The small intestine, the villus and the large intestine

1A tennis court, or half a badminton court?

For decades, textbooks said that the inside surface of the human gut, spread out flat, would cover a tennis court, about 260 to 300 m². In 2014 two Swedish scientists measured it again, using microscope images of real gut linings, and found about 32 m²: not a tennis court, but about half a badminton court. The old figure was wrong, but even 32 m² is enormous for a tube folded into your abdomen, and almost all of it is in the small intestine. This lesson shows why that surface is needed, and how it is built.

2In the duodenum: bile and pancreatic juice

Acid chyme from the stomach meets two juices in the duodenum:

• Bile, from the gall bladder. Its salts emulsify fats, and it is alkaline, so it helps neutralise the stomach acid.
• Pancreatic juice, from the pancreas. It contains sodium hydrogencarbonate, which neutralises acid, and three kinds of enzyme: pancreatic amylase (starch → maltose), trypsin, a protease (proteins and polypeptides → shorter peptides), and lipase (fats → fatty acids and glycerol).

The enzymes of the small intestine work best in slightly alkaline conditions, which is why the acid must be neutralised.

3In the ileum: digestion finished

The epithelial cells lining the ileum carry the last enzymes on their surface: maltase, sucrase and lactase split the double sugars into single sugars, and peptidases split peptides into amino acids. Now every food has been broken into molecules small enough to absorb.

Many adults around the world make much less lactase after childhood. Undigested lactose then reaches the large intestine, where bacteria ferment it, causing gas, bloating and sometimes diarrhoea after drinking fresh milk: this is lactose intolerance. Fermented milk, such as ikivuguto, often causes less trouble, because its bacteria have already used up some of the lactose.

4Absorption: built for the job

Digested food passes through the wall of the ileum into the blood. The ileum is suited to absorption in these ways:

• It is long: in a living adult, about two-thirds of the 5 m gut is small intestine.
• Its lining has folds, the folds are covered in villi, and each cell on a villus carries microvilli: a very large surface area.
• The villus wall is one cell thick, so the distance for diffusion is short.
• Each villus has a dense network of blood capillaries. The blood keeps carrying the absorbed food away, so the concentration stays lower inside, and diffusion keeps going.
• Each villus has a lacteal to take up fats.

Figure 1: One villus. Glucose and amino acids enter the blood capillaries (by diffusion and active transport, which you met in Senior 2). Fatty acids and glycerol enter the lacteal and reach the blood later, through the lymph.

5After absorption: assimilation

Blood from the villi goes first to the liver, in the hepatic portal vein. The liver stores extra glucose as glycogen and releases it when needed; body cells use glucose in respiration. Amino acids are built into new proteins, for growth and repair; amino acids that are not needed are broken down in the liver, and their nitrogen leaves as urea, as you saw in Senior 2. Fats are used for energy, stored under the skin, and used to build cell membranes.

6The large intestine

What reaches the large intestine is mostly water, fibre that humans cannot digest, dead cells from the gut lining and bacteria. The colon absorbs much of the remaining water and mineral salts, turning the liquid into semi-solid faeces. Its bacteria make some vitamins, such as vitamin K, which are also absorbed. Fibre from beans, vegetables, fruit and whole grains gives the faeces bulk, so the muscles of the colon can grip them and move them along by peristalsis. The faeces are stored in the rectum and egested through the anus.