 The organ where the bulk of the digestion and absorption of nutrition occurs is the small intestine. It's a really long, really skinny tube, hence small. The really large surface area of the small intestine enables the efficient absorption of molecules. The average small intestine surface area is about 30 square metres. If that's laid flat out, it's about the area of a handball court. To increase the surface area in a small space, the intestine has many folds. The intestinal tissue is a mass of finger-like shapes called villi, or in the singular, villus. To get into our bodies, the nutrients have to cross the cell membranes into the intestinal tissue and so the greater the surface area of cells, the more nutrients can be brought in. If we zoom in further, we find that the surface of each villus is made up of hundreds of epithelial cells. Each of those cells has an increased surface area by having microvilli on the side that meets the inside of the intestine. Microvilli are more of these finger-like shapes that provide lots of surface area. They increase the opportunities for transport across the cell membrane. Collectively, the microvilli of many cells on the villi is called the brush border. A previous video talked about active and passive transport across membranes. Some nutrients, such as the sugar fructose, move into the epithelial cells by passive diffusion. Others, like amino acids, glucose and vitamins, require active transport. Sugars and amino acids are moved into the capillaries that protrude into each villi and so immediately enter the bloodstream. Other molecules, like fats, are absorbed into the epithelial cells, process further and then transported by exocytosis into lacteals in the villi. These join up with the lymphatic system. Later on, the molecules in the lymphatic system are added to the blood. Without the specialised microvilli surface of the epithelial cells in the small intestine, and without the shape of the small intestinal tissue as a tube with a ginormous surface area, the absorption of nutrients by the digestive system would be really inefficient. Likewise, if the food hadn't already been broken down to small pieces by the time it hit the small intestine, the large surface area wouldn't really help the absorption of nutrients. Each of the parts of the digestive system, the organs, the tissues and the cells, have evolved an important structure that helps its specific function play its part as part of the whole.