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What is the primary function of the digestive system?
Converts foods into raw materials that build and fuel our body's cells.
Which of the following organs are part of the alimentary canal?
Identify the accessory digestive organs:
What are the six essential activities involved in the processing of food?
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What is the function of the peritoneum?
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Peritonitis is inflammation of the peritoneum usually caused by a ruptured appendix.
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Which layer of the alimentary canal is in contact with the luminal contents?
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What are the main components of saliva?
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Which type of teeth are primarily used for cutting food?
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Gingivitis is a serious condition that cannot be reversed.
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What is the role of the tongue in digestion?
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What is the primary function of salivary amylase in the mouth?
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What type of epithelium lines the pharynx?
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Which muscle layers are found in the pharynx?
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The esophagus is a muscular tube approximately 10 inches long that connects the laryngopharynx to the stomach.
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The part of the stomach surrounding the cardial orifice is called the ______.
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What is the function of the pyloric sphincter?
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Which cells in the stomach secrete hydrochloric acid?
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Pepsinogen is activated by hydrochloric acid to pepsin, which begins protein digestion in the stomach.
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What regulates the secretion of gastric juice?
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What is the main purpose of the intrinsic factor in the stomach?
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Which organ produces bile?
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Vomiting can be caused by extreme stretching of the GI tract.
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What is the main bile pigment?
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What is the primary function of bile salts and phospholipids?
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Enterohepatic circulation refers to the recycling of bile salts.
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Which organ is primarily involved in the production of pancreatic juice?
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What are the two main types of cells found in the pancreas?
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Which of the following enzymes is NOT produced by the pancreas?
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The small intestine is the body's major site for digestion and absorption.
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What are the three subdivisions of the small intestine?
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What triggers the release of bile into the duodenum?
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The large intestine extends from the ileocecal valve to the ______.
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What is the major digestive function of the large intestine?
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What condition results from inflammation of the liver?
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Gallstones are caused by too few bile salts.
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What is the common treatment for a patient with appendicitis?
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What part of the pancreas produces bicarbonate-rich fluid to neutralize acidic chyme?
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What is the role of enteroendocrine cells in the intestinal mucosa?
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What type of epithelium lines the large intestine?
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The large intestine contains circular folds and villi.
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What lubricates the passageway in the large intestine?
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What condition can occur from a low-fiber diet in the colon?
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Which of the following is a common cause of antibiotic-associated diarrhea?
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What procedure can be performed if the colon is removed?
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What reflex initiates defecation?
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Which of the following increases during the mass movements in the large intestine?
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Irritable bowel syndrome (IBS) is an anatomical disorder.
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What is the primary function of the large intestine?
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What increases the absorption of calcium in the intestines?
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The absorption process moves substances from the lumen of the gut into the ___.
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Which vitamins are absorbed in the large intestine from bacterial metabolism?
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Diarrhea occurs when the large intestine has too much time to absorb water.
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What are the steps observed during lipid digestion in the small intestine?
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Study Notes
Deglutition (Swallowing)
- Deglutition, or swallowing, is the process of moving food or liquid from the mouth to the stomach.
- The process of swallowing is divided into three stages:
- Oral stage: voluntary movement of food to the back of the mouth.
- Pharyngeal stage: involuntary movement of food from the pharynx to the esophagus.
- Esophageal stage: involuntary movement of food from the esophagus to the stomach.
The Stomach Temporarily Stores Food and Begins Protein Digestion
- The stomach serves as a temporary storage tank for ingested food.
- The stomach physically and chemically breaks down food.
- The stomach produces a paste-like substance called chyme.
- The stomach delivers chyme to the small intestine.
Gross Anatomy of the Stomach
- The empty stomach has a volume of approximately 50 ml, expanding to 4 L when fully distended.
- The stomach is composed of four major regions:
- Cardia.
- Fundus.
- Body.
- Pyloric part.
- The pyloric part is further divided into the:
- Pyloric antrum.
- Pyloric canal.
- The pyloric sphincter controls the passage of food from the stomach into the small intestine.
