Digestive System

• Adult Human Dental Formula:

$$\mathbf{\frac{I:2, \; C:1, \; PM:2, \; M:3}{I:2, \; C:1, \; PM:2, \; M:3} \times 2 = 32\text{ teeth}}$$

• Child Milk Dentition Formula:

$$\mathbf{\frac{I:2, \; C:1, \; PM:0, \; M:2}{I:2, \; C:1, \; PM:0, \; M:2} \times 2 = 20\text{ teeth}} \quad (\text{Premolars are completely absent})$$

• Four Types of Teeth & Functions:
1. Incisors (8): Chisel-shaped front teeth for cutting and biting food.
2. Canines (4): Sharp, pointed conical teeth for tearing flesh and tough food.
3. Premolars (8): Two-cusped flat teeth for grinding and chewing.
4. Molars (12): Broad-crowned teeth with multiple cusps for crushing and fine mastication.

1. Activation of Enzymes: Creates a strongly acidic medium ($\text{pH} \approx 1.5-2.0$) that activates the dormant proenzyme inactive pepsinogen into active digestive enzyme Pepsin.
   2. Antimicrobial Action: Kills harmful bacteria, germs, and living pathogens ingested along with food.
   3. Stops Salivary Action & Softens Food: Acidifies the food chyme, halting the action of salivary ptyalin, and dissolves connective tissue fibers to soften fibrous meat and bone particles.

• Despite containing zero enzymes, bile performs two vital physiological functions:
1. Emulsification of Fats: Bile salts (Sodium taurocholate and glycocholate) act as biological detergents that break large, insoluble fat globules into millions of microscopic droplets (emulsion), enormously increasing the surface area for pancreatic lipase (steapsin) to digest fats.
2. Neutralization of Acidic Chyme: Acidic food leaving the stomach would destroy intestinal enzymes. The alkaline bile salts neutralize $\text{HCl}$, creating an alkaline medium ($\text{pH} \approx 8.0$) essential for pancreatic and intestinal enzymes to operate.

1. Trypsin: Converts Proteins, Peptones, and Proteoses into smaller Peptides and Amino Acids in an alkaline medium.
   2. Amylopsin (Pancreatic Amylase): Converts Remaining Starch into the disaccharide Maltose.
   3. Steapsin (Pancreatic Lipase): Hydrolyzes Emulsified Fats into absorbable Fatty Acids and Glycerol.

1. Enormous Length ($pprox 6\text{ meters}$): Provides an extensive transit duration, ensuring complete digestion and prolonged contact time for absorption.
   2. Millions of Villi and Microvilli: The inner mucosal lining is folded into millions of tiny finger-like villi, expanding the internal absorptive surface area over $600\text{ times}$ ($pprox 250\text{ m}^2$, the area of a tennis court).
   3. Rich Dual Vascularization: Each villus possesses an extensive network of blood capillaries (for absorbing glucose and amino acids) and a central lacteal (lymph vessel for absorbing fatty acids and glycerol), with an ultra-thin single-cell epithelium for rapid diffusion.

• Peristalsis: The involuntary, rhythmic wave of alternating muscular contraction and relaxation of the circular and longitudinal muscles of the alimentary canal wall that propels the food bolus or chyme forward.
• Occurrence: Occurs throughout the entire length of the alimentary canal—most prominently in the oesophagus, stomach, small intestine, and large intestine.

• (i) Carbohydrates: Broken down into Simple Monosaccharides (primarily Glucose, plus Fructose and Galactose).
• (ii) Proteins: Broken down into Amino Acids.
• (iii) Fats: Broken down into Fatty Acids and Glycerol.
• Primary Site of Absorption: The Ileum of the Small Intestine across the microscopic villi.

• Bread is rich in insoluble, tasteless starch (a complex polysaccharide).
• During prolonged mastication, saliva thoroughly mixes with the bread.
• The enzyme Salivary Amylase (Ptyalin) in saliva catalyzes the hydrolysis of tasteless starch into the sweet-tasting disaccharide sugar Maltose:

$$\text{Starch} + \text{Water} \xrightarrow{\text{Ptyalin}} \mathbf{\text{Maltose (Sweet)}}$$

• As maltose dissolves on the taste buds of the tongue, it tastes sweet.