Human Anatomy - Kotsan I. Y. 2009

Splanchnology
Digestive System
Age-Related Characteristics of the Small Intestine

The length of the Small Intestine in a newborn is 1.2–2.8 m. By 10–11 years of age, it reaches the length of an adult's intestine.

In newborns, the duodenum is ring-shaped, and its flexures form later. The duodenal glands (Brunner's glands) in a newborn are small and less branched than in adults. These glands develop most intensively During the first years of a child's life. The beginning and end of the duodenum in a newborn are located at the level of the first lumbar vertebra. By the age of 7, the descending part of the duodenum descends to the level of the second lumbar vertebra or even lower. The initial section of the mesenteric part of the small intestine in children is positioned higher (sometimes at the level of the first lumbar vertebra), whereas the terminal section (the ileocecal junction) in newborns is located high up under the Liver; with age, it descends, and in elderly individuals, it may even be located in the pelvic cavity.

The mucous membrane of the small intestine in a newborn is thin, highly vascularized, and exhibits high permeability, especially during the first months of life; consequently, incompletely digested products, toxins, and even microbes can enter the bloodstream. The folds and villi of the mucous membrane are poorly developed. The intestinal glands (crypts of Lieberkühn) are larger in newborns than in adults.

The muscular layer of the small intestine in a newborn is poorly developed, particularly its longitudinal layer.