Nutrient Assimilation in the GI Tract

Assimilation of Nutrients in the GI Tract

• Mastication of food in the mouth creates a bolus to be swallowed, while saliva lubricates and provides enzymes for digestion, and is under nervous control
• Food travels through the esophagus in 10 seconds
• Food stays in the stomach for 1-4 hours and mixes, grinding food, with smaller particles passing through the pyloric sphincter
• Exocrine stomach secretions dilute food, and gastric acid dissolves food
• Entry of food into the duodenum coordinates with exocrine secretions from the pancreas/liver
• Pancreas is essential for digestive enzymes
• The pancreas secretes HCO3− to neutralize stomach acid
• The gallbladder delivers stored bile to the intestine
• Food moves through the small intestine in 7-10 hours
• Food is mixed with digestive enzymes, where significant absorption of nutrients occurs
• The large intestine transports fluids, electrolytes, and fermented insoluble fiber for 12-24 hours
• Elimination of fecal waste occurs 1-3 days after ingestion

Three Main Functions of the GI Tract

• The GI tract churns to enhance digestion and nutrient absorption
• Propulsion is the caudal movement of food and waste
• Acts as a reservoir facilitated by sphincters

Sphincters of the GI Tract

• Seven sphincters are in the GI tract
• Upper esophageal sphincter
• Lower esophageal sphincter
• Pyloric sphincter
• Sphincter of Oddi
• Ileocecal sphincter
• Internal anal sphincter
• External anal sphincter
• It features specialized circular muscles, acting as one-way valves
• It regulates antegrade and retrograde movement, and proximal stimuli cause sphincteric relaxation as distal stimuli induce sphincteric contraction
• It has separate low-pressure organs, maintaining positive resting pressure
• Coordination with adjacent organs occurs by intrinsic and neurohumoral stimuli

Mastication and Deglutition

• Most muscles of mastication are innervated by the motor branch of CN V
• Chewing is important for cellulose membranes, where digestive enzymes act on the food particle surface
• Food stimulates sensitive epithelial swallowing receptor areas around the opening of the pharynx, especially tonsillar pillars
• Afferently via CNs IX and X to the deglutition center of the brainstem and efferently via CNs V, IX, X, and XII initiate automatic pharyngeal muscle reflex contractions
• A bolus of food in the mouth causes the jaw to drop, leading to a stretch reflex and rebound contraction
• 25 chews are commonly sufficient for a food bolus to pass through the esophagus without pain
• Voluntary Stage 1: Food bolus is pushed into the oropharynx by elevating the tongue against the hard palate
• Pharyngeal Stage 2 (Involuntary): The soft palate is pulled up to close posterior nares to prevent food reflux
• Palatopharyngeal folds impede large particles from passing and the larynx is pulled upward/anteriorly
• Vocal cords are approximated, and the epiglottis covers the laryngeal inlet because 3-4 cm of the upper esophageal wall of the cricopharyngeus muscle (upper esophageal sphincter) relaxes
• Pharyngeal peristalsis forces the bolus into the upper esophageal sphincter, and respiration is momentarily halted
• CVA may affect the pharyngeal stage because this stage requires a lot of coordination
• Esophageal Stage 3 (Involuntary): When upright, gravity assists swallowing ahead of peristalsis, and primary peristalsis is caused by stretching of the muscular wall
• Some cells release VIP in response to stretch, inhibiting smooth muscle and relaxing in front of the bolus
• The myenteric initiates secondary peristalsis if needed - vagovagal reflex, swallowing possible in any position

Structures of the Esophagus

• The Upper Esophageal Sphincter (UES) consists of the cricopharyngeus muscle upper 3-4 cm of the esophageal wall; it's controlled by extrinsic cranial nerves and is composed of skeletal muscle
• The UES remains constricted but relaxes during the pharyngeal stage of deglutition
• The Esophageal Body consists of skeletal and smooth muscle that remains relaxed but displays no slow waves, and its mucosa only secretes mucus
• The Lower Esophageal Sphincter (LES), which is the cardiac sphincter/esophagogastric junction with circular smooth muscle 3 cm above the juncture and it's controlled by the ENS
• The three parts of the LES are the intrinsic sphincter (esophageal muscle), the extrinsic sphincter (fibers of crural portion of diaphragm), and the oblique fibers of the stomach that are a flap valve that closes when gastric pressure is high
• It's tonically constricted with an intraluminal pressure of 30 mmHg and it relaxes when swallowing
• Tonic activity prevents reflux of gastric contents into the esophagus
• Acetylcholine constricts while NO and VIP from interneurons cause it to relax

