Digestive System Overview

• The digestive system includes the digestive tract and associated glands.
• The digestive tract extends from the mouth to the anus, encompassing the pharynx, esophagus, stomach, small intestine, large intestine, and rectum.
• Glands like the salivary glands, liver, and pancreas secrete substances into the digestive tract aiding digestion.
• The roles of the digestive system includes ingesting, digesting, absorbing nutrients, water, and eliminating indigestible waste.
• The digestive system is open at both ends.
• A complex immune system acts as a barrier against pathogens and maintains internal balance.

Layers of the digestive tract wall

• The mucosa is the innermost layer, consisting of epithelial cells.
  • Its characteristics vary along the tract.
  • It comprises the epithelium, lamina propria, and a thin muscular layer (muscularis mucosae).
• The submucosa is a connective tissue layer containing blood and lymphatic vessels.
• The muscular layer facilitates movement with circular and longitudinal layers.
  • A nerve plexus, the myenteric plexus, lies between these layers.
• The serosa or adventitia is the outermost layer, which is part of the peritoneum in the abdominal region.
• The abdominal cavity is lined by the peritoneum, which covers the viscera and secretes lubricating peritoneal fluid.
  • The peritoneum has visceral and parietal layers.

The Digestive Tract

• The oral cavity is lined with stratified squamous epithelium.
  • It contains the teeth and tongue, which are crucial for chewing.
  • Lingual papillae are located on the dorsal surface of the tongue
• Salivary glands secrete saliva in the oral cavity
  • This saliva moistens food for swallowing
  • Major salivary glands are distinguished from minor glands dispersed in the oral mucosa.
• The pharynx serves as a common pathway for the respiratory and digestive systems.
  • It is divided into the nasopharynx, oropharynx (mesopharynx), and laryngopharynx (hypopharynx).
• The esophagus is a tube passing through the thorax and diaphragm, leading to the stomach in the abdominal cavity.
  • It is lined with non-keratinized stratified squamous epithelium.
• The stomach is a dilated section with folds and ridges, containing numerous glands.
  • It is marked by two sphincters: the cardiac sphincter (prevents gastroesophageal reflux) and the pyloric sphincter (prevents duodenogastric reflux).
• Mucus and bicarbonate-secreting glands are near both sphincters in the gastric area.

Stomach regions and their functions

• The fundus contains glands that secrete intrinsic factor, bicarbonate, hydrochloric acid, pepsinogen, and lipase.
• The body has abundant fundus-like glands and produces the largest volume of gastric juice.
• The antrum has fewer glands, primarily functioning in mechanical digestion.
• The small intestine is 6-7 meters long, extending from the pylorus to the ileocecal valve
  • It consists of the duodenum, jejunum, and ileum.
• The duodenum is the initial 30 cm, C-shaped segment around the pancreas' head.
  • It features the ampulla of Vater, where ducts from the liver and pancreas enter.
• The jejunum is the small intestine's middle segment.
  • The first 2/5 is considered jejunum, and the remaining 3/5 is ileum.
• The ileum is the last segment, ending at the ileocecal valve.
• Intestinal mucosa has a simple columnar epithelium from the cardia to the anus, with distinct features in the small intestine.
• Epithelial cells (enterocytes) possess microvilli, aiding in absorption and enzyme release.
• Villi are projections formed by the mucosa, covered with enterocytes and containing connective tissue with immune cells.
  • Crypts of Lieberkühn are deep areas between the villi.
• Villi and microvilli increase the surface area, facilitating nutrient absorption.
• Enterocytes form the main cells of the intestinal mucosa, along with other cells.
• Goblet cells secrete protective mucus on the surface.
• Paneth cells at the villi base defend against and control intestinal flora.
• Enteroendocrine cells release intestinal hormones.

