Digestive system

Dr. BP Kumar

Introduction 

  1. The alimentary system, also known as the digestive system, is responsible for the breakdown of food, absorption of nutrients, and elimination of waste.
  2. It consists of a long, muscular tube called the gastrointestinal (GI) tract and various accessory digestive organs that aid digestion.
  3. This system plays a crucial role in maintaining overall health by ensuring the body gets essential nutrients, such as carbohydrates, proteins, fats, vitamins, minerals, and water, required for energy production, growth, and repair.

The major functions of the alimentary system include:

  1. Ingestion: Intake of food.
  2. Digestion: Breakdown of food into simpler substances (mechanical and chemical digestion).
  3. Absorption: Transfer of nutrients into the bloodstream.
  4. Assimilation: Utilization of absorbed nutrients by body cells.
  5. Excretion: Elimination of undigested and waste materials.

Components of the Alimentary System

The alimentary system is divided into two parts:

  • Gastrointestinal (GI) Tract: A continuous hollow tube that extends from the mouth to the anus.
  • Accessory Digestive Organs: Glands that produce digestive enzymes and fluids.

A. Gastrointestinal (GI) Tract

Mouth (Oral Cavity)

The mouth is the first organ in the digestive system, responsible for mechanical and chemical digestion.

Structures in the Mouth:

  • Teeth: Mechanically break food into smaller pieces (mastication).
  • Tongue: Helps in food manipulation and pushing food towards the throat.
  • Salivary Glands: Secrete saliva, which contains enzymes for digestion.
  • Saliva Components:
    • Water (99.5%): Moistens food.
    • Mucus: Lubricates food for easy swallowing.
    • Enzymes:
      • Salivary Amylase (Ptyalin): Breaks down starch into maltose.
      • Lingual Lipase: Begins fat digestion.

Functions of the Mouth:

  • Ingestion of food.
  • Mastication (chewing) for mechanical breakdown.
  • Salivation and enzyme action for chemical digestion.
  • Swallowing (Deglutition): Moving food into the pharynx.

Pharynx (Throat)

The pharynx is a muscular passage that is a common route for food and air. It has three parts:

  1. Nasopharynx – Air passage.
  2. Oropharynx – Connects to the mouth.
  3. Laryngopharynx – Leads to the esophagus.

Function:

  • Swallowing (Deglutition): Moves food from the mouth to the oesophagus.
  • Epiglottis Action: Prevents food from entering the trachea (windpipe).

Esophagus

The oesophagus is a muscular tube (~25 cm long) that connects the pharynx to the stomach.

Movement of Food:

  • Peristalsis: A wave-like muscle contraction that pushes food down.
  • Lower Esophageal Sphincter (Cardiac Sphincter): Prevents stomach acid reflux.

Function:

  • Transportation of food to the stomach.
  • Prevention of acid reflux through the lower oesophagal sphincter.

Stomach

The stomach is a J-shaped muscular organ that stores food and breaks it down into chyme.

Regions of the Stomach:

  1. Cardia – Connects with the oesophagus.
  2. Fundus – Stores undigested food.
  3. Body – Main region for digestion.
  4. Pylorus – Regulates emptying into the small intestine.

Gastric Secretions:

  • Hydrochloric Acid (HCl): Provides acidity (pH 1.5-3.5) to kill microbes and activate enzymes.
  • Pepsinogen → Pepsin: Digests proteins into peptides.
  • Gastric Lipase: Begins fat digestion.
  • Mucus: Protects the stomach lining.
  • Intrinsic Factor: Required for vitamin B12 absorption.

Functions of the Stomach:

  • Stores food for controlled release.
  • Mechanical digestion through churning.
  • Chemical digestion of proteins and fats.
  • Secretes hormones (Gastrin) to regulate digestion.
  • Chyme formation for easy absorption.

Small Intestine

Most digestion and absorption occur in the small intestine (~6 meters long).

Sections of the Small Intestine:

  1. Duodenum (25 cm): Receives bile from the liver and enzymes from the pancreas.
  2. Jejunum (2.5 m): Major site of nutrient absorption.
  3. Ileum (3.5 m): Absorbs remaining nutrients and vitamin B12.

