Digestive System Overview

Overview of the Digestive System

• The digestive system breaks down food into absorbable nutrients which are then delivered to cells via the bloodstream.
• Organs of the digestive system are located from the head to the abdominopelvic cavity.
• The digestive system includes the alimentary canal organs and accessory organs.
• Alimentary canal organs, also known as the gastrointestinal (GI) or digestive tract, include the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory organs are not part of the alimentary canal, however, they assist in digestion.
• Accessory organs include teeth, tongue, salivary glands, liver, gallbladder and pancreas.

Basic Digestive Functions and Processes

• The digestive system's main function is to break down food into nutrients that can be used by body cells.
• The digestive system is critical for fluid, electrolyte, and acid-base homeostasis.
• Plays a role in homeostasis via the diet and delivering them to the blood.
• Other functions include ingesting vitamins and minerals, producing hormones, and excreting metabolic wastes.
• Basic processes carried out include ingestion, secretion, propulsion, peristalsis, mechanical and chemical digestion, absorption, and defecation.

Organization of the Abdominopelvic Digestive Organs

• Digestive organs in the abdominopelvic cavity share serous membranes, blood vessels, and nerves.
• The peritoneal membrane is the largest serous membrane in the body, consisting of parietal and visceral peritoneum.
• The peritoneal cavity, which contains serous fluid, is located between the two peritoneal layers.
• The visceral peritoneum forms structures called mesenteries around the small intestine, which supports and binds organs while maintaining the shape of the small intestine within the abdominopelvic cavity.
• The greater omentum and lesser omentum are two prominent mesenteries.
• The splanchnic circulation is the blood vessels that supply and drain the abdominal digestive organs.
• Nerves the digestive system supplied with nerves from the autonomic nervous system, these are divided into sympathetic and parasympathetic divisions.
• The nerve supply to digestive organs is located in the celiac plexus, superior mesenteric plexus, and inferior mesenteric plexus.

Histology of the Alimentary Canal

• Alimentary canal organs follow a general tissue pattern like other hollow organs.
• The mucosa is the innermost layer facing the lumen and consists of lamina propria, muscularis mucosae, and regenerative epithelial cells.
• The submucosa consists of dense irregular connective tissue with blood and lymphatic vessels and submucosal glands.
• The submucosal plexus (Meissner's plexus), part of the enteric nervous system, regulates secretion and blood flow.
• Muscularis externa is typically a thick layer of smooth muscle composed of inner circular and outer longitudinal layers.
• The myenteric plexus (Auerbach's plexus), part of the enteric nervous system, regulates muscularis externa motility.
• Serosa is the outer layer covering the organs within the peritoneal cavity, composed of simple squamous epithelium and loose connective tissue.
• Adventitia is the outer layer for organs outside the peritoneal cavity

Regulation of Motility by the Nervous and Endocrine Systems

• Motility or movement is a key process in the alimentary canal.
• Skeletal muscle controls motility in the oral cavity, pharynx, upper esophagus, and large intestine.
• Forms of motility include swallowing, churning, peristalsis, and defecation.
• The nervous system and/or endocrine system regulate motility.
• The autonomic nervous system (ANS) regulates motility, sympathetic activity generally inhibits digestive processes, and parasympathetic activity stimulates them.
• The enteric nervous system (ENS) also regulates motility.
• The endocrine system regulates digestive processes through hormones.

