Jejunum

The jejunum is the second part of the small intestine. It is found in humans and most higher vertebrates. This group includes mammals, reptiles, and birds. The jejunum plays a vital role in the digestive system. It sits between the duodenum and the ileum. Its main job is to absorb nutrients from food. This process is essential for keeping the body healthy.

To understand how it works, we must look at the process of absorption. First, enzymes in the duodenum digest food into small molecules. These molecules then enter the jejunum. The lining of the jejunum is made of specialized cells called enterocytes. These cells grab the small nutrient particles. Some nutrients, like the sugar fructose, move through passive transport. Other nutrients, such as vitamins and glucose, use active transport. Active transport means the cells use energy to move these molecules. Once absorbed, most nutrients enter the enterohepatic circulation. They then travel to the liver via the hepatic portal vein.

The structure of the jejunum is built specifically for this work. The interior surface is covered in many tiny projections called villi. These villi look like small, finger-like growths. They greatly increase the surface area available for absorption. The cells on these villi also have even smaller microvilli. Additionally, the submucosa contains large circular folds called plicae circulares. These folds are best developed in the jejunum. By having many folds and villi, the jejunum can absorb much more than a smooth tube could.

There are several ways to distinguish the jejunum from other parts of the intestine. While there is no clear anatomical line between the jejunum and the ileum, there are subtle differences. The jejunum typically has a larger diameter than the ileum. It also has less fat inside its mesentery. The villi in the jejunum are much longer than those in the duodenum or the ileum. Furthermore, the jejunum contains very few Brunner's glands or Peyer's patches. Peyer's patches are lymphoid nodules found mostly in the ileum. However, the jejunum does have some lymph nodes in its mesentery.

The history of the name "jejunum" is quite interesting. It comes from the Latin word jējūnus, which means "fasting." Early observers noticed this part of the intestine was often empty. This was frequently seen in bodies following death. The area was void of food because of its intense activity. This movement is called peristalsis. Peristalsis is a process where circular and longitudinal smooth muscles contract. These muscles work together to push food along the tract. Because the jejunum is so active, it often clears itself quickly.

In terms of scale, the jejunum is a significant portion of the digestive tract. In adult humans, the small intestine is quite long. The jejunum makes up about two-fifths of that total length. The environment inside the jejunum is also very specific. The pH level usually stays between 7 and 8. This means the environment is neutral or slightly alkaline. To keep the bowel mobile within the abdomen, it is suspended by the mesentery. This tissue allows the intestine to move freely while staying in place.

While the term "jejunum" is standard for higher vertebrates, other animals use different names. In fish, the divisions of the small intestine are not as clear. Scientists may use the terms "middle intestine" or "mid-gut" instead. This shows how different biological systems adapt to their environments. Regardless of the name, the function of absorbing nutrients remains a core part of life. The jejunum serves as a highly specialized bridge in the complex system of digestion.