Lower Esophageal Sphincter and Barrier Pressure

• Barrier pressure opposes gastric regurgitation/aspiration of gastric contents
• The likelihood of gastroesophageal reflux is determined by barrier pressure
• Barrier pressure (BrP) = LES pressure – Intragastric pressure, where a higher barrier pressure lowers the risk of reflux/aspiration
• To prevent aspiration, a positive gradient maintains the LES tone
• Pharmacologic prophylaxis for aspiration aims to increase gastric pH, lower gastric volume, and reduce the incidence of emesis
• Barrier pressure is reduced by decreased LES tone, increased intragastric pressure, anticholinergics, pregnancy (decreases LES and increases intragastric pressure), and cricoid pressure
• Barrier pressure is increased by metoclopramide and succinylcholine has no effect - no change increases LES and IGP
• Patients with heartburn have a lower barrier pressure
• GERD occurs when the hiatal hernia- gastroesophageal junction dysfunction interferes with barrier pressure
• Achalasia is a pathological condition that results in a failure of smooth muscle motility and a failure of the LES to relax
• It prevents the transmission of food into the stomach
• Esophagitis is inflammation of the esophagus caused by infection (bacterial or viral), or acid reflux from the stomach
• Barrett's esophagitis is GI metaplasia (transformation of epithelium) of the lower esophagus in response to chronic GERD
• Carcinoma of the upper esophagus is typically in response to smoking/alcohol

Motor Functions of the Stomach

• During fasting, the stomach is quiescent, but with ingestion, vagovagal (receptive) relaxation allows up to 1.5 L
• Food causes mixing waves initiated by basic electrical rhythm, forming semifluid paste (chyme)
• It slowly empties chyme from the stomach at a rate suitable for proper digestion
• Intense antral peristaltic contractions (systole) create 50-70 cm H2O pressure for a pumping action
• Mechanically and chemically digest- the pyloric pump forces milliliters of chyme into the duodenum
• Postprandially, hunger contractions (pangs) begin 12-24 hours post-prandially with greatest intensity in 3-4 days
• The pyloric sphincter is tonically contracted but remains open enough for water to empty into the duodenum with ease
• Receptive relaxation of the stomach is vagally mediated and is accommodated with food with minimal increase in pressure
• The stomach emptying is regulated by signals from the duodenum and stomach, where the antrum, pylorus, and upper duodenum function as a unit
• Stomach emptying is promoted by stretching the stomach wall that elicits myenteric reflexes thus accentuating activity of the pyloric pump and inhibiting the pylorus
• For emptying liquids, volume is the most important determinant of the rate of emptying, so large volumes will empty faster than small ones
• Gastrin causes secretion of highly acidic gastric juice that stimulates motor functions, particularly enhancing the activity of the pyloric pump
• Stomach emptying is inhibited by duodenal distention & CCK
• Chyme is monitored and can inhibit gastric emptying if excessively acidic, it contains too much unprocessed protein or fat, or it's hypotonic or hypertonic
• Isotonicity is fast, while hyper/hypotonicity empties slowly during gastric emptying and NPO guidelines
• Sugars/starch empty fast, protein empties intermediately, and fats empty slowly during gastric emptying and NPO guidelines
• Liquids empty fast, semi-solids empty intermediately, and solids empty slowly during gastric emptying and NPO guidelines

Movements of the Small Intestine

• Segmentation movements "chop" the chyme, and peristalsis propels chime (with small peristaltic waves)
• Migrating motor complexes (MMCs) interrupt at 90-minute intervals to move indigestible components from the stomach through the small intestine
• Nervous signals greatly increase peristalsis after meals
• Gastrin, CCK, insulin, and motilin hormone levels affect intestinal motility that comes from the circular SM
• The mucosal muscle contracts, causing short folds to appear in the intestinal mucosa, and its movement stimulates digestion
• Individual fibers extend into the intestinal villi to “milk” the villi so lymph flows freely from central lacteals into the lymphatic system

Ileocecal Valve

• Ileocecal valve prevents backflow from the colon to the small intestine, protruding into the cecum and forcefully closing when excess pressure builds in the cecum, resisting reflux
• Normally, 1500-2000 ml of chyme empties into the cecum each day
• It features feedback control that contracts the ileocecal sphincter and controls the intensity of peristalsis in the terminal ileum by reflexes from the cecum
• The pressure and chemical irritation relax the sphincter and excite peristalsis that promote emptying, while the fluidity of contents promotes emptying
• Pressure or chemical irritation in the cecum inhibits peristalsis in the ileum and excites the sphincter

Movements of the Colon

• The main functions of the colon absorption of water and electrolytes from the chime, progressing from fluid to solid along the colon, until fecal matter is stored
• Mixing movements, caused by haustrations, slowly dig into and roll over fecal material in the large intestine, exposing all fecal material to the mucosal surface
• Another is large circular contractions plus teniae coli resulting in mass Movements by propulsive movements forcing feces into the rectum and it is stimulated by the gastrocolic and duodenocolic reflexes
• It is stimulated by distention of the stomach and duodenum: Occurs 1-3 times a day (ulcerative colitis all day) and especially during/following breakfast
• Defecation is the process that involves a rectum that is usually empty of feces, which are stopped by the dribble of fecal matter through the anus.
• It is prevented by tonic constriction of the internal and external and sphincters
• Intrinsic myenteric reflex is weak, and must be fortified by a parasympathetic defecation reflex