Gastrointestinal Tract Overview

Subdivisions of the GIT System

• Alimentary Canal: The primary pathway for food and waste, which includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory Organs: These are connected to the alimentary canal but do not have food or waste passing through them; they include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Gastrointestinal Tract Function

• Initial mechanical breakdown and mixing of food with digestive fluids occurs through mechanical homogenization.
• Digestive enzymes (more than 30g protein daily) hydrolyze macromolecules into smaller molecules like oligomers and monomers.
• Electrolytes are secreted to maintain a conducive environment for enzymatic activity.
• Bile acids act as detergents, aiding in lipid digestion and absorption.
• Final digestion of nutrients occurs in the intestine via enzymes on the intestinal surface.
• Nutrient molecules are transported from the intestinal lumen into the bloodstream.

Specialized Glands and Epithelia

• The GIT has specialized glands such as salivary glands, gastric mucosa, pancreas, liver, and intestinal linings for digestion and absorption.
• Salivary glands and the pancreas have specialized cells that produce and deliver enzymes (exocrine function).
• The stomach initiates protein hydrolysis and assists in digestion through its mechanical actions.
• The pancreas and small intestine are critical for nutrient assimilation, with substantial reserve capacities for enzyme secretion.

Intestinal Surface Area

• The total surface area of the human intestine is around 180 m², comparable to a tennis court (195.7 m²).

Fluid Balance in the GI Tract

• Daily ingestion is approximately 2 liters.
• Absorption in the GI tract totals around 8.8 liters, while secretion amounts to about 7 liters.
• Fecal output is around 0.2 liters daily.

Saliva Composition and Production

• Saliva originates from three major glands: parotid, sublingual, and submandibular.
• A healthy individual produces about 1 liter of saliva daily, with a pH of 6.0-7.0.
• Contains primarily water (99%), ions (e.g., HCO3-, K+, Cl-), and 1% solid proteins, primarily mucin.
• Digestive enzymes in saliva include salivary amylase and lingual lipase.

Functions of Saliva

• Lubricates food and initiates digestion of polysaccharides and lipids.
• Mild antibacterial properties through components like muramidase, lactoferrin, and immunoglobulin IgA.
• Neutralizes food acids and regurgitated stomach acid.
• Maintains teeth health by replenishing minerals lost during bacterial fermentation.

Control of Salivary Secretion

• Primarily controlled by the autonomic nervous system, rather than GIT hormones.
• Parasympathetic stimulation from the salivatory nuclei enhances secretion through taste and mechanical input.
• Sympathetic stimulation increases salivary secretion slightly via beta-adrenergic receptors.
• Xerostomia (dry mouth) can result from drug side effects or systemic diseases, leading to discomfort and infections.

Stomach Anatomy

• The stomach is divided into three regions: cardia, fundus and body, and antrum.
• Surface mucous cells produce neutral mucins and HCO3- for protective mucous gel formation.
• Endopeptidase in the stomach works on peptide bonds, producing peptones, which stimulate gastrin release.

Gastric Acid and Protection

• HCl production involves an alkaline tide, balancing excess HCO3- entering the bloodstream.
• Regulation of HCl secretion is influenced by gastrin, histamine, and acetylcholine; absence of any factor affects acid secretion.
• Gastric mucosal barrier protects against acidic contents through mucus and alkaline secretions, maintaining a near-neutral pH for epithelial cells.

Pancreas Functions

• The pancreas serves both exocrine (digestive enzymes) and endocrine (hormone production) functions.
• The acini produce pancreatic juice, while the islets of Langerhans secrete insulin and glucagon.
• Exocrine function accounts for 99% of pancreas activity; hormones support metabolic regulation of glucose, lipids, and proteins.