Most small molecules produced by the digestion of food enter the blood from the small intestine. Some small soluble molecules produced by digestion in the stomach and duodenum are absorbed through the gut lining, but this represents only a small proportion of the overall level of absorption that takes place in the ileum.
The epithelial lining has an enormous surface area and has developed particularly for the process of absorption.
It has folds in the inner surface of the intestinal wall, folded projections called villi on the surface of the intestinal cells and microscopic projections called microvilli on the cell surface membranes.
Glucose is actively transported into the cells of the small intestine and across the walls of the blood capillaries that run close by. Amino acids and small peptide molecules are also actively transported along the same route, although by different carrier molecules.
Products of lipid digestion do not enter the blood directly. Bile salts and lipase enzymes break up complex lipids into monoglycerides and free fatty acids and these combine with bile salts to form micelles, which diffuse into the intestinal epithelial cells. After conversion to chylomicrons, they then enter the lymphatic system, not the blood.