Food is a fundamental requirement for all living organisms, providing energy and organic materials necessary for growth, repair of tissues, and metabolic processes. The main components of our food are biomacromolecules like carbohydrates, proteins, and fats, along with smaller quantities of vitamins and minerals. Water is also vital for metabolic activities and hydration.

Biomacromolecules in food are complex and cannot be directly used by the body. They must be broken down into simpler, absorbable forms through the process of digestion, which is carried out by the digestive system using both mechanical and biochemical methods.

The human digestive system consists of the alimentary canal and associated digestive glands (Figure 16.1).

The Digestive System

The human digestive system comprises the alimentary canal and associated digestive glands.

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Alimentary Canal

The alimentary canal is a long tube extending from the mouth (anterior opening) to the anus (posterior opening).

Parts of the alimentary canal:

1. Mouth: The anterior opening, leading into the buccal cavity.
2. Buccal cavity (Oral cavity): Contains teeth and a muscular tongue.
   • Teeth: Embedded in sockets of the jaw bone (thecodont attachment). Humans form two sets of teeth during life (diphyodont): temporary milk teeth replaced by permanent teeth. Adults have 32 permanent teeth of four different types (heterodont dentition): Incisors (I), Canine (C), Premolars (PM), and Molars (M). The dental formula for each half of the upper and lower jaw is $\frac{\textsf{2123}}{\textsf{2123}}$. The hard enamel surface helps in chewing (mastication).
   • Tongue: A freely movable muscular organ attached by the frenulum to the floor of the oral cavity. Upper surface has small projections called papillae, some bearing taste buds.
3. Pharynx: A short common passage for food and air, connecting the oral cavity to the oesophagus and trachea. A cartilaginous flap, the epiglottis, covers the glottis (opening of trachea) during swallowing to prevent food entry.
4. Oesophagus: A thin, long tube passing through the neck, thorax, and diaphragm, leading to the stomach. Its opening into the stomach is regulated by the gastro-oesophageal sphincter.
5. Stomach: A 'J' shaped bag-like structure located in the upper left abdominal cavity (Figure 16.3). Has four main parts:
   • Cardiac: Oesophagus opens into this part.
   • Fundic: Upper dome-shaped region.
   • Body: Main central region.
   • Pyloric: Lower region, opening into the duodenum. The opening is guarded by the pyloric sphincter.
6. Small intestine: The longest part, highly coiled. Divided into three regions:
   • Duodenum: A 'C' shaped first part, receiving ducts from the liver and pancreas.
   • Jejunum: Long coiled middle portion.
   • Ileum: Highly coiled last part, opening into the large intestine.
7. Large intestine: Consists of:
   • Caecum: A small blind sac, hosting symbiotic microorganisms. The vermiform appendix, a vestigial organ, is a narrow tubular projection from the caecum.
   • Colon: Divided into ascending, transverse, descending, and sigmoid parts.
   • Rectum: The final section, which opens out through the anus.
8. Anus: The posterior opening for egestion of faeces.

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Wall of the Alimentary Canal: From the oesophagus to the rectum, the wall has four concentric layers (Figure 16.4):

1. Serosa: Outermost layer, made of thin mesothelium (epithelium of visceral organs) with some connective tissue.
2. Muscularis: Formed by smooth muscles, usually arranged in an inner circular and outer longitudinal layer. An oblique layer may be present in some regions (e.g., stomach).
3. Sub-mucosa: Formed of loose connective tissue containing nerves, blood vessels, and lymph vessels. Glands (Brunner's glands) are also present in the duodenum submucosa.
4. Mucosa: Innermost layer, lining the lumen of the alimentary canal. Forms irregular folds (rugae) in the stomach and small finger-like projections called villi in the small intestine (Figure 16.5).

Modifications in the small intestine mucosa:

• Villi: Increase the surface area for absorption.
• Microvilli: Microscopic projections on the cells lining the villi, giving a brush border appearance and further increasing surface area enormously.
• Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal (important for fat absorption).
• Goblet cells: Mucosal epithelial cells that secrete mucus for lubrication.
• Gastric glands: Glands in the stomach mucosa.
• Crypts of Lieberkuhn: Glands located in between the bases of villi in the intestine.

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Digestive Glands

Associated digestive glands secrete digestive juices into the alimentary canal. These include:

1. Salivary glands: Three pairs located just outside the buccal cavity: parotids (cheeks), sub-maxillary/sub-mandibular (lower jaw), and sub-linguals (below the tongue). They secrete saliva into the buccal cavity.
2. Liver: The largest gland (1.2-1.5 kg in adults), located below the diaphragm in the abdominal cavity. Has two lobes. Structural and functional units are hepatic lobules containing hepatic cells (liver cells) arranged in cords. Each lobule is covered by a thin connective tissue sheath called Glisson's capsule. Hepatic cells secrete bile, which passes through hepatic ducts. Bile is stored and concentrated in the gall bladder. The duct of the gall bladder (cystic duct) joins the hepatic duct to form the common bile duct (Figure 16.6).
3. Pancreas: A compound gland (both exocrine and endocrine), elongated, situated between the limbs of the duodenum.
   • Exocrine portion: Secretes alkaline pancreatic juice containing digestive enzymes.
   • Endocrine portion: Secretes hormones insulin and glucagon.

