GIT DIGESTIVE

Overview of the Gastrointestinal Tract

Learning Objectives

Understand the functional structures of the GIT.
Discuss the secretory activities of the GIT.
Explore the motor activity of the GIT.
Outline the digestion and absorption of foodstuffs.
Explain the role of accessory organs in digestion.
Comprehend the pathophysiology of GIT disorders.

Functions of the GIT

Ingestion and Mastication: The oral cavity is responsible for the initial intake and mechanical breakdown of food.
Propulsion and Mixing: The pharynx and esophagus facilitate the movement of food towards the stomach through peristalsis.
Digestion and Secretion: The stomach and intestines secrete digestive enzymes and acids to break down food.
Absorption: Primarily occurs in the small intestine, where nutrients are absorbed into the bloodstream.
Elimination: The second half of the large intestine is responsible for the excretion of waste products.
Erythropoiesis Support: The GIT secretes intrinsic factor necessary for vitamin B12 absorption, crucial for red blood cell production.

Histology and Structure of the GIT

Layers of the GIT

Mucosa: The innermost layer consisting of three parts: mucous epithelium, lamina propria, and muscularis mucosae.
Submucosa: Contains blood vessels, glands, and nerve plexus, providing structural support and innervation.
Muscularis: Composed of circular and longitudinal smooth muscle layers, responsible for peristalsis and segmentation.
Serosa: The outermost layer that reduces friction between organs, consisting of the visceral and parietal peritoneum.
Mesenteries: Support and stabilize abdominal organs, including the mesentery proper, lesser omentum, and greater omentum.

Enteric Nervous System

Overview: The GIT has its own intrinsic nervous system, the Enteric Nervous System (ENS), which regulates digestive processes independently.
Myenteric Plexus (Auerbach’s Plexus): Located between muscle layers, it controls gut motility and contraction intensity.
Meissner's Plexus: Found in the submucosa, it regulates secretion and blood flow within the GIT.
Autonomic Innervation: The GIT receives parasympathetic (excitatory) and sympathetic (inhibitory) signals, influencing digestive activity.

Electrical Activity and Muscle Contraction in the GIT

Electrical Activity of GIT Smooth Muscle

Slow Waves (Basic Electrical Rhythm): Generated by interstitial cells of Cajal, these waves dictate the rhythm of contractions in the GIT.
Spike Potentials: True action potentials that trigger muscle contractions when membrane potential exceeds -40 mV, leading to peristalsis.

Comparison of Skeletal and Smooth Muscle Contraction

Skeletal Muscle: Characterized by rapid cycling of myosin bridges, low energy requirement, and a well-developed sarcoplasmic reticulum.
Smooth Muscle: Exhibits slower cycling, requires more ATP, and relies on extracellular calcium for contraction.

Functional Movements and Digestive Processes

Types of GIT Movements

Mixing Movements: Continuous mixing of contents through peristaltic and local contractions.
Propulsive Movements: Forward movement of food along the tract, primarily through peristalsis.
Peristalsis: A coordinated contraction that propels food, stimulated by the myenteric plexus and external signals.

Secretion of Saliva

Salivary Glands: Major glands include parotid, submandibular, and sublingual, each contributing to saliva composition.
Types of Secretions: Saliva contains serous secretion (ptyalin for starch digestion) and mucous secretion (mucin for lubrication).
Role of Saliva: Facilitates digestion and oral hygiene by breaking down food and providing a medium for taste.

Salivary Secretion and Functions

Composition of Saliva

Saliva is primarily composed of water, electrolytes, and enzymes, with mucin being a key component for lubrication.
The parotid glands secrete a serous type of saliva, while submandibular and sublingual glands produce both serous and mucous secretions.
Saliva contains high levels of potassium and bicarbonate ions, which help maintain oral pH and neutralize acids.
Sodium and chloride ions are present in lower concentrations, contributing to the overall ionic balance of saliva.

Functions of Saliva

Mechanical Functions: Saliva keeps the mouth moist, aids in speech, and facilitates the mastication process by lubricating food.
Digestive Functions: Contains the enzyme ptyalin, which initiates the breakdown of starch into maltose.
Excretory Functions: Saliva helps excrete waste products such as urea, heavy metals, and certain drugs.
Oral Hygiene: Saliva washes away food particles and bacteria, and contains thiocyanate and lysozyme, which have bactericidal properties.
Taste Sensation: Saliva is essential for taste perception as it dissolves food substances, allowing them to interact with taste buds.

Anatomy of the Oral Cavity

Lips: Composed mainly of the orbicularis muscle, with keratinized stratified epithelium on the outer surface and stratified squamous epithelium on the inner margin.
Cheeks: Contain buccinator muscles that help flatten the cheeks against the teeth during mastication.
Tongue: Attached to the floor of the mouth by the frenulum, with the anterior two-thirds covered in papillae that house taste buds.