The Oral Cavity, Pharynx, and Esophagus

• The digestive system begins at the oral cavity (mouth)
• Four digestive processes occur: ingestion, secretion, chemical and mechanical digestion, and propulsion in the mouth.
• The oral cavity is posterior to the teeth and bounded by the palate and tongue, structural features include the cheeks, lips, gums, vestibule, oral cavity proper, and palate (hard and soft).
• Teeth are key organs for mastication.
• Teeth are in bony sockets called alveoli, within the mandible and maxilla, held in place by the periodontal ligament.
• Incisors, canines (cuspids), premolars, and molars are the three types of teeth classified by their shape.
• Primary dentition or deciduous teeth are "baby teeth", there are 20 deciduous teeth: 4 incisors, 2 canines, and 4 molars in both the mandible and maxilla.
• Lower central incisors are the first deciduous teeth to erupt, usually around 6 months, erupting at a rate of one pair of teeth per month until 24 months, where all 20 teeth are usually present.
• Secondary dentition, or permanent teeth, are situated above the primary dentition in the maxilla and below them in the mandible, consisting of 32 permanent teeth: 4 incisors, 2 canines, 4 premolars, and 6 molars in both the mandible and maxilla.
• By age 12, deciduous teeth fall out and the third set of molars erupt which are known as wisdom teeth, which erupt between ages 17 and 21.
• The crown is the visible portion of the tooth above the gum line, while the root is embedded within the alveolus.
• Other parts of tooth structure include enamel, pulp, cementum, dentin, pulp cavity, and root canal.
• The tongue consists of skeletal muscle and is covered with stratified squamous epithelium, attached to the floor of the oral cavity by a lingual frenulum.
• This prevents the tongue from moving too far posteriorly.
• Papillae are small projections covering the tongue epithelium.
• Filiform, fungiform, circumvallate, and foliate papillae; all contain taste buds except filiform papillae and these papillae detect chemicals associated with different taste sensations.
• Two groups of skeletal muscles control tongue movement: extrinsic and intrinsic.
• Saliva is secreted through ducts into the oral cavity by three pairs of salivary glands, containing water, enzymes, mucus, and other solutes; saliva moistens, lubricates, and cleanses the oral mucosa.
• Salivation is the process of secretion from the three pairs of salivary glands, controlled by the parasympathetic nervous system in a reflex arc.
• The pharynx (throat) is posterior to the nasal and oral cavities, and consists of three divisions: the nasopharynx, oropharynx, and laryngopharynx.
• Only the oropharynx and laryngopharynx are part of the alimentary canal.
• Both the oropharynx and laryngopharynx are lined with stratified squamous epithelium to protect against abrasion by food.
• The primary function of the pharynx is propulsion in the form of swallowing.
• The esophagus is a 25 cm long muscular tube posterior to the trachea, transporting a bolus from the pharynx to the stomach.
• The esophageal mucosa is lined with stratified squamous nonkeratinized epithelium and contains esophageal glands that secrete mucus.
• The gastroesophageal sphincter regulates the passage of the bolus into the stomach which prevents the contents of the stomach from re-entering the esophagus.
• Propulsion and a small amount of mucus secretion are the primary function of the esophagus.
• Swallowing or deglutition pushes a bolus of food from the oral cavity through the pharynx and esophagus to the stomach.
• The swallowing process consists of three phases: voluntary, pharyngeal, and esophageal which is almost entirely neural.
• The cerebral cortex controls the voluntary phase.
• The medulla and enteric nervous system control the remaining two phases.
• In the voluntary phase, the tongue pushes the bolus posteriorly toward the oropharynx.
• During the pharyngeal phase, the bolus enters the oropharynx, the soft palate, and epiglottis seal off the nasopharynx and larynx.
• Peristaltic waves move the bolus down the esophagus to the stomach during the esophageal phase.

The Stomach

• The esophagus pierces the diaphragm through the esophageal hiatus, emptying into the J-shaped stomach; the stomach sits primarily in the left upper quadrant just inferior to the diaphragm.
• The stomach has five anatomical regions: cardia, fundus, body, pyloric antrum, and pylorus.
• The pylorus contains the pyloric sphincter, it controls the flow of ingested food between the stomach and the small intestine.
• Rugae are folds on the interior of the stomach that allow it to expand considerably.
• The stomach has the same four tissue layers as the rest of the alimentary canal: mucosa, submucosa, muscularis externa, and serosa that are modified to suit the stomach's functions.
• The stomach has an additional inner layer of smooth muscle in the stomach's body with fibers oriented obliquely.
• This allows the stomach to perform churning, which pummels food into a liquid called chyme.
• Gastric glands form from the heavily indented mucosa of the stomach.
• Gastric glands at the base of the gastric pits are multiple branched glands containing endocrine cells and exocrine cells.
• Endocrine cells secrete hormones into the bloodstream and exocrine cells secrete an acidic, enzyme-containing fluid (gastric juice) into the lumen of the stomach.
• Enteroendocrine cells are near the bottom to secrete hormones that influence digestion.
• G cells are a type of enteroendocrine cell that secretes the hormone gastrin.
• Gastrin stimulates secretions from other cells known as parietal cells.
• Chief cells secrete the inactive precursor enzyme pepsinogen.
• When pepsinogen encounters an acidic pH, it becomes the active enzyme pepsin, which begins protein digestion in the stomach.
• Parietal cells secrete hydrochloric acid (HCl), responsible for the acidic pH of gastric juice which activates pepsinogen, destroys many disease-causing organisms, and stimulates parietal cell production of the chemical intrinsic factor which is required for intestinal absorption of vitamin B12.
• Mucous neck cells are near the top to secrete acidic mucus that prevents the neutralization of the acid produced by the parietal cells.
• The stomach's three primary functions are secretion, propulsion, and digestion.
• Gastric glands secrete multiple exocrine products including hydrochloric acid (HCl) released by parietal cells.
• During eating, gastric acid secretion changes from the basal rate under the influence of the parasympathetic nervous system and multiple hormones.
• Secretion is divided into the cephalic phase, gastric phase, and intestinal phase.
• The stomach's motility allows it to receive food from the esophagus, churn the incoming bolus into chyme, and control the rate at which chyme empties into the small intestine.

