Topics to be Learn :

  • Introduction
  • Human Digestive System
  • Histological Structure of Alimentary Canal
  • Digestive Glands
  • Physiology of Digestion
  • Absorption, Assimilation and Egestion
  • Nutritional Disorders and Disorders of
  • Digestive System
Introduction
  • Nutrition and Respiration provide essential energy for the body.
  • Human dietary needs include Carbohydrates, Proteins, Fats, Vitamins, Minerals, Water, and Fibres in adequate amounts.
Nutrition
  • Definition: Nutrition is the process of consuming and utilizing food substances.
  • WHO defines it as the intake of food in relation to the body’s dietary needs.
Steps in Nutrition
  • Ingestion: Intake of food through the mouth.
  • Digestion: Conversion of complex, non-diffusible, non-absorbable substances into simple, absorbable forms using enzymes.
  • Absorption: Diffusion of digested nutrients into blood and lymph.
  • Assimilation: Use of absorbed nutrients to synthesize cell protoplasm.
  • Egestion: Removal of undigested food from the body.
Importance of Digestion
  • Function: Transforms complex nutrients (carbs, proteins, lipids, vitamins) into simple, absorbable compounds.
  • Outcome: Provides the body with essential nutrients in a usable form.
Human Digestive System
 
Structure: Consists of the alimentary canal and associated digestive glands.
Alimentary Canal: Long tube (8-10 meters), running from mouth to anus.
  • Mouth: Entry for food; includes lips, cheeks, palate, tongue, and mucous membrane. Contains salivary glands.
Teeth: Adult human has 32 teeth, described as Thecodont, Diphyodont, and Heterodont.
  • Thecodont: Each tooth is in a separate jaw socket.
  • Diphyodont: Two sets of teeth in a lifetime – milk teeth and permanent teeth.
  • Heterodont: Four types of teeth – Incisors (I), Canines (C), Premolars (PM), and Molars (M).
Teeth Classification Table

Dental Formula:
  • I 2/2, C 1/1, PM 2/2, M 3/3=2+1+2+3×2=32 teeth
Structure of Tooth

Tooth Components:

  • Crown: Visible part above the gum; covered by enamel (hardest body substance, made of calcium phosphate and calcium carbonate).
  • Root: Embedded projections in gum (usually two or three).
  • Neck: Connects crown to root.
  • Dentin: Calcified tissue forming the tooth's basic shape, surrounding the pulp cavity (contains blood vessels and nerves).
  • Root Canal: Extension of pulp cavity within the root.
  • Cementum: Bone-like layer covering root dentin, attaching root to gum socket.

Function: Teeth masticate (chew) food, breaking it into smaller particles and mixing it with saliva.

Human Dentition

Types:

  • Thecodont: Teeth anchored in jaw sockets by gomphosis joints.
  • Diphyodont: Humans develop two sets of teeth – milk teeth and permanent teeth.
  • Heterodont: Four types of teeth (incisors, canines, premolars, molars).

Definition: Dentition is the study of teeth concerning their number, arrangement, and development.

Tongue
  • Description: Muscular, fleshy, triangular organ on the buccal cavity floor.
Functions:
  • Papillae: Projections on upper surface containing taste buds (detect flavors).
  • Bolus Formation: Mixes saliva with food for easy swallowing.
  • Assists in swallowing and speech.
Pharynx

Location: Short passage from buccal cavity, common to food and air.

  • Glottis: Opening into the trachea.
  • Epiglottis: Cartilaginous flap that closes during swallowing (deglutition) to prevent food from entering trachea.
  • Oropharynx: Lower part of the pharynx, opening to oesophagus through the gullet.
Function: Pharynx guides food toward the oesophagus.

Controlled by the medulla oblongata. Phases:

  • Oral Phase (voluntary)
  • Pharyngeal Phase (involuntary)
  • Oesophageal Phase (involuntary)
Oesophagus

Description: Thin, muscular tube (approximately 25 cm long), lying behind the trachea.

Structure:

  • Lined by mucus cells for lubrication.
  • Composed of longitudinal and circular muscles.

Function: Uses peristalsis (rhythmic muscle contractions) to push food toward the stomach.

Stomach
  • Location: Upper left side of abdominal cavity.
  • Shape: Muscular, sac-like 'J'-shaped organ, 25-30 cm long.
Regions:
  • Cardia: First part; where oesophagus meets stomach. Has cardiac sphincter to prevent backflow.
  • Fundus: Dome-shaped area above and left of cardia.
  • Body: Main central section, stores food.
  • Pylorus: Narrow end that connects to the duodenum (beginning of small intestine). Has pyloric sphincter to control food flow.
Function: Temporarily stores food; churns and mixes it with gastric juice.
Small Intestine

Description: Long, narrow tube (~6 meters, 2.5 cm wide) coiled in the abdominal cavity.