Mastication and Swallowing

Process of Mastication

Mastication involves the grinding of food by posterior teeth (molars) and cutting by anterior teeth (incisors).
The jaw muscles exert a force of approximately 55 pounds to close the teeth during chewing.
The chewing reflex is initiated by the presence of food, leading to rebound contractions that aid in the process.

Phases of Swallowing (Deglutition)

Voluntary Stage: Initiates the swallowing process as food is pushed to the back of the mouth.
Pharyngeal Stage: An involuntary phase where food passes through the pharynx into the esophagus, involving the nasopharynx, oropharynx, and laryngopharynx.
Esophageal Stage: Involuntary phase that promotes the passage of the bolus through the esophagus to the stomach, utilizing peristalsis.

Gastric Function and Secretion

Motor Functions of the Stomach

The stomach stores food temporarily, mixing it with gastric secretions to form chyme, a semi-fluid mixture essential for digestion.
Mixing waves occur every 20 seconds, controlled by the basic electrical rhythm (BER), facilitating the mixing of food and gastric juices.
Powerful peristaltic constrictor rings propel the contents toward the pylorus, mixing and pushing food into the small intestine.

Phases of Gastric Secretion

Cephalic Phase: Triggered by the sight, smell, or thought of food, stimulating gastric secretions even before food enters the stomach.
Gastric Phase: Activated by food in the stomach, leading to the secretion of gastric juices that aid in digestion.
Intestinal Phase: Occurs when food enters the small intestine, signaling a decrease in gastric secretions to slow down digestion.

Gastric Glands and Enzymes

Types of Gastric Glands

Single-cell Mucous Glands: Located throughout the stomach, secreting mucus for protection.
Oxyntic (Gastric) Glands: Found in the body and fundus of the stomach, responsible for secreting hydrochloric acid (HCl) and intrinsic factor.
Pyloric Glands: Located in the antrum, secreting mucus and gastrin to regulate gastric function.

Enzymatic Activity in Gastric Juice

Pepsinogen: Activated by HCl into pepsin, which has proteolytic activity essential for protein digestion.
Gastric Lipase: Enzyme that digests butterfat, contributing to fat digestion.
Gastric Amylase: Minor role in starch digestion, complementing salivary amylase.

Anatomy of the Small Intestine

Length and Structure

The small intestine is approximately 6 meters long, divided into three main sections:
Duodenum: 12 inches (30 cm) long, the first part of the small intestine where most chemical digestion occurs.
Jejunum: 2.5 meters long, primarily involved in nutrient absorption.
Ileum: 3.5 meters long, absorbs vitamin B12 and bile salts, contains Peyer patches for immune function.

Modifications for Absorption

The small intestine has several adaptations to increase surface area for absorption:
Circular Folds: Permanent folds of the mucosa and submucosa that slow down chyme movement and increase surface area.
Villi: Finger-like projections of the mucosa that contain blood vessels and lymphatics for nutrient transport.
Microvilli: Tiny projections on the surface of villi that further increase surface area and contain enzymes for digestion.

Movements of the Small Intestine

Mixing Contractions (Segmentation)

Segmentation involves localized contractions that mix chyme with digestive juices, enhancing nutrient absorption.
These contractions are triggered by the distension of the intestinal wall, creating a segmented appearance.

Propulsive Movements (Peristalsis)

Peristalsis is a wave-like muscle contraction that propels chyme through the intestine.
Peristaltic Rush: A rapid peristaltic movement triggered by irritation of the intestinal mucosa, often leading to diarrhea.

Enzymatic Activity in the Small Intestine

Enzymes Secreted by the Small Intestine

Enzymes are secreted by epithelial cells of the intestinal mucosa, particularly those covering the villi.

Key enzymes include:

Enterokinase: Activates trypsinogen to trypsin, initiating protein digestion.
Proteolytic Enzymes: Break down proteins into amino acids.
Carbohydrate-splitting Enzymes: Such as lactase, sucrase, and maltase, which break down disaccharides into monosaccharides.

Absorption Mechanisms

Water Absorption: 99% of water is absorbed by osmosis.
Sodium Absorption: 25-35 grams/day through active transport and diffusion.
Glucose and Galactose: Absorbed via Na-glucose co-transport; fructose is absorbed by facilitated diffusion.

Functions and Movements of the Colon

Functions of the Colon

Absorbs water and electrolytes from chyme to form solid feces.
Stores fecal matter until expulsion, with 80-200 ml expelled daily.
Produces vitamins K and B, contributing to gut health.

Movements of the Colon

Mixing Movements: Known as haustration, which helps in the absorption process.
Propulsive Movements: Mass movements occur every 8-12 hours, forcing fecal matter towards the rectum.

