Stages of Human Digestion, Absorption, and Waste Elimination

Stage B: The Digestion Process

Digestion is the second stage of the digestive process, where food is transformed into nutrients that the body can absorb.

Digestion primarily takes place in the stomach (a large cavity in the digestive tract with muscular walls) and the first portion of the small intestine, the duodenum.

Steps of Digestion

1. The bolus reaches the stomach, entering through the cardiac sphincter (the entrance door of the stomach). This sphincter acts as a double-way door.
2. Once in the stomach, the bolus is mixed with two substances secreted by the stomach walls:
   • Pepsin: An enzyme used to break down proteins.
   • Hydrochloric Acid (HCl): Lowers the pH to activate pepsin and destroy pathogens.

   This mixing process is aided by the movements of the stomach's muscular walls.

3. Once the bolus is thoroughly mixed with the stomach juices (a process taking approximately 2–2.5 hours), the resulting acidic mixture is called Chyme.
4. The chyme exits the stomach through the pyloric sphincter and enters the first part of the small intestine: the Duodenum.
5. In the duodenum, chyme is mixed with intestinal juices, pancreatic juice, and Bile (produced by the liver and stored in the gall bladder). This mixture transforms into Chyle.

Key Notes on Digestion

• Chyme is highly acidic.
• Intestinal juices contain sodium carbonate to neutralize the acid.
• Bile helps to digest fats and oils by forming microscopic bubbles (emulsification).
• Chyle is a mixture of biomolecules and undigested food.

Stage C: Nutrient Absorption

Absorption takes place primarily in the second part of the small intestine, the Jejunum. It consists of the transportation of biomolecules from the small intestine into the blood or lymph system.

Structure for Absorption: Villi and Microvilli

The wall of the jejunum has finger-like projections called villi, which are covered with numerous microscopic folds (microvilli). This structure significantly increases the surface area available for absorption.

Villi and microvilli are densely supplied with blood vessels and lymphatic vessels, where micromolecules are absorbed and transported.

Stage D: Egestion and Waste Elimination

After nutrient absorption, undigested products (and most remaining water) move into the large intestine through the ileum (the last part of the small intestine).

Pathway through the Large Intestine

The waste products pass through the following structures:

Cecum > Colon > Rectum > Anus

Processes: Water is absorbed here. Undigested and unabsorbed products stay in the large intestine for up to 32 hours before elimination.

The Respiratory System: Function and Gas Exchange

By breathing, we obtain O₂ necessary for cellular respiration and expel CO₂ (carbon dioxide).