  • Duodenum: First part (26 cm), U-shaped, beneath the stomach.
  • Jejunum: Middle part (2.5 meters), coiled and narrower.
  • Ileum: Last part (3.5 meters), highly coiled, opens into caecum (beginning of large intestine).
Function: Site of digestion and absorption of nutrients.
Large Intestine

Description: Shorter (~1.5 meters) and wider than the small intestine.

  • Caecum: Small, blind sac at junction of ileum and colon; hosts symbiotic microorganisms. Has appendix (vestigial in humans).
  • Colon: Divided into ascending, transverse, and descending sections; lined with mucosal cells.
  • Rectum: Last part of large intestine; stores faeces until egestion.
Function: Absorbs water and minerals; stores and compacts waste.
Anus
  • Description: Terminal opening of the alimentary canal.
  • Function: Expels waste (fecal matter) through egestion/defecation; guarded by sphincter muscles.
Comparison: Small Intestine vs. Large Intestine

Histological Structure of Alimentary Canal

Layers of the Gastrointestinal Tract

The gastrointestinal tract has four basic layers from inside to outside:
  • Mucosa
  • Submucosa
  • Muscularis
  • Serosa
These layers may modify based on the location and function of the organ.
Serosa
  • Outermost layer of the alimentary canal.
  • Composed of: Mesothelium: Layer of squamous epithelium. Connective tissue: Inner layer.
Muscularis
  • Composed of smooth muscles.
Arranged in three concentric layers:
  • Outermost: Longitudinal muscles.
  • Middle: Circular muscles.
  • Innermost: Oblique muscles (absent in the intestine).
Thickness varies:
  • Wider in the stomach.
  • Thinner in the intestines.
Submucosa
  • Made of loose connective tissue.
  • Contains: Blood vessels, Lymph vessels, Nerves.
  • Duodenal submucosa: Contains glands.
Mucosa
  • Lines the lumen of the alimentary canal.
  • Contains goblet cells that secrete mucus, aiding in lubrication.
Modifications include:
  • Stomach: Irregular folds called rugae.
  • Gastric glands: Secrete gastric juice.
Villi
  • Finger-like foldings in the mucosa of the small intestine.
  • Lined by: Brush border: Epithelial cells with microvilli.
  • Supplied with: Capillaries, Lymph vessels (called lacteals).
  • Crypts of Lieberkuhn: Intestinal glands located between villi bases.
Significance of Glandular Epithelium of Mucosa

Goblet cells secrete mucus:

  • Lubricates the lumen of the alimentary canal.
  • Facilitates the movement of food through the gastrointestinal tract.

Digestive Glands

  • Types of Digestive Glands: Salivary Glands, Liver, Pancreas.
Salivary Glands

Three pairs of salivary glands open into the buccal cavity:

  • Parotid glands: Located in front of the ear.
  • Submandibular glands: Located below the lower jaw.
  • Sublingual glands: Located below the tongue.
Composed of two types of cells:
  • Serous cells: Secrete a fluid containing the digestive enzyme salivary amylase.
  • Mucous cells: Produce mucus that lubricates food and aids in swallowing.
Liver
  • Largest gland in the body, dark reddish-brown color.
  • Weight: 1.2 to 1.5 kg in adults.
  • Location: Right upper portion of the abdominal cavity, below the diaphragm.
Structure:
  • Divided into two lobes: right and left.
  • Covered by Glisson’s capsule (connective tissue sheath).
  • Contains hepatic lobules (functional units) composed of hepatocytes (liver cells).
  • Each lobule has a central vein and triangular portal area (contains branches of hepatic artery, hepatic portal vein, and bile duct).
  • Sinusoids: Spaces between hepatic cells where blood flows, containing Kupffer cells (phagocytic cells).
Functions:
  • Secretes bile (aids in digestion).
  • Synthesizes vitamins A, D, K, and B12.
  • Produces blood proteins like prothrombin and fibrinogen.
  • Stores excess glucose as glycogen.
  • Involved in deamination of excess amino acids.
Bile Ducts
  • Bile duct: Carries bile from the gall bladder to the intestine.
  • Common hepatic duct: Drains bile from the liver and aids in waste transportation.
Pancreas
  • Shape: Leaf-shaped, heterocrine gland.
  • Location: Gap formed by the bend of the duodenum under the stomach.
Exocrine part:
  • Composed of acini.
  • Acinar cells secrete alkaline pancreatic juice containing digestive enzymes.
  • Pancreatic juice is transported to the duodenum via the pancreatic duct.
  • Joins the common bile duct to form the hepato-pancreatic duct, which opens into the duodenum, guarded by the sphincter of Oddi.
Endocrine part:
  • Composed of islets of Langerhans.
  • Cell types: Alpha cells: Secrete glucagon, Beta cells: Secrete insulin, Delta cells: Secrete somatostatin (inhibits glucagon and insulin secretion).
Importance of Liver
  • Kupffer cells destroy toxins, dead cells, and microorganisms.
  • Bile juice emulsifies fats and makes food alkaline.
  • Stores excess glucose as glycogen.
  • Deamination of excess amino acids occurs here.
  • Synthesis of vitamins (A, D, K, B1) and blood proteins.
  • Acts as a hemopoietic organ during early development.

