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Digestive System - PPT/PDF

Download notes (PPT/PDF) providing a comprehensive overview of the Digestive System. These notes cover the functions, anatomy, and physiology of the digestive system, including detailed discussions of the alimentary canal (mouth, pharynx, esophagus, stomach, small intestine, large intestine), accessory digestive organs (teeth, salivary glands, pancreas, liver, gallbladder), and the activities occurring in each segment of the digestive tract. Suitable for B.Pharm and related health science programs.

Keywords: Digestive System, Functions, Anatomy, Physiology, Alimentary Canal, Mouth, Pharynx, Esophagus, Stomach, Small Intestine, Large Intestine, Teeth, Salivary Glands, Pancreas, Liver, Gallbladder, Digestion, Absorption, B.Pharm, PPT, PDF, Download.

Digestive System - Detailed Explanation

1. Functions of the Digestive System

The digestive system breaks down food into smaller molecules that can be absorbed into the bloodstream and used by the body's cells. Key functions include:

• Ingestion: Taking food into the mouth.
• Mechanical Breakdown: Physically breaking down food into smaller pieces (e.g., chewing, churning in the stomach).
• Chemical Breakdown (Digestion): Using enzymes to break down large food molecules (carbohydrates, proteins, fats) into smaller molecules that can be absorbed.
• Propulsion: Moving food through the alimentary canal (e.g., swallowing, peristalsis).
• Absorption: The movement of digested nutrients from the lumen of the GI tract into the blood or lymph.
• Defecation: The elimination of indigestible substances from the body as feces.

2. Anatomy of the Digestive System

The digestive system consists of the alimentary canal (GI tract) and accessory digestive organs.

• Organs of the Alimentary Canal: A continuous muscular tube that extends from the mouth to the anus.
  • Mouth (Oral Cavity):
    • Bounded by the lips, cheeks, hard palate, and soft palate.
    • Contains the teeth, tongue, and openings of the salivary glands.
    • Functions:
      • Ingestion.
      • Mechanical breakdown (chewing/mastication).
      • Chemical breakdown (salivary amylase begins carbohydrate digestion).
      • Propulsion (swallowing).
  • Pharynx (Throat):
    • A common passageway for food, liquids, and air.
    • Divided into three parts: nasopharynx, oropharynx, and laryngopharynx.
    • Involved in swallowing (deglutition).
  • Esophagus:
    • A muscular tube that connects the pharynx to the stomach.
    • Located posterior to the trachea.
    • Passes through the diaphragm to enter the abdominal cavity.
    • Uses peristalsis (waves of muscle contraction) to propel food to the stomach.
    • The upper esophageal sphincter and lower esophageal sphincter (cardiac sphincter) control the passage of food.
  • Stomach:
    • A J-shaped, expandable organ located in the upper left quadrant of the abdomen.
    • Regions:
      • Cardia: Where the esophagus connects.
      • Fundus: The dome-shaped superior portion.
      • Body: The main, central portion.
      • Pylorus: The inferior portion that connects to the small intestine. The pyloric sphincter controls the release of chyme (partially digested food) into the duodenum.
    • Wall has four layers:
      • Mucosa: Contains gastric glands that secrete gastric juice (hydrochloric acid, pepsinogen, mucus, intrinsic factor).
      • Submucosa: Contains blood vessels, lymphatic vessels, and nerves.
      • Muscularis Externa: Has *three* layers of smooth muscle (outer longitudinal, middle circular, inner oblique) for mixing and churning.
      • Serosa: The outermost layer, part of the visceral peritoneum.
    • Functions:
      • Mechanical breakdown (churning).
      • Chemical breakdown (pepsin begins protein digestion; hydrochloric acid activates pepsinogen and kills bacteria).
      • Storage of food.
      • Formation of chyme.
      • Limited absorption (e.g., some water, alcohol, certain drugs).
  • Small Intestine:
    • The longest part of the GI tract (about 6 meters), where most digestion and absorption occur.
    • Divided into three sections:
      • Duodenum: The first, shortest section (about 25 cm). Receives chyme from the stomach, bile from the liver and gallbladder, and pancreatic juice from the pancreas. Contains Brunner's glands, which secrete alkaline mucus.
      • Jejunum: The middle section (about 2.5 meters).
      • Ileum: The final section (about 3.5 meters). Joins the large intestine at the ileocecal valve.
    • Structural modifications to increase surface area for absorption:
      • Plicae Circulares (Circular Folds): Permanent folds of the mucosa and submucosa.
      • Villi: Finger-like projections of the mucosa. Each villus contains blood capillaries and a lacteal (lymphatic capillary).
      • Microvilli: Tiny projections on the surface of the absorptive cells (enterocytes) that form the "brush border." Brush border enzymes complete the digestion of carbohydrates and proteins.
    • Functions:
      • Completes the chemical breakdown of carbohydrates, proteins, and fats.
      • Primary site of nutrient absorption.
      • Propulsion (peristalsis and segmentation).
  • Large Intestine:
    • Shorter than the small intestine (about 1.5 meters) but wider in diameter.
    • Regions:
      • Cecum: A pouch-like structure at the beginning of the large intestine. The appendix is attached to the cecum.
      • Colon: The longest part. Divided into:
        • Ascending Colon: Travels upward on the right side of the abdomen.
        • Transverse Colon: Crosses the abdomen horizontally.
        • Descending Colon: Travels downward on the left side of the abdomen.
        • Sigmoid Colon: An S-shaped portion that connects to the rectum.
      • Rectum: Stores feces before elimination.
      • Anal Canal: The final portion, with the internal anal sphincter (smooth muscle, involuntary) and the external anal sphincter (skeletal muscle, voluntary).
    • Distinctive features:
      • Teniae Coli: Three bands of longitudinal smooth muscle.
      • Haustra: Pouches or sacculations that give the colon its segmented appearance.
      • Epiploic Appendages: Small, fat-filled pouches of peritoneum.
    • Functions:
      • Absorption of water and electrolytes.
      • Formation and storage of feces.
      • Houses a large population of bacteria (gut microbiota) that ferment undigested carbohydrates and synthesize some vitamins (K and some B vitamins).

