The small intestine is a major digestive organ which absorbs vital nutrients. It is the longest part of the gastrointestinal tract and can be separated into three parts - the duodenum, jejunum and the ileum . The duodenum is responsible for the breakdown of food. It has the widest lumen of the small intestine and itself is split in to four parts, the superior part, the descending part, the inferior part and the ascending part . The jejunum is smaller in diameter than the duodenum and absorbs carbohydrates and sugars. The ileum is smaller still than the jejunum and absorbs what is not digested by the jejunum, such as vitamin B12.
Diffusion is the main process for the absorption of micromolecules from the lumen into the epithelial cells. Generally there is a greater concentration of micromolecules (particularly glucose) in the lumen than in the blood, this creates a concentration gradient for which the molecules can diffuse down, from the lumen into the bloodstream. This concentration gradient is maintained as the blood is in constant circulation and so the recently absorbed glucose will never affect the gradient as is instantly transported around the body.
As diffusion can only function up until a point when the concentrations are (at best) equal, acvtive transport must be employed to uptake the remainder of macromolecules into the blood. Using glucose as an example: Glucose is co-transported into the blood via active transport with sodium ions, that have been removed from the cell by the sodium-potassium pump, via a co-transport molecule. The glucose concentration is then much greater in the epithelial cell than it is in the blood and so glucose is able to move, via facilitated diffusion, using a carrier protein .