Effects of Alcohol on Liver

  • Alcoholism can lead to: Steatosis (fatty liver), Alcoholic hepatitis, Fibrosis, Cirrhosis (distorted/scarred liver)
  • Most alcohol is metabolized in the liver by alcohol dehydrogenase, converting ethanol to acetaldehyde (toxic).
  • Overconsumption can lead to liver failure.
Physiology of Digestion

Involves mechanical and biochemical methods:
Mechanical Digestion:
  • Involves movements of the alimentary canal.
  • Includes: Mastication (chewing), Churning in the stomach, Peristaltic movements of the gastrointestinal tract.
Chemical Digestion: A series of catabolic reactions that hydrolyze food.
Digestion in the Mouth (Buccal Cavity)
  • Both mechanical and chemical digestion occur.
Mastication:
  • Involves teeth and tongue.
  • Teeth crush and grind food; tongue manipulates food.
  • Food is moistened by saliva, aiding crushing.
Saliva:
  • Contains mucus that lubricates and binds food into a mass called bolus.
  • Bolus is swallowed via deglutition.
  • The tongue pushes the bolus into the pharynx and down the oesophagus.
Chemical Digestion:
  • Salivary amylase converts starch (polysaccharide) into maltose (disaccharide).
  • About 30% starch is converted to maltose in the mouth.
Passage of food:
  • Controlled by the gastrooesophageal sphincter.
  • Bolus moves through the oesophagus by peristalsis into the stomach.

Constituents of Saliva

  • Composition: 98% water, 2% electrolytes (sodium, potassium, calcium, chloride, bicarbonates), salivary amylase, and lysozyme (antibacterial agent).
Digestion in the Stomach
  • Structure: Muscular, sac-like, ‘J’ shaped organ.
  • Duration: Stores food for 4-5 hours.
Mechanical Digestion:
  • Involves churning by the thick muscular wall.
  • Breaks down food particles and mixes with gastric juice.
Chemical Digestion:

Mucosa has gastric glands with:

  • Mucus cells: Secrete mucus.
  • Peptic (chief) cells: Secrete pepsinogen (inactive enzyme).
  • Parietal (oxyntic) cells: Secrete HCl (hydrochloric acid) and intrinsic factor (for B12 absorption).
Gastric Juice Components: Mucus, pepsinogen, HCl, intrinsic factor.
Functions:
  • Mucus protects stomach lining from HCl.
  • HCl makes food acidic, stops salivary amylase activity, and kills germs.
  • Pepsinogen converts to pepsin in acidic conditions, breaking proteins into peptones and proteoses.
End Product:
  • Food becomes a semi-fluid acidic mass called chyme.
  • Chyme is pushed into the small intestine via pyloric sphincter for further digestion.
Role of Rennin in Infants

Rennin (found in gastric juice of infants):

  • Acts on casein (milk protein).
  • Causes curdling of milk proteins with calcium.
  • Curdled protein is further digested by pepsin.
Note: Rennin is absent in adults.
Digestion in the Small Intestine

Overview

  • Digestive Juices: In the small intestine, intestinal juice, bile juice, and pancreatic juice mix with food.
  • Peristaltic Movements: Muscularis layer facilitates mixing of digestive juices with chyme.

Key Processes

  • Bile and Pancreatic Juice: Both juices are poured into the duodenum through the hepato-pancreatic duct.
  • Functions of Bile: Bile Salts: Neutralize acidic chyme, making it alkaline; emulsify fats.
Digestive Enzymes:

Pancreatic Juice: Contains enzymes like:

  • Pancreatic amylases: Break down starch.
  • Lipases: Convert fats into fatty acids and diglycerides.
  • Inactive enzymes: Trypsinogen → Trypsin (activated by Enterokinase). Chymotrypsinogen → Chymotrypsin (activated by Trypsin).
  • Nucleases: Digest nucleic acids.
Intestinal Juice:
  • Contains: Dipeptidases, lipases, disaccharidases (maltase, sucrase, lactase). Goblet Cells: Produce mucus.
Mucus and Bicarbonates: Protect intestinal mucosa and provide alkaline medium for enzymatic action.

