DIGESTIVE SYSTEM: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
The digestive system helps our body take in food, break it down into smaller nutrients, and absorb them for energy and growth. It transfers nutrients from the outside environment (food we eat) to the inside environment (our blood and cells). The circulatory system then distributes these nutrients to all body cells.
Dpharmguru’s exam insights:
In my years of teaching gastrointestinal physiology, I have observed that students often confuse the alimentary canal with the accessory organs. Remember: The alimentary canal is the tube from mouth to anus (food passes through it). The accessory organs (salivary glands, pancreas, liver, gall bladder) help with digestion but food does not pass through them. This is a very common exam question!
ANATOMY OF THE GASTROINTESTINAL TRACT (GIT)
The alimentary canal is a long muscular tube that begins at the mouth and ends at the anus. Food passes through different organs for digestion and absorption in this order: Oral cavity (mouth), Pharynx (throat), Oesophagus (food pipe), Stomach, Small intestine, Large intestine.
ORAL CAVITY (MOUTH)
The mouth is the first part of the digestive tract. It is surrounded by lips (front), cheeks (sides), tongue and floor muscles (below), hard and soft palate (above), and pharynx (back). The roof of the mouth is called the palate, divided into the hard palate (bony front part) and soft palate (muscular back part ending in the uvula).
Functions of the Mouth
- Takes in food (ingestion)
- Tastes food
- Mixes and lubricates food with saliva
- Starts digestion with salivary enzymes
- Helps in speech
- Used for some medicines (sublingual route)
Teeth
Teeth help in chewing (mastication) — breaking food into smaller pieces for easy swallowing and digestion.
| Type | Number | Function |
|---|---|---|
| Incisors | 8 | Cutting and biting food |
| Canines (Cuspids) | 4 | Tearing food |
| Premolars (Bicuspids) | 8 | Grinding food |
| Molars | 12 | Crushing and grinding food |
Tongue
The tongue is a muscular organ located on the floor of the mouth. It helps in chewing, swallowing, and speaking. It is rich in nerves and blood supply, which allow movement and taste sensation.
Dpharmguru’s exam insights:
Remember the taste zones on the tongue: Sweet at the tip, Salty on the sides (front), Sour on the sides (middle), and Bitter at the back. This is a classic exam question!
OESOPHAGUS (FOOD PIPE)
The oesophagus is a long muscular tube (~25 cm) connecting the pharynx to the stomach. It lies behind the trachea and passes through the diaphragm to join the stomach at the cardiac sphincter.
Functions of Oesophagus
- Carries food from mouth to stomach
- Produces mucus for smooth passage
- Prevents food from entering the windpipe
- Pushes food using peristaltic contractions
STOMACH
The stomach is a J-shaped, muscular bag located on the left side of the upper abdomen, between the oesophagus and the small intestine. It temporarily stores food, mixes it with gastric juices, and begins the chemical digestion of proteins.
Parts of the Stomach
- Cardia: Upper part that connects the oesophagus to the stomach.
- Fundus: Dome-shaped upper part that collects gases.
- Body (Corpus): The middle and largest part; main site for mixing food with gastric juice.
- Pylorus (Antrum): Lower part that connects to the duodenum; controls food passage through the pyloric sphincter.
Gastric Juice and Its Composition
The stomach secretes gastric juice, which is acidic (pH 1–3) and contains water, hydrochloric acid (HCl), enzymes (Pepsin, Lipase, Rennin), mucus, intrinsic factor, and mineral salts.
| Component | Function |
|---|---|
| Pepsin | Begins protein digestion; converts proteins into peptides. |
| HCl (Hydrochloric acid) | Kills bacteria, activates pepsin, maintains acidic pH, stimulates bile and pancreatic juice flow. |
| Gastric Lipase | Breaks down fats into fatty acids and glycerol. |
| Rennin (in infants) | Curds milk for easier digestion. |
| Mucus | Protects the stomach wall from self-digestion by acid. |
| Intrinsic Factor | Helps in absorption of Vitamin B₁₂. |
Dpharmguru’s exam insights:
Intrinsic factor is essential for Vitamin B12 absorption. A deficiency of intrinsic factor leads to pernicious anaemia. This is a very common exam question in haematology and gastroenterology!
SMALL INTESTINE
The small intestine is a long, coiled tube about 6–7 meters in length and 2.5–3 cm in diameter. It is the main site for digestion and absorption of nutrients. Its inner wall has folds called villi and microvilli, which greatly increase the surface area for absorption.
Parts of Small Intestine
- Duodenum: The first and shortest part (~25 cm). Receives bile and pancreatic juice. Major site of chemical digestion.
- Jejunum: The middle portion (~2.5 m). Has many folds and villi for nutrient absorption.
- Ileum: The last and longest part (~3.5 m). Joins the large intestine at the ileocecal valve. Absorbs remaining nutrients and vitamins.
Functions of Small Intestine
- Digestion: Completes digestion of carbohydrates, proteins, and fats using enzymes.
- Absorption: Absorbs 90% of nutrients, minerals, and water.
- Secretion: Produces intestinal juice and hormones like secretin and cholecystokinin (CCK).
