Digestive System

Food does not pass through every digestive organ directly; it travels down one continuous tube, the alimentary canal, while certain glands pour their juices into that tube from the side. The main canal runs mouth → pharynx → oesophagus → stomach → small intestine → large intestine → rectum → anus, and into it empty the salivary glands, the liver (through the gall bladder) and the pancreas, which are called accessory glands because food never actually flows through them.

The key idea to fix early is that digestion begins in the mouth and is completed in the small intestine, while the liver and pancreas help the small intestine without ever touching the food's path directly. The digestive system breaks food into simpler substances which are then absorbed and used for energy, growth and repair.

Digestion starts the moment food enters the mouth, where it is both physically and chemically broken down. The teeth chew and grind food into small pieces, the muscular tongue mixes it with saliva and helps in tasting and swallowing, and the three pairs of salivary glands pour in saliva at pH 6.8–7.0 carrying the enzyme salivary amylase, also called ptyalin — the only digestive enzyme that acts in the mouth.

A common exam trap is to list "maltose" as an enzyme; in fact maltose is the sugar produced, not an enzyme, and the only enzyme of saliva is salivary amylase (ptyalin) acting on starch. The wisdom teeth are simply the last (third) molars to appear, and the swallowed food is driven to the stomach by wave-like contractions called peristalsis.

From the oesophagus the food reaches the stomach, a J-shaped muscular bag that can expand to hold up to 1.5 litres of food and churns it into a paste called chyme. The wall secretes gastric juice, whose key component is hydrochloric acid (HCl), making the stomach strongly acidic at pH 1.5–3.5. This acidic medium is essential: the protein-digesting enzyme pepsin can work only in acid, which is why no protein digestion happens in the mouth.

The single fact most worth remembering is that pepsin acts only in an acidic medium, supplied by HCl. The stomach mainly digests protein — final digestion of fat is completed in the small intestine, not the stomach — and after 2–4 hours the acidic chyme is released into the small intestine.

The liver is the largest gland in the human body and one of its busiest organs, yet it never receives food directly. Its main digestive product is bile, a yellowish-green fluid made in the liver, stored and concentrated in the gall bladder, and poured into the small intestine through the bile duct when fatty food arrives.

The most examined point here is that bile contains no enzyme; it only emulsifies fat, breaking large globules into tiny droplets so that the fat-digesting enzyme lipase (from the pancreas) can act on a far larger surface area. Diagram labels for this region: Liver, Bile Duct, Gall Bladder.

The pancreas pours pancreatic juice — a mix of powerful enzymes plus alkaline bicarbonate — into the small intestine, where the bicarbonate neutralises the acid arriving from the stomach. It is unusual because it does two jobs: it is both a digestive gland and a hormone-producing (endocrine) gland.

Because it carries enzymes for all three food types, pancreatic juice is often called the complete digestive juice. The pancreas also produces two hormones that act on blood glucose in opposite directions.

A standard trap reverses these: remember that insulin lowers blood glucose (storing glucose as glycogen) while glucagon raises it, and a lack of insulin causes the disease diabetes mellitus.

The small intestine is the longest part of the alimentary canal, coiled and about 6–7 metres long, and it is here that digestion is completed and most nutrients are absorbed. It receives food from the stomach through the duodenum, and receives bile from the liver and enzymes from the pancreas.

The lining is folded into millions of finger-like villi, which increase the absorbing surface nearly 600 times. Each villus holds blood capillaries that pick up sugars and amino acids and a lacteal (a lymph vessel) that picks up the products of fat (villus diagram labels: Epithelial Cells, Lacteal, Blood Capillaries, Villus). The nutrients absorbed here pass into the blood, and most digestion is completed in the small intestine with the help of bile and pancreatic juice.

Whatever the small intestine cannot digest passes into the large intestine, made up of the colon, rectum and anus. No real digestion happens here; its main job is to absorb water and electrolytes from the undigested material and form faeces.

Food takes 24–72 hours to pass through the large intestine (parts labelled: Colon, Rectum, Anus). The small worm-like appendix opens here too: in humans it is a vestigial / waste organ (linked with chlorophyll/cellulose-rich diets), and the corresponding caecum is the largest organ in cows, goats and buffaloes where it helps digest plant matter.

The end of the canal handles waste removal. The rectum stores faeces temporarily, and when it fills, nerve signals reach the brain so we feel the urge to pass stool.

• The anus removes waste from the body (defecation).

Some organs aid digestion without food passing through them — these are the accessory organs.

Diagram labels for this cluster: Liver, Gall Bladder, Pancreas.

It helps to see every digestive juice in one place — where it is made, what enzymes it carries and what it does — because exam questions often mix these up.

The two stand-out points are that bile carries no enzyme and that pancreatic juice carries enzymes for all three food groups.

The acidity or alkalinity of each region is fixed to suit the enzymes working there, and the swing from the acidic stomach to the alkaline intestine is a favourite exam fact.

• Absorption of most nutrients occurs in the small intestine.
• Water absorption mainly occurs in the large intestine.

The stomach is acidic because pepsin needs acid; the small intestine is made alkaline by the bicarbonate of pancreatic juice and bile.

A quick picture of which job happens where, from intake to egestion.

Once food is broken into simple molecules, those molecules must cross the gut wall into the body — this is absorption, happening almost entirely in the small intestine (water being the exception).

So water-soluble products go via blood capillaries → hepatic portal vein → liver, while fat products go via lacteals → lymph → thoracic duct before joining the blood.

The final stage clears out everything the body could not use.

• Faeces move to the rectum and are stored.
• When the rectum is full, signals are sent to the brain and we feel the urge to pass stool.
• Waste is removed from the body through the anus.
• Did You Know: the colon absorbs around 1.5 litres of water daily.

The food we swallow does not move at one speed; it lingers longest in the intestines.

A handy memory aid is IDAAE — Ingestion, Digestion, Absorption, Assimilation, Egestion.

Finally, the table examiners return to again and again — every major digestive enzyme, its source, what it acts on and what it produces.

Note that HCl is not an enzyme — it is an acid that kills germs and creates the medium pepsin needs — and that bile has no enzyme at all, so neither appears as a true "enzyme".