Digestion & Swallowing: Key Processes and Implications

Gastrointestinal Tract Functions

• The gastrointestinal tract (GIT) transfers digested nutrients, minerals, and water from the external environment to the internal environment, involving digestion and absorption.
• Digestion forms absorbable molecules from food through GIT motility, pH changes, and enzymes, which are mainly produced by the pancreas by breaking down food.
• Absorption moves digestive food from the intestine into the blood or the lymphatic system.
• Excretion removes non-absorbable food components, bacteria, intestinal cells, hydrophobic molecules (drugs), cholesterol, and steroids.
• The GIT acts as a host defense by forming a barrier with the outside environment and a developed immune system.
• The GIT can inactivate harmful bacteria and microorganisms in the large intestine.

Gastrointestinal Tract Components

• The GIT includes the mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine.
• Accessory organs include the pancreas, liver and gallbladder.

Structure of the Gastrointestinal Tract

• The GIT is a long muscular tube stretching from the mouth to the anus.
• From the mid-esophagus to the anus, the composition is similar.
• Circular folds are where the entire inner surface of the tube folds in on itself.
• The top third of the human esophagus is made of skeletal muscle.
• The rest of the GIT is composed of smooth muscle.
• The lumen is the inside of the tube, and it contains many folds and processes to increase surface area.
• Villi project into the lumen of the tube and have crypts or invaginations below the surface.

Layers of the Gastrointestinal Tract

• The mucosa contains three subsections: epithelium, lamina propria, and muscularis mucosa.
• Other layers include the submucosa, muscularis externa (outer muscular layer), and serosal layer (connective tissue layer).

Layers of the Mucosa

• The mucosa has three layers: epithelium, lamina propria, and muscularis mucosa.
• Epithelium lines all body cavities and surfaces.
• Epithelial cells have a basolateral surface and an apical surface, meaning they are polarized.
• The apical surface inserts into the inside of the tube or the lumen.
• The basolateral surface faces away from the tube, closest to the blood surface.
• The polarized epithelial layer has different transport proteins at the apical surface compared to the basolateral surface.
• Transport proteins are restricted to different cell surfaces due to tight junctions.

Epithelial Layer

• The epithelial layer consists of a single layer of cells that selectively uptake nutrients, electrolytes, and water while preventing harmful substances from passing through.
• It has an amplified surface area due to villi and crypts.
• Villi contain a single layer of epithelial cells, including microvilli.
• Crypts are regions of invagination into the lamina propria.
• Stem cells in the crypts divide to produce differentiated daughter cells, including absorptive and mucous-producing cells.
• Stem cells migrate up the villus, reaching the top and sloughing off at the end of their life.
• The epithelial cell layer is replaced every 5 days in the small intestine.
• Rapidly dividing epithelial cells are susceptible to anticancer drugs, which can result in diarrhea or bloody stool.

Selective Transport Across the Epithelium

• The epithelial layer is selective, allowing specific nutrients to cross the intestinal epithelium into the body.
• Chemicals or molecules can use two pathways to cross the epithelial layer: paracellular and transcellular.
• Paracellular pathways allow chemicals to move between cells across cell junctions, but are limited by tight junctions, restricting the diffusion of water and small ions; not many substances can pass through this pathway.
• Transcellular pathways cross the cell and require transport proteins; many nutrients must use this two-step process.

Lamina Propria

• The lamina propria includes everything above the muscle layer.
• Its structures include connective tissue, small blood vessels (arterioles and capillaries), nerve fibers, lymphatic vessels, and immune and inflammatory cells for immune protection.
• It contains a lacteal or lymphatic vessel.

Muscularis Mucosa

• It is a thin layer of smooth muscle that is not involved in GIT contraction, but may function in moving the villi.

Submucosa

• The submucosa is beneath the mucosa layer.
• This submucosal layer contains blood vessels, lymphatic vessels, the submucosal nerve plexus, and connective tissue.
• The submucosal nerve plexus relays information to and from the mucosa.

Muscularis Externa

• The muscularis externa includes circular muscle, the myenteric nerve plexus, and longitudinal muscle.
• Circular muscle (thick inner layer) fibers are oriented in a circular pattern and contract and relax to close and open the tube (causing narrowing of the lumen).
• The myenteric nerve plexus (myo = "of muscle") regulates the muscle function of the GIT.
• Longitudinal muscle lengthens and shortens to control the length of the tube, but does not change the diameter.

Serosa

• It is a thin connective tissue layer that encases the intestine and forms connections with the intestine and the abdominal wall.

Blood Supply for Digestive Processes

• The GIT's blood supply carries away many nutrients and other absorbed components from the diet.
• The blood supply transports water-soluble nutrients and other molecules.
• Lacteals (lymphatic system; lamina propria in the mucosa layer) are important for fat absorption.
• Blood enters the GIT highly oxygenated (red) but loses that oxygen as it perfuses the intestine (blue).
• Blood that perfuses the intestine goes to the liver via the portal vein and does not flow directly from the GIT back to the heart.

Process of Digestion

• Food is digested, starting in the stomach.
• Absorption and secretion occur in the small intestine.
• Processing occurs in the colon.
• The elimination of feces containing indigestible material happens next.