The bile duct and the pancreatic duct join to form the common hepato-pancreatic duct, which opens into the duodenum and is guarded by the sphincter of Oddi.

Digestion Of Food

Digestion is carried out by mechanical and chemical processes.

In the Oral Cavity:

• Mechanical: Mastication (chewing) of food by teeth, mixing with saliva by the tongue. Mucus in saliva lubricates and binds food into a bolus for swallowing.
• Chemical: Saliva contains electrolytes (Na$^+$, K$^+$, Cl$^-$), salivary amylase, and lysozyme.
  • Salivary amylase (Ptyalin): Carbohydrate-splitting enzyme. Hydrolyzes about 30% of starch into maltose (disaccharide) at optimum pH 6.8.
  • Lysozyme: Acts as an antibacterial agent, preventing infections.

  Starch $\xrightarrow{\textsf{Salivary Amylase, pH 6.8}}$ Maltose

Swallowing (Deglutition): Bolus is moved to the pharynx, then oesophagus by peristalsis (wave-like muscular contractions). Gastro-oesophageal sphincter controls entry into the stomach.

In the Stomach:

• Mechanical: Muscular wall churns food with gastric juice, forming chyme.
• Chemical: Gastric glands in the stomach mucosa secrete gastric juice. Glands have three types of cells:
  • Mucus neck cells: Secrete mucus.
  • Peptic (Chief) cells: Secrete proenzyme pepsinogen.
  • Parietal (Oxyntic) cells: Secrete HCl and intrinsic factor (for vitamin B12 absorption).

  HCl provides acidic pH (optimal pH 1.8) for pepsin action and kills bacteria. Mucus and bicarbonates protect mucosal epithelium from HCl.

  • Pepsin: Activated from pepsinogen by HCl. Proteolytic enzyme, digests proteins into proteoses and peptones (peptides).
  • Rennin: Proteolytic enzyme in infants' gastric juice, helps digest milk proteins.
  • Small amounts of gastric lipases may be present.

Stomach stores food for 4-5 hours.

In the Small Intestine:

• Mechanical: Muscularis layer generates movements for thorough mixing of food with secretions.
• Chemical: Receives bile, pancreatic juice, and intestinal juice (succus entericus). Pancreatic juice and bile enter via the hepato-pancreatic duct.
• Pancreatic juice: Contains inactive enzymes: trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases, nucleases. Enterokinase (secreted by intestinal mucosa) activates trypsinogen to trypsin, which activates other pancreatic enzymes.
• Bile: Secreted by liver, stored in gall bladder. Contains bile pigments (bilirubin, biliverdin), bile salts, cholesterol, phospholipids. No enzymes.
  • Emulsification of fats: Bile salts break large fat globules into smaller micelles, increasing surface area for lipases.
  • Activates lipases.
• Intestinal juice (Succus entericus): Secreted by goblet cells (mucus) and brush border cells of mucosa (enzymes). Contains disaccharidases (maltase, lactase, sucrase), dipeptidases, lipases, nucleosidases, etc. Provides alkaline medium (pH 7.8) for enzymatic activity (aided by mucus and pancreatic bicarbonates) and protects mucosa from acid.

Digestion of major food components in the small intestine:

• Proteins: Proteoses, peptones, and dietary proteins are acted upon by pancreatic proteolytic enzymes (Trypsin, Chymotrypsin, Carboxypeptidase) to form dipeptides.

Proteins, Peptones, Proteoses $\xrightarrow{\textsf{Trypsin/Chymotrypsin, Carboxypeptidase}}$ Dipeptides

• Carbohydrates: Polysaccharides (starch) are hydrolyzed by pancreatic amylase into disaccharides.

Polysaccharides (starch) $\xrightarrow{\textsf{Amylase}}$ Disaccharides

• Fats: Broken down by lipases (with bile's help) into di- and monoglycerides.

Fats $\xrightarrow{\textsf{Lipases}}$ Diglycerides $\rightarrow$ Monoglycerides

• Nucleic Acids: Acted upon by pancreatic nucleases to form nucleotides and nucleosides.