Anatomy of the Stomach (1 of 2)
- The stomach has two curvatures:
- Greater curvature.
- Lesser curvature.
Microscopic Anatomy of the Stomach
- The stomach wall consists of four tunics:
- Mucosa.
- Submucosa.
- Muscularis externa.
- Serosa.
- The stomach's muscularis externa allows churning, mixing, and movement of chyme.
- The stomach's mucosa is composed of simple columnar epithelium entirely composed of mucous cells.
- The stomach's mucosa is dotted with gastric pits, which lead into tubular gastric glands that produce gastric juice.
Microscopic Anatomy of the Stomach
- The stomach's gastric glands contain five types of cells:
- Mucous neck cells.
- Parietal cells.
- Chief cells.
- Enteroendocrine cells.
- Surface mucous cells.
Microscopic Anatomy of the Stomach
- The mucosal barrier protects the stomach from its own acid and proteolytic enzymes.
- The mucosal barrier is formed by:
- A thick coating of bicarbonate-rich mucus.
- Tight junctions between epithelial cells of the mucosa.
- Rapid replacement of damaged epithelial mucosal cells.
Digestive Processes in the Stomach
- The stomach performs several functions:
- Mechanical breakdown: Churning action of peristaltic waves.
- Digestion of proteins: HCl denatures proteins, and pepsin breaks down proteins into smaller polypeptides.
- Absorption of lipid-soluble substances: Alcohol and aspirin.
Regulation of Gastric Secretion
- The regulation of gastric secretion involves neural and hormonal mechanisms.
-
Neural mechanisms:
- Long reflexes: Stimulated via sympathetic and vagus nerves.
- Short reflexes: Stimulated by local events in the stomach.
-
Hormonal mechanisms:
- Gastrin: Stimulates HCL and gastric secretion.
- Enterogastrones: Inhibit gastric secretion.
Regulation of Gastric Secretion
- Gastric secretion is divided into three phases:
- Cephalic phase: Stimulated by the sight, smell, and thought of food.
- Gastric phase: Stimulated by the presence of food in the stomach.
- Intestinal phase: Stimulated by the presence of chyme in the small intestine.
Mechanism of HCl Secretion
- Parietal cells secrete HCl into the stomach lumen.
-
HCl is formed by:
- The movement of H+ from carbonic acid into the stomach lumen via proton pumps.
- The movement of Cl- into the stomach lumen.
- Bicarbonate ions (HCO3-) are exported into the blood.
Regulation of Gastric Motility and Emptying
- The stomach responds to filling by stretching to accommodate incoming food.
- Two factors maintain constant pressure as the stomach fills:
- Receptive relaxation.
- Gastric accommodation.
Regulation of Gastric Motility and Emptying
- Gastric contractile activity is most powerful in the pyloric part of the stomach.
- The pyloric region holds about 30 ml of chyme, and each peristaltic wave pushes approximately 3 ml of chyme into the duodenum.
Regulation of Gastric Motility and Emptying
- Gastric emptying is typically complete within 4 hours, but can take longer, especially if the chyme is high in fat.
- The emptying rate of carbohydrate-rich chyme is quicker than fat-rich chyme.
- The duodenum controls the rate of gastric emptying to prevent overfilling.
The Liver Secretes Bile; The Pancreas Secretes Digestive Ensymes
- The liver, gallbladder, and pancreas are accessory organs of the digestive system.
- The liver: Produces bile, which helps in fat emulsification.
- The gallbladder: Stores and concentrates bile.
- The pancreas: Secretes digestive enzymes and bicarbonate ions, which neutralize stomach acid in the duodenum.
The Liver
- The liver is the largest gland in the body, weighing approximately 3 pounds.
- The liver is composed of four lobes:
- Right lobe.
- Left lobe.
- Caudate lobe.
- Quadrate lobe.
- The gallbladder rests in a recess on the inferior surface of the right lobe.
The Liver
- The liver receives blood from the hepatic artery proper and the hepatic portal vein.
- Bile leaves the liver via the left and right hepatic ducts, which fuse to form the common hepatic duct.