Digestive Enzymes in the Small Intestine:

  • Carbohydrate Digestion:
    • Pancreatic Amylase: Breaks down starch.
    • Maltase, Sucrase, Lactase: Break down sugars.
  • Protein Digestion:
    • Trypsin & Chymotrypsin: Break proteins into peptides.
    • Peptidases: Convert peptides into amino acids.
  • Fat Digestion:
    • Pancreatic Lipase: Breaks fats into glycerol and fatty acids.
    • Bile Salts: Emulsify fats.

Absorption of Nutrients:

  • Carbohydrates → Glucose → Bloodstream.
  • Proteins → Amino Acids → Bloodstream.
  • Fats → Fatty Acids & Glycerol → Lymphatic System (Lacteals).

Functions of the Small Intestine:

  • Final digestion of food.
  • Absorption of nutrients through villi and microvilli.
  • Neutralization of acidic chyme using bicarbonate.

Large Intestine (Colon)

The large intestine (~1.5 meters long) absorbs water, electrolytes, and vitamins and forms faeces.

Parts of the Large Intestine:

  1. Cecum: The first part contains the appendix.
  2. Ascending Colon: Absorbs water and minerals.
  3. Transverse Colon: Fermentation of undigested materials.
  4. Descending Colon: Stores solid waste.
  5. Sigmoid Colon: Moves waste to the rectum.

Functions of the Large Intestine:

  • Absorption of Water and Electrolytes.
  • Fermentation by Gut Bacteria (produces Vitamin K and B12).
  • Formation and Storage of Feces.
  • Excretion of Waste.

Rectum and Anus

The rectum stores faeces before elimination. The anus controls defecation.

Anus Control:

  • Internal Anal Sphincter (Involuntary).
  • External Anal Sphincter (Voluntary).

Function:

  • Elimination of waste through the defecation reflex.

B. Accessory Digestive Organs

Salivary Glands

The salivary glands are exocrine glands that produce saliva, which moistens food and contains enzymes that start digestion.

Types of Salivary Glands:

  1. Parotid Glands:
    • Largest salivary glands, located near the ears.
    • Secrete watery, enzyme-rich saliva.
    • Produces most of the salivary amylase (breaks down starch).
  2. Submandibular Glands:
    • Located beneath the lower jaw.
    • Secrete both watery and mucus-rich saliva.
  3. Sublingual Glands:
    • Smallest salivary glands, located under the tongue.
    • Produce mucus-rich saliva for lubrication.

Functions of Saliva:

  • Moistens food for easy swallowing.
  • Lubricates oral cavity (prevents dryness).
  • Begins digestion:
    • Salivary Amylase (Ptyalin): Converts starch into maltose.
    • Lingual Lipase: Starts fat digestion (activated in the stomach).
  • Antibacterial Properties: Contains lysozyme and antibodies to kill microbes.

Liver

The largest internal organ in the upper right abdomen is the liver, weighing about 1.5 kg. It is essential for bile production, metabolism, detoxification, and nutrient storage.

Liver Structure:

  • Divided into right and left lobes.
  • Functional units: Lobules containing hepatocytes (liver cells).
  • Blood supply:
    • Hepatic Artery: Brings oxygen-rich blood.
    • Hepatic Portal Vein: Brings nutrient-rich blood from the intestines.

Digestive Functions of the Liver:

  1. Bile Production:

    • The liver produces bile (500–1000 mL/day) stored in the gallbladder.
    • Bile Components:
      • Bile salts: Help in fat emulsification.
      • Bilirubin: A byproduct of RBC breakdown.
      • Cholesterol and Phospholipids.

  2. Metabolism of Nutrients:

    • Carbohydrates: Converts glucose into glycogen for storage.
    • Proteins: Synthesizes plasma proteins (albumin, clotting factors) and converts ammonia into urea for excretion.
    • Fats: Regulates lipid metabolism and cholesterol levels.

  3. Detoxification:

    • Breaks down toxins, drugs, and alcohol.
    • Converts harmful substances into less toxic forms for excretion.

  4. Storage of Nutrients:

    • Stores glycogen, vitamins (A, D, B12), and iron.

Non-Digestive Functions of the Liver:

  • Produces blood clotting factors.
  • Regulates hormones and cholesterol levels.
  • Supports the immune system by removing bacteria and toxins from the blood.

Gallbladder

The gallbladder is a small, pear-shaped organ located beneath the liver. It stores and releases bile into the small intestine.

Functions of the Gallbladder:

  1. Storage of Bile:

    • Bile is stored and concentrated (5-10 times) in the gallbladder.

