Human Digestive System — REET Notes

Structure of the digestive system — the alimentary canal

The human digestive system mainly has two parts — the alimentary canal and the digestive glands. The alimentary canal is a long tube that starts at the mouth and runs through the oesophagus, stomach, small intestine, large intestine, and rectum, ending at the anus. Food keeps moving forward through this same sequence, getting digested and absorbed along the way. Every organ along this tube — from the mouth down to the anus — plays a specific role in either breaking food down physically and chemically or absorbing the nutrients that are released, before whatever remains is finally expelled from the body.

What happens in each part of the digestive tract

The small intestine is the longest part of the alimentary canal, and most digestion and absorption happens right here.

Digestive glands — salivary glands, liver, and pancreas

The salivary glands secrete saliva in the mouth, which contains an enzyme called salivary amylase (ptyalin) that starts breaking down starch into maltose. The liver is the body's largest gland, and it makes bile; bile collects in the gallbladder and is released into the small intestine, where it helps digestion by breaking fat into tiny droplets (emulsification), though it isn't itself an enzyme.

The pancreas makes pancreatic juice, which contains enzymes such as trypsin (for protein), pancreatic amylase (for starch), and lipase (for fat), which are released into the small intestine and carry digestion further. Between them, these three glands — the salivary glands, the liver, and the pancreas — supply the saliva, bile, and pancreatic juice that, together with the intestinal juice made by the small intestine itself, carry digestion forward from the mouth all the way to the point where nutrients can be absorbed.

Major digestive enzymes and their roles

Salivary amylase (ptyalin)

Breaks starch into maltose in the mouth

Pepsin

Breaks protein into peptones in the stomach's acidic medium (HCl)

Trypsin

Breaks protein/peptones into smaller peptides in the small intestine's alkaline medium

Lipase

Breaks fat into fatty acids and glycerol

Intestinal juice

Secreted in the small intestine, completes the final stage of digestion

Absorption and the role of the large intestine

The inner wall of the small intestine has millions of finger-like projections called villi; these greatly increase the absorption area and absorb digested nutrients (glucose, amino acids, and so on) into the blood. The undigested leftover reaches the large intestine, where extra water and some salts are absorbed, forming solid faeces, which collects in the rectum and is expelled from the body through the anus (egestion). This is also why the small intestine, already the longest part of the alimentary canal, is where the bulk of digestion and absorption actually takes place, while the large intestine's job is comparatively simple — recovering water and some salts from what is left before it leaves the body.

Types of teeth and their function

Chewing food into smaller pieces (mechanical digestion) is the first step of digestion, which starts in the mouth, and the teeth play the main role in it. Humans have four types of teeth — incisors, which cut food; canines, which help tear food; and premolars and molars, which grind food into fine pieces. Taken together, these four types of teeth divide the work of mechanical digestion between them — cutting, tearing, and grinding food down before it is swallowed — which is exactly the first physical step of the digestion process described above.

Chewing food well doesn't just make food particles smaller — it also lets more saliva mix with the food, which lets salivary amylase do its job better and makes further digestion easier.

The tongue also plays a supporting role in digestion — it moves food around between the teeth while chewing, helps with swallowing, and, through its taste buds, is what lets us recognise the taste of food.

How to teach this to students

Practice questions

Frequently asked questions

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