Passive, paracellular absorption occurs in the jejunum and ileum, and, to a much lesser extent, in the colon when dietary calcium levels have been moderate or high. In this case, ionized calcium diffuses through tight junctions into the basolateral spaces around enterocytes, and hence into blood. Such transport depends on having higher concentrations of free calcium in the intestinal lumen than in blood. Phosphorus Phosphorus is predominantly absorbed as inorganic phosphate in the upper small intestine. Iron Iron homeostasis is regulated at the level of intestinal absorption, and it is important that adequate but not excessive quantities of iron be absorbed from the diet.
Inadequate absorption can lead to iron-deficiency disorders such as anemia.
On the other hand, excessive iron is toxic because mammals do not have a physiologic pathway for its elimination. Iron is absorbed by villus enterocytes in the proximal duodenum. Efficient absorption requires an acidic environment, and antacids or other conditions that interfere with gastric acid secretion can interfere with iron absorption. This transporter is not specific for iron, and also transports many divalent metal ions. Once inside the enterocyte, iron follows one of two major pathways.