Digestive system: Organs and GI Tract Layers

Digestive System Overview

• The digestive tract is approximately 16 feet long and includes various organs and accessory organs
• Main structures include the oral cavity, esophagus, stomach, small and large intestines
• Accessory organs such as the pancreas, liver, and gallbladder aid the digestive and absorptive processes by providing or storing secretions

Layers of the Gastrointestinal Tract

• The gastrointestinal tract has four main layers: the mucosa, submucosa, muscularis externa, and serosa

The Mucosa

• It is the innermost layer and consists of three sublayers: the mucosal membrane, the lamina propria, and the muscularis mucosa
• Epithelial cells line the lumen of the gastrointestinal tract, serving as membrane of the mucosa
• Exocrine cells secrete enzymes and juices into the lumen
• Endocrine cells secrete hormones into the blood
• The lamina propria consists of connective and lymphoid tissue and contains immune cells like macrophages and lymphocytes
• The muscularis mucosa is composed of smooth muscle

The Submucosa

• Connective tissue, blood and lymphatic vessels, lymphoid tissue, and the submucosal plexus are what comprises this layer
• The submucosal plexus (plexus of Meissner) controls gastrointestinal secretions and local blood flow
• Lymphoid tissue protects against ingested foreign substances and it connects the mucosal layer to the muscularis externa

Muscularis Externa

• This layer contains inner circular and outer longitudinal smooth muscles that facilitate motility
• The myenteric plexus (plexus of Auerbach) is located here and controls the frequency and strength of muscular contractions

The Serosa

• The outermost layer consists of mesothelial cells that produce lubricating fluids
• It is continuous with the peritoneum, a two-layered membrane within the abdominal cavity for many parts of the digestive system

Immune System Protection

• The entire gastrointestinal tract contains Gut-associated Lymphoid Tissue, which is especially in the mucosa and submucosa layers of the small intestine
• Leukocytes, T- and B-lymphocytes, plasma cells, natural killer cells, macrophages, microfold cells, and dendritic cells carry out immunoprotection within these layers
• Peyer's patches, aggregates of lymphoid tissue, are often found in the mucosa and submucosa

Secretory Functions

• Plasma cells produce secretory IgA to bind antigens and inhibit bacterial growth and translocation
• Tissue macrophages secrete cytokines for immunoprotective effects
• M-cells transport foreign antigens to Peyer's patches or lymphocytes to initiate an immune response
• Dendritic cells destroy foreign substances and stimulate lymphocyte activity as antigen-presenting cells

The Oral Cavity's Role

• The digestive process that involves both the oral cavity and the pharynx, begins with chewing of food and mixing with saliva, which is secreted by three pairs of salivary glands which constitutes about 1-2 L/day
• Saliva mostly is made of water (99.5%) and contains proteins, enzymes, mucus, antiviral/antibacterial proteins, electrolytes, urea, phosphates, and bicarbonate, which dissolves foods

Digestive Enzymes In Saliva

• Salivary α-amylase (ptyalin) hydrolyzes internal α (1-4) bonds within starch
• Lingual lipase, secreted by lingual serous glands, hydrolyzes dietary triacylglycerols, especially in infants

Additional Components and Functions of Saliva

• Mucus lubricates food and protects the oral mucosa
• Antibacterial and antiviral proteins such as antibody IgA and lysozyme protect against bacteria
• R-protein enhances vitamin absorption in the stomach
• Bicarbonate neutralizes acids, with saliva having a pH of about 7

Xerostomia and Saliva Production

• Saliva is released at a rate of 0.3–0.5 mL/minute during rest and increases to 2 mL/minute with eating
• Insufficient saliva leads to xerostomia and increases the risk of dental caries and gum disease

Swallowing Process

• Food mixed with saliva forms a bolus and enters the esophagus through the pharynx
• Swallowing involves voluntary, pharyngeal, and esophageal stages and is regulated by the medulla in the brain
• The esophageal sphincter relaxes to allow food to enter the esophagus, while the larynx moves upward to close the glottis

