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ANATOMY AND PHYSIOLOGY
DIGESTIVE SYSTEM
− Also known as the Gastrointestinal Tract (GIT) − A complex set of organs, glands, and ducts that work together to transform food into nutrients for cells
FUNCTIONS
o INGESTION – consumption of solid or liquid food, usually through the mouth o MASTICATION – the breakdown of large food particles into many small ones o PROPULSION o MIXING o SECRETION o DIGESTION – breakdown of large organic molecules into smaller molecules that can be absorbed; occurs through mechanical & chemical means o ABSORPTION – movement of molecules out of the digestive tract &b into the blood or lymphatic system o ELIMINATION – removal of undigested material, such as fiber from food, plus other waste products from the body as feces
OVERVIEW OF THE DIGESTIVE SYSTEM
A. DIGESTIVE TRACT/ALIMENTARY CANAL
- Oral Cavity (Mouth)
- Pharynx
- Esophagus
- Stomach
- Small Intestine
- Large Intestine
- Rectum
- Anal Canal
- Anus
B. ACCESSORY ORGANS
- Salivary Glands
- Teeth
- Tongue
- Liver
- Pancreas
- Gallbladder
HISTOLOGY
I. MUCOSA
o Innermost layer or tunic o Consists of 3 layers: (1) inner mucous epithelium , (2) a loose connective tissue called the lamina propria , & (3) a thin outer layer of smooth muscle called the muscularis mucosae
II. SUBMUCOSA
o Lies just outside the mucosa o A thick layer of connective tissue containing nerves, blood vessels, & small glands o An extensive network of nerve cell processes forms a plexus within this tunic
III. MUSCULARIS
o Digestive Tract – consists of an inner layer of circular smooth muscle & an outer layer of longitudinal smooth muscle o Nerve plexus, innervated by the ANS lies between the 2 muscle layers o Nerve plexuses of the muscularis & submucosa compose the Enteric Nervous System (ENS)
IV. SEROSA/ADVENTITIA
o Outermost layer of the digestive tract o Either a serosa or an adventitia o Serosa – consists of the peritoneum, a smooth epithelial layer & its underlying connective tissue o Adventitia – covers the regions of the digestive tract which isn’t covered by peritoneum; connective tissue layer; continuous with the surrounding connective tissue
NEURAL INNERVATION OF THE
GASTROINTESTINAL TRACT
1) ENTERIC NERVOUS SYSTEM
o Myenteric plexus – plexus of Auerbach o Submucosal plexus – plexus of Meissner
- AUTONOMIC NERVOUS SYSTEM
CHEMICAL REGULATION OF THE DIGESTIVE
SYSTEM
- ACETYLCHOLINE – stimulates GI tract motility & secretions
- NOREPINEPHRINE – inhibits GI tract motility & secretions
- SEROTONIN – stimulates GI tract motility
- GASTRIN – stimulates gastric glands to secrete large amounts of gastric juice
- SECRETIN – stimulates the flow of pancreatic juice that is rich in bicarbonate (HCO 3 - ) ions
PERITONEUM
− Largest serous membrane of the body − Consist of simple squamous epithelium (mesothelium) with an underlying areolar connective tissue − Contains large folds that weave between the viscera − Consists of the: (1) Parietal Peritoneum & the (2) Visceral Peritoneum
5 MAJOR PERITONEAL FOLDS
1) MESENTERY
o Fan-shaped fold of the peritoneum o Largest peritoneal fold; laden with fat o Consist of 2 layers of serous membranes with a thin layer of connective tissue between them o Hold many of the organs in place within the abdominal cavity o Provides a route for blood vessels & nerves from the abdominal wall to the organs
2) GREATER OMENTUM
o Mesentery extending as a fold from the greater curvature & then to the transverse colon o Also known as the fatty apron o Omental Bursa – cavity within the greater omentum
3) FALCIFORM LIGAMENT
o Attaches the liver to the anterior abdominal wall & diaphragm
4) LESSER OMENTUM
o Mesentery connecting the lesser curvature of the stomach & the proximal end of the duodenum to the liver & diaphragm
5) MESOCOLON
− Bind the transverse colon (transverse mesocolon) & sigmoid colon (sigmoid mesocolon) of the large intestine to the posterior abdominal wall
ORGANS
I. ORAL CAVITY (MOUTH)
− 1 st^ part of the digestive tract
a) CHEEKS − Form the lateral walls of the oral cavity − Contains the buccinator muscles which flatten the cheeks against the teeth − Play a role in mastication − Help form words during mastication b) LIPS − Muscular structures mostly formed by the orbicularis oris − Covered by keratinized stratified epithelium & is continuous with the moist stratified squamous epithelium of the mucosa in the oral cavity c) TEETH − There are 32 teeth in the normal adult mouth located in the mandible & the maxillae − Teeth of adults are called permanent teeth or secondary teeth − These replace your primary/deciduous teeth or milk/baby teeth , which consists of 20 teeth − Each tooth has 3 regions: (1) the crown , (2) neck , & (3) the root ➢ Crown – has one or more cusps (points); the visible portion of a tooth
➢ The lower esophageal sphincter is sometimes called the cardiac sphincter
DEGLUTITION
STRUCTURE ACTIVITY RESULT
