stomach
Stomach
shape ?
what does it do ?
• J- shaped organ extends between levels of T7 and L3
o occupies left hypogastric, epigastric and portions of umbilical and left lumbar regions of abdominopelvic cavity (Fig 25.11)
• allows bulk storage of ingested food
• mechanically breaks down food
• chemical digestion is initiated
• chyme
stomach
resultant food composition from actions of stomach = viscous, acidic and soupy mixture
• short lesser curvature
– forms medial surface of organ (Fig 25.10)
o lesser omentum = small pocket of mesentery between lesser curvature and liver
stomach
• long greater curvature
forms lateral surface
o greater omentum = large pouch of mesentery hangs from greater curvature of stomach
• divided into four regions
o cardia = superior and medial portion at the junction of the esophagus and stomach
o fundus = region superior to gastroesophageal junction that contacts the inferior and posterior surface of the diaphragm
o body = largest regions important for mixing ingested food with gastric secretions
o pylorus – divided into pyloric antrum – connected to body of stomach, and pyloric canal – connects to the duodenum (proximal segment of small intestine)
pyloric sphincter – regulates release of chyme into duodenum
• rugae
prominent longitudinal folds of mucosa in relaxed stomach – flattens as stomach expands
• muscularis mucosa and muscularis externae
contain an extra muscular layer
o outer longitudinal layer, middle circular layer, inner oblique layer – strengthens and aids in mixing and churning
• simple columnar epithelium
lines all regions of stomach
o secrete mucus that forms a sheet like covering of the luminal surface to protect the epithelium from acids and enzymes in the gastric lumen
• gastric pits
small depressions of the mucosa (Fig 25.12)
o mucous necks cells and at neck (top of) gastric pits
regenerative stem cells replace mucous cells that are continuously shed into the lumen
• gastric glands (
continuations of gastric pits into the lamina propria of the fundus and body of stomach
o simple branched tubular glands that contain 4 types of secretory cells
mucous neck cells
mucous neck cells
at the entry to the gland = simple columnar epithelial cells
• secrete mucus
parietal (oxyntic) cells
parietal (oxyntic) cells
– along the sides of the gland = pyramid-shaped cells
• secrete hydrochloric acid and intrinsic factor
o HCl lower pH of gastric juice, kills microorganisms and activates secretions of chief cells
o intrinsic factor – protein required to protect and help absorb vitamin B12
chief (zymogen) cells
columnar cells at the base of the gland
• secrete pepsinogen – an inactive enzyme that is activated to pepsin by the low pH in the lumen
• initiates protein digestion
enteroendocrine cells
– scattered between parietal and chief cells
• produce a variety of secretions depending on the region
• G cells – predominant in pyloric region – secrete hormone gastrin
o stimulates secretory activity of parietal and chief cells and promotes smooth muscle contraction
• S cells – in duodenal mucosa – secrete secretin = hormone that stimulates liver, pancreas and duodenal glands allowing release of HCO3- (bicarb) that neutralizes the acid
• I cells – in duodenal mucosa - secrete cholecystokinin (CCK) – inhibits gastric secretion and motility; enhances intestinal motility and secretions from liver and pancreas
• production of acid and enzymes by gastric mucosa controlled:
o directly by autonomic nervous system (increased by parasympathetic activity and decreased by sympathetic activity)
o indirectly by local hormones released in response to food in the stomach (e.g. gastrin)
• gastritis
inflammation of the gastric mucosa
o can develop after swallowing drugs, alcohol, aspirin; severe emotional or physical distress, bacterial infections, ingestion of strong acid or alkaline chemicals
Helicobacter pylori
bacterium associated with ulcer formation = distinct erosion of the stomach lining
symptoms = burning pain and bleeding
responsible for 50-80% of gastric ulcers and 95% of duodenal ulcers
treated by antacids (neutralize acid), drugs that inhibit acid production, dietary restrictions (limit intake of acidic beverages and eliminate foods that promote acid production (caffeine, spices) and limit alcohol consumption (damage mucosal cells))