Digestion Overview: Most digestion is completed in the small intestine.

Bile
  • Appearance: Dark green fluid.
Components:
  • Bile Pigments: Bilirubin and biliverdin.
  • Bile Salts: Na-glycocholate and Nataurocholate.
  • Other Components: Cholesterol, phospholipids.
Functions:
  • Neutralization: Bile salts neutralize chyme acidity.
  • Emulsification: Break down fats into smaller droplets.
  • Activation: Activates lipid-digesting enzymes (lipases).
Coloration: Bile pigments contribute to the color of fecal matter.
Constituents of Pancreatic Juice 

Enzymes:

  • Pancreatic Amylases: Break down carbohydrates.
  • Lipases: Break down fats.
  • Inactive Enzymes: Trypsinogen and Chymotrypsinogen (proteins).
  • Nucleases: Digest nucleic acids.
Hunger Hormone

Ghrelin:

  • Produced mainly by the stomach and small intestine, pancreas, and brain.
  • Functions: Stimulates appetite, increases food intake, promotes fat storage.
Action of Pancreatic Juice

Action of Intestinal Juice
  • Large Intestine's Role in Digestion:
Converts:
  • Proteins → Amino Acids
  • Fats → Fatty Acids and Monoglycerides
  • Nucleic Acids → Sugar and Nitrogenous Bases
  • Carbohydrates → Monosaccharides
  • End product of digestion is called chyle.
Chyle: Alkaline slurry containing nutrients ready for absorption. Nutrients are absorbed; undigested remains move to the large intestine.
Large Intestine Functions:
  • Mucosa produces mucus (no enzymes).
  • Some carbohydrates and proteins enter the large intestine.
Bacteria in the large intestine digest some carbohydrates and proteins.
  • Carbohydrate Fermentation: Produces hydrogen, carbon dioxide, and methane gas in the colon.
  • Protein Digestion: Produces indole, skatole, and H2S (causes faecal odor).
  • Bacteria synthesize vitamins like B vitamins and Vitamin K.
Pancreatitis: Inflammation of the pancreas.
  • Causes: Alcoholism, chronic gallstones, high calcium levels, and high fats in blood.
  • Main cause in 70% of cases is alcoholism.
Regulation of Gastric Function

  • Importance: Digestive enzymes and juices must be produced in the correct sequence and timing.

Control Mechanisms:

Neurohormonal control triggered by:
  • Sight, smell, and thought of food → triggers saliva secretion.
  • Tenth cranial nerve stimulates gastric juice secretion.
  • Hormone gastrin also stimulates gastric juice production.
Hormones Produced by Intestinal Mucosa:
Secretin:
  • Inhibits gastric juice secretion.
  • Stimulates bile juice, pancreatic juice, and intestinal juice secretion.
Cholecystokinin (CCK):
  • Similar actions to secretin.
  • Induces satiety (feeling of fullness).
Gastric Inhibiting Peptide (GIP):
  • Inhibits gastric secretion.
Absorption, Assimilation, and Egestion

Absorption:

  • Definition: Passage of end products of digestion through the mucosal lining into blood and lymph.

Methods of Absorption:

  • Simple diffusion
  • Osmosis
  • Facilitated transport
  • Active transport
Location of Absorption:
  • 90% in small intestine; remaining in mouth, stomach, and large intestine.

Sites of Absorption:

  • Mouth: Absorption through mucosa; e.g., some drugs (painkillers).
  • Stomach: Gastric mucosa is mostly impermeable; little water, electrolytes, alcohol, and some drugs (like aspirin) are absorbed.
  • Small Intestine: Absorbs glucose, fructose, galactose, amino acids, minerals, and water-soluble vitamins into blood capillaries. Absorbs lipids and fat-soluble vitamins (A, D, E, K) into lacteals.
  • Large Intestine: Absorbs water, electrolytes (like sodium), and some vitamins.
Mechanisms of Absorption of Compounds
  • Simple Diffusion: Absorption of glucose, amino acids, and electrolytes (like chloride ions) based on concentration gradient.
  • Facilitated Transport: Some amino acids and fructose absorbed via carrier ions (e.g., Na+).
  • Active Transport: Some ions (e.g., sodium) absorbed against concentration gradient; requires energy.
  • Water Absorption: Water absorbed along the concentration gradient.
Transportation Mechanism for Monoglycerides and Fatty Acids 