Peristalsis

• Striated and smooth muscles in the esophagus stretch and are stimulated by the nervous system to move through the esophagus into the stomach
• Peristalsis is a wavelike motion usually carried out in less than 10 inces
• This process is in the entire digestive tract, from esophagus to colon

Gastroesophageal Sphincter

• This is also called the lower esophageal sphincter, and can be found at the distal end of the esophagus and just above the juncture between the esophagus and the stomach
• It relaxes to allow food to pass into the stomach and the decrease in pressure within allows for the food to pass

Gastroesophageal Reflux

• This burning sensation comes from stomach acid passing up the esophagus and may last for hours and is known as heartburn or pyrosis
• Repeated gastroesophageal reflux experiences result in GERD (gastroesophageal reflux disease), also known as acid reflux disease
• Meds and dietary changes are the main ways to heal this such as avoiding high fat foods, alcohol, and caffeine

The Stomach

• Bolus enters the stomach through gastroesophageal sphincter which is a J-shaped organ located on the left side of the upper abdomen
• Extends from gastroesophageal sphincter to duodenum, the upper section of the small intestine
• Four main regions: the cardia, fundus, body, and antrum

Gastric Regions

• The cardia receives food from the esophagus
• The fundus lies adjacent to and above the cardia
• The body serves as the main reservoir for swallowed food and produces gastric juice
• The antrum grinds and mixes food with gastric juices through strong peristalsis to form chyme

Gastric Motility

• Circular, longitudinal, and oblique smooth muscles mix food with gastric juices, acid and enzymes
• The stomach expands from an empty volume of 50 mL to a filled volume of 1-1.5 L
• Rugae (folds) in the stomach disappear as it fills
• Receptive relaxation allows gastric expansion with minimal pressure impact

Gastric Glands

• Cardiac glands, found near the esophagus
• Oxyntic glands, found in the fundus and body
• Pyloric glands, located in the antrum

Cells within Gastric Glands

• Neck (mucous) cells secrete mucus
• Parietal (oxyntic) cells secrete hydrochloric acid and intrinsic factor
• Chief (peptic) cells secrete pepsinogen and gastric lipase
• Enteroendocrine cells secrete hormones
• Cardiac glands lack parietal cells, and pyloric glands lack chief cells

Gastric Juice Composition

• Gastric juice contains water, electrolytes, hydrochloric acid, enzymes, mucus, and intrinsic factor
• The juice has about 2 L of secretion daily.

Hydrochloric Acid

• Gastric juice has an abundance of hydrochloric acid, which is responsible for low pH of about 2

Functions of Hydrochloric Acid

• Converts pepsinogen to pepsin for activating protein digestion
• Denatures proteins to expose interior peptide bonds
• Releases nutrients from organic complexes
• Acts as a bactericidal agent

Gastric Enzymes

• Pepsin functions as a protease, hydrolyzing interior peptide bonds in proteins and it is most active at pH 3.5
• Gastric lipase hydrolyzes fatty acids from triacylglycerols
• Salivary α-amylase originating from saliva retains some activity in the stomach

Mucus in Gastric Juice

• Mucus is secreted by neck cells and mucosal epithelial cells and is composed of mucins which consist of a network of glycoproteins
• The gastric mucus layer coats and protects the gastric mucosal membrane, creating a local pH of about 6–7
• Prostaglandins, vagal nerve stimulation, acetylcholine, and hormones enhance mucus production

Regulation of Gastric Secretions

• Gastric secretions are divided into three categories based on when they occur to food which are the cephalic, gastric, and intestinal phases with both neural and chemical functions
• Eating, tasting, thinking, seeing, or smelling food trigger secretions in the cephalic phases
• Vagal stimulation promotes secretion of acetylcholine and gastrin, which stimulate histamine release
• Acetylcholine and gastrin trigger hydrochloric acid secretion by parietal cells and promote enzyme release by chief cells
• The gastric phase initiates when food reaches the stomach and is enhanced of distension of the stomach

