Absorption is the movement of small, digested food molecules and ions through the walls of the intestine into the blood or lymphatic system.
The small intestine is where nutrients are absorbed.
A common misconception is that water is only absorbed by the large intestine. Most water is actually absorbed in the small intestine alongside dissolved nutrients. The colon (large intestine) also absorbs some of the remaining water to compact undigested material into solid feces.
To absorb massive quantities of nutrients efficiently before food passes out of the body, the small intestine requires a huge internal surface area. This is achieved through thousands of finger-like projections called villi(sg. villus).
The villi are folds in the inner lining (mucosa) of the small intestine. They multiply the inner surface area by a factor of many hundreds.
The microvilli are microscopic foldings on the cell surface membranes of the individual epithelial cells that cover each villus.
By having both villi and microvilli, the small intestine dramatically increases its surface area : volume ratio. This structural adaption ensures a significantly faster rate of diffusion and active transport, allowing nutrients to move out of the gut cavity and into the bloodstream as quickly as possible.
The outer layer of the villus is only one cell later thick, providing a short diffusion distance for food molecules to pass through, accelerating the rate of absorption.
A rich network of microscopic blood vessels runs directly beneath the epithelium.
They absorb water, soluble carbohydrates (glucose), amino acids, mineral ions, and water-soluble vitamins.
The continuous flow of blood rapidly carries these absorbed nutrients toward the liver (via the hepatic portal vein), maintaining a steep concentration gradient so diffusion can continue uninterrupted.
A small tube located right in the center of the villus, which is a part of the lymphatic system.
Lipids (fats and oils) are too large and hydrophobic to safely enter standard blood capillaries directly. Instead, digested fatty acids and glycerol are recombined into small fat droplets inside the epithelial cells and absorbed straight into the lacteal. The lymphatic system later empties these lipids into the main bloodstream.