Monoglycerides and Fatty Acids:

  • Cannot be absorbed directly into blood.
  • Dissolve in micelles (spherical aggregates formed by bile salts).
  • Micelles enter intestinal villi, reforming into chylomicrons.
  • Chylomicrons: Small protein-coated fat globules.
  • Transported into lymph vessels (lacteals) and then to the bloodstream.
Assimilation
  • Definition: Absorbed food material reaches tissues and becomes part of the protoplasm.
Egestion
  • Undigested waste is converted to faeces in the colon and reaches the rectum.
  • Composition of Faeces: Water, inorganic salts, sloughed-off mucosal cells, bacteria, and undigested food.
  • Distension of the rectum stimulates pressure-sensitive receptors initiating a neural reflex for defecation (egestion).
  • It is a voluntary process occurring through the anal opening, guarded by sphincter muscles.
Nutritional Disorders and Disorders of the Digestive System

Nutrition-Related Disorders

  • Disorders arise from excess or deficiency of nutrition.
  • Categorized based on dietary intake and organ/gland dysfunction in the digestive system.

Protein Energy Malnutrition (PEM)

  • Definition: Inadequate intake of proteins, often with insufficient vitamins and minerals.
  • Associated Diseases: Kwashiorkor & Marasmus
Kwashiorkor
  • Description: Protein deficiency disorder mainly in children (1-3 years).
  • Symptoms: Underweight, stunted growth, poor brain development, loss of appetite.
  • Physical Signs: Anaemia, protruding belly, slender legs, bulging eyes, oedema of lower legs and face, changes in skin and hair color.

Marasmus

  • Description: Prolonged protein energy malnutrition in infants (<1 year).
  • Symptoms: Protein deficiency combined with low caloric intake.
  • Physical Signs: Impaired growth, thin limbs, prominent ribs, dry and wrinkled skin. No oedema; loss of weight, digestive gland atrophy halts digestion and absorption.

Major Causes of Kwashiorkor and Marasmus

  • Causes: Unavailability of Nutritious Food.
  • Contributing Factors: Poverty, Large family size, Improper spacing of children, Early termination of breastfeeding, Over-diluted milk.
Indigestion
  • Definition: Discomfort from overeating, inadequate enzyme secretion, or consuming spicy foods.
  • Causes: Overeating, Inadequate enzyme secretion, Spicy food, Anxiety, Improperly digested food or food poisoning.
  • Symptoms: Loss of appetite, Acidity (acid reflux), Heartburn, Regurgitation, Dyspepsia (upper abdominal pain), Stomach pain.
  • Preventive Measures: Avoid large meals, Do not lie down after meals, Avoid spicy, oily, junk food, smoking, and alcohol.
Constipation
  • Definition: Reduced defecation frequency (less than once per week).
  • Symptoms: Difficulty in defecation, Abdominal pain and distortion, Rarely, perforation.
  • Causes: Affected colonic mobility (e.g., neurological dysfunction like spinal cord injury). Low fiber diet. Inadequate fluid intake. Inactivity.
  • Management: Increase roughage in diet. Ensure sufficient fluid intake. Regular exercise.
Diarrhoea
  • Definition: Passing loose, watery stools more than three times a day.
  • Symptoms: Blood in stool, Nausea, Bloating,  Fever (depending on cause and severity).
  • Causes: Infections from contaminated food and water. Disorders like ulcers, colitis (inflammation of the intestine), or irritable bowel syndrome.
Jaundice
  • Definition: Disorder marked by yellowing of conjunctiva of eyes and skin, and whitish stools.
  • Causes: Abnormal bilirubin metabolism and excretion. Excessive breakdown of red blood cells. Increased bilirubin levels exceeding liver capacity. Obstruction of bile flow from liver to duodenum.
  • Types of Bilirubin: Water-soluble and fat-soluble. Fat-soluble bilirubin is toxic to brain cells.
  • Treatment: No specific treatment; supportive care and proper rest are recommended.
Hepatitis
  • Definition: Inflammation of the liver.
  • Causes: Infections, Alcohol consumption, Immune system disorders.
Vomiting
  • Definition: Ejection of stomach contents through the mouth due to reverse peristaltic movements.
  • Control: Managed by the non-vital vomiting center in the medulla.
  • Associated Symptoms: Typically accompanied by feelings of nausea.

Know This:

  • Gross Calorific Value: Amount of heat released by complete combustion of 1g of food in a bomb calorimeter.
  • Physiological Value: Actual energy produced by 1g of food.
  • Energy Content: Expressed in terms of heat energy in animals.