Accessory Organs of the Digestive System

The Pancreas

The pancreas secretes digestive enzymes in response to chyme in the small intestine.
Pancreatic Juice: Contains enzymes for digesting proteins, carbohydrates, and fats, with a daily secretion of about 1000 ml and a pH of 8.0.

Enzymes of Pancreatic Juice

Proteolytic Enzymes: Such as trypsinogen and chymotrypsinogen, activated in the intestine to digest proteins.
Carbohydrate-splitting Enzyme: Pancreatic amylase breaks down starch into maltose.
Lipolytic Enzymes: Pancreatic lipase splits fats into fatty acids and monoglycerides.

Hormonal Regulation of Digestion

Key Hormones and Their Functions

Secretin: Secreted in response to acidic chyme, it stimulates the pancreas to release a fluid rich in bicarbonate (HCO3) to neutralize stomach acid.
Cholecystokinin (CCK): Released in response to fats and peptides, it promotes the secretion of digestive enzymes from the pancreas and stimulates gallbladder contraction to release bile.
Gastrin: Released during the gastric phase, it stimulates gastric acid secretion and promotes gastric motility.
Role of Hormones: These hormones coordinate the digestive process, ensuring that enzymes and bile are released at the right time to aid in digestion.

Mechanisms of Action

Secretin Mechanism: Bicarbonate secretion helps to neutralize the acidic chyme entering the small intestine, creating an optimal pH for enzyme activity.
CCK Mechanism: CCK not only stimulates enzyme secretion but also enhances the digestion of fats by promoting bile release, which emulsifies fats for better absorption.
Gastrin Mechanism: Gastrin increases gastric acid secretion, which is crucial for protein digestion and acts as a defense mechanism against pathogens.

Gallbladder and Bile Function

Structure and Function of the Gallbladder

The gallbladder is a pear-shaped organ that stores and concentrates bile produced by the liver.
Functions of Bile: Bile emulsifies fats, excretes waste products, stimulates intestinal motility, and neutralizes acidic chyme.
Bile Composition: Bile consists of water, inorganic salts, bile salts, bile pigments (bilirubin and biliverdin), and lipids (cholesterol, lecithin).
Daily Secretion: The liver secretes 700 to 1200 ml of bile daily, with a pH range of 7.7 to 8.6.

Bile's Role in Digestion

Emulsification: Bile salts break down large fat globules into smaller micelles, enhancing fat absorption in the intestine.
Excretion: Bile serves as a route for the excretion of heavy metals, toxins, and waste products from the body.
Stimulation of Peristalsis: Bile salts promote intestinal motility, aiding in the movement of food through the digestive tract.

Liver Functions and Metabolism

Major Functions of the Liver

The liver plays a crucial role in metabolism, including carbohydrate, protein, and fat metabolism.
Glycogen Storage: The liver stores glucose as glycogen and releases it when needed to maintain blood sugar levels.
Detoxification: The liver detoxifies harmful substances and drugs, converting ammonia to urea for excretion.
Protein Synthesis: It synthesizes important plasma proteins, including clotting factors and albumin.

Metabolic Pathways in the Liver

Gluconeogenesis: The liver can produce glucose from non-carbohydrate sources such as amino acids and glycerol.
Glycogenolysis: The breakdown of glycogen into glucose when blood sugar levels drop.
Glycogenesis: The conversion of excess glucose into glycogen for storage.

GIT Disorders and Pathophysiology

Common GIT Disorders

Achalasia: A condition where the lower esophageal sphincter fails to relax, preventing food from entering the stomach, often due to the absence of the myenteric plexus.
Gastritis: Inflammation of the gastric mucosa, which can lead to gastric atrophy and conditions like achlorhydria and pernicious anemia.
Peptic Ulcer: An excoriated area in the mucosa caused by excess acid and pepsin secretion, often exacerbated by stress and certain medications.

Symptoms and Effects of GIT Disorders

Appendicitis: Inflammation of the appendix, characterized by crampy abdominal pain and potential obstruction.
Constipation: Slow movement of feces leading to hard stools, often due to a low-fiber diet.
GERD: Gastroesophageal reflux disease, where stomach acid flows back into the esophagus, causing heartburn and potential complications like esophagitis.

GIT Irritation and Symptoms

Causes and Effects of GIT Irritation

Vomiting: Reflex loss of upper gastrointestinal contents due to irritation or over-distention.
Nausea: A sensation that often precedes vomiting, caused by various factors including motion sickness.
Flatus: Gas in the GIT can result from swallowed air or bacterial action, leading to discomfort.

Management of GIT Symptoms

Dietary Adjustments: Increasing fiber intake can alleviate constipation and improve overall digestive health.
Medications: Antacids and proton pump inhibitors can help manage symptoms of GERD and gastritis.
Hydration: Maintaining adequate fluid intake is crucial for preventing dehydration, especially in cases of diarrhea.