Digestive system: absorption

After digestion, nutrients are still inside the digestive tube, which is technically outside your body. Absorption is the step that moves them across the wall of the tube and into the blood or lymph. Most absorption happens in the small intestine.

The small intestine is narrow but long, and its inner surface is built to be enormous. The lining is folded into ridges, and the ridges are covered with millions of tiny fingerlike projections called villi. The cells on each villus have even smaller projections called microvilli. Each fold, villus, and microvillus adds surface area, so the small intestine has a surface area of many times that of a smooth tube. More surface means more places for nutrients to cross.

Each villus is a miniature absorbing machine. Its wall is one cell thick, and inside it is a network of capillaries and a small lymph vessel called a lacteal. Sugars and amino acids pass into the capillaries and enter the blood. Fatty acids and glycerol enter the cells, are repackaged into fat-carrying particles, and move into the lacteal, which carries them to the blood through the lymphatic system. Vitamins and minerals are also absorbed here.

Blood from the small intestine does not go straight to the rest of the body. It first travels through the hepatic portal vein to the liver. The liver checks the incoming blood, stores some glucose, releases it later when blood sugar falls, and removes harmful substances. Only then does the blood continue to the heart.

What cannot be digested, such as fiber, moves on to the large intestine. This wider, shorter tube absorbs most of the remaining water and some salts, which turns the liquid waste into solid feces. Trillions of helpful bacteria live in the large intestine. They break down some of the leftovers and make vitamin K and some B vitamins that the body uses. Feces are stored in the rectum and eliminated through the anus. Fiber helps because it adds bulk and keeps material moving; if too little water is absorbed, the result is diarrhea, and if too much, constipation.

Common mistakes: thinking nutrients are absorbed in the stomach, and thinking the large intestine is where food gets digested. Most absorption occurs in the small intestine, and the large intestine mostly handles water and waste. Next lesson: the urinary system, which removes a different kind of waste.