13. DIGESTIVE SYSTEM

DIGESTIVE SYSTEM

The digestive system transfers nutrients from the external environment (in the form of food) to the internal environment (via ingestion of food). In the internal environment, nutrients are distributed to different cells of the body via the circulatory system. The digestive system includes the alimentary canal (gastrointestinal tract), accessory digestive glands, and digestive processes for absorption of digested food.

PARTS OF ALIMENTARY CANAL

• Oral cavity (mouth)
• Pharynx
• Oesophagus
• Stomach
• Small intestine
• Large intestine

ORAL CAVITY (MOUTH)

Functions of Oral Cavity

1. Ingestion of food
2. Perception of taste of food
3. Lubrication of food
4. Mechanical digestion with salivary enzymes
5. Aids in speech
6. Source for delivery of drugs

TEETH

Types of Teeth and Functions

• Incisors (8): Biting-off large pieces of food
• Cuspids/Canines (4): Grasping and tearing food
• Bicuspids/Premolars (8): Grinding food (appear after 9 years)
• Molars (12): Largest teeth; grinding food

TONGUE

Functions of Tongue

1. Houses taste buds; perception of taste
2. Helps in movement of food within oral cavity for chewing and grinding
3. Helps in formation of bolus for swallowing
4. Helps in speech and phonetics

OESOPHAGUS

Functions of Oesophagus

1. Links oral cavity to stomach through thoracic cavity
2. Uses muscle contractions to move food
3. Restricts food from entering windpipe (epiglottis)
4. Facilitates instant passage of food through rhythmic relaxations and contractions
5. Assists breakdown of food into bolus

STOMACH

Regions of Stomach

• Cardia: Continuous from oesophagus
• Fundus: Dome-shaped upper curvature
• Body (Corpus): Central region
• Pylorus (Antrum): Lower region continuing into duodenum

Gastric Juices

Gastric juice is strongly acidic (pH 1-3). Composition: water (99.45%), inorganic solids (NaCl, KCl, CaCl, bicarbonates), organic solids (mucin, intrinsic factor), enzymes (pepsinogen, gastric lipase, renin), and hydrochloric acid.

Phases of Gastric Secretion

1. Cephalic Phase (Reflex Phase): Activated by smell, taste, sight, or thought of food (before food enters stomach).
2. Gastric Phase (3-4 hours): Initiated by local neural and hormonal mechanisms when food enters stomach.
3. Intestinal Phase: Contains excitatory and inhibitory elements; duodenum regulates stomach emptying.

Functions of Stomach

1. Provides highly acidic environment (pH 1-3) for food metabolism
2. Secretes 2-3 litres of gastric acid per day
3. Pepsinogen converts to pepsin for protein digestion
4. Produces intrinsic factor for vitamin B12 absorption

SMALL INTESTINE

Small intestine is a long, highly convoluted tube (about 7m in length, 2.5-3cm in diameter). It absorbs about 90% of nutrients from ingested food.

Parts of Small Intestine

• Duodenum: Initial and smallest part; connects stomach to jejunum; majority of chemical digestion
• Jejunum: Middle part; attached to back of abdominal wall by mesentery
• Ileum: Lower end; continues into large intestine (caecum)

Functions of Small Intestine

1. Passage for food movement via peristalsis
2. Receives pancreatic and liver secretions for digestion
3. Metabolises carbohydrates, proteins, and fats
4. Secretes cholecystokinin (CCK) and secretin
5. Site for absorption of nutrients
6. Provides protection from microbes (lymph follicles)

LARGE INTESTINE

Large intestine extends from caecum to anus (about 1.5m in length, 6-7cm in diameter). It absorbs water and vitamins while converting digested food into faeces.

Parts of Large Intestine

• Caecum: First part; large pouch continuous with ileum; appendix extends from it
• Colon: Largest part (ascending, transverse, descending, sigmoid colon)
• Rectum: Terminal portion (12cm); stores faeces temporarily
• Anus (Anal Cavity): External opening; surrounded by internal (involuntary) and external (voluntary) sphincters

Functions of Large Intestine

1. Secretions lubricate faeces
2. Houses bacteria (E. coli, S. faecalis, C. welchii) that synthesise vitamin K and folic acid
3. Absorption of water, glucose, and salt
4. Excretion of excess calcium, iron, and heavy metal drugs
5. Segmental contractions mix contents; peristaltic contractions push matter to rectum

ACCESSORY DIGESTIVE GLANDS

Salivary Glands

• Parotid Glands: Largest; located anterior to ear; serous secretions; opens near upper second molar
• Submandibular Glands: Second largest; below mandible; opens adjacent to frenulum of tongue
• Sublingual Glands: Smallest; mucus secretion; opens through 10-12 small ducts

Pancreas

Pancreas is an elongated gland (6-10 inches, 65g) located behind the stomach. It is a mixed gland producing exocrine (pancreatic juice with digestive enzymes) and endocrine (insulin, glucagon, somatostatin) secretions.

Functions of Pancreas

• Exocrine: Secretes inactive enzymes activated in duodenum; bicarbonates neutralise stomach acid
• Endocrine: Insulin and glucagon regulate blood glucose; somatostatin regulates insulin and glucagon secretion

Liver

Liver is the largest gland of the human body (1.4kg, reddish-brown). Located inferior to diaphragm on the right side of the abdomino-thoracic region. Divided into two major lobes (left and right) and two minor lobes (caudate and quadrate).

Functions of Liver

1. Carbohydrate metabolism (maintains blood glucose, glycogen storage)
2. Lipid metabolism (stores triglycerides, synthesises cholesterol and bile salts)
3. Protein metabolism (deamination, urea synthesis, plasma protein synthesis)
4. Processing of drugs and hormones (detoxification)
5. Excretion of bilirubin
6. Synthesis of bile salts
7. Storage of vitamins (A, B₁₂, D, E, K) and minerals (iron, copper)
8. Phagocytosis (Kupffer cells engulf aged RBCs, WBCs, bacteria)
9. Activation of vitamin D

Gall Bladder

Gall bladder is a pear-shaped organ (8cm × 4cm) on the inferior surface of the liver. Comprises fundus, body, and neck. Wall has three layers: mucosa (rugae), muscularis (smooth muscle), and serosa.

Functions of Gall Bladder

1. Storage of bile
2. Anti-oxidant (removes toxins)
3. Acid neutralisation (alkaline bile neutralises HCl)
4. Emulsification of lipids
5. Facilitates absorption of fats and fat-soluble molecules
6. Elimination of bilirubin waste

PHYSIOLOGY OF DIGESTION AND ABSORPTION

Five Steps of Digestion

1. Ingestion: Food enters mouth; chewed and mixed with saliva; bolus formed
2. Propulsion: Swallowing (voluntary) and peristalsis (involuntary – alternating waves of contraction/relaxation)
3. Mechanical and Chemical Digestion: Mechanical (chewing, churning, segmentation) and chemical (enzymatic breakdown)
4. Absorption: Occurs mainly in small intestine via active transport, passive diffusion, facilitated diffusion, co-transport, and endocytosis
5. Defecation: Voluntary removal of undigested materials as faeces