Pharynx Pharyngeal stage of deglutition Moves bolus from oropharynx to laryngopharynx & into esophagus; closes air passageways Esophagus
- Relaxation of upper esophageal sphincter
- Esophageal stage of deglutition (peristalsis)
- Relaxation of lower esophageal sphincter
- Secretion of mucus
- Permits entry of bolus from laryngopharynx into esophagus
- Pushes bolus down esophagus
- Permits entry of bolus down esophagus
- Lubricates esophagus for smooth passage of bolus
IV. STOMACH
− Primarily houses food for mixing with hydrochloric acid & other secretions − An enlarged segment of the digestive tract in the left superior part of the abdomen − Internal volume of about 50mL when empty − 1.0 – 1.5L after a typical meal − Up to 4L when extremely full & will extend nearly as far as the pelvis − Takes approximately 4 hours to clear a meal − Antrum holds 30mL − 3mL of chyme is released into the duodenum per contraction − Receives parasympathetic fibers from vagus & sympathetic fibers from the celiac ganglia − Protected from the harsh acidic & enzymatic environment it creates through 3 ways: (1) mucous coat , (2) tight junctions , & (3) epithelial cell replacement − Divided into 4 regions: ➢ Cardiac Region (Cardia) – around the gastroesophageal opening; small area within about 3cm of the cardiac orifice
➢ Fundic Region (Fundus) – most superior part of the stomach; dome-shaped portion superior to esophageal attachment ➢ Body (Corpus) – makes up the greatest part of the stomach; turns to the right, forming a greater curvature & a lesser curvature ➢ Pyloric Region – narrower pouch at the inferior end i. Pyloric Opening – opening into the small intestine ii. Pyloric Sphincter – thick ring of smooth muscle surrounding the pyloric opening
HISTOLOGY
− The muscularis layer of the stomach is different from the other regions of the stomach in that it has 3 layers: (1) an outer longitudinal layer , (2) a middle circular layer , & (3) an inner oblique layer − These muscular layers produce a churning action in the stomach − The submucosa & mucosa of the stomach are thrown into large folds called rugae when the stomach is empty − The rugae allow the mucosa & submucosa to stretch & the folds disappear as the stomach is filled − Stomach is lined with simple columnar epithelium − Mucosal surface forms numerous tube-like gastric pits which are the openings for the gastric glands − There are 5 groups of epithelial cells in the stomach: 1) Surface mucous cells – coats & protects the stomach lining 2) Mucous neck cells – produce mucus 3) Parietal cells – produce hydrochloric acid & intrinsic factor 4) G cells/Endocrine cells – produce regulatory chemicals; gastrin 5) Chief cells – produce pepsinogen
SECRETIONS OF THE STOMACH
− As food enters the stomach, the food is mixed with stomach secretions to become a semifluid mixture called chyme − Stomach secretions from the gastric glands include: ➢ Hydrochloric Acid – produces a pH of about 2.0 in the stomach; kills microorganisms & activates the enzyme, pepsin ➢ Pepsin – converted from its inactive form pepsinogen; breaks covalent bonds of proteins to form smaller peptide chains; exhibits optimum enzymatic activity at a pH of about 2. ➢ Mucus – lubricates the epithelial cells of the stomach wall & protects them from the damaging effect of the acidic chyme & pepsin ➢ Intrinsic Factor – binds with vitamin B 12 & makes it more readily absorbed in the small intestine
MOVEMENT
− 2 types of stomach movement aid digestion& help move chyme through the digestive tract: (1) mixing waves & (2) peristaltic waves − Both result from smooth muscle contractions in the stomach wall − Contractions occur every 20 seconds ➢ Mixing Waves – from relatively weak contractions; thoroughly mix ingested food with stomach secretions to form chyme ➢ Peristaltic Waves – from stronger contractions; force the chyme toward & through the pyloric sphincter; pyloric sphincter usually remains closed because of mild tonic contraction , but each peristaltic contraction is sufficiently strong to cause partial relaxation of this & to pump a few millimeters of chyme through it & into the duodenum; increased by low blood glucose levels ➢ Hunger Pangs – caused by peristaltic waves when the stomach is empty; occur for about 2 – 3 minutes & can build in strength; begin 12 – 24 hours after the previous meal; growling − If the stomach empties too fast, the efficiency of digestion & absorption in the small intestine is reduced − If the rate of emptying is too slow, the highly acidic contents of the stomach may damage the stomach wall
REGULATION OF STOMACH SECRETION