Intestinal and Secretions Phase

• Distension stimulates chemoreceptors that initiate submucosal plexus nerve activity
• Protein and several substances initiate enhance gastric secretions
• Somatostatin is released by D cells to diminish parietal cell, G cell, and enterochromaffin-like cell sections with reduction in chyme volume and lower pH of gastric juice
• Receptors in the duodenal bulb sense distension, osmolarity, nutrient content, and acidity of chyme to control gastric emptying
• Distension, hypertonic chyme, and dietary fat intake reduce gastric emptying
• Hormones like cholecystokinin, secretin, and gastric inhibitory reduce gastric emptying

Gastric Motility

• It is strongest in the lower body and antral sections and involves with peristaltic waves for digestive contents with retropulsion in the antrum Pacemaker cells near the myenteric plexus initiate a basic electrical rhythm

Factors Affecting Gastric Emptying

• Volume and fluidity of chyme affect peristaltic contractions
• Gastric distension, gastrin release, and fluidity increase motility
• Duodenal bulb receptors affect sensations within

Intestine and Delayed Emptying

• Distension by hypertonic chyme and acidic chyme affect gastric emptying
• Neural reflexes result in the ileogastric reflex elicited by distension in the ileum to diminish gastric movements
• Emotions impact digestive system smooth muscles through autonomic controls
• Physical disintegration of solid foods into liquid results in most nutrients not being absorbed within the body

Stomach Absorbency

• Only alcohol and small quantities of water and some minerals are absored by the stomach
• 1-5 mL of chyme enters the duodenum twice per minute and the Gastric emptying takes between 1 and 4 hours
• Gastric residuals should be monitored with tube feeding patients.

Peptic Ulcer Disease

• Ulcers that are commonly found in the mucosa and submucosa layers in the stomach, duodenum, and lower esophagus which can form from gastric disease from many factors
• Zollinger-Ellison is a extremely copious secretion of gastrin in the blood H. pylori bacterial can also cause them leading to alcohol usage
• Similar treatment measures are for gastroesophageal reflux disease and dietary restriction modifications

Drug Measures

• Histamine receptor blockers help treat PUD and inhibit hydrogen production in the parietal region
• Protein pump inhibitors on the other hand work best to help reduce stomach acid secretion
• Stomach removal may be needed if non of the treatment options work leading complications such as dumping syndrome

Dumping Syndrome

• Chyme which is hyperosmolar will release "too rapidly" with gastric dumping from about 30 mins to 3 hours which causes the pulling of fluid into the lumen
• This leads to a series of conditions and symptoms such as tachycardia and diarrhea and hypoglycemia

Small Intestine

• Chime enters through pyloric sphincter which begins the small intestine and the main sit for absorption and digestion
• Divided into Duodenum (1ft), Jejunum (8ft), and Ileum (11/2ft)
• The Treitz is important for location ligament on the site of the duodenum and the jejunum for easier identification from an endoscopy

Aspects of Nutrient Absorbtion

• Enchanced with its structure of layers, the small intestine absorbs the nutrients by having its lining as a sidewall across 3 football fields
• Circular large folds from the Mucosa can be identified via Kerkring Finger like projections, called Villi, contain millions of intestinal cells calle denterocytes for absortion

Membranes

• Microvilli extensions or plasma, increases surface area and are covered in Gylcocalyx
• Enterocyte's is a barrier that serves to protect membranes, brush border and the A pical which can have absorption from the tight junctions
• These enxymes will hydolize partially digested nutrients found in the Glycocalyx, while protein is further used in the cytosol

Tissue and Cells

• Stem cells can be found in Pockets Lieberkuhn within the instestine
• Paneth Cells, antimicrobial secretion
• Goblet Cells - Antifungal and mucus
• Brunners glands, duodenum protection cells
• pancreatic serctions will alkalize and bring enzymes