o peptic ulcer
erosion by digestive juices and enzymes in the proximal portions of the small intestine
due to excessive exposure to acid or inadequate production of alkaline mucus (that neutralizes acidic chyme)
Small Intestine
• approximately 6 m (20 ft) in length – position stabilized by mesenteries attached to dorsal body wall
o blood vessels, nerves and lymphatics reach segments of small intestine through mesenteries
o parasympathetic innervation via vagus nerve; sympathetic innervation via fibers from superior mesenteric ganglion
primary role of small intestine
• primary role = digestion and absorption of nutrients
o 90% of nutrient absorption occurs in small intestine
small intestine contains
o circular folds (plicae circularis) = series of transverse folds of mucosa that greatly increases surface area (Fig 25.14, 25.15)
o intestinal villi = finger-like projections of mucosa (Fig 25.14)
covered by simple columnar epithelium with scattered mucus secreting goblet cells
microvilli = extensions of the plasma membrane on the apical surface of epithelial cells
o intestinal crypts = indentations at the bases of the villi
base contains stem cells that continuously divide to replace cell on the surface that are continuously shed
enteroendocrine cells – produce several enzymes and hormones
o lamina propria – contains numerous lymphatic cells, extensive network of capillaries, nerve endings and a terminal central lacteal
lacteal = wide terminal lymphatic vessel that transports large lipid-protein complexes (chylomicrons) that cannot enter the capillaries
3 subdivisions of small intestine
o duodenum
o jejunum
o ileum
o duodenum
shortest (25 cm / 10 in) and widest portion of small intestine that is connected proximally to the pylorus of the stomach
entry of chyme into the duodenum regulated by the pyloric sphincter
hepatopancreatic ampulla = muscular chamber formed from bile duct (delivers bile from the liver) and pancreatic duct (delivers digestive enzymes from the pancreas) that enters ½ way along the length of the duodenum (Fig 25.21)
submucosa contains duodenal submucosal (Brunner’s) glands (Fig 25.15) – produce alkaline mucous that protects epithelium and neutralizes acidic chyme
o jejunum
2.5 m (8 ft) between duodenum and ileum
performs bulk of chemical digestion and nutrient absorption
o ileum
3.5 m (12 ft) that ends at the ileocaecal valve – control flow of materials from small intestine into large intestine
aggregated lymphoid nodules (Peyer’s patches) = large masses of lymphatic tissue in the submucosa (Fig 25.15)
• especially abundant at distal end of ileum to protect against migration of resident E.coli in the lumen of the large intestine
• neural reflexes involving
small intestine
submucosal and myenteric plexuses control movement of small intestine
o influenced by parasympathetic and sympathetic innervation
what control the secretory output of the small intestine
• hormonal and CNS
Large Intestine
- 1.5 m (5 ft) begins at the ileum and ends at the anus
- reabsorbs water and electrolytes
- compacts contents into feces
- absorbs important vitamins produced by bacterial action
- stores fecal matter prior to defecation
- wall thinner than small intestine and lacks villi
- intestinal crypts deeper than small intestine and dominated by goblet cells
- large lymphoid nodules scattered throughout lamina propria and extend into submucosa
- materials move very slowly from cecum to transverse colon via peristaltic contractions and haustral churning
what is more abundant than small intestine
• goblet cells more abundant than small intestine
• mass movements
powerful contractions from transverse colon to rectum that happens a few times per day
large intestine divided into 3 parts
cecum
colon
rectum
o cecum
expanded pouch at entry into large intestine
appendix (vermiform appendix) – attached to posteriomedial surface
• muscosa and submucosa dominated by lymphoid nodules
o colon
haustra = series of pouches in the wall of the colon
taeniae coli = 3 longitudinal bands of smooth muscle
omental appendices = sacs of fat along the serosa of the colon
colon subdivied in4 regions