The Small Intestine

• The small intestine (or small bowel) is the longest part of the alimentary canal at 6 meters (almost 20 feet).
• Four main processes occur: secretion, digestion, absorption, and propulsion; cells known as enterocytes produce multiple digestive enzymes, hormones, and mucus.
• The small intestine consists of the duodenum which is the initial segment of the small intestine and it begins at the pylorus; the jejunum, and the ileum.
• The duodenum is the shortest of the three divisions (about 25 cm or 10 in. long).
• The duodenum houses the major duodenal papilla, where secretions from the gallbladder and pancreas enter the small intestine.
• Duodenal (Brunner's) glands are in the duodenal submucosa producing alkaline mucus to protect the duodenum from acidic chyme.
• The jejunum begins at the duodenojejunal flexure and sits within the peritoneal cavity.
• The jejunum measures about 2.5 meters (7.5 feet) in length and is the most active site for chemical digestion and absorption.
• The ileum is the small intestine's final segment, measuring 3.6 meters (10.8 feet) in length, terminating at the cecum (portion of the large intestine).
• The ileocecal valve is a sphincter which controls the movement of materials from the ileum into the cecum, preventing materials in the large intestine from flowing backward into the ileum.
• The internal surface of the small intestine has three progressively smaller types of folds, most absorption in the alimentary canal occurs in these area's
• The surface are is made available for absorption at 400 to 600 times because of the small intestines folds.
• Visible ridges on the wall is the largest type of folds known as circular folds or plicae circulars.
• Increased the surface area is circular folds which involve both the mucosa and submucosa of the small intestine.
• Slower transit of chyme through the small intestine allows more time to more time for digestion of nutrients, the cells, called enterocytes, have more time to absorb nutrients.
• Villi are projections into which the mucosa folds, each villus has a layer of enterocytes surrounding a central core of blood capillaries and a lymphatic vessel.
• The lymphatic vessel in each villus is called a lacteal.
• Between villi the mucosa forms intestinal crypts, these house glands with both enteroendocrine cells and mucus-secreting goblet cells.
• Microvilli are the smallest folds, found in the plasma membrane of the enterocytes.
• Brush border is the appearance that 3000 microvilli gives each enterocyte.
• The enzymes produced and secreted by enterocytes catalyze reactions that break down disaccharides, and peptidases catalyze reactions that break down peptides.
• Small intestinal motility differs during fasting and eating.
• The small intestine has slow, rhythmic contractions during fasting along its length in a pattern called the migrating motor complex which clears material.
• Contractions push digesting food from the duodenum to the ileocecal valve and takes about 2 hours.
• The small intestine undergoes peristalsis and segmentation during eating and the vagus nerve regulates both.

The Large Intestine

• The large intestine surrounds the small intestine and other abdominal organs like a frame.
• The large intestine receives undigested and unabsorbed material from the small intestine, and it is a passageway for feces to exit the body.
• The large intestine absorbs water and electrolytes, critical for maintaining fluid, electrolyte, and acid-base homeostasis.
• The tasks also include secretion (primarily mucus), propulsion, and defecation.
• The different regions are: cecum with vermiform appendix, colon composed of (ascending, transverse, descending, and sigmoid), and rectum and anal canal completes the regions in the large intestine.
• The mucosa lacks villi and microvilli.
• The muscularis externa has a unique discontinuous longitudinal layer gathered into three bands called taeniae coli.
• Haustra are pockets formed when taeniae coli are under constant tension.
• Epiploic appendages are fat-filled pouches located on the serosa or visceral peritoneum.
• The number of the bacteria are hosted staggering in the large intestine, bacteria make up as much as 60% of the dry mass of feces.
• Proximal and distal are the two functional segments in the large intestine.
• The proximal large intestine consists of the ascending and transverse colon while the distal includes the descending and sigmoid colon, rectum, and anal canal.
• Water and electrolyte absorption and also bacterial activity are the main primary sites in the proximal large intestine
• The two types of motility exhibited are segmentation/churning and mass movement/mass peristalsis.
• The distal large intestine's main role is the storage of fecal material
• The distal performs a small amount of water absorption.
• The mass movement of fecal material into the rectum initiates the parasympathetic-mediated defecation reflex.
• Stretch receptors transmit rectal distention to the spinal cord.
• Parasympathetic neurons contract the sigmoid colon and rectum and relax the internal anal sphincter.
• Cerebral cortex impulses relax the external anal sphincter and contract the levator ani muscle.
• The motility of the large intestine influences the amount of water in feces.