Microscopic Anatomy of the Liver
- The liver is composed of hepatocytes, which are responsible for:
- Bile production.
- Nutrient processing.
- Fat-soluble vitamin storage.
- Detoxification.
The Liver
- Bile is composed of bile salts, bile pigments, cholesterol, triglycerides, phospholipids, and electrolytes.
- Bile salts and phospholipids aid in the digestion of fats.
- Bilirubin is the main bile pigment, a product of heme (hemoglobin) metabolism.
The Liver
- Enterohepatic circulation is a recycling mechanism that conserves bile salts.
- Bile salts are reabsorbed in the ileum and returned to the liver via the hepatic portal vein.
The Gallbladder
- The gallbladder is a thin-walled muscular sac located on the inferior surface of the liver.
- The gallbladder stores and concentrates bile by absorbing water and ions.
The Pancreas
- Primarily located behind the peritoneum, near the stomach
- The head of the pancreas is surrounded by the duodenum, while the tail sits next to the spleen
- Consists of endocrine and exocrine components
- Exocrine function involves producing pancreatic juice.
- Acinar cells form clusters that produce zymogen granules, containing inactive digestive enzymes.
- Ducts transport secretions from acinar cells to the duodenum.
- Duct cells contribute water and bicarbonate ions to the pancreatic juice.
- Endocrine function oversees the secretion of insulin and glucagon by the pancreatic islets.
Structure of the Enzyme-Producing Tissue of the Pancreas
- Pancreatic juice is a watery, alkaline solution with a pH of approximately 8.
- Plays a vital role in neutralizing acidic chyme from the stomach.
- Composed of electrolytes (primarily bicarbonate ions), and digestive enzymes.
- Proteases for protein digestion, initially secreted in inactive forms to prevent self-digestion within the pancreas.
- Amylase for carbohydrate digestion.
- Lipases for lipid digestion.
- Nucleases for nucleic acid digestion.
- Proteases are activated in the duodenum for their digestive activity.
- Enteropeptidase, an enzyme attached to the duodenal epithelial cells, activates trypsinogen to trypsin.
- Trypsin subsequently activates more trypsinogen, as well as procarboxypeptidase to carboxypeptidase, and chymotrypsinogen to chymotrypsin.
Bile and Pancreatic Secretion into the Small Intestine
- Bile duct and main pancreatic duct converge within the duodenal wall, forming the hepatopancreatic ampulla.
- The ampulla opens into the duodenum through the major duodenal papilla.
- The hepatopancreatic sphincter controls the flow of bile and pancreatic juice into the duodenum.
- An accessory pancreatic duct exists, smaller and positioned proximally, emptying directly into the duodenum.
The Small Intestine: Major Site of Digestion and Absorption
- Plays the primary role in digestion and absorption within the body.
- Digestion is completed with the help of pancreatic enzymes and bile.
- Nearly all nutrient absorption occurs in the small intestine.
Gross Anatomy of the Small Intestine
- Approximately 6-7 meters in length in cadavers, with a diameter of 2.5-4 cm.
- Divided into three major segments:
- Duodenum: approximately 25 cm in length, located close to the pancreas.
- Jejunum: approximately 2.5 meters long, extending from the duodenum.
- Ileum: approximately 3.6 meters long, connecting the jejunum to the large intestine.
- The jejunum and ileum are attached to the posterior abdominal wall by a mesentery.
- The terminal ileum joins the large intestine through the ileocecal valve.
Blood Supply and Innervation of the Small Intestine
- Receives oxygenated blood from the superior mesenteric artery.
- Nutrient-rich venous blood flows through veins, emptying into the superior mesenteric vein, which connects to the hepatic portal vein (leading to the liver).
- Innervated by both parasympathetic (vagus nerve) and sympathetic (thoracic splanchnic nerves) systems.
Microscopic Anatomy of the Small Intestine
- Multiple adaptations enhance surface area for nutrient absorption.
- Circular folds, villi, and microvilli contribute significantly to the overall surface area expansion.
- Circular folds: permanent mucosal and submucosal folds, forcing chyme to spiral through the lumen, slowing its movement and increasing absorption time.