3. Accessory Digestive Organs

These organs contribute to digestion but are not part of the continuous alimentary canal.

• Teeth:
  • Located in the sockets (alveoli) of the mandible and maxilla.
  • Function: Mechanical breakdown of food (mastication/chewing).
    • Types of teeth:
      • Incisors: Chisel-shaped for cutting.
      • Canines (Cuspids): Cone-shaped for tearing.
      • Premolars (Bicuspids): Have two cusps for grinding.
      • Molars: Have multiple cusps for grinding.
    • Humans have two sets of teeth:
      • Deciduous Teeth (Baby Teeth): 20 teeth.
      • Permanent Teeth: 32 teeth.
• Salivary Glands:
  • Produce saliva, which moistens food, begins carbohydrate digestion, and has antibacterial properties.
  • Major salivary glands:
    • Parotid Glands: Largest; located near the ears. Secrete a watery saliva rich in amylase.
    • Submandibular Glands: Located below the mandible. Secrete a mixed serous (watery) and mucous saliva.
    • Sublingual Glands: Located under the tongue. Secrete a primarily mucous saliva.
  • Saliva contains:
    • Water: Moistens food and helps dissolve it for taste.
    • Salivary Amylase: An enzyme that begins the breakdown of starch.
    • Mucus: Lubricates food for swallowing.
    • Lysozyme: An enzyme that kills bacteria.
    • Immunoglobulin A (IgA): An antibody that helps prevent infection.
• Pancreas:
  • A retroperitoneal organ located posterior to the stomach.
  • Has both exocrine and endocrine functions.
  • Exocrine Function: Produces pancreatic juice, which contains enzymes for digesting carbohydrates, proteins, and fats, and bicarbonate ions to neutralize acidic chyme. Pancreatic juice is secreted into the duodenum via the pancreatic duct.
  • Endocrine Function: Pancreatic islets (Islets of Langerhans) produce hormones, including insulin and glucagon, which regulate blood glucose levels.
  • Pancreatic juice contains:
    • Pancreatic Amylase: Digests carbohydrates.
    • Pancreatic Lipase: Digests fats.
    • Trypsin, Chymotrypsin, Carboxypeptidase: Digest proteins. These are secreted in inactive forms (zymogens) and activated in the duodenum.
    • Nucleases: Digest nucleic acids (DNA and RNA).
    • Bicarbonate Ions (HCO3-): Neutralize acidic chyme entering the duodenum from the stomach.
• Liver:
  • The largest internal organ, located in the upper right quadrant of the abdomen.
  • Produces bile, which is essential for fat digestion and absorption.
  • Has numerous other metabolic and regulatory functions (see previous "Function of the Liver" responses for details).
• Gallbladder:
  • A small, pear-shaped sac located on the inferior surface of the liver.
  • Stores and concentrates bile produced by the liver.
  • Releases bile into the duodenum via the cystic duct and common bile duct.

4. Physiology of the Digestive System

• The physiological process of digestive system include ingestion, propulsion, digestion ( mechanical and chemical), absorption and elimination.

5. Activities Occurring in the Mouth, Pharynx, and Esophagus

• Mouth:
  • Ingestion.
  • Mechanical breakdown (chewing).
  • Chemical breakdown (salivary amylase begins carbohydrate digestion).
  • Formation of a bolus (a mass of chewed food mixed with saliva).
• Pharynx and Esophagus:
  • Swallowing (Deglutition): A complex process involving both voluntary and involuntary phases.
    • Buccal Phase (Voluntary): The tongue pushes the bolus into the oropharynx.
    • Pharyngeal-Esophageal Phase (Involuntary): Controlled by the swallowing center in the medulla oblongata. The soft palate rises to close off the nasopharynx, the larynx rises, and the epiglottis covers the glottis, preventing food from entering the trachea. The upper esophageal sphincter relaxes, allowing the bolus to enter the esophagus. Peristaltic waves move the bolus down the esophagus to the stomach. The lower esophageal sphincter relaxes to allow the bolus to enter the stomach.

