Digestive System Overview

• Digestion is the mechanical and chemical process of breaking down complex food substances into simpler substances.

Digestion Time of Various Foods

• Approximate digestion times:

• Nuts take around 4 hours.

• Fruits and vegetables take 0.5 hours.

• Water takes 0.3 hours.

• Milk products take 1.5 hours.

• Meat, chicken, and fish take 3 hours.

• Cooked vegetables take 1.5 hours.

• Hydrochloric acid (HCl) is the primary component of gastric acid in the stomach.

• HCl is a strong acid that helps with digestion and kills bacteria.

• HCl prepares food for digestion by breaking it down and activating enzymes.

• Enzymes break down food into nutrients that the body can absorb.

Digestive Processes

• Ingestion: Taking in food material.
• Digestion: Breaking down food into smaller pieces.
• Absorption: Movement of nutrients, water, and electrolytes from the small intestine into cells, and then into the blood.
• Elimination: Undigested material passes out of the system.

Ingestion

• This is facilitated by the mouth.
• This is the starting point of digestion with food intake through drinking or eating.
• Teeth mechanically break down food (mastication), while saliva moistens it.
• Chewing (mastication) breaks food into smaller pieces during mechanical digestion in the mouth.
• Saliva moistens food and begins carbohydrate breakdown during chemical digestion in the mouth.

Esophagus

• The bolus (food ball) is swallowed and travels via peristalsis.
• The esophagus transports food to the stomach.
• No significant digestion occurs in the esophagus.
• Peristalsis is used to move the food down to the stomach.

Stomach

• Stomach muscles churn and mix food with gastric juices which forms chyme (a semi-liquid paste).
• Chemical digestion occurs in the stomach
  • Hydrochloric acid (HCl) activates pepsin and kills pathogens.
  • Pepsin breaks proteins into peptides.

Small Intestine

• Major digestion and absorption occur here.
• Pancreatic enzymes contribute to digestion
  • Lipase breaks fats into fatty acids.
  • Proteases (trypsin) further digest proteins.
  • Amylase continues carbohydrate breakdown.
  • Bile (from the liver/gallbladder) emulsifies fats, increasing enzyme efficiency.
• Villi and microvilli are finger-like projections in the intestinal lining that absorb nutrients into the bloodstream.
• Nutrients (glucose, amino acids, fatty acids) enter circulation for energy and repair.

Large Intestine

• It absorbs water, electrolytes (e.g., sodium), and vitamins (produced by gut bacteria).
• Gut microbiota ferment indigestible fibers, producing vitamins (e.g., vitamin K).
• Remaining material becomes feces.
• Feces are stored in the rectum until excretion.

Excretion (Defecation)

• Feces are stored in the rectum.
• Defecation expels waste through the anus.
• Feces exit the body through the anus via coordinated muscle relaxation.

The Digestion Process

• The steps are as follows: Ingestion -> Mechanical/Chemical Digestion (Mouth) -> Propulsion (Esophagus) -> Stomach Breakdown -> Small Intestine Digestion/Absorption -> Large Intestine Water Removal -> Excretion.