Portal Circulation

• Portal circulation is a circulation that drains blood from the digestive tract to the liver.
• Blood in the portal circulation is nutrient-rich.
• It’s important for the liver removes harmful substances and processes nutrients with its enzymes, detoxification.
• The liver is an exception to the rule that most organs in the body are perfused by arterial blood because the source is fully oxygenated.
• When you have not been eating, about 30% of the blood in the liver comes from an arterial source.
• After eating, this is reduced to 10%.
• The hepatic artery contains fully oxygenated blood that perfuses the liver and the hepatic portal vein carries blood to the liver that has already perfused the stomach, pancreas, small intestine, and large intestine.
• These two blood supplies mix, so that the liver is perfused with nutrient-rich blood coming from the GIT organs with a poor oxygen content.

Series vs Parallel Circulation

• Most organs are perfused in parallel within the systemic circulation.
• The liver is perfused in series because the liver is perfused predominantly by blood that has already perfused another organ; blood flows from the digestive organs to the liver
• Some blood comes from the heart, but most comes from digestive organs (stomach, pancreas, intestines) and it flows in series between them

Regulation of GI Processes

• GI processes are primarily controlled through the volume and composition of what is inside the intestinal tract rather than by the CNS.
• GI processes include secretion and motility, or the movement of the GI tract.

Reflexes Regulating GI Processes

• Reflexes which regulates Gl processes are initiated by Distension of the GIT wall by the volume, Osmolarity of whats contained (pH).
• The concentration of the specific digestion contents, such as monosaccharides,fatty acids, peptides and amino acids initiates different regulatory pathways
• These reflexes are propagated by various receptors
• Distension of the wall, osmolarity, pH and specific contents trigger the initiate reflexes by acting through various receptors in the wall.
• Mechanoreceptors are activated by mechanical stimuli (pressure and stretch); eating a lot and stretching out the stomach.
• Osmoreceptors are activated by a change in osmolarity such as eating salty food.
• Chemoreceptors are activated by specific chemicals.

Intrinsic Neural Regulation of GI

• Processes are performed to control motility and secretion.
• Intrinsic is neural regulation contained wholly within the organ, occurs with nerve plexi located in the GIT wall.
• Nerve plexi branching networks of intersecting nerves ( Enteric nervous system, intrinsic nerve regulation) controls the activity of the secretomotor, neurons which has a role in secretion and motility.
• A large number of neurons"Brain of the gut" in the walls of the contained Enteric nervous system
• Involuntary digestive functions are critical as the system functions independently of the CNS allowing us to digest our food without having to think about it.

Myenteric Plexus

• Network of nerves located between the circular and longitudinal muscle layers of the muscularis externa.
• Responsible for influencing and regulating the smooth muscle.

Submucosal Plexus

• Network of nerves located predominatley in the submucosa.
• Predominantly influences secretion.
• Has Nerves that extend from the submucosal, plexus to the mucosa to control secretion

Plexi Interactions in the Digestive System

• The neural activity in one plexus influences the activity in the other.
• The Communication between the two layers occurs so that motility and secretion of digestive enzymes work together , assisting mobile activity.
• An Extrinsic process is outside of the gut wall and extrensic regulation occurs through autonomi nervous system ANS.
• Nerve fibers from the parasympathetic and sympathetic pathways enter the intestinal tract and synapse with neurons in both plexuses.
• The CNS can influence motility and secretion through the autonomic nervous system as its used of food sends signals through the brain to the GIT start to g.ets eady.
• Emotional states create Different emotional states in the brain, influence appetite.

Autonomic Pathways in the GI System

• The "Rest and digest" Parasympathetic response stimulates the flow of a large volume of watery saliva, Stimulates peristalsis
• “Fright, flight, fight" Sympathetic response Stimulates the small, Volume small, thick saliva inhibits peristalsis Inhibits Secretion.
• Reflexes which use a Long long neural extisic neural or short or short intrinsic pathways.

Gastroenteric Stimuli

• Are short reflexes that are intrinsic pathway modify modify and secretion , stimulus activated by aReceptors , nerves and action
• These stimuli includes eating a meal activates, Receptors in GIT wall ( mechanoreceptors, or somo receptores chemoreceptors_ stimulus froim receptors for into nervous tostimuales and muscle, to causes response. Or SMELL food hungry/emotion to stimulus and the CNS, then action of the stomach,

Chemical Messenger Regulation

• Has chemical messenger regulation and, includes a 4 categories, Hormone-secreting cells release into through bas lateral into one or many parts of the body, which releases must action.
• A Neurocrne regulation, nerve cell prounce electrical signals release the neurontransmitter, which travels action cell both neuton and the ccells, messeger reeache the acts synapse, act on Synpatic cell.
• Paracrine the region to acts on the body and or autocrine.

HOrmond control

• The endcorine is a form stimulous and it, is a production with eputhlium
• Interoendochrine is a form function where hormones
• Hrmpnal is forrm glucogen.

Hormonal Control of GI Activity

• These are all peptide hormones released by endocrine cells that Participate to perform with regualtion, for a some aspect with Gi rumen.

CCK Feedback

• For in example in cck , realse stimulate the presence and releashes the panceas.
• Absoption of acts controls the releasses or control.

Intestinal Motility

• Stim by contraction and relax, outer action the GIT, causes the contant action.
• Peristasis maintcevesd.
• and Circular muscles the area.
• Important for mixing foods, segments with lottle of net work,, mostly, allow mixed and Slow

Basic Electrical Rhythm

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Phases of Gastrointestinal Control

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Explore the role of lingual lipase in neonates, radiation-induced xerostomia, and the mechanics of swallowing. Understand the liver's dual blood supply, its importance in nutrient processing, and the unique features of the esophagus. Also, learn the importance of amylase and lingual lipase during some diseases.