Intestinal Mobility

• Chyme is propelled through constractions, that respond from nervous and hormones such as IntestinalPolypeptide and the intestinal mobility peptide that regulates the contractions that digest meals from meal to meal during digestives
• Contrations prevent bacterial growth with three hours of trans time

Endocrine Pancreas

• the pancreas, liver, and gallbladder support the intestine absorb and digestive, these are around 2m islet
• Cells that make up regions are Langerhans, mainly B cells for Insulin, glucagons d cells for all, while exoxcince produces pacretic juices from ducts that attach to the small ducts
• The juices produce alkali and and will released though Wirsung Duct

Enzymes

• This is needed to assist proper digestion in the small intestine with juice volume of two liters
• contains sodium, chloride and calcium to assist in activity with enzymes digest around 50% carbs, protein an 90% fat
• the pancreas protects from tripsin with zymogens and a special protein inhibitor to digest properly

Pnacetitis

Activated zymogens digest and inflam the panceat tissue

Regulation

Secretin that releases juice neutrilzing and Cholecystoknin secreting enzymes

The Liver

• the largester organ of the body contains Lobe Cells called hepatocyets that radiaite out like plates toward central veins The plates transport blood to cells. Sinusoids phagocyte kupffer cells and produce bile

• The Liver produces the the yellowish water to help transport and help produce triglycerides

• bile acids from oxidizing reactions can be treated

• the liver helps water and ions. to produce micelles for digestion

Gallbladder

• Located where the gallbladder concentrate to about 40-50l to assist the liver with digestion fat laden help the response to empty bile to assist with fatty tissues Gallstones and chooecithis can inflam the gallbladder.

Circulation and Recriculation of bile

Bile salts secreted in intestine that are more than 90% that that reabsorbed in the lleum from albumin. where amino acids and synthesized with duedenem secretion with about two mills and is the conversion deoxycholic which is recycled

Hypo- Cholesterolmia and Bile

• Cholysteramine bind a treatment to promote high drug excretion of cholestorl
• Plany Sterlons also reduce choleristoal
• The secretion from Small Intestine assist in the absoprtion process and with this food is ready to move to the cells and digest in the intestinal Most digestion occurs in the proximal from small intestines or duodenum to break downt

Nutrients Absorption

• Absorption process is through diffusion, facilitated diffusion, transport and endocytosis
• Proteins help regulate the absorbtion of nutrients through the tract

Intestional Content Unbsorbtion

• Through it helps balance a bacterial the the area and maintain water levels and is then transported through three Muscular Bande
• Intetional contraction occurs at the point it takes to take nutrients and absorptions. the mucus is helped by sodim adn chloride
• Anitaric helps reatin hormone vasopression secreted byt eh petuitary from sodiun and glucotics increase it

Microbiota Gut

• Tracts such as the intestine bacteria contain over 1000 species help regulate different functions and contains anaerobic bacteria to help with resistiance _ Bacteria gains different nutrition like carbs from enzyme to assist in functions short chanin fatty produce an important function to the host bacteria.
• The gut is enhanced with immune systems to reduce pathogen

Cholonic Bacteria

• Bateria metabolizes the energy produced , while other bacteria is used ot metabolize the carbon of the colon and lactose

Intestial Cordination and protiobitcs

• Gut microbiota Imbalance has linked to inflammatory corditin that has encouraged use fo Lacto bacteria and the consumtion helped to maintain good gut health and balance bacteria
• Sympathetic decreases functions and the parasympathetic the gut bacteria help mainitain hormone activity

Explore the digestive system's 16-foot tract, including the oral cavity, esophagus, stomach, and intestines. Accessory organs like the pancreas, liver, and gallbladder support digestion. The GI tract features four layers: mucosa, submucosa, muscularis externa, and serosa. The mucosa layer contains sublayers such as the mucosal membrane, the lamina propria, and the muscularis mucosa.