Saliva, Swallowing and Esophagus

• L6: Digestive System Part 1, covering human physiology

Digestive System Overview

• The digestive system provides nutrients for cell maintenance and growth.
• The Gastrointestinal Tract (GIT) is organized into two main parts: the alimentary canal and accessory digestive organs.
• The alimentary canal (digestive tract), includes the mouth, pharynx, esophagus, stomach, small and large intestines, rectum, and anal canal.
• Accessory digestive organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Activities of the Digestive System

• The digestive system's activities are categorized into six processes: ingestion, propulsion, digestion, secretion, absorption, and defecation.
• Ingestion: Food enters the digestive tract through the oral cavity, where mastication (chewing) occurs.
• Propulsion: Includes both swallowing and peristalsis.
• Peristalsis: Involuntary alternating waves of muscle contraction and relaxation in organ walls.
• Segmentation: Regions of the small intestine contract and relax independently for efficient digestion and absorption.
• Digestion: Includes both mechanical and chemical digestion.
• Mechanical digestion: Muscular movements physically break down food.
• Chemical digestion: Hydrolysis reactions, aided by enzymes from the stomach and small intestine,break down food into nutrient molecules.
• Vitamins, free amino acids, minerals, cholesterol, and water are absorbed without being digested.
• Secretion: Involves water, electrolytes, and other organic constituents released into the digestive tract lumen, triggered by neural or hormonal stimulation.
• Absorption: Nutrient molecules are taken up into the epithelial cells of the digestive tract, then transferred into the blood or lymph.
• Defecation: Waste products are excreted from the body as feces.

Mouth & Oral Cavity

• Food is mechanically digested into smaller particles via mastication.
• Salivary amylase initiates chemical digestion by breaking down polysaccharides into disaccharides.
• The tongue, composed of skeletal muscle, manipulates food and contains taste buds.
• Mastication mixes food with saliva, forming a moist lump called a bolus for swallowing.

Salivary Glands

• There are three paired salivary glands that produce 1-1.5 liters of saliva per day, with a pH between 6 and 7.
• Parotid glands
• Submandibular or submaxillary glands
• Sublingual glands

Composition of Saliva

• Saliva is mostly water (99.5%).
• Saliva has a low ionic content of Na+ and Cl-, but high in K+ and HCO3-.
• Saliva contains the enzymes lingual lipase and α-amylase (ptyalin).
• Saliva contains mucins.
• Saliva contains the proteolytic enzymes known as lysozymes
• Contains immunoglobulin A (IgA)

Functions of Saliva

• Digestive functions: Amylase breaks down starch, and lingual lipase breaks down triglycerides (active in the stomach).
• Lubrication: Keeps the mouth moist for speech and moistens food to aid swallowing.
• Protection: Cleanses the mouth and teeth of debris, exhibits antibacterial action, and buffers acidic gastric secretions.

Pharynx and Esophagus

• Swallowing:
• Is motility associated with the pharynx and esophagus.
• Begins when the bolus is voluntarily forced to the rear of the mouth into the pharynx.
• Can be initiated voluntarily, but it cannot be voluntarily stopped.
• Occurs in two stages: the oropharyngeal and esophageal stages.
• Esophagus:
• Is a straight muscular tube.
• Extends between the pharynx and the stomach.
• Peristaltic waves move food through the esophagus.
• Secretes mucus entirely.
• Esophagus Function:
• Swallowing, or deglutition, moves food passes from the mouth to the stomach.
• No digestion or absorption occurs in the esophagus.
• Secretes mucus.
• Esophagus Features:
• It Contains sphincters at each end.
• Pharyngoesophageal sphincter.
  • Keeps the entrance closed to prevent air from entering the esophagus and stomach during breathing.
• Gastroesophageal sphincter.
  • Prevents reflux of stomach contents.

Swallowing Reflex

• Swallowing is initiated voluntarily but is then controlled by a reflex.
• Receptors: Touch receptors in the pharynx.
• Afferent: Sensory impulses from the receptors.
• Centre: Swallowing center in the brain stem.
• Efferent: Parasympathetic nerves to the muscles of the pharynx and esophagus.

Mechanism of Swallowing

• There are three stages: oral, pharyngeal, and esophageal.
• Oral (Voluntary): The bolus passes into the pharynx as the tongue moves upward and backward against the palate, stimulating touch receptors.
• Pharyngeal: The bolus passes involuntarily through the pharynx into the esophagus, with respiratory passageways closed and respiration inhibited.
• Esophageal: The bolus passes involuntarily from the esophagus to the stomach via peristaltic movements.

Stomach Overview

• The stomach is a pouch-like organ designed for food storage (2-4 hours).
• Mechanical and chemical digestion occurs within the stomach.
• Contains sphincters at both ends to regulate food movement: the cardiac and pyloric sphincters.
• It is divided into four regions: cardiac, fundic, body, and pyloric.
• The stomach lining has thick folds called rugae, which provide a larger surface area for expansion, secretion, and digestion.

Major Functions of the Stomach

• Storage of ingested food.
• Mechanical breakdown of ingested food.
• Disruption of chemical bonds in food material via acids and enzymes.
• Production of intrinsic factor (glycoprotein needed for vitamin B12 absorption in the small intestine).

Gastric Secretory Cells

• Gastric secretion averages 3 L/day and is highly acidic (pH 1-2).
• Mucous cells secrete mucus and alkaline substances to neutralize HCl.
• Parietal cells secrete HCl and "intrinsic factor".
• Chief cells secrete pepsinogen (inactive enzyme).
• D cells produce somatostatin, which inhibits gastric acid secretion.
• G cells secrete gastrin, which stimulates parietal cells and overall gastric secretion.

HCI Functions

• Kills ingested bacteria.
• Aids protein digestion by activating pepsinogen into pepsin.
• Provides the optimum pH for pepsin action.
• Stimulates hormones to promote bile and pancreatic juice flow.

Protective Mechanisms of Stomach

• The stomach is exposed to harsh conditions.
• It Has a mucosal barrier.
• Alkaline mucus contains HCO3-.
• Epithelial cells linked by tight junctions.
• Parietal and chief cells are impermeable to HCl.
• Damaged epithelial cells are quickly replaced.

Chemical Digestion & Absorption in the Stomach

• Carbohydrate digestion continues with gastric amylase via disaccharides.
• Protein digestion begins with pepsin via peptides activation by HCl.
• Lipid digestion commences using gastric lipases, which break down certain lipids into fatty acids.
• Absorption in the stomach is limited to water, alcohol, aspirin, and certain drugs.
• The resulting yellowish paste of all the mixing, digestion, secretion, and absorption is called chyme, which is passed on to the small intestine.

Phases of Gastric Secretion

• Cephalic phase: Activated by the thought, taste, smell, and sight of food, mediated by cholinergic/vagal mechanisms.
• Gastric phase: Due to the chemical effects of food and distension of the stomach; gastrin is the major mediator.
• Intestinal phase: Accounts for a small proportion of the acid secretory response to a meal.

Questions about the functions of saliva (bolus formation, ionic composition, digestive functions), the swallowing process (stages, initiation, protective mechanisms), and the esophagus (mucus secretion, sphincter functions).