1) Cephalic Phase − Stomach secretions are increased in anticipation of incoming food − Vagus nerves carry parasympathetic action potentials to the stomach where enteric plexus neurons are activated − Postganglionic neurons stimulate secretion by parietal & chief cells & stimulate gastrin & histamine secretion by endocrine cells − Gastrin is carried through the blood back to the stomach, along with histamine ➢ Gastrin – stimulates additional secretory activity ➢ Histamine – most potent stimulator of hydrochloric acid secretion 2) Gastric Phase − Greatest volume of gastric secretion occurs; activated by presence of food in the stomach − Distention of stomach stimulates mechanoreceptors & activates a parasympathetic reflex; action potentials generated by the mechanoreceptors are carried by the vagus nerves to the medulla oblongata − Medulla oblongata increases action potentials in the vagus nerves that stimulate secretions by parietal & chief cells & stimulate gastrin & histamine secretion by endocrine cells − Distention of the stomach also activates local reflexes that increase stomach secretions − Gastrin is carried through the blood back to the stomach, along with histamine 3) Intestinal Phase − Primarily inhibits gastric secretions; controlled by the entrance of acidic chyme into the duodenum − Chyme in the duodenum with a pH less than 2 or containing lipids inhibits gastric secretions by 3 mechanisms − Chemoreceptors in the duodenum are stimulated by H+^ (low pH) or lipids; action potentials generated are carried by the vagus nerves to the medulla oblongata where they inhibit parasympathetic action potentials, thereby decreasing gastric secretions − Secretin & cholecystokinin produced by the duodenum decrease gastric secretions in the stomach
VII. PANCREAS
− Located retroperitoneal, posterior to the stomach in the inferior part of the left upper quadrant − Has a head near the midline of the body & a tail that extends to the left where it touches the spleen − Composed of both endocrine & exocrine tissues that perform several functions ➢ Pancreatic islets (islets of Langerhans) – contained in the endocrine glands ➢ Islet cells – produce the hormones insulin & glucagon ➢ Acinar gland – the exocrine part ➢ Acini – produce digestive enzymes; connected by small ducts which join to form larger ducts, & the larger ducts join to form the pancreatic duct
FUNCTIONS
− The increased pH resulting from the secretion of HCO 3 -^ stops pepsin digestion but provides the proper environment for the function of pancreatic enzymes − Exocrine secretions include bicarbonate ions (HCO 3 - ) & digestive enzymes called pancreatic enzymes ➢ Bicarbonate ions – neutralize the acidic chyme that enters the small intestine from the stomach ➢ Pancreatic enzymes – are important in digesting all major classes of food − The major protein-digesting (proteolytic) enzymes are: (1) trypsin , (2) chymotrypsin , & (3) carboxypeptidase ➢ Pancreatic amylase – continues the polysaccharide digestion that began in the oral cavity ➢ Nucleases – enzymes that degrade DNA & RNA to their component nucleotides ➢ Lipase – a lipid-digesting enzyme
VIII. LIVER
− Produces bile − Largest internal organ of the body − Weighs about 1.36kg − Located in the upper right quadrant of the abdomen − Consists of 2 major lobes: (1) right lobe , (2) left lobe − The 2 lobes are separated by a connective tissue septum called the falciform ligament − 2 smaller liver lobes: the (1) caudate lobe & the (2) quadrate lobe can be seen from the inferior view − The porta , the gate through which blood vessels, ducts, & nerves enter or exit the liver, can also be seen from the inferior view
HISTOLOGY
− Many delicate connective tissue septa divide the liver into lobules with portal triads at their corners − The portal triads contain 3 structures: (1) the hepatic artery , (2) hepatic portal vein , & (3) the hepatic duct − Hepatic cords , formed by plate-like groups of liver cells called hepatocytes , are located between the center & the margins of each lobule − The hepatic cords are separated from one another by blood channels called the hepatic sinusoids − The sinusoid epithelium contains phagocytic cells that help remove foreign particles from the blood ➢ Bile canaliculus – is a cleft-like lumen between the cells of each hepatic cord − Bile flows through the bile canaliculi to the hepatic ducts in the portal triads − Macrophages are called Kupffer cells
FUNCTIONS
o Bile Production o Detoxification of Drugs o Carbohydrate Metabolism o Lipid Metabolism o Protein Metabolism o Storage o Phagocytosis o Activation of Vitamin D
BILE
− Secreted by the liver at the rate of 600 – 1000mL a day − Dilutes & neutralizes stomach acid − Dramatically increases the efficiency of fat digestion & absorption
− Bile salts emulsify fats, breaking the fat globules into smaller droplets − Also contains excretory products, such as cholesterol, fats, & bile pigments ➢ Bilirubin – a bile pigment that results from the breakdown of hemoglobin ➢ Gallstones – may form if the amount of cholesterol secreted by the liver becomes excessive & is not able to be dissolved by the bile salt
IX. GALLBLADDER
− A small sac on the interior surface of the liver − Stores concentrated bile
