Duodenum

The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In mammals it may be the principal site for iron absorption. The duodenum precedes the jejunum and ileum and is the shortest part of the small intestine.

The duodenum is divided into 4 parts, delimited by different flexures:

• The cranial part of duodenum (Pars cranialis duodeni) is relatively short. From the pylorus, it goes to the right, more or less caudally and dorsally depending on the species, and ends at the cranial duodenal flexure (Flexura duodeni cranialis). In many species, just after the pylorus, it presents a more or less distinct dilation: the duodenal ampulla (Ampulla duodeni) or duodenal bulb. Well defined in the Horse, voluminous in Camels, this ampulla is less pronounced or absent in other domestic mammals.

• The descending part of duodenum (Pars descendens duodeni) begins at the cranial duodenal flexure and runs caudally, more or less far depending on the species, along the right border of the lumbar region or in the right flank.

  It is short in Humans, Equids, and Pigs, where it barely surpasses the level of the right kidney’s symphysis, but is much longer in Carnivores and even more so in Ruminants and Rabbits, where it reaches near the iliac region. It usually ends at the caudal duodenal flexure (Flexura duodeni caudalis).

• The transverse part of the duodenum (Pars transversa duodeni) begins after the caudal duodenal flexure and extends to the left, but its length and shape vary greatly.

  It is longest in Humans and Equids, where it clearly crosses transversely over the ventral side of the caudal vena cava and the aorta to reach the left lumbar region. It is much shorter in Carnivores, where it forms a curve that connects seamlessly with the ascending part. In Ruminants, it is so short that it merges with the caudal flexure and loses its individuality. As a result, the third flexure of the duodenum (between transverse part and ascending part) is often difficult to identify and has not been named in these species.

• The ascending part of duodenum (Pars ascendens duodeni) is the terminal segment of duodenum. It is very long in Ruminants and Rabbits, somewhat shorter in Carnivores, it is short in Humans and barely formed in Equids. In all species it lies to the left of the mesenteric root, reaching the ventral surface of the left kidney or its vicinity, and, attached to the lumbar region by a narrow mesentery, ends at the duodenojejunal flexure (Flexura duodenojejunalis), generally abrupt and oriented ventro-caudally.