Nucleic acids $\xrightarrow{\textsf{Nucleases}}$ Nucleotides $\rightarrow$ Nucleosides

Final digestion steps (near mucosal epithelial cells) convert end products into absorbable forms using enzymes in succus entericus:

• Dipeptides $\xrightarrow{\textsf{Dipeptidases}}$ Amino acids
• Maltose $\xrightarrow{\textsf{Maltase}}$ Glucose + Glucose
• Lactose $\xrightarrow{\textsf{Lactase}}$ Glucose + Galactose
• Sucrose $\xrightarrow{\textsf{Sucrase}}$ Glucose + Fructose
• Nucleotides $\xrightarrow{\textsf{Nucleotidases}}$ Nucleosides $\xrightarrow{\textsf{Nucleosidases}}$ Sugars + Bases
• Di and Monoglycerides $\xrightarrow{\textsf{Lipases}}$ Fatty acids + Glycerol

Most digestion is completed in the duodenum. The simple absorbable substances are then absorbed in the jejunum and ileum.

In the Large Intestine:

• No significant digestive activity.
• Functions: Absorption of some water, minerals, and certain drugs. Secretion of mucus for lubrication and adhering waste.

Undigested, unabsorbed substances (faeces) enter the caecum via the ileo-caecal valve (prevents backflow) and are temporarily stored in the rectum before defaecation.

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Regulation of digestive activities: Controlled by neural and hormonal mechanisms.

• Neural: Sight, smell, or presence of food stimulates saliva/gastric/intestinal secretions. Muscular activity of the gut is moderated by local and CNS neural mechanisms.
• Hormonal: Local hormones from gastric and intestinal mucosa control secretion of digestive juices.

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Calorific Value:

Energy content of food is measured in calories or joules. 1 kcal is the energy to raise 1 kg of water by 1°C. Gross calorific value (measured by bomb calorimeter) is the heat from complete combustion of 1g food. Physiological value is the actual energy available to the body.

• Carbohydrates: Gross 4.1 kcal/g, Physiological 4.0 kcal/g
• Proteins: Gross 5.65 kcal/g, Physiological 4.0 kcal/g
• Fats: Gross 9.45 kcal/g, Physiological 9.0 kcal/g

Absorption Of Digested Products

Absorption is the process where the simple end products of digestion pass through the intestinal mucosa into the blood or lymph. It occurs via passive, active, or facilitated transport.

• Simple Diffusion: Small amounts of monosaccharides (glucose), amino acids, and electrolytes (chloride ions) are absorbed down concentration gradients.
• Facilitated Transport: Some glucose and amino acids are absorbed with the help of carrier proteins.
• Osmosis: Water absorption depends on the osmotic gradient.
• Active Transport: Nutrients like amino acids, monosaccharides (glucose), and electrolytes (Na$^+$) are absorbed against concentration gradients, requiring energy (ATP).

Fatty acids and Glycerol: Being insoluble, they are first incorporated into small droplets called micelles. Micelles move into the intestinal mucosa, where they are re-formed into very small protein-coated fat globules called chylomicrons. Chylomicrons are transported into the lymph vessels (lacteals) in the villi. Lymph vessels eventually release these substances into the bloodstream.

Absorption occurs in different parts of the alimentary canal, but maximum absorption takes place in the small intestine.

Summary of Absorption (Table 16.1):

Part	Substances Absorbed
Mouth	Certain drugs in contact with mucosa and lower side of tongue absorbed into blood capillaries.
Stomach	Absorption of water, simple sugars, and alcohol etc. takes place.
Small Intestine	Principal organ for absorption. Digestion completed here. Final products (glucose, fructose, fatty acids, glycerol, amino acids) absorbed through mucosa into blood stream and lymph.
Large Intestine	Absorption of water, some minerals and drugs takes place.

Assimilation: The process where absorbed substances finally reach tissues and are utilized for their activities.

Egestion (Defaecation): The removal of undigested digestive wastes (faeces) from the rectum through the anus. It's a voluntary process initiated by a neural reflex, involving mass peristaltic movement in the colon.

Disorders Of Digestive System

Common ailments affecting the digestive system, often due to bacterial or viral infections, or parasitic infestations (tapeworm, roundworm, etc.), include:

• Jaundice: Liver dysfunction leads to deposit of bile pigments, causing yellowing of skin and eyes.
• Vomiting: Forceful ejection of stomach contents through the mouth, controlled by the vomit center in the medulla. Preceded by nausea.
• Diarrhoea: Increased frequency and liquidity of bowel movements, reducing food absorption.
• Constipation: Irregular bowel movements and retention of faeces in the colon.
• Indigestion: Incomplete digestion of food, causing a feeling of fullness. Causes include inadequate enzymes, anxiety, food poisoning, overeating, spicy food.

Protein-Energy Malnutrition (PEM): Widespread deficiency of dietary proteins and total food calories, common in underdeveloped countries, especially during famine or turmoil. Affects infants and children, causing Marasmus and Kwashiorkor.

• Marasmus: Caused by simultaneous deficiency of proteins and calories. Found in infants <1 year. Impairs growth, causes extreme emaciation, dry wrinkled skin, reduced growth rate, impaired brain development.
• Kwashiorkor: Caused by protein deficiency without calorie deficiency. Found in children >1 year. Wasting of muscles, thinning limbs, growth/brain development failure. Unlike marasmus, some fat remains, but extensive oedema (swelling) is prominent.

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