Pancreas, Liver, and Gallbladder

• The pancreas, liver, gallbladder are accessory organs and exocrine glands secreting products into the alimentary canal.
• The pancreas has both endocrine and exocrine functions.
• The pancreas, which is mainly in the left upper quadrant, has three regions: head, body, and tail.
• The main pancreatic duct runs through the middle of the pancreas, receiving secretions from acinar cells.
• Pancreatic juice is a mix of secretions from pancreatic acinar and duct cells.
• Pancreatic juice contains water, digestive enzymes, other proteins, and bicarbonate ions (a base) which neutralize acidic chyme.
• The pancreatic secretion is a basal rate occurring between meals.
• Pancreatic secretion is increased due to parasympathetic and hormonal stimulation during eating.
• Cholecystokinin (CCK) is a hormonal mediator.
• Duodenal enteroendocrine cells produce and released CCK in response to lipids and partially digested proteins and it stimulates acinar cells to secrete digestive enzymes and proteins.
• Secretin is stimulated by duodenal cells that are released in response with acids and lipids.
• Bicarbonate ions duct cells are trigger to secrete by secretin.
• The liver is located and pyramid-shaped in the right upper quadrant just inferior to the diaphragm with the gallbladder being a small sac on the posterior side.
• The liver and gallbladder have close anatomical and functional relationships.
• The liver is covered by a thin tissue capsule and visceral peritoneum, it is composed of four lobes: right, left, caudate, and quadrate.
• The liver lobule is the basic unit which are separated by septa branching from the connective tissue capsule.
• Hepatocytes are liver cells arranged in flattened plates in a hexagonal shape, stacked with a small central vein.
• At each six corners of the lobule, the portal triad in each corner include a hepatic arteriole, portal venule, and small bile duct.
• Hepatic sinusoids are large, leaky capillaries that pass blood flows slowly between rows of hepatocytes materials are exchanged between the blood. Bile flows in the opposite direction, from hepatocytes into bile canaliculi, and then into a bile duct. The functions of the liver include a diverse set of functions in the body in which it releases endocrine and exocrine secretions, and it converts harmful chemicals into nontoxic substances.
• The liver's main digestive function is to produce bile (containing water, electrolytes, and organic compounds).
• Bile serves one critical function: It is required for the digestion and absorption of lipids.
• Emulsification is the mechanical digestion necessary for the chemical digestion and absorption of lipids.
• Bile also functions as the mechanism by which the liver excretes wastes and other substances that the kidneys cannot excrete.
• Other functions include nutrient metabolism, detoxification, and excretion.
• The gallbladder receives most bile from the common hepatic duct.
• Bile release is stimulated by CCK, causing the gallbladder to release bile into the cystic duct.
• The cystic duct then joins the common hepatic duct to form the common bile duct, which joins the main pancreatic duct near the duodenum to form the hepatopancreatic ampulla.
• The hepatopancreatic sphincter surrounds the ampulla, it controls the emptying of bile and pancreatic fluids into the duodenum.
• The contents of the hepatopancreatic ampulla then empty into the duodenum at the major duodenal papilla.
• Secretion occurs in response to gallbladder contraction and sphincter relaxation at the hepatopancreatic ampulla.
• CCK and the vagus nerve accomplish this: the most potent stimulus for bile production and release is bile itself, specifically bile salts.

