Digestion and Absorption

Food gives us energy and the raw materials to build and repair the body, but most of what we eat is in the form of large, insoluble molecules that cannot pass into our cells. Digestion is the process of breaking down this complex food into simple, soluble molecules that the body can absorb and use.

The human digestive system has two parts: the alimentary canal (a long tube from mouth to anus) and the associated digestive glands. The alimentary canal runs: mouth → pharynx → oesophagus (food pipe) → stomach → small intestine → large intestine → rectum → anus.

• The mouth has teeth (for chewing) and a tongue, and receives saliva from the salivary glands.
• The oesophagus pushes food to the stomach by waves of muscle contraction called peristalsis.
• The stomach is a muscular bag that churns food and mixes it with gastric juice.
• The small intestine is the main site of digestion and absorption; it is very long and its inner lining is folded into tiny finger-like villi.
• The large intestine absorbs water and forms the faeces.

The chief associated glands are the salivary glands, the liver (makes bile, stored in the gall bladder) and the pancreas (makes pancreatic juice with several enzymes).

Digestion happens step by step as food moves along the canal, with each region adding its own juice and enzymes. Enzymes are biological catalysts that speed up the breakdown of each type of food.

• In the mouth: teeth chew the food and saliva (from the salivary glands) moistens it. Saliva contains the enzyme salivary amylase (ptyalin), which begins the digestion of starch into maltose.
• In the stomach: the food mixes with gastric juice, which contains hydrochloric acid (makes the medium acidic and kills germs) and the enzyme pepsin, which begins the digestion of proteins into peptones. (Pepsin works best in acid.)
• In the small intestine: this is where digestion is completed. Bile from the liver (no enzymes) emulsifies fats — breaks large fat drops into tiny droplets so enzymes can act faster. Pancreatic juice and the intestinal juice supply enzymes that finish the job: trypsin (proteins → amino acids), pancreatic amylase (starch → maltose/glucose), and lipase (fats → fatty acids + glycerol).

By the end of the small intestine, carbohydrates are broken into glucose, proteins into amino acids, and fats into fatty acids and glycerol — all simple and ready to be absorbed.

Once food is digested into simple molecules, it must be taken into the blood — this is absorption. Most absorption happens in the small intestine, which is beautifully designed for it: it is very long, and its inner wall is folded and covered with millions of tiny finger-like projections called villi. The villi (and even smaller microvilli) give a huge surface area for absorbing nutrients, and each villus has a rich blood supply and a lymph vessel (the lacteal).

• Glucose and amino acids pass into the blood capillaries of the villi.
• Fatty acids and glycerol are absorbed into the lacteals (lymph) of the villi.

The absorbed glucose, amino acids and fats are then carried by the blood to the liver and the rest of the body, where they are used to release energy (assimilation), build tissues and store reserves. The large intestine then absorbs most of the remaining water, and the undigested waste is passed out as faeces (egestion).

Several common digestive disorders can affect this system: indigestion (food not properly digested, often from overeating or anxiety), constipation (hard stools and difficulty passing them, often from too little fibre or water), diarrhoea (frequent watery stools, dangerous because it causes dehydration), vomiting, and jaundice (yellowing of the skin and eyes due to a liver problem). Eating a balanced diet with enough fibre and water, and good hygiene, helps keep the digestive system healthy.