- ascending colon – ascends along right lateral and posterior abdominal wall to inferior surface of liver
- transverse colon – curves anteriorly from right to left – passes inferior to great curvature of stomach
- descending colon – begins near the spleen and proceeds inferiorly along left side of abdomen
- sigmoid colon = S-shaped curve that begins at the left iliac fossa and end at the rectum passing posterior to urinary bladder
o rectum
= last 15 cm (6 in) of digestive tract
anal canal – at distal end and contains small longitudinal folds
• hemorrhoids = veins in lamina propria and submucosa that become distended
anus – contains stratified keratinized epithelium
• internal anal sphincter = thickened circular smooth muscle in muscularis externa – under involuntary control
• external anal sphincter = ring of skeletal muscle encircling distal end of anal canal – under voluntary control
Liver
= largest visceral organ
• mainly in right hypochondriac and epigastric regions
• ability to regenerate after injury depending on appropriate vascular supply
• liver important in metabolic regulation
o regulates circulating levels of carbohydrates, lipid and amino acids
o removes circulating toxins and metabolic waste products
o fat-soluble vitamins (A, D, K, and E) absorbed and stored in liver
• liver important in hematological regulation
o large blood reservoir (receives 25% of cardiac output)
o removes old or damaged RBCs
o contributes to osmotic regulation by producing albumin
o makes many of the clotting factors, plasma transport proteins and complement proteins
• synthesis and secretion of bile
o made by hepatocytes of liver and stored in gallbladder
o excreted into duodenum
= water, minor amount of ion, bilirubin (bile pigment derived from hemoglobin), lipids (known as bile salts)
what is deep to visceral perineum
• tough fibrous capsule deep to visceral perineum
how does the posterior wall looks like
- anterior surface smooth and follows curve of body wall
* posterior surface contains impressions of stomach, small intestine, right kidney and large intestine
liver : 4 lobes
right, left caudate and quadrate
• falciform ligament
liver
division between left and right lobe
o round ligament - thickening along lower margin = remnants of fetal umbilical vein
• blood delivery
= hepatic portal vein and hepatic artery proper
• blood return
– hepatic veins drain into inferior vena cava
• liver lobule
series of irregular plates of hepatocytes arranged like spokes of a wheel
o bile canaliculi = narrow channels between cells
o portal areas (hepatic triads) = locations where blood enters sinusoids from small branch of hepatic portal vein and hepatic artery and bile leaves via small branch of bile duct
o central vein – receives blood from sinusoids between plates
o Kupffer cells = phagocytic cells that along with endothelial cells line the sinusoids
• right and left hepatic ducts
liver
collect bile from all the bile ducts of liver lobes join to form
• common hepatic duct
leaves liver and can flow into
o common bile duct that empties into duodenum
o enter cystic duct that leads to gallbladder
Gallbladder
- hollow, pear-shaped muscular organ that stores and concentrates bile
- contains mucosa with narrow folds (mucosal crypts), lamina propria and muscularis externa
Pancreas
- posterior to stomach extending laterally from duodenum toward spleen
- covered by thin, transparent connective tissue capsule
- has rich blood supply
head, tail and body of pancreas
• head lies within the loop formed by the duodenum, body extends toward spleen, tail short and rounded
pancreas :
what kind of secretion ?
• primary exocrine organ producing digestive enzymes
o compound tubuloacinar gland
• partitions of connective tissue divide tissue into distinct lobules
o ducts branch repeatedly within lobule and end in pancreatic acini
lined with simple cuboidal epithelium
secrete mixture of water, ions and pancreatic digestive enzymes
• lipases digest lipids, carbohydrases (pancreatic amylase) digest sugars and starches, nucleases breakdown nucleic acids, and proteolytic enzymes (proteinases, peptidases) degrade proteins
o pancreatic islets
o pancreatic islets = endocrine portion scattered between acini = 1% of gland
• pancreatic duct
delivers secretions to duodenal ampulla
