Maarg Manthan · Topic 3.6

Digestive System and Human Dietary Patterns

Science & Technology › Human Physiology – Organ Systems · Topic 3.6

The digestive system breaks down food into small molecules that the body can absorb and use, and removes the undigested part. It consists of the alimentary canal, which runs from the mouth through the pharynx, oesophagus, stomach, small intestine and large intestine to the anus, and the associated glands: the salivary glands, liver and pancreas. Digestion is both mechanical, by chewing and churning, and chemical, by enzymes. Carbohydrates are broken down to glucose, proteins to amino acids and fats to fatty acids and glycerol. Most absorption takes place in the small intestine, which is about 6 to 7 metres long and has finger-like villi that increase its surface area. Humans are omnivores and need a balanced diet.

5 min read · UPSC Prelims · Science & Technology · Human Physiology – Organ Systems

At a Glance

Small intestine About 6 to 7 m; main absorption
Largest gland Liver
Stomach acid Hydrochloric acid and pepsin
Teeth 32 in adults
Energy Fat 9 kcal; carbs and protein 4 kcal per g
Exam link UPSC Prelims, Science & Technology: Human Physiology – Organ Systems

Where Does It Fit?

This is the sixth topic of the macro on human physiology and organ systems. It describes digestion and links the biomolecules in the first macro to nutrition and dietary patterns. Previous topic: Osmoregulation, ADH and Formation of Urine; Thermoregulation. Next topic: Immune System and Immunity.

What Does Each Part of the Digestive System Do?

Organ	Role and enzymes
Mouth	Chewing and mixing with saliva. Salivary amylase, also called ptyalin, starts digestion of starch
Oesophagus	Pushes food down by wave-like contractions called peristalsis
Stomach	Churns food with gastric juice containing hydrochloric acid, which kills microbes, pepsin which digests protein, and mucus that protects the lining
Liver	The largest gland. Makes bile, which emulsifies fats, stores glycogen, makes proteins and removes toxins
Pancreas	Makes pancreatic juice with amylase, trypsin and lipase, and the hormones insulin and glucagon
Small intestine	Duodenum, jejunum and ileum. Final digestion by intestinal enzymes and absorption through villi
Large intestine	Absorbs water and salts, hosts gut bacteria that make vitamin K, and forms faeces

What Are the Nutrients and a Balanced Diet?

Carbohydrates: the main source of energy, from cereals, millets, roots and sugar.
Proteins: for growth and repair, from pulses, milk, eggs, fish and meat.
Fats: store energy and carry fat-soluble vitamins.
Vitamins and minerals: needed in small amounts for many body functions, as in Chemical Building Blocks of Life (Biomolecules).
Fibre and water: fibre from vegetables and whole grains helps bowel movement, and water supports all processes.
Balanced diet: contains all nutrients in the right amounts for age, sex and activity, and the ICMR-NIN dietary guidelines released in 2024, reported as 17 guidelines, advise a diverse diet with less salt, sugar and oil.
Malnutrition: includes undernutrition such as stunting and wasting, and overnutrition such as obesity. Kwashiorkor is protein deficiency and marasmus is lack of both energy and protein.

What Else Should You Remember?

Teeth: adults have 32 teeth, in a pattern of 2 incisors, 1 canine, 2 premolars and 3 molars on each side of each jaw, and children have 20 milk teeth.
Enzyme matching: amylase acts on starch, pepsin on proteins in acid, trypsin on proteins in the intestine, lipase on fats, and lactase on lactose.
Bile: stored in the gall bladder and made in the liver, with no enzymes, and it breaks fat into small droplets and neutralises stomach acid.
Absorption: glucose and amino acids enter the blood in the villi, while fats enter the lymph vessels called lacteals.
Peptic ulcer: a sore in the lining of the stomach, often caused by the bacterium Helicobacter pylori, for which Barry Marshall and Robin Warren received the Nobel Prize in 2005.
Jaundice and hepatitis: liver conditions in which bile pigment builds up and the skin turns yellow, and hepatitis viruses spread through water and blood.
Diarrhoea and dehydration: treated by oral rehydration solution, as in Osmoregulation, ADH and Formation of Urine; Thermoregulation.
Lactose intolerance: lack of the enzyme lactase, causing discomfort after drinking milk.
Appendix: a small pouch at the start of the large intestine whose inflammation is appendicitis.
Alimentary canal length: the alimentary canal is about 9 metres long in an adult, from the mouth to the anus, and its inner lining is protected by mucus.
Digestive enzymes in summary: digestive enzymes are made by the salivary glands, stomach, pancreas and small intestine, and each works best at a particular pH, such as pepsin in acid and trypsin in alkaline conditions.
Gut bacteria: trillions of bacteria in the large intestine help to ferment fibre and protect against harmful microbes.

