Final answer:
The interior of ductal structures is lined by epithelial tissue. In the digestive system, villi and microvilli in the small intestine's lumen increase the surface area for nutrient absorption. Other ducts, like those in the lungs and mammary glands, are also lined with specialized epithelial cells for their respective functions.
Step-by-step explanation:
The interior of ducts and smaller ductal structures is primarily lined by a type of tissue known as epithelium. In the example of the small intestine, which is called the lumen, the lining consists of a mucosal epithelium that is highly specialized to increase surface area for absorption.
This is achieved through finger-like projections called villi, which are covered by even smaller projections called microvilli. These structures are integral for absorptive efficiency, providing vast surface area for nutrient uptake.
In other contexts, such as in alveoli (the tiny sacs in our lungs), an alveolar duct leading to these sacs is lined with smooth muscle and connective tissue. In the mammary glands, the alveolar lumen is lined with cells that produce milk, and the secretion is ejected into the ducts during breastfeeding.
The gallbladder's mucosa has a simple columnar epithelium for concentrating bile. All these structures serve to facilitate the transport or processing of various substances within the body.