Digestive System: Function and Processes

Primary Function of the Digestive System

• Transfers nutrients, water, and electrolytes from ingested food into the body's internal environment.

Four Digestive Processes

• Motility: Muscular contractions that mix and move contents.
• Secretion: Release of water, electrolytes, and organic constituents into the digestive tract lumen, which are then reabsorbed into the blood.
• Digestion: Biochemical breakdown of complex foodstuffs into smaller, absorbable units through enzymatic hydrolysis.
  • Carbohydrates break down into monosaccharides.
  • Proteins break down into amino acids.
  • Fats break down into glycerol and fatty acids.
• Absorption: Transfer of small units resulting from digestion, along with water, vitamins, and electrolytes, from the digestive tract lumen into blood or lymph.

Types of Motility

• Propulsive Movements: Push contents forward through the digestive tract.
• Mixing Movements: Mix food with digestive juices, promoting digestion and absorption by exposing all parts of intestinal contents.

Digestive Tract and Accessory Organs

• Digestive Tract: Includes the mouth, throat, esophagus, stomach, small intestine, large intestine, and anus.
• Accessory Digestive Organs: These include the salivary glands, exocrine pancreas, and biliary system.
• The digestive tract wall has a consistent structure from the esophagus to the anus.

Four Major Tissue Layers of the Digestive Tract Wall

• Mucosa: The innermost layer.
• Submucosa and Muscularis Externa
• Serosa: The outer layer.

Regulation of Digestive Motility and Secretion

• Regulated by autonomous smooth muscle function, intrinsic nerve plexuses, extrinsic nerves, and gastrointestinal hormones.

Autonomous Smooth Muscle Function

• Involves self-induced electrical activity such as slow-wave potentials
• Interstitial cells of Cajal (pacemaker cells) are involved in this function.

Intrinsic Nerve Plexuses

• Includes submucosal and myenteric plexuses of nerve fibers within the digestive tract wall.
• These plexuses are collectively called the enteric nervous system.

Extrinsic Nerves

• Nerve fibers originate from both branches of the Autonomic Nervous System (ANS).
• Influence digestive tract motility and secretion by modifying the activity of the enteric nervous system and altering hormone secretion levels.
  • The sympathetic system inhibits digestion
  • The parasympathetic system stimulates digestion.

Sensory Receptors of the Digestive Tract

• The three types of receptors respond to local changes.
  • Chemoreceptors
  • Mechanoreceptors
  • Osmoreceptors.

The Oral Cavity

• The lips help procure, guide, and contain food
• They are important in speech and have well-developed tactile sensation.
• The palate forms the roof and separates the mouth from nasal passages
• The uvula helps seal off nasal passages during swallowing.
• The tongue helps form the floor and is composed of skeletal muscle
• Tongue movements aid in chewing, swallowing, and speech
• Taste buds are contained within the tongue.
• The pharynx is a cavity at the rear of the throat acting as a common passageway for digestive and respiratory systems and includes the tonsils.

The Teeth

• Responsible for chewing (mastication)
• It is also the first step in the digestive process to break food into smaller pieces, mix it with saliva, and stimulate taste buds.

Saliva Composition and Functions

• Major pairs of salivary glands produce saliva.
• Saliva includes 99.5% water but also 0.5% electrolytes and proteins like amylase, mucus, and lysozyme.
• Function:
  • Salivary amylase begins the digestion of carbohydrates.
  • Facilitates swallowing by moistening food.
  • Provides lubrication with mucus.
  • Offers antibacterial action as the lysozyme destroys bacteria.
  • Acts as a solvent for molecules that stimulate taste buds
  • it aids speech and keeps the mouth and teeth clean.
  • Has bicarbonate buffers.

Salivary Reflexes

• Salivary secretion that is continuous can be reflexively increased
• The simple reflex occurs when food occupies mouth
• The conditioned reflex happens when seeing/smelling food.
• Both reflexes activate the brain stem's salivary centre, thus triggering the Autonomic Nervous System(ANS) to promote saliva production.

Reflex of Swallowing

• Complex action that can't be stopped upon beginning.
• The first stage is oropharyngeal and only takes 1 second.
• Esophageal stage then moves bolus mouth to esophogus, into pharynx.

The Esophagus

• Fairly straight muscular tube extending to stomach from pharynx.

Primary Peristaltic Wave

• Ring-like contractions pushes food(bolus) down, and involves smooth circular muscle.

Secondary Peristaltic Wave

• Caused by esophagus distention, does not involve swallowing center

Sphincters at Each End

• Pharyngoesophageal Sphincter: Located at the upper esophagus,
  • Kept close to prevent air entering esophagus, stomach during breath.
• Gastroesophageal Sphincter: Located at the lower esophagus,
  • Prevents gastric contents to reflux.
  • Opened wave to assist movement so bolus can enter stomach.
  • If not closed properly, result in irritation/heartburn (gastric reflux)

Esophageal Secretion

• Mucus that is protective; is secreted all throughout to minimize edges damages.

Stomach

• Sac-like chamber shaped like a letter "J" that lies between SI and esophagus; connected with pyloric sphincter that serves border for SI and stomach.
• Has 3 sections: Antrum, Fundus, and Body.

Functions of the Stomach

• The stomach stores food.
• It secretes both Hydrochloric Acid(HCl) and protein digestive enzymes.
• Converts food into chyme.

Functions of Gastric Filling

• It involves receptive relaxation, triggering of eating which is mediated by the vagus nerve so that there is increased food accomodation and pressure.