Additional structures found in the intestines

• Brunner's glands in the duodenum secrete mucus that empties into Lieberkühn's crypts.
  • This action protects the intestine from stomach acid.
• Peyer's patches in the jejunum and ileum are lymphoid aggregates similar to tonsils.
• The large intestine lies between the ileocecal valve and anus, with a wider diameter than the small intestine
  • Three longitudinal bands (“taeniae coli”) cause it to form pouches (haustra).
• The cecum is a pouch containing the ileocecal valve
  • The appendix extends from here, marking the start of the colon.
• The colon has the ascending colon (right abdomen), transverse colon (upper abdomen), descending colon (left abdomen), and sigmoid colon (S-shaped segment).
• Hepatic and splenic flexures are also present.
• The rectum, located at the large intestine’s end, features internal and external sphincters for controlling waste elimination.
  • The rectal ampulla and anal canal are parts of the rectum, surrounded by hemorrhoidal plexuses.
• Digestive glands, also known as accessory glands, primarily function in digestion and are connected to the digestive system
  • Salivary glands, the liver, and the pancreas fit into this group.
• Minor salivary glands include the 600-1000 small glands that empty into the oral mucosa
• Major salivary glands are anatomically distinct with lobular structures, long, branching excretory ducts, and include three pairs of glands.
  • Parotid glands (preauricular areas)
  • Submaxillary glands (below the mandible)
  • Sublingual glands (mouth floor)
• The liver is the body's largest organ, primarily under the right hypochondrium, epigastrium, and part of the left hypochondrium.
  • It is divided into right and left lobes, each with its own arterial supply, venous drainage, and biliary drainage, separated by the falciform ligament.
• Hepatic arteries supply oxygenated blood, about 30% of the blood that the liver receives.
• The portal vein supplies nutrient-rich blood absorbed from the intestine, about 70% of liver blood flow, to be metabolized.
• At the microscopic level, the liver has hepatic lobules or acini formed by hepatocytes around the central vein.

The Gallbladder and Pancreas

• The gallbladder is located behind the liver to store bile for fatty digestion.
• Bile goes from the liver through the common hepatic duct and cystic duct, then flows via the common bile duct to the duodenum.
  • Bile empties into the duodenum at the ampulla of Vater, regulated by the sphincter of Oddi.
• The pancreas is situated in the upper and middle abdomen's epigastric and left hypochondriac areas.
  • It has a head, body, and tail.
  • The head is framed by the duodenum, and the body and tail extend toward the left hypochondrium, with the tail contacting the spleen.
• The Wirsung duct is a central duct that collects exocrine secretions from all pancreatic ducts and transports them to the duodenum.
• The pancreas functions as both an exocrine and endocrine gland.
  • The exocrine gland comprises most of the organ, while the endocrine part is located inside the islets of Langerhans.
• Exocrine secretion enters the duodenum and includes all critical nutrient-degrading enzymes.
• Endocrine secretion goes into the bloodstream and features hormones like insulin and glucagon, which are essential for glucose regulation.

Physiology of Digestion

• The digestive system's primary function is nutrition, though it supports protection and endocrine functions.
• Alimentation is the voluntary act of obtaining and ingesting food.
• Nutrition is the involuntary process of obtaining, transforming, and using chemical substances in food.
• Digestion is the involuntary process where the digestive system breaks down food into absorbable nutrients.
• The stages of the digestive process include the cephalic, oral, esophageal, gastric, intestinal, and colonic phases.
• The cephalic phase occurs before ingesting food and involves preparation for ingestion.
  • Thinking about food etc originates parasympathetic reactions, such as digestive secretions like saliva and gastric juices.
• The oral phase involves ingested food developing into a swallowable bolus.
  • During mastication mechanical processes are primarily handled by teeth.
  • The chemical processes (initial salvation) are handled by glands.
• Tongue aids in food breakdown and lubrication (resulting in an easily swallowable bolus).
• Food travels through the pharynx and into the esophagus in a process called "deglutition"
  • The epiglottis prevents the bolus from entering the larynx.
• Saliva is the first secretion in the digestive tract that lubricates the food bolus.
• Saliva allows for carb digestion using ptyalin, an amylase, and contains immunoglobulin A.
• Bolus travels through the esophagus propelled by peristaltic movements caused by the muscular layer in its walls.

