Final answer:
The middle segment of the small intestine is the jejunum, which is primarily responsible for nutrient absorption, including carbohydrates, amino acids, and fatty acids, into the bloodstream.
Step-by-step explanation:
The correct answer to which part of the small intestine is the middle segment is A) jejunum. The small intestine is a vital component of the digestive system, where most of the digestion and absorption of food occurs. It is divided into three parts: the duodenum, jejunum, and ileum. The jejunum is the second part, situated between the duodenum and the ileum. It is about 0.9 meters (3 feet) long when measured in a living body and is primarily responsible for the absorption of nutrients into the bloodstream. The name 'jejunum' is derived from the Latin word for 'empty' because it was frequently found empty after death in ancient times. The absorption process in the jejunum involves different mechanisms such as simple diffusion, facilitated diffusion, and active transport to transfer nutrients like sugars, amino acids, and fatty acids into the blood. It has a rich blood supply and plays a significant role in chemical digestion and nutrient absorption.