Can you believe your digestive system is as long as a bus? Yep, it's about 9 meters long, all wrapped up inside your body. A lot of things go on in your digestive system, not just digestion. What else do you think happens? Digestion goes hand in hand with absorption, which happens in your small intestine, which is what we're going to look at in this video. Digestion is the breakdown of food, from large insoluble molecules into small molecules, which can be used by your body. Digestion involves many organs before it reaches the small intestine. For more information on the roles of these organs in the digestive system, watch this video. Enzymes and other chemicals such as bile break down food. For more on the role of enzymes and bile in digestion, watch these videos. As they move through the digestive system, carbohydrates are broken down into sugars. Proteins are broken down into amino acids. And fats are broken down into fatty acids and glycerol. But the job of the digestive system doesn't stop at digestion. For our body to make use of these small, soluble molecules, we need to absorb them into our bloodstream. This happens in a small intestine. The small intestine is made up of lots of tiny finger-like projections called villi. The villi increase the surface area of the small intestine. The larger the surface area, the more absorption that can take place. Each villis has a tiny blood vessel called a capillary, and another vessel called a lactil. The food molecules pass from the villi of the small intestine into either the blood capillary or the lactil. Some have absorbed into the blood vessel in the small intestine that digestive food molecules can then be transported around the body to where they are needed. So why did those large molecules need to be broken down in the first place? The small intestine is kind of like a sieve. It has small holes in it. Small molecules are able to pass through those holes, whilst large molecules cannot. So carbohydrates, proteins and fats are all too large to pass through the holes in the small intestine. Such sugars, amino acids, fatty acids and glycerol are all small enough to pass through. Sugar and amino acids pass through the holes into the blood capillary, while fatty acids and glycerol pass into the lactil of the villi. These small molecules move by diffusion, moving from a high to a low concentration. For example, the small intestine has a high concentration of sugar molecules, whereas the blood supply in the small intestine has a low concentration of sugar. So the sugar moves down its concentration gradient from a high concentration in the small intestine through the holes in the villi into the blood capillary where it's in low concentration. Sugar, amino acids, fatty acids and glycerol all use diffusion to move into the blood within the small intestine. So after digestion breaks down large molecules into small molecules, absorption of the small molecules happens inside our small intestine by diffusion into the blood vessels. What an amazing system!