Digestion Phases - Gastric and Beyond

• Most digestion occurs during the gastric phase.
• Bolus enters the stomach through the cardia which prevents gastroesophageal reflux.
• Inside the stomach, gastric juice acts on the bolus, mixing and agitating it via stomach contractions.
• The food bolus converts into a pasty substance called chyme facilitated by gastric juices.
• Gastric juice includes water, hydrochloric acid for an acidic pH of around 2 for enzyme action, digestive enzymes, intrinsic factor for vitamin B12 absorption, mucus &bicarbonate to protects against stomach pH, and hormones which mostly control pH.
• Hydrochloric acid provides an acidic pH for enzymes to do their work.
• Digestive enzyme examples include gastric lipase that breaks down lipids & pepsin that breaks down proteins.
• Hormone examples include Gastrin, histamine and somatostatin.
• Gastrin stimulates HCl secretion, histamine stimulates HCl liberation & somatostatin inhibits gastrin functioning as a negative controller and activates when the pH is at 2 to prevent overcorrection.
• The stomach mixes its contents and propels digestion due to gastric mobility which is the result of periodic contractions.
  • The cardiac sphincter controls eructation and prevents reflux.
  • The antrum is much stronger in mobility and propels substances for further digestion.
  • Duodenum activity is regulated by the pyloric sphincter which is a muscular ring that enables the regulated passage of substances.
• The exit of chyme occurs via antrum contractions to open the pylorus, allowing for small portions to be processed because the duodenum can only handle so much at once.
• Duodenum fullness prompts pancreas contraction and bile secretions to accelerate digestion.

Later Digestion and Liver Functions

• Intestinal phases complete digestion and absorb nutrients.
• Digestion gets completed as a result of pancreatic secretions and bile action that go into the duodenum via the Vater ampulla location.
• Intestinal mucosa is adapted to increases and specializes its surface via villi & microvilli.
• The liver has a role in digestion from the bile which is a yellow/green liquid that helps primarily digesting fats.
• Bile gets stored in the gallbladder then reaches the duodenum for digestion processes via the duct or common bile duct.
• Glucid, lipid and protein metabolization.
• Vitamin storage.
• Depuration/elimination such as bilirubin, biliverdin, cholesterol from waste.
• Bile is an alkaline and liquid substance product of the liver containing bile salts/lecithin that fat absorption.
• Bactericidal actions for elimination.
• Bicarbonate neutralizes chyme acidity that gets secreted when pH is high.
• Bilirubin & biliverdin are pigments resulting from hemoglobin waste.

Pancreatic Juice and Nutrient Absorption

• The pancreas produces juice using exocrine function to promote digestion by discharging into the duodenum.
• Water is its main component and enzymes include amylases, lipolytic-type enzymes along with protease-precursors made of trypsinogen products that break down proteins.
• Enzymes get stored as inactive forms which activates once at duodenum to prevents the pancreas from digesting itself.
• Pancreatic juice has bicarbonate for pH regulation to protects from acid.
• Intestinal absorption is function of enterocytes where carbohydrates are absorbed as monosaccharides.
• Carbohydrates is breakdown into smaller pieces using amylases for the most part.
• Proteins break down into amino acids to then get passed to the blood via transporters.
• Lipids converts into fatty acids which gets transported via thoracic duct.
• Water makes up roughly 65% of human body content by weight, essential for many functions like transport, and lubrication.
• The digestive system regulates this water content to maintain fluid levels which it aids with via absorption and elimination.
• Roughly 200ml of water gets eliminates via feces per day, remaining water gets absorbed during process.
• Volemia is parameter that depends on water, where a drop can cause shock.

