Subject: Integrated Science | Level: JHS (Basic 7 – 9) | Topic: The Human Digestive System | Difficulty: Intermediate
Introduction
Every time you eat kenkey and fish, jollof rice, or a bowl of porridge, your body sets to work breaking down that food into tiny pieces that can be absorbed into the blood and used by your cells. This process is called digestion. The organs that work together to carry out digestion make up the human digestive system, also known as the alimentary canal or gastrointestinal tract. Understanding this system is one of the most important and frequently examined topics in JHS Integrated Science.
1. Overview of Digestion
Digestion involves two types of processes:
- Mechanical (physical) digestion: Food is physically broken into smaller pieces by chewing, churning, and muscular action. The food does not chemically change.
- Chemical digestion: Enzymes break down large food molecules into small, soluble molecules that can be absorbed into the blood.
2. The Organs of Digestion and Their Functions
The Mouth
Digestion begins in the mouth. Food is mechanically broken down by the teeth (chewing). The tongue mixes food with saliva, which is produced by the salivary glands. Saliva contains:
- Water and mucus (to lubricate and soften the food)
- Salivary amylase – an enzyme that begins the chemical digestion of starch into maltose (a simpler sugar)
The chewed food, now a soft ball called a bolus, is swallowed and enters the oesophagus.
The Oesophagus (Food Pipe)
The oesophagus is a muscular tube that connects the mouth to the stomach. Food is moved down the oesophagus by rhythmic muscle contractions called peristalsis. No digestion occurs here; it is purely a passageway.
The Stomach
The stomach is a muscular, J-shaped bag that holds food and continues digestion. It carries out:
- Mechanical digestion: The muscular walls churn the food, mixing it thoroughly
- Chemical digestion: Gastric juice is secreted, containing:
- Hydrochloric acid (HCl): kills bacteria in food and creates the acidic pH needed for the stomach enzyme
- Pepsin: an enzyme that begins protein digestion, breaking proteins into shorter chains called peptides
- Mucus: protects the stomach wall from the acid
After about 2-4 hours, the churned food (now called chyme) passes into the small intestine through the pyloric sphincter.
The Small Intestine
The small intestine is a long, coiled tube (about 6-7 metres in a human adult). This is where most digestion is completed and where almost all nutrient absorption occurs.
Sections of the small intestine:
- Duodenum: The first section. Receives secretions from the pancreas and bile from the liver (stored in the gall bladder). These continue chemical digestion.
- Jejunum: Main site of further digestion and absorption
- Ileum: Final section; completes absorption of nutrients
Digestive secretions entering the duodenum:
- Pancreatic juice (from the pancreas): Contains amylase (digests starch), lipase (digests fats), and protease (digests proteins)
- Bile (made in the liver, stored in the gall bladder): Does not contain enzymes but emulsifies fats – it breaks large fat droplets into smaller ones, increasing the surface area for lipase to work on
Absorption in the small intestine:
The inner wall of the small intestine is covered with millions of tiny finger-like projections called villi (singular: villus). Each villus contains:
- A network of blood capillaries that absorb glucose and amino acids into the blood
- A lacteal (a lymph vessel) that absorbs fatty acids and glycerol
Villi greatly increase the surface area of the small intestine, making absorption highly efficient.
The Large Intestine
The large intestine (colon) receives undigested material from the small intestine. Its main functions are:
- Absorption of water from the remaining material back into the blood
- Absorption of mineral salts
- Formation of faeces from the remaining undigested waste
The large intestine contains billions of harmless bacteria that help break down remaining material and produce some vitamins.
The Rectum and Anus
Faeces are stored in the rectum until they are ready to be expelled. They exit the body through the anus. This final stage is called egestion (not excretion – egestion is the removal of undigested material, not waste products of metabolism).
3. Associated Organs
The Liver
The liver is the largest internal organ and performs many functions beyond digestion:
- Produces bile for fat digestion
- Stores excess glucose as glycogen
- Detoxifies harmful substances (alcohol, drugs, toxins) in the blood
- Makes blood proteins (including clotting factors)
- Regulates blood glucose levels
The Pancreas
The pancreas produces and releases pancreatic juice into the duodenum through the pancreatic duct. It also produces hormones insulin and glucagon that regulate blood glucose levels.
4. Summary Table: Digestion of Food Types
| Food Type | Enzyme | Where Made | Where it Acts | Product |
|---|---|---|---|---|
| Starch | Salivary amylase | Salivary glands | Mouth | Maltose |
| Starch | Pancreatic amylase | Pancreas | Duodenum | Maltose |
| Protein | Pepsin | Stomach wall | Stomach | Peptides |
| Protein | Protease (trypsin) | Pancreas | Duodenum | Amino acids |
| Fats | Lipase | Pancreas | Duodenum | Fatty acids + glycerol |
Summary
- Digestion begins in the mouth (salivary amylase digests starch).
- The stomach produces HCl and pepsin; churns food into chyme.
- The small intestine is the main site of digestion and absorption; villi increase surface area.
- Bile from the liver emulsifies fats; pancreatic enzymes complete digestion.
- The large intestine absorbs water; faeces are expelled through the anus.
- Glucose and amino acids are absorbed into blood capillaries; fatty acids into lacteals.
Practice Questions
- Where does digestion first begin, and which enzyme is involved?
- What is the role of hydrochloric acid in the stomach?
- Describe TWO ways in which the small intestine is adapted for efficient absorption of nutrients.
- Explain the difference between egestion and excretion.
- A student ate a meal containing rice, fish, and groundnut soup. Name the enzymes that would digest each food type and state where each enzyme acts.
Answers: (1) Mouth; salivary amylase (2) Kills bacteria; provides acidic pH for pepsin to work (3) Very long (6-7m) for maximum absorption time; villi increase surface area; good blood supply in villi (4) Egestion is removal of undigested material (faeces) through the anus; excretion is removal of metabolic waste products (e.g. CO₂, urea) from body cells (5) Rice (starch): salivary/pancreatic amylase in mouth/duodenum; Fish (protein): pepsin in stomach, protease in duodenum; Groundnut soup (fat): lipase in duodenum)
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