Final answer:
Iron and calcium are primarily absorbed in the duodenum. This part of the small intestine assists in chemical digestion and adjusts absorption based on the body's needs, particularly iron through active transport. The jejunum and ileum also play roles in nutrient absorption, but for different vitamins and remnants.
Step-by-step explanation:
Iron and calcium are absorbed mostly by the duodenum, which is the first part of the small intestine and the shortest, where most chemical digestion takes place. Specifically, iron absorption occurs here in amounts that meet the body's current requirements. Iron is absorbed into mucosal cells via active transport and, once inside, binds to the protein ferritin. When the body requires iron, such as during bleeding or in the case of women during menstruation, there is an increased uptake of iron from the intestine. The jejunum is the second part of the small intestine and is where most nutrients, other than some specific minerals like iron and calcium, are absorbed into the blood. While the ileum is the last part of the small intestine, it absorbs a few remaining nutrients, such as vitamin B12 and bile salts.