Question

Describe In detail Anatomy of the small and large intestines— how does it change in the 3 parts?

Answer 1

Anatomically the small intestine extends from the pylorus to the ileocaecal junction. It is about 6 m long and divided into three parts: duodenum, jejunum, and ileum. The
proximal fixed part which is about 10 inches (25 cm) long is called duodenum. The remaining long part which is freely mobile is divided into two parts—jejunum and ileum.

The duodenum is principally retroperitoneal and fixed. It
receives chyme from the stomach, bile from the gallbladder, and pancreatic juice from the pancreas. The fixation of duodenum is essential because mobile duodenum will cause twisting and kinking of common bile duct (CBD) and pancreatic ducts which open into it. It is intimately related with pancreas and extra hepatic biliary apparatus. The duodenum is 10 inches (25 cm) long. It begins at the pylorus which lies on the transpyloric plane about 2.5 cm to the right of the median plane and ends at the duodenojejunal junction which lies about 2.5 cm to the left of the median plane and little below the transpyloric plane.The duodenum is retroperitoneal except the proximal 2.5 cm, which is suspended above by the lesser omentum and below by the greater omentum.The main function of the duodenum is digestion. It receives chyme from the stomach which is mixed with bile and pancreatic enzymes here.   
Duodenum is divited into four parts.
The upper half of the duodenum develops from the foregut
and the lower half from the midgut. Therefore, the arterial
supply of the upper half is derived from the coeliac trunk
(artery of the foregut) and that of the lower half from the
superior mesenteric artery (artery of the midgut).   
The small intestine proper extends from the duodenojejunal flexure to the ileocaecal junction. Its upper two-fifth forms the jejunum and its lower three-fifth forms the ileum. However, there is no definite line of demarcation. The jejunum and ileum are suspended from the posterior abdominal wall by a large fold of peritoneum called the mesentery of small intestine; hence, the small intestine enjoys a considerable mobility. The structure of the small intestine corresponds to its functional requirements.

The large intestine is about 1.5 m long and extends from the caecum in the right iliac fossa to the an...us in the perineum. Apart from the transverse colon and sigmoid colon, it is more fixed in position than the small intestine.

For descriptive purposes, the large intestine is divided into
the following four parts:
1. Caecum and appendix.
2. Colon.   
The colon is further divided into four parts: ascending
colon, transverse colon, descending colon, and sigmoid
colon .
3. Rectum.
4. An...al canal.