Nutrient Digestion and Absorption

• Digestion breaks food down into nutrient molecules.
• Mechanical digestion physically breaks down food, while chemical digestion breaks bonds between food molecules.
• Digestive enzymes catalyze hydrolysis reactions that use water to break bonds, speeding up reactions via enzymatic hydrolysis.
• Absorbed molecules must enter the body.
• Digestion begins with salivary amylase in the mouth, breaking long polysaccharides into shorter oligosaccharides.
• In the small intestine, chemical digestion continues when polysaccharides and oligosaccharides encounter pancreatic amylase.
• Enzymes (lactase, maltase, sucrase) complete oligosaccharide digestion, and enterocytes on the brush border cause reaction.
• Carbohydrates, that have been digested into monosaccharides such as glucose, galactose, and fructose are ready to be absorbed and delivered to the liver via the hepatic portal vein for processing.
• Glucose and galactose transport across the enterocyte's apical membrane via the Na+/glucose cotransporter (secondary active transport).
• Fructose crosses the apical enterocyte membrane by binding a channel that mediates its facilitated diffusion.
• Chemical digestion doesn't begin until the protein reaches the stomach.
• Pepsin encounters here chemical digestion begins.
• Chief cells produce the inactive precursor pepsinogen that becomes pepsin at pH2 and inactivated at pH7.
• Proteins in the stomach digest smaller polypeptides, oligopeptides, and some free amino acids with digestion by the use of activated pepsin-catalyzed reactions.
• Most protein digestion happens in the small intestine with pancreatic enzymes and brush border enzymes, which are released as inactive precursors.
• Trypsinogen is converted to trypsin on the intestinal brush border which then converts other pancreatic enzymes in addition to activating more trypsinogen.
• Trypsin catalyzes reactions that digest proteins and polypeptides into oligopeptides and free amino acids.
• Multiple brush border enzymes catalyze the digestion of oligopeptides into free amino acids.
• Proteins must be broken down into small oligopeptides and free amino acids.
• A free sodium ion gradient is established by secondary transport, this is used to move oligopeptides and free amino acids cross the enterocyte apical membrane which are then used by Na+/K+ pump; oligopeptides are broken down to free amino acids inside the enterocyte.
• Free amino acids eventually move with facilitated diffusion into the capillaries of villus following the completion facilitated diffusion.
• Like carbohydrates the amino acids are delivered to the liver for processing,

Digestion and Absorption of Lipids

• Triglycerides constitutes the majority (90%) of dietary lipids, with consists of three fatty acid molecules bound to a glycerol core molecule.
• Nonpolar lipids stick together forming huge globules.
• Mechanical breaks down food particles to be more tiny in the mouth, the intestine, and stomach .
• An emulsion is formed as polar parts interact with surrounding watery fluid is made by nonpolar parts interacting with the lipids, which mixes the lips with bile salts breaks them down into tiny pieces, which also gives digestive enzymes more surface for it to be able to digest the lipids better.
• The stomach churns and breaks apart the lipids the same process happens with gastric lipase.
• Emulsification from fat happens with bile sales in the intestine.
• Pancreatic lipase digests lipids breaks into into free fatty acids after an reaction.
• Micelles are formed by bile sales .
• Several face water based berries that stop them from going threw the mucous in the intestines due to them being nonpolar.
  • Micelles escort lipids to the enterocyte plasma membrane.
  • Lipids are released by an phospholipid and passes threw and cell body.
  • The lipids assemble, by reassembling, in order that fat is transported in what is know as chylomicrons.
  • Finally fats, also know as chylomicrons, is released a liquid that allows them to move easier. Then they enter what is called a lacteal.
• Lipids are not delivered directly to the liver via the hepatic portal vein after absorption; the hepatic portal vein delivers the leftover bile salts to the liver, where they are used to make new bile.

Nucleic Acids

• Nucleic acids in food undergo chemical digestion in the small intestine with pancreatic enzymes (nucleases)
• The hepatic portal system delivers these substances to the liver for metabolic processing.

Absorption of Water, Electrolytes, and Vitamins

• Alimentary canal absorbs large amounts of water, electrolytes, and vitamins.
• Absorption of Water: Over 9 liters enter the small intestine each day and about 8 liters are absorbed into the intestines. In fact most of the water is absorbed by the small intestines leaving a tiny amount for excretion in the feces.
• Water absorption occurs exclusively by osmosis, which requires a concentration gradient.
• Absorption of Electrolytes: Electrolytes are taken in from the diet and present in secretions from digestive organs.
• Absorption of Vitamins: Vitamins are chemicals generally provided by the diet that are involved in a host of metabolic reactions.
• Water-soluble vitamins are polar molecules.
• Fat-soluble vitamins (A, D, E, and K) are lipid-based and mostly nonpolar.