Human Digestive System.

What is Digestion?

 Digestion is the process by which complex food molecules (like carbohydrates, proteins, fats) are broken down into simpler, absorbable forms (like glucose, amino acids, fatty acids).

•Food → Mouth → Esophagus → Stomach → Small Intestine → Large Intestine → Anus.

•Digestion is essential for energy release, growth, and repair of the body.

  Types of Digestion.

•There are two types of digestion;

1.Mechanical type.

In this type,Physical breakdown happens (e.g., chewing, churning).

2. Chemical type.

Breakdown by enzymes and digestive juices

Human Digestive System .

 Organs Involved;

A. Alimentary Canal (Food Pipe)

1.Mouth (Buccal Cavity)

2.Pharynx

3.Oesophagus (Food Pipe)

4.Stomach

5.Small Intestine (Duodenum, Jejunum, Ileum)

6.Large Intestine (Colon)

7.Rectum and Anus

B. Accessory Digestive Organs.

1.Salivary Glands

2.Liver

3.Gall Bladder

4.Pancreas

Process of Digestion Step-by-Step;

Step 1: In the Mouth.

A.Teeth: it's work is to Chew and crush food (mechanical digestion)

B.Saliva (from salivary glands):

Types of salivary gland;

*Contains salivary amylase.

1.Parotid glands – Near the ears

2.Submaxillary / Submandibular glands – Below the jaw.

3.Sublingual glands – Under the tongue.

*Composition of Saliva:

Water (98%) – Makes food moist.

Mucus – Softens food and aids swallowing

Salts – Maintain pH and ionic balance.

Enzyme: Salivary Amylase (also called Ptyalin) – Begins starch digestion

Lysozyme – Kills bacteria in the mouth

Mucus helps in lubricating food.

•Function;

Salivary glands initiate digestion in the mouth.

Mechanical role: Moisten and soften food.

Chemical role: Start carbohydrate digestion via salivary amylase.

Protective role: Saliva cleanses the mouth and kills germs.

C.Tongue: Mixes food and pushes to pharynx.

Step 2: Oesophagus.

.Food moves by Peristalsis: Wave-like muscular contraction.

.Contraction & relaxation process.

Step 3: Stomach.

The stomach is a muscular, J-shaped organ where food stays for 3–4 hours.

It churns and mixes the food with digestive juices to make it semi-liquid (chyme).

Food enters via cardiac sphincter

Gastric Juice is released from the inner lining of the stomach.

It contains 3 main substances:

1. HCl (Hydrochloric Acid) - Kills germs, makes food acidic, activates enzymes.

2.Pepsin Enzyme Starts protein digestion: Protein → Peptides.

3.Mucus Protects stomach wall from strong acid.

•Important points.

Only proteins are digested here.

Pepsin works best in acidic condition created by HCl.

Carbohydrates and fats are not digested in the stomach.

Stomach muscles churn food, mix it with gastric juice, and slowly send it to the small intestine.

What's churning?

The muscles of the stomach wall contract and relax in a rhythmic way.

This movement mixes food with gastric juice (which contains acid and enzymes).

It turns the food into a semi-liquid paste called chyme.

 Purpose of churning.

Breaks food into smaller pieces.

Ensures better mixing with digestive juices.

Helps enzymes work effectively on the food

Step 4: Small Intestine.

The small intestine is the main site of digestion and absorption in our body.

 It is a long, coiled tube about 6 meters long and has three parts:

Duodenum

Jejunum

Ileum.

Function of Small intestine.

1.Complete digestion of food.

2.Absorption of nutrients into the blood.

Digestion process in small intestine;

 1.Food enters from the stomach as chyme.

2. Digestive juices are added from three sources:

A.Liver (via gall bladder)- Bile Juice -Breaks large fat into small droplets (emulsification).

B. Pancreas -Pancreatic Juice- Contains enzymes to digest proteins, fats, and carbs.

C.Intestinal Wall - Intestinal Juice - Final breakdown of food into simple forms

 Important Enzymes in Small Intestine:

A.Pancreatic Juice .

1.Amylase- Starch -Maltose

2.Trypsin -Proteins- Peptides

3.Lipase- Fats Fatty acids + Glycerol.

B.Intestinal Juice.

1. Maltase - Maltose - Glucose.

2. Peptidase - Peptides -Amino acids.

3. Lactase - Lactose - Glucose + Galactose

Final Products of Digestion;

Carbohydrates → Glucose

Proteins → Amino Acids

Fats → Fatty Acids + Glycerol

Step 5: Absorption.

What is Absorption?

Absorption is the process by which the digested food (nutrients) is taken into the bloodstream or lymph from the digestive tract, mainly through the walls of the small intestine

* Small Intestine Glucose, amino acids, fatty acids, vitamins, water

*Large Intestine Water, minerals (salts).

How Absorption happens in small intestine?

Absorption in Small Intestine – Main Site

The inner wall of the small intestine has millions of villi (singular: villus).

Each villus has blood capillaries and lymph vessels.

How it works?

1.Glucose --- Blood capillaries --- To all body cells for energy.

2.Amino Acids --- Blood capillaries --- For growth and repair.

3. Fatty acids + Glycerol --- Lymph vessels (lacteals) --- Then into blood later.

4.Water & Minerals ---Blood --- For body functions.

Step 6: Large Intestine.

The large intestine is the last part of the digestive system. Its main job is not digestion but the absorption of water and the formation of feces (waste).

Function & Explanation.

1. Water Absorption.

Absorbs water from undigested food, making it solid.

2. Mineral Absorption.

   Absorbs some salts (minerals).

3. Formation of Feces.

  Converts undigested food into semi-solid waste (feces).

4. Bacterial Action Good bacteria help make vitamins (like Vitamin K) and digest some fibers

Step 7: Egestion.

Removal of feces through anus.

 Importance of Digestion.

A.Supplies energy to the body

B.Provides building blocks for growth and repair

C.Helps in maintaining body metabolism

Disorders of Digestive System.

Acidity

Ulcers

Constipation

Diarrhea

Liver Cirrhosis