- Villi: finger-like mucosal projections, each containing a lacteal (lymphatic capillary for lipid absorption) and a dense capillary bed.
- Microvilli: cytoplasmic extensions of enterocyte apical surfaces, forming the brush border.
- Intestinal wall histology reflects its digestive functions.
- Epithelium of villi and intestinal crypts contains five primary cell types:
- Enterocytes: absorptive cells, forming the majority of epithelium.
- Goblet cells: mucus-secreting cells.
- Enteroendocrine cells: secrete enterogastrones, including CCK and secretin.
- Paneth cells: located deeper in crypts, release antimicrobial agents.
- Stem cells: continuously divide and differentiate to produce other cell types.
- Mucosa-associated lymphoid tissue (MALT) provides immune defense.
- Includes individual lymphoid follicles and Peyer's patches (aggregated lymphoid nodules).
- Submucosa of the duodenum contains duodenal glands that secrete alkaline mucus to neutralize acidic chyme.
Digestive Processes in the Small Intestine
- Chyme entering the small intestine contains partially digested carbohydrates and proteins, along with mostly undigested fats.
- Digestion is aided by enzymes and bile from the liver and pancreas, and by brush border enzymes.
- Strict control of chyme entry into the duodenum due to its hypertonic nature and potential for water loss from the blood.
- Feedback mechanisms (enterogastric reflex and enterogastrones) regulate food movement into the duodenum.
Motility of the Small Intestine
- Composed of two main motility patterns:
- Segmentation: primary motility after a meal, promoting mixing of chyme.
- Migrating motor complex (MMC): peristaltic movements, dominant pattern between meals.
- After meal:
- Intrinsic pacemaker cells initiate segmentation.
- Intensity regulated by long and short reflexes, and hormones.
- Parasympathetic stimulation increases motility, while sympathetic stimulation decreases it.
- Between meals:
- True peristalsis, initiated by motilin, occurs every 90-120 minutes.
- Peristaltic waves move from duodenum to ileum, completing a cycle in approximately 2 hours.
Ileocecal Valve Control
- Ileocecal valve relaxes to allow chyme entry into the cecum, triggered by:
- Gastroileal reflex: enhances segmentation and relaxes the valve.
- Gastrin: stimulates ileal motility and relaxes the valve.
- The valve closes when chyme exerts backward pressure, preventing regurgitation into the ileum.
The Large Intestine: Water Absorption and Waste Elimination
- Extends from the ileocecal valve to the anus, shorter than the small intestine but larger in diameter.
- Primarily responsible for:
- Absorbing remaining water from indigestible food residues.
- Absorbing metabolites produced by resident bacteria.
- Storing waste temporarily before elimination as feces.
Gross Anatomy of the Large Intestine
- Consists of the cecum, appendix, colon, rectum, and anal canal.
- Cecum: initial sac-like segment of the large intestine.
- Appendix: contains lymphoid tissue (part of MALT).
- Colon: divided into four main regions:
- Ascending colon: travels upward on the right side of the abdominal cavity.
- Transverse colon: crosses the abdominal cavity.
- Descending colon: descends on the left side of the abdominal cavity.
- Sigmoid colon: S-shaped segment that connects to the rectum.
- Rectum: runs just in front of the sacrum, containing rectal valves to prevent gas expulsion with feces.
- Anal canal: final segment of the large intestine, opening to the exterior at the anus.
- Controlled by two sphincters: internal anal sphincter (smooth muscle) and external anal sphincter (skeletal muscle).
Relationship of the Large Intestine to the Peritoneum
- Most of the colon is retroperitoneal, except for the transverse and sigmoid colon.
- These regions are intraperitoneal and attached to the posterior abdominal wall by mesocolons.
- The rectum is also retroperitoneal.
Microscopic Anatomy of the Large Intestine
- Lined with simple columnar epithelium, except for the anal canal which has stratified squamous epithelium.
- Distinguishable from the small intestine by:
- Lack of circular folds, villi, and brush border.
- Thicker mucosa with deeper crypts and a higher concentration of goblet cells.
