Once food has been turned into chyme in the stomach it is squirted into the small intestine. Here, in a frenzy of chemical activity, it is broken down further and finally absorbed by the blood. Each day, around 11.5 litres (20 pints) of food, liquids and digestive juices pass through the small intestine.
Organs in concert
To help it digest, the small intestine gets help from three other organs; the pancreas, which makes enzymes; the liver, which makes bile; and the bile-storing organ, the gallbladder.
One of the liver’s many jobs is to produce bile – a bitter liquid that turns fats into more readily digestible fatty droplets. Once produced, bile is then stored in the gallbladder.
When food leaves the stomach bile leaves the gallbladder and heads for the small intestine. There it mixes with incoming enzymes from the pancreas.
The pancreas produces three main enzymes; amylase, which turns carbohydrates into sugars; protease, which turns proteins into amino acids; and lipase, which turns fatty droplets into fatty acids.
For 3-5 hours bile and enzymes work together, reducing nutrients to simpler, absorbable forms. Absorption takes place in the intestine wall, which is lined by thousands of finger-like projections. These projections, called villi, greatly increase the surface area of the intestine and so its capacity for absorbing nutrients.
Into the Blood
Villi absorb nutrients and channel them into the blood. Once there, they are taken to the liver and distributed around the body. Meanwhile, the remaining chyme enters the final part of the gut. Also, fat digestion has another complication. On entering the villi, the fatty acids take a trip through the lymph system before finally entering the bloodstream.
“Around 95 per cent of all absorption takes place in the small intestine – the rest takes place in the colon”.
—CHEWING THE FAT—
Fats are particularly hard to digest. Even after being drenched in hydrochloric acid in the stomach, they are still not fit for enzyme consumption. This is where bile comes in. In a process called emulsion, bile turns fats into fatty droplets, which are then small enough for enzymes to attack.