Intestinal System Mobility

• Completion must occur for digestion while undigested residues need to continue circulating within digestive pathway by intestinal motility.
• Segmental movements assists mixing while peristaltic ones assist pushing residue in colon's direction.
• Colon activities includes three functions of intestinal activity: reabsorption of fluids while storing the feces while fermenting certain ingredients via bacteria.
• Gas levels increase at the same time as stool amount that reaches colon locations.

Colonic Phase Regulation

• Gastrogastric reflex where eating leads intestinal mobility to evacuates.
• Orthocolic reflex causes urge to be defecated upon waking.

Intestinal System Microflora

• Microflora consists of one hundred billion bacteria and has essential functions like nutritional, protective, immune assistance for infections for the system.
• Fiber, made almost mostly from cellulose, goes undigested but helps the work that certain enzymes make.

Digestive Problems

• Esophagitis = swelled esophagus with causes like infection, irritation or inflammation with risky behaviors that result in symptoms like vomiting, acid or difficulties in digesting.
• Hernia results in a slide into stomach due to hiatal openings.
• Barrett disease results from damage is made acids of gastric juices.
• Esophageal cancers have forms associated to alcohol and other substances which triggers Barrett type cases.
• Gastric juice acidity has a burning type sensation by showing it to the esophagus which may cause disorders such as discomfort on stomach or chest.

Dyspepsia and Gastric Issues

• Dyspepsia includes disorders that disturb digestion.
• Main causes includes peptic issues, bulimia or another type of related systematic disease.
• Some causes includes alcohol, related substance, stress or high smoking volume.
• Gastric inflammation is referred to as gastritis causes such as alcohol usage, NSAIDs.
• Peptic disorders can result from aggressive balance resulting is lack defensive qualities.
• Gastric problems are related from a certain level to Helicobacter where infections happens often to result in cancers.
• It is important to get treatments with medication and other aid is provided in order to help treat stomach.
• Pancreatitis can be an inflammation if it starts activating from with which may result from problems or disorders which have their conditions treated with fluid and antibiotics.

Cancer and Intestinal Problems

• Cancer can occur where tumors made by cells will trigger problems resulting in problems such as weight loss.
• Tumors have two types being the exocrine or another tumor can affect hormones.
• Tumors have there treatment of medications and with surgeries plus therapies to alleviate symptoms.
• Stones forms is gallbladder leading to the bile where symptoms includes throwing or belly pains.
• This stone removal can happen if surgery is performed in order to help prevent the problem which may get severe.
• Hepatitis occurs when there is irritation or inflamed which make digestive consequences which also leads to immune conditions.
• Chronic hepatitis can damage body without symptoms, needing treatments from sensitive medications to fix disorder.

Cirrhosis, Liver and Malabsorption Problems

• Tissue becomes scarred damaging progress functions which liver cannot heal.
• Tumors gets made causes from other areas as a result as well which require action to happen in order to help.
• Malnutrition happens but difficult in absorption of nutrients can be general specific causing issues in nutrient volume intake to also become a problem, thus needing to check to receive the treatment by checking problems that exist.
• Celia disorder occurs when the system cannot get used to wheat but treatments involve avoiding it due to reaction.
• Chromic causes inflammation creating autoimmune problems that disturb regular processing of nutrients that happens often that there is a disorder in the intestinal area.
• Colitis occurs with irritation where digestive problem and treatment happen so that problems won't to expand to have treatments, this infection results from contamination of bad product intake as well.
• Diverticulitis has inflamed formation resulting if bacteria forms so that problems do not worsen and have better treatments through specific medications

Infections, Tumors and Health Complications

• Infections leads to pain while tumors results formation.
• Appendicitis is infections that can gets triggered resulting with bowel movement to be blocked, needing medical assessment.
• Intestinal cancers start forming on the area to make an adenonomas to treat by conducting certain chemotherapy, medical or another process that helps
• Checking is import to conduct to reduce the likelihood that the patient will be affected in the future so that action, therapy can be administered.