Gastric Functions

• Takes place in:
  • Stomach body - Gastric storage
  • Antrum - Gastric mixing
• largely controlled by the duodenum - Gastric emptying

Gastric Juice

• Mucosa has 2 areas responsible:
  • Body & Fundus -Oxyntic Mucosa
  • Antrum - Pyloric Gland Area (PGA)
• Gastric Pits are at gland base in Oxyntic Mucosa

Gastric Exocrine Secretory Cells

• Line Gastric pits & glands entrance - Mucous Cells
  • Secrete thin, watery mucus which acts a protecionary lubricant.
• Secretes Pepsinogen which requires HCL to convert into Pepsin - Chief Cells
• Secrete HCL and VItamin B12 absorptive factor - Parietal Oxyntic Cells.
  • B12 factors helps by breaking down fibers and protein.
  • Kills Microorganisms as result.

Gastric Secretion Control

• Cephalic phase:
  • Occurs before digestion
  • Involves Increased HCL and pepsinogen secretion, due to "brain" stimuli
• Gastric Phase:
  • Requires food to enter stomach
  • Gastric Secretion is increased - Due to protein presense.
• Intestinal phase:
  • Flow of gastric juices gets switched off -As chyme begins to flow into SI

Pancreas

• Elongated gland behind & under the stomach - Made of endocrine cells mixutre of tissues.

Exocrine function of Pancreas

• Contains active pancreatic enzymes, or pancreatic juice of: amylase, protease, lipase.
• Acinar cells secrete the Enzymes.
• Sodium Bicarbonate is contained in the fluid, actively secreted by the ducts.

3 Protein Digestive Enzymes:

• Active form: Procarboxypeptidase with Carboxypeptidase, Chymotrypsin and Trypsin from chymotrypsinogen

Pancreatic Secretions

• Polysaccharides changed into disaccharide maltose by Pancreatic amylase
• Throughout digestions the only enzyme that digests Fat is Pancreatic Lipase.
• Break down a bond (Peptides) so convert proteins to Amino Acids - Pancreatic Protease

Liver

• It is both important Metabolic organ + Body's key Biochemical factor and its secretion system is bile salts.
• Has function beyond secretions, such as:
  • Synthesis Plasma Proteins with Carbs, Fats, Proteins - Metabolic Processing,
  • Stores Glycogen, Fats, Iron and copper including Vitamins,
  • Grades Waste, Hormones, + and other toxins by detoxifying and degrading them.
  • Remove aged out red blood cells.
  • Gets rid Cholesterol including Bilirubin.

Liver Blood Flow

• Originates from 2 source:
  • O2 (Arterial Blood)'s Liver supply, including hepatitis which metabolizes with borne Blood -Hepatic Artery.
  • Newly Absorbed Nutrients Process/Storage + digestive tract drainage (Venous Blood) and Hepatic Portal Vein.
  • Blood that out streams the liver from Hepatic Vein.

Bile Description and Function

• Actively secreted by the gallbladder and liver between meals.
• Composed of: salts, cholesterol, bilirubin, and lecithin.
• When meals are complete, bile inters duodenum.
• Derivative of cholesterol to convert Fats to liquid emulsion > get the blood reabsorb for fat digestion.

Micelle

• A lipid (hydrophilic) and water (hydrophobic) mixture of biliary salts, cholesterol and lecithin aggregates for digestion
• Fats enter the SI and dissolve to lipid-soluble core with assistance from micelles dissolving fats for digestion.

Bile Storage

• Store in Gallbladder after secretion between meals
• Is transportive of water which concentrates bile.
• Trigged with digestion or fat prescence (Cholecystokinin[CKK])

Small Intestine:

• Where absorption and digestion takes place and is divided into : duodeum, jejunum and lileum.
• Has two motilities:
  • Segmentation + migrating motility complex

Segmentation of Small Intestines

• The SI uses segmented ring-liked constrictions over length for motility.
• Pacemaker cells initiate segmetation due to their basal rhythm while responding to gastin, distension, and external nerve activities.
• Action mixes chime across lumens in SI

Small Entestine

• Most enzymes that digest are in salt with mucus but Small INtestines don't contain chemical that breaks things down for digestive use . -Aminopeptidases, dIscaccharidases are needed for Enterokinase digestion.

Digest Enzymes

• Brush/Border of protein and Carbo - Pancreatic,
• In SI itself the fats broken down by pancreatic lipase and brush borders.

The Small Intestine and Absorption

• Most action occurs in the duodenum and jejunum.
• Inside contains Microscopic, with Villi & folds that's brushed with epithelial lining that only lasts 3 days.
• The Digested Fats transforms into absorptives and go straight to the lymph, after which goes through lymphatic absorptions.

Colon Functions

• Liquid transformation with solid chyme results w/Water elctrolytes being used up> waste is transformed through surface.
• Bacteria fermentation and the storage/elimination with feces help with overall functions and heath.

Diarrhea and its 2 Classifications:

• Too frequent, bowel discharges where death is frequent for children.
• Frequent from drugs, bacteria and poor eating habits.
• There can be high vs low volume discharges. The two (volume)-categorizations: ---osmotic : response from electrolyte and water and poor nutrient intake ---SEcretory: bowel increments with infection cases as frequent responses --- Treatment: is electrolytes or fluid balances, medication (Motlity drugs) and absorbents

Constipation

• infrequent and difficult or incomplete evacuations; from fiber and fluid
• Response from lack of drive to get urge through; or inactivity or related ailments like pregnancy and drug use/hemorrhoids
• Treatment, exercise or laxatives or fluids.

Digestive Hormones

• stomach (Gastrin) stimulated by amino distending, acids, psns, clear food > actions
• Duodenum (Secretin;) stimulated by Fat\pH - > actions
• CCK(ChloecysotoKinin) for intestines and stimulation by fats/animo to release satiety secretions etc