  2. Release of Bile:

    • When fatty food enters the duodenum, the hormone cholecystokinin (CCK) signals the gallbladder to release bile.
    • Bile emulsifies fats, increasing their surface area for lipase action.

  3. Regulation of Fat Digestion:

    • Helps in fat absorption and cholesterol excretion.

Gallbladder Disorders:

  • Gallstones (Cholelithiasis): Solid deposits of cholesterol or bilirubin.
  • Cholecystitis: Inflammation due to gallstone blockage.

Pancreas

The pancreas is a dual-function organ that acts as an endocrine and exocrine gland. It is located behind the stomach and secretes digestive enzymes and hormones.

Pancreatic Functions:

(A) Exocrine Function (Digestive Role):

The pancreatic acini cells secrete pancreatic juice (1.5 L/day) into the duodenum via the pancreatic duct.

Components of Pancreatic Juice:

  • Enzymes for digestion:

    1. Carbohydrate Digestion:
      • Pancreatic Amylase → Breaks starch into maltose.
    2. Protein Digestion:
      • Trypsinogen → Trypsin: Breaks proteins into peptides.
      • Chymotrypsin: Further breaks peptides into amino acids.
      • Carboxypeptidase: Acts on peptide bonds.
    3. Fat Digestion:
      • Pancreatic Lipase: Breaks triglycerides into fatty acids and glycerol.
    4. Nucleic Acid Digestion:
      • Deoxyribonuclease (DNase) & Ribonuclease (RNase): Break down DNA & RNA.

  • Bicarbonate (HCO₃⁻):

    • Neutralizes stomach acid.
    • Provides an optimal pH (~7.5) for enzyme activity.

(B) Endocrine Function (Hormonal Role):

The Islets of Langerhans in the pancreas secrete hormones into the bloodstream to regulate metabolism:

  1. Insulin (from Beta Cells):
    • Lowers blood glucose by promoting glucose uptake in cells.
  2. Glucagon (from Alpha Cells):
    • Increases blood glucose by stimulating glycogen breakdown.
  3. Somatostatin (from Delta Cells):
    • Inhibits insulin and glucagon secretion.

Pancreatic Disorders:

    • Pancreatitis: Inflammation due to gallstones or alcohol.
    • Diabetes Mellitus: Caused by insulin deficiency or resistance.

Mechanism of Digestion

A. Mechanical Digestion

  • Physical breakdown of food.
  • Occurs in:
    • Mouth: Chewing.
    • Stomach: Churning.
    • Small Intestine: Segmentation.

B. Chemical Digestion

  • Breakdown of food molecules using enzymes.
  • Occurs in:
    • Mouth: Salivary amylase starts carbohydrate digestion.
    • Stomach: Pepsin digests proteins.
    • Small Intestine:
      • Pancreatic and brush border enzymes complete digestion.
      • Bile aids in fat digestion.

Absorption of Nutrients

Absorption occurs mainly in the small intestine through villi and microvilli.

A. Carbohydrate Absorption

  • Monosaccharides (Glucose, Fructose, Galactose) are absorbed via active transport and facilitated diffusion.
  • Transported to the bloodstream.

B. Protein Absorption

  • Amino acids are absorbed via active transport into blood capillaries.

C. Fat Absorption

  • Fatty acids and monoglycerides are absorbed via lacteals (lymphatic vessels).
  • Converted into chylomicrons and transported via lymph.

D. Water and Electrolyte Absorption

  • Water absorption: Occurs via osmosis.
  • Electrolytes (Na+, K+, Cl-, Ca2+) are absorbed via active transport.

Functions of the Large Intestine

  • Water and electrolyte absorption.
  • Fermentation of undigested carbohydrates by gut bacteria.
  • Formation and elimination of faeces.

Regulation of Digestion

A. Nervous Control (Autonomic Nervous System)

  • Parasympathetic (Vagus Nerve): Increases digestion.
  • Sympathetic: Inhibits digestion.

B. Hormonal Control

  1. Gastrin: Stimulates gastric juice secretion.
  2. Secretin: Stimulates bile and bicarbonate release.
  3. Cholecystokinin (CCK): Stimulates bile and pancreatic enzyme release.
  4. Ghrelin: Increases appetite.
  5. Leptin: Regulates satiety.

Leave a Reply

Your email address will not be published. Required fields are marked *