Human Digestive System: Organs, Enzymes, Digestion & Absorption

TAKEAWAYS:

• The human digestive system is a tube-within-a-tube with two openings (mouth and anus) and accessory organs (liver, pancreas, gallbladder) that secrete enzymes/digestive juices.
• Digestion involves mechanical breakdown (teeth, peristalsis) and chemical breakdown (enzymes like amylase, pepsin, lipase) to convert food into absorbable nutrients.
• The small intestine is the primary site for nutrient absorption, while the large intestine absorbs water and forms feces.
• Enzymes (e.g., amylase, trypsin, lipase) are protein catalysts that speed up digestion but are not consumed in the process.
• Disorders like indigestion, constipation, and diarrhea occur when digestion or absorption is disrupted.
• NEB exams test labeling diagrams, matching organs to functions, and explaining digestion steps—practice tracing food’s journey!

1. Structure of the Human Digestive System

The digestive system is a long tube (alimentary canal) + accessory organs that work together to break down food, absorb nutrients, and eliminate waste.

1.1 The Alimentary Canal (Digestive Tube)

A muscular tube ~9 meters long, divided into 6 main parts:

flowchart TD
    A["Mouth"] --> B["Pharynx"]
    B --> C["Esophagus"]
    C --> D["Stomach"]
    D --> E["Small Intestine"]
    E --> F["Large Intestine"]
    F --> G["Rectum"]
    G --> H["Anus"]

Label the organs and accessory glands. (Image: Mariana Ruiz, Jmarchn, Public domain, via Wikimedia Commons)

1.2 Accessory Organs (Not Part of the Tube but Help Digestion)

2. Digestion: Mechanical vs. Chemical

Digestion happens in two ways:

1. Mechanical Digestion – Physical breakdown of food (chewing, churning, peristalsis).
2. Chemical Digestion – Enzymes break down food into smaller molecules.

2.1 Mechanical Digestion

• Mouth: Teeth cut and grind food; tongue mixes it with saliva.
• Stomach: Muscular walls churn food into a semi-liquid called chyme.
• Small Intestine: Peristalsis (wave-like muscle contractions) moves food forward.

2.2 Chemical Digestion (Enzymes Break Down Food)

Why enzymes are important?

• They speed up digestion without being used up.
• They work at specific pH levels (e.g., pepsin works in acidic stomach, trypsin in alkaline small intestine).

3. Step-by-Step Digestion Journey

Let’s trace a carbohydrate-rich meal (e.g., roti) through the digestive system:

Step 1: Mouth (Ingestion & Mechanical Digestion)

• Teeth break roti into smaller pieces.
• Saliva (from salivary glands) contains amylase, which starts breaking starch → maltose.
• Bolus (chewed food) is formed and swallowed.

Step 2: Esophagus (Transport)

• Peristalsis (muscle contractions) pushes the bolus down to the stomach (~6-8 seconds).

Step 3: Stomach (Protein Digestion & Chyme Formation)

• Gastric juice (HCl + pepsin) breaks down proteins → peptides.
• Mucus protects the stomach lining from acid.
• Food turns into chyme (thick, semi-liquid paste).

Step 4: Small Intestine (Main Digestion & Absorption)

• Duodenum (first part) receives:
  • Pancreatic juice (enzymes: trypsin, amylase, lipase).
  • Bile (from liver, stored in gallbladder) emulsifies fats.
• Villi & Microvilli increase surface area for absorption.
• Nutrients absorbed:
  • Carbohydrates → Glucose → Blood.
  • Proteins → Amino acids → Blood.
  • Fats → Fatty acids + glycerol → Lymph (lacteals).

Step 5: Large Intestine (Water Absorption & Feces Formation)

• Water & electrolytes (Na⁺, K⁺) are absorbed.
• Gut bacteria help digest undigested food (e.g., cellulose in vegetables).
• Feces (solid waste) is formed and stored in the rectum before elimination.

Step 6: Rectum & Anus (Egestion)

• Feces are expelled through the anus via defecation.

4. Absorption of Nutrients

The small intestine is the main absorption site due to:

1. Villi (finger-like projections).
2. Microvilli (tiny projections on villi).
3. Large surface area (~200 m², like a tennis court!).

5. Common Digestive Disorders

6. Worked Example: Tracing Digestion of a Burger

Question: Where does each part of a burger (bread, beef, cheese, lettuce) get digested? Solution:

7. NEB-Style Questions & Answers

Question 1: Label the Diagram

Diagram: Human digestive system (mouth to anus). Answer:

Mouth → Pharynx → Esophagus → Stomach → Small Intestine (Duodenum → Jejunum → Ileum) → Large Intestine (Cecum → Colon → Rectum) → Anus
Accessory Organs: Salivary glands, Liver, Gallbladder, Pancreas

Question 2: Short Answer

Q: Why is bile important in digestion? A:

• Bile is not an enzyme but emulsifies fats (breaks large fat globules into smaller droplets).
• Increases surface area for lipase to act efficiently.
• Produced in the liver, stored in the gallbladder, released into the duodenum.

Question 3: Match the Column

Match the organ to its function:

Question 4: Essay Type

Q: Explain the role of enzymes in digestion with examples. A: Enzymes are biological catalysts that speed up digestion without being used up.

1. Amylase (in saliva and pancreatic juice) breaks starch → maltose.
2. Pepsin (in stomach) breaks proteins → peptides (works in acidic pH).
3. Trypsin (from pancreas) breaks peptides → amino acids (works in alkaline pH).
4. Lipase (from pancreas) breaks fats → fatty acids + glycerol. Example:

• If you eat rice (starch), amylase in saliva starts digestion in the mouth, and pancreatic amylase continues in the small intestine to produce glucose for absorption.

Exam Tip

✅ For NEB exams, focus on:

1. Labeling diagrams (mouth, stomach, small intestine structure).
2. Matching organs to functions (e.g., liver → bile, pancreas → enzymes).
3. Explaining digestion steps (trace food from mouth to anus).
4. Enzyme actions (what they digest and where they work).
5. Absorption sites (small intestine for nutrients, large intestine for water).

🚫 Avoid memorizing long lists—understand how digestion happens step-by-step.

Compare the length of the small and large intestines. (Image: Mariana Ruiz, Jmarchn, Public domain, via Wikimedia Commons)