• Activities of the Stomach:
  • Storage: The stomach can store food for several hours.
  • Mixing: Peristaltic waves mix the food with gastric juice, forming chyme.
  • Chemical Digestion:
    • Protein Digestion: Pepsin (formed from pepsinogen in the presence of HCl) begins the breakdown of proteins into smaller peptides.
    • Limited Fat Digestion: Gastric lipase plays a minor role in fat digestion.
  • Secretion: Gastric glands secrete gastric juice, which contains:
    • Hydrochloric Acid (HCl): Produced by parietal cells. Creates an acidic environment (pH 1.5-3.5) that:
      • Activates pepsinogen to pepsin.
      • Kills bacteria.
      • Denatures proteins.
    • Pepsinogen: Produced by chief cells. The inactive precursor to pepsin.
    • Mucus: Protects the stomach lining from the acidic environment.
    • Intrinsic Factor: Produced by parietal cells. Essential for the absorption of vitamin B12 in the small intestine.
  • Regulation of Gastric Secretion and Motility: Controlled by neural and hormonal mechanisms. Three phases:
    • Cephalic Phase: Occurs before food enters the stomach. Triggered by the sight, smell, taste, or thought of food. Stimulates gastric secretion and motility via the vagus nerve.
    • Gastric Phase: Occurs when food enters the stomach. Distension of the stomach and the presence of peptides stimulate gastrin release (from G cells in the stomach), which further stimulates gastric secretion and motility.
    • Intestinal Phase: Occurs when chyme enters the duodenum. Initially, there is a brief stimulatory effect. However, as the duodenum distends and becomes more acidic, inhibitory mechanisms are triggered (enterogastric reflex, release of secretin and cholecystokinin (CCK)), which slow down gastric emptying and reduce gastric secretion.
  • Gastric Emptying: The pyloric sphincter controls the release of chyme from the stomach into the duodenum. The rate of gastric emptying is regulated by factors in both the stomach and the duodenum.

• Activities of the Small Intestine:
  • Mixing and Propulsion:
    • Segmentation: Localized contractions that mix chyme with digestive juices and bring it into contact with the absorptive surface. The primary type of movement in the small intestine.
    • Peristalsis: Wave-like contractions that propel chyme along the small intestine. Weaker than in the esophagus.
  • Digestion:
    • Carbohydrates: Pancreatic amylase continues the breakdown of starch. Brush border enzymes (e.g., maltase, sucrase, lactase) break down disaccharides into monosaccharides.
    • Proteins: Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) continue the breakdown of proteins into smaller peptides. Brush border peptidases break down peptides into amino acids.
    • Fats: Bile emulsifies fats (breaks them into smaller droplets). Pancreatic lipase breaks down triglycerides into fatty acids and monoglycerides.
    • Nucleic Acids: Pancreatic nucleases break down DNA and RNA into nucleotides.
  • Absorption: Most nutrient absorption occurs in the small intestine.
    • Monosaccharides (glucose, fructose, galactose): Absorbed by facilitated diffusion and active transport into the blood capillaries of the villi.
    • Amino Acids: Absorbed by active transport into the blood capillaries of the villi.
    • Fatty Acids and Monoglycerides: Combine with bile salts to form micelles, which facilitate their absorption into the enterocytes (intestinal cells). Inside the enterocytes, they are reassembled into triglycerides and packaged into chylomicrons, which enter the lacteals (lymphatic capillaries) of the villi.
    • Water: Absorbed by osmosis.
    • Electrolytes: Absorbed by various mechanisms (diffusion, active transport).
    • Vitamins: Fat-soluble vitamins (A, D, E, K) are absorbed along with fats. Water-soluble vitamins are absorbed by diffusion or active transport.

• Activities of the Large Intestine:
  • Propulsion:
    • Haustral Contractions: Slow, segmenting movements that mix the contents and facilitate water absorption.
    • Mass Movements: Powerful, peristaltic contractions that occur a few times a day, propelling feces towards the rectum.
  • Absorption: Primarily water and electrolytes (sodium, chloride).
  • Bacterial Action: The large intestine houses a large population of bacteria (gut microbiota) that:
    • Ferment undigested carbohydrates, producing gases (methane, hydrogen sulfide).
    • Synthesize some vitamins (vitamin K and some B vitamins).
  • Defecation: The elimination of feces from the body. The defecation reflex is triggered by distension of the rectum. It involves relaxation of the internal anal sphincter (involuntary) and, if the time is appropriate, voluntary relaxation of the external anal sphincter.

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