- Anal sinuses that release mucus when compressed by feces.
Digestive Processes in the Large Intestine
- Food residue remains in the large intestine for 12-24 hours.
- Main function is propulsion of feces towards the anus for defecation.
- Digestion is limited to bacterial action.
- Absorption includes salts, water, and vitamins produced by bacterial metabolism.
Motility of the Large Intestine
- Characterized by two main motility patterns:
- Haustral contractions: slow segmenting movements, primarily in the ascending and transverse colon.
- Mass movements: long, slow, powerful contractile waves that propel feces towards the rectum.
- Mass movements typically occur after meals due to the gastrocolic reflex.
Defecation
- Mass movements push feces into the rectum, triggering the defecation reflex.
- The reflex initiates contraction of the sigmoid colon and rectum.
- The internal anal sphincter relaxes.
- Voluntary control enables contraction and relaxation of the external anal sphincter.
- Rectal muscle contraction expels feces, aided by:
- Valsalva's maneuver: increasing intra-abdominal pressure through glottis closure, diaphragmatic contraction, and abdominal wall muscle contraction.
- Levator ani muscle contraction elevates the anal canal.
Digestion and Absorption
- Digestion breaks down food into smaller molecules for absorption.
- Enzymatic hydrolysis is the main mechanism of digestion.
- Enzymes, produced by the body, break down macromolecules into monomers.
- Absorption moves digested nutrients from the gut lumen into the body.
- Most substances pass through epithelial cells, entering through the apical membrane and exiting through the basolateral membrane.
- Nonpolar lipids are absorbed passively through simple diffusion.
- Other substances require membrane transport mechanisms, mainly active transport.
Carbohydrate Digestion and Absorption
- Carbohydrate digestion begins in the mouth with salivary amylase.
- Pancreatic amylase further breaks down carbohydrates in the small intestine.
- Brush border enzymes complete the breakdown of carbohydrates into monosaccharides.
- Monosaccharides are absorbed via active transport and facilitated diffusion.
Protein Digestion and Absorption
- Protein digestion begins in the stomach with pepsin.
- Pancreatic proteases further break down proteins in the small intestine.
- Brush border enzymes complete the breakdown of proteins into amino acids.
- Amino acids are absorbed via active transport.
Lipid Digestion and Absorption
- Emulsification breaks down large fat globules into smaller droplets, increasing surface area for digestion.
- Bile salts aid in emulsification.
- Pancreatic lipase breaks down triglycerides into monoglycerides and fatty acids.
- Micelles form, allowing lipids to cross the apical membrane via simple diffusion.
- Chylomicrons are formed and transported into lacteals (lymphatic vessels).
Nucleic Acid Digestion and Absorption
- Nucleic acids are broken down into nucleotides by pancreatic nucleases.
- Brush border enzymes further break down nucleotides into pentose sugars, phosphate, and nitrogenous bases.
- These components are absorbed via active transport.
Vitamin Absorption
- Fat-soluble vitamins (A, D, E, K) are absorbed with lipids via simple diffusion.
- Water-soluble vitamins (B and C) are absorbed via passive or active transport.
- Vitamin B12 requires intrinsic factor for absorption via endocytosis.
Electrolyte Absorption
- Most ions are actively transported along the small intestine.
- Sodium, chloride, and bicarbonate are absorbed using active and passive transport mechanisms.
- Potassium absorption is influenced by water absorption.
Water Absorption
- Water is absorbed by osmosis, driven by concentration gradients created by solute transport.
- Most water is absorbed in the small intestine, with the remaining absorbed in the large intestine.
Developmental Aspects of the Digestive System
- Primitive gut is formed from endoderm and mesoderm.
- Oral membrane forms the mouth.
- Cloacal membrane forms the anus.
- By week 5, the alimentary canal is a continuous tube.
- Accessory organs bud from the mucosa shortly after.
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Test your knowledge on the human digestive system with this quiz. Explore the functions of various organs involved in digestion, including the alimentary canal and accessory organs. Discover essential activities and the role of different components in the digestive process.