Why Does It Matter for the Exam?

Enzyme questions: which enzyme acts where and on which food.
Organ questions: functions of the liver and pancreas, which are also endocrine and detox organs.
Nutrition policy: national schemes for fortified rice, the Poshan Abhiyaan and school meals link to this topic.
Health link: obesity, diabetes and diet-related diseases are growing in India.
Common errors: believing that digestion is complete in the stomach or that bile contains enzymes.

As an analytical point, digestion turns the biomolecules of food into units the body can use. Healthy eating depends as much on the balance of nutrients as on the quantity of food.

Why Is This Topic Important?

Facts Enzymes and organs asked
Concept Balanced diet
Policy Nutrition schemes

What Came Next?

The next topic covers the immune system and immunity. Next topic: Immune System and Immunity.

Key People and Terms

Peristalsis Wave-like contraction of gut
Bile Liver secretion that emulsifies fats
Villi Finger-like folds for absorption
Amylase Starch-digesting enzyme
Pepsin Protein-digesting enzyme in stomach
Malnutrition Imbalance of nutrients

Exam Corner

Points to Remember

Liver is the largest gland.
Bile has no enzymes.
Small intestine 6 to 7 m.
Marshall and Warren: H. pylori, Nobel 2005.
Adults have 32 teeth.

Do Not Confuse With

Pepsin and trypsin Stomach versus intestine protein enzyme
Kwashiorkor and marasmus Protein lack versus lack of energy and protein
Digestion and absorption Breakdown versus entry into blood

Memory Hook

Mouth amylase, stomach pepsin, bile from the liver; pancreas gives trypsin, villi absorb and deliver.

Mains Angle

Questions link digestion with nutrition and public health. These points add depth.

Process Organs and enzymes
Diet Balanced diet
Burden Malnutrition and obesity

A Question You May Face

An original practice question, not a past paper question.

“Describe the process of digestion and absorption in humans and discuss the importance of a balanced diet.”

How to Answer

Organs Canal and glands.
Enzymes Action.
Absorption Villi.
Diet Balance.

GS Relevance

Prelims: organs, enzymes and their action, liver and pancreas, balanced diet, disorders. GS Paper 3: science and technology, health, nutrition.

Frequently Asked Questions

What are the main organs of the digestive system?

The alimentary canal consists of the mouth, pharynx, oesophagus, stomach, small intestine and large intestine ending at the anus, and the associated glands are the salivary glands, liver and pancreas. Food is broken down by mechanical action and by enzymes, and the nutrients are absorbed mostly in the small intestine.

What is the role of bile?

Bile is made in the liver, stored in the gall bladder and released into the duodenum. It contains no enzymes but breaks large fat droplets into small ones, called emulsification, which gives enzymes a larger surface to work on, and it also neutralises acid from the stomach.

Where does absorption of food take place?

Most absorption of digested food takes place in the small intestine, which is about 6 to 7 metres long and has millions of finger-like villi that increase its surface area. Glucose and amino acids enter the blood capillaries, while fatty acids and glycerol enter the lymph vessels called lacteals.

What is a balanced diet?

A balanced diet contains carbohydrates, proteins, fats, vitamins, minerals, fibre and water in the right proportions for a person's age, sex and activity. The ICMR-NIN dietary guidelines released in 2024 advise a varied diet with more pulses, vegetables and whole grains and less salt, sugar and oil.

What is the difference between kwashiorkor and marasmus?

Kwashiorkor is a form of severe malnutrition caused mainly by a lack of protein, with swelling of the body, while marasmus is caused by a lack of both energy and protein and shows severe wasting of the body. Both mainly affect young children and are prevented by adequate and varied food.

PYQ Practice — Statement Analysis

1 The liver is the largest gland in the human body.

True

It makes bile.

2 Bile contains digestive enzymes.

False

It only emulsifies fats.

3 Pepsin digests proteins in the stomach.