- Peristalsis: Moves chyme forward using smooth muscle contractions.
- Immunity: Has Peyer’s patches (lymph tissues) that defend against harmful microbes.
LARGE INTESTINE
The large intestine is about 1.5 meters long and 6–7 cm wide. It absorbs water and salts, forms and stores faeces, and helps in vitamin production.
Parts of Large Intestine
- Caecum: First part; a small pouch connected to the ileum. Has the appendix (contains lymph tissue).
- Colon: Major part with four sections: Ascending colon, Transverse colon, Descending colon, Sigmoid colon.
- Rectum: Final straight portion (~12 cm). Stores faeces temporarily.
- Anus: External opening for waste removal. Controlled by two sphincters – internal (involuntary) and external (voluntary).
Functions of Large Intestine
- Absorbs water, electrolytes, and vitamins (especially vitamin K and folic acid).
- Houses friendly bacteria like E. coli that help in vitamin synthesis.
- Lubricates faeces with mucus for smooth passage.
- Stores waste and helps in its removal (defecation).
- Performs segmental movements to mix and move contents.
ACCESSORY GLANDS OF DIGESTION
The accessory glands help in digestion by producing enzymes or digestive juices, but food does not directly pass through them. These glands include: Salivary glands, Pancreas, Liver, and Gall bladder.
Salivary Glands
- Parotid Glands: Largest salivary glands. Present in front of each ear. Secrete watery fluid containing salivary amylase.
- Submandibular Glands: Second largest glands, present below the jawbone. Secrete both mucus and watery secretions.
- Sublingual Glands: Smallest pair of glands, located under the tongue. Secrete mainly mucus.
Pancreas
The pancreas is a soft, leaf-shaped gland about 6–10 inches long. It lies behind the stomach and is connected to the duodenum. It is a mixed gland — having both exocrine and endocrine parts.
- Exocrine Function: Produces pancreatic juice containing enzymes that digest carbohydrates, proteins, and fats. Also releases bicarbonate to neutralize stomach acid.
- Endocrine Function: Secretes insulin (lowers blood sugar), glucagon (increases blood sugar), and somatostatin (controls hormone secretion).
Dpharmguru’s exam insights:
Insulin lowers blood sugar, and glucagon raises blood sugar. This is the most important concept in diabetes mellitus. Remember: Insulin is produced by beta cells, and glucagon by alpha cells of the Islets of Langerhans.
Liver
The liver is the largest gland and the second largest organ (after skin) of the human body. It weighs about 1.4 kg and is located on the right side under the diaphragm.
Functions of Liver
- Carbohydrate Metabolism: Converts glucose to glycogen and back when needed.
- Lipid Metabolism: Stores fats and makes cholesterol and lipoproteins. Produces bile salts from cholesterol.
- Protein Metabolism: Removes amino group from amino acids and converts ammonia to urea. Produces plasma proteins like albumin and fibrinogen.
- Detoxification: Breaks down drugs, alcohol, and hormones.
- Excretion of Bilirubin: Removes bilirubin from old red blood cells into bile.
- Synthesis of Bile Salts: Needed for fat digestion and absorption.
- Storage Function: Stores vitamins (A, D, E, K, B₁₂) and minerals.
- Phagocytosis: Kupffer cells destroy worn-out RBCs, WBCs, and bacteria.
Gall Bladder
The gall bladder is a small pear-shaped sac under the right lobe of the liver. It is about 8 cm long and 4 cm wide, and stores bile produced by the liver.
Functions of Gall Bladder
- Storage of Bile: Stores bile until needed for digestion.
- Emulsification of Fats: Bile breaks large fat globules into smaller droplets.
- Neutralization of Acid: Bile helps neutralize stomach acid in the small intestine.
- Absorption Aid: Bile helps absorb fats and fat-soluble vitamins (A, D, E, K).
- Waste Removal: Removes bilirubin and toxins from the body.
PHYSIOLOGY OF DIGESTION AND ABSORPTION
The process of digestion involves five main steps:
- Ingestion: Food enters through the mouth. Chewing and mixing with saliva forms a soft mass called bolus.
- Propulsion: Bolus is pushed from mouth to oesophagus by swallowing. Peristalsis moves food through the digestive tract.
- Mechanical and Chemical Digestion: Mechanical digestion involves chewing and churning. Chemical digestion uses enzymes to break large food molecules into smaller absorbable ones.
- Absorption: Digested nutrients move from the intestine into the blood and lymph. Methods include Active Transport, Passive Diffusion, Facilitated Diffusion, Co-transport, and Endocytosis.
- Defecation: Undigested material and waste are eliminated as faeces through the anus.
Dpharmguru’s exam insights:
The digestive system is responsible for breaking down food and absorbing nutrients. In exams, focus on the functions of each organ, the role of digestive enzymes, and the absorption mechanisms in the small intestine. Remember: The liver produces bile, the pancreas produces digestive enzymes and insulin, and the gall bladder stores bile. These concepts are almost guaranteed to appear in exams!
REFERENCES AND FURTHER READING
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
- Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
- Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
- Hall, J. E., & Guyton, A. C. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
- National Institutes of Health (NIH). (2022). Digestive System Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
