How Your Body Turns Food Into Energy, Step by Step

Food passes through the human digestive system in this order: buccal cavity (mouth), pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine, rectum, and anus. Along this path, several digestive glands help break down food: the salivary glands (in the mouth), liver, pancreas, and glands in the wall of the stomach and small intestine. Each part plays a special role. The buccal cavity starts digestion by chewing and mixing food with saliva. The pharynx connects the mouth to the oesophagus, which moves food to the stomach. The stomach churns and digests food further. The small intestine completes digestion and absorbs nutrients, while the large intestine absorbs water. The rectum stores undigested waste, which is finally passed out through the anus. For example, when you eat a chapati, it is first chewed in your mouth, then travels down this entire path before the nutrients reach your blood. Many students forget the order of the small intestine's parts—remember: duodenum, jejunum, ileum.

Digestion in the mouth starts with the teeth breaking food into smaller pieces, the tongue mixing it, and saliva moistening it. Saliva contains an enzyme called salivary amylase, which begins breaking down starch into maltose (a simpler sugar). The tongue helps roll the chewed food into a soft ball called a bolus, which is then swallowed. The bolus passes through the pharynx into the oesophagus. The oesophagus moves the bolus to the stomach by a wave-like muscular movement called peristalsis. Peristalsis pushes food down even if you are lying down or upside down, because it does not depend on gravity. For example, when you eat a biscuit, it starts to taste sweet after chewing because salivary amylase is changing starch into maltose right in your mouth. Some students think food simply drops into the stomach, but peristalsis is always at work.

In the stomach, food is mixed with gastric juice, which contains hydrochloric acid and an enzyme called pepsin. Hydrochloric acid makes the stomach acidic and helps pepsin start breaking down proteins into simpler substances. The partly digested food then moves to the small intestine, where three digestive juices act on it: bile from the liver (stored in the gall bladder), pancreatic juice from the pancreas, and intestinal juice from glands in the small intestine wall. Bile breaks up large fat droplets into smaller ones (emulsification), making it easier for enzymes to act. Pancreatic juice contains enzymes that digest proteins, carbohydrates, and fats. Intestinal juice completes the digestion, turning all food into simple, absorbable forms. For example, the oil in a fried samosa is broken into tiny droplets by bile, so enzymes can digest it. Some students think bile digests fat, but it only breaks it into smaller droplets; enzymes do the actual digestion.

Nutrients from digested food are absorbed mainly in the small intestine. The inner wall of the small intestine has tiny finger-like projections called villi, which increase the surface area for absorption. The villi take up simple sugars, amino acids, fatty acids, and glycerol into the blood. These nutrients are then carried to all body cells, where they are used for energy, growth, and repair—a process called assimilation. Water and some minerals are absorbed in the large intestine. The remaining undigested waste is stored in the rectum and finally passed out through the anus (egestion). For example, after you eat dal and rice, the digested nutrients enter your blood from the small intestine and help your body grow and stay active. A common misconception is that all absorption happens in the stomach, but most of it occurs in the small intestine.