Clase 45 Fisiología Gastrointestinal - Propulsión y Mezcla Parte 1 (1/2) (IG:@doctor.paiva)

Introduction to Gastrointestinal Physiology

Overview of Class Topics

• The 45th class focuses on gastrointestinal physiology, specifically propulsion and mixing in the digestive process.

• Key topics include food ingestion, mastication (chewing), dilution, voluntary and involuntary phases of digestion, stomach motor functions, storage, mixing, propulsion, and gastric emptying.

Importance of Proper Digestion

• For effective processing in the digestive tract, food must remain in each section for an adequate time and be mixed properly.

Mastication: The Process of Chewing

Mechanical Processes Involved

• Mastication involves mechanical processes where incisors cut food with a force up to 25 kg while molars crush it with a force up to 100 kg.

• The chewing muscles (temporalis and masseter) are innervated by the trigeminal nerve (cranial nerve V).

Reflex Mechanisms

• Mastication is initiated by a reflex called the masticatory reflex triggered by food presence in the mouth.

• This reflex leads to alternating contractions that compress the bolus against the palate and initiate further chewing cycles.

Role in Digestion

• Effective mastication is crucial for digesting raw fruits and vegetables containing indigestible cellulose; it breaks down cell walls for better nutrient absorption.

Deglutition: The Swallowing Process

Phases of Deglutition

• Deglutition consists of two main phases: voluntary (oral phase) and involuntary (pharyngeal phase).

• The oral phase begins when the tongue pushes the bolus against the hard palate; this action transitions into an automatic swallowing process.

Pharyngeal Phase Mechanics

• During swallowing:

• Soft palate closes off nasal passages.

• Pharyngeal folds close to prevent aspiration.

• Vocal cords approximate while epiglottis rises to block trachea entry.

Esophageal Functionality

Movement Types in Esophagus

• The esophagus primarily conducts food from pharynx to stomach using peristaltic movements—primary waves initiated at pharynx continue toward stomach.

Duration of Peristalsis

• Primary peristaltic waves last about 8–10 seconds as they transport bolus rapidly due to gravity's effect.

Esophageal Function and Gastric Motility

Esophageal Mechanics

• Primary zones of the esophagus fail to move food bolus to the stomach, causing it to become stagnant. This leads to secondary peristaltic waves initiated by distension.

• The food is propelled by primary waves but stops due to esophageal distension, triggering secondary peristaltic waves.

• The upper third of the esophagus consists of striated muscle innervated by the pharyngeal and vagus nerves, while the lower two-thirds are smooth muscle controlled by the vagus nerve.

• The lower esophageal sphincter (LES), or gastroesophageal sphincter, maintains a resting tone of approximately 30 mmHg and relaxes during peristaltic waves.

• The LES prevents gastric acid reflux into the esophagus through a valvular mechanism that closes under increased intra-abdominal pressure.

Gastric Functions

Anatomical Division

• The stomach is anatomically divided into fundus, body, antrum, cardia, pylorus, greater curvature, and lesser curvature.

Physiological Division

• Functionally divided into oral and caudal portions; key motor functions include storage, mixing of food into chyme, and emptying into the duodenum.

Storage Mechanism

• Food enters the stomach in concentric circles; older food remains near walls while newer food occupies central space.

• Gastric distension triggers a vagovagal reflex that relaxes the stomach for accommodation up to 1.5 liters without increasing internal pressure.

Mixing and Propulsion

• Digestive juices secreted from gastric glands mix with food upon contact; weak peristaltic contractions begin in response to food presence.

• Distension generates constrictive waves called "mixing waves," occurring at about three times per minute (every 20 seconds).

Chyme Formation

• Increased frequency of mixing waves leads to strong constrictive potentials known as "peristaltic rings."

Retropropulsion Mechanism

• As mixing occurs in conjunction with retropropulsion at the pylorus—constriction hinders emptying towards duodenum for better mixing.

Hunger Contractions

• After 12–24 hours without food intake, sustained contractions occur lasting 2–3 minutes known as hunger contractions; these can cause discomfort termed "hunger pangs."

Gastric Emptying and Its Regulation

Mechanisms of Gastric Contraction

• Normal hypoglycemia increases contractions in individuals undergoing prolonged fasting, reaching maximum intensity after 3 to 4 days before gradually weakening.

• Rhythmic contractions begin in the mid-stomach and move towards the pylorus, generating a pressure of 50 to 70 cm of water—six times greater than mixing waves.

• Each contraction pushes several milliliters of chyme into the duodenum, functioning as a "pyloric pump" at the distal stomach orifice.

Pyloric Functionality

• The circular smooth muscle at the pylorus thickens by 50% to 100%, acting as a sphincter that restricts solid food passage while allowing liquids through.

• The consistency of chyme influences its ability to pass; liquid-like substances can flow more easily compared to solids.

Factors Influencing Gastric Emptying

Stimulating Factors

• Gastric emptying is stimulated by increased food volume leading to gastric distension, which activates local enteric reflexes that relax the pyloric sphincter.

• The release of gastrin hormone due to gastric distension also promotes gastric secretion and has a mild effect on enhancing gastric emptying.

Inhibiting Factors

• Enterogastric reflexes originating from the duodenum inhibit gastric emptying by affecting both the pyloric pump and sphincter function.

• Distension, irritation of mucosa, acidity (pH < 4), and hypertonicity in chyme all contribute to inhibiting gastric emptying for protective reasons.

Protective Mechanisms Against Acidity

• When chyme's pH drops below 4, reflexes are activated to prevent further acidic material from entering the duodenum until alkaline secretions neutralize it.

• Alkaline secretions from pancreatic and gallbladder sources help protect intestinal lining from acidic damage caused by low pH chyme.

Role of Hormones in Gastric Regulation

• Fats arriving in the duodenum trigger specialized epithelial receptors that secrete hormones like cholecystokinin (CCK), which inhibits pyloric activity and slows down gastric emptying.

• Other hormones such as secretin and gastric inhibitory peptide also play roles in regulating gastric motility alongside CCK.

Summary of Gastric Emptying Control

• Overall control involves feedback mechanisms including enteric nervous system signals and hormonal responses primarily driven by CCK.

• Key factors limiting gastric emptying include excessive chyme volume, high acidity levels, unprocessed proteins or fats, hypertonicity, or irritative properties within chyme.