Small Intestine

The stomach opens through the pyloric sphincter into the 6 meter long small intestine. Due to its length, it is highly folded and occupies a large portion of the abdominal cavity. The small intestine is composed of the following parts:

1. Duodenum

2. Jejunum

3. Ileum

As food passes through the stomach and the small intestine, digestive enzymes secreted by the glands of the small intestine’s lining mucosa work on it.

The mucosal lining of the small intestine is folded into minute finger-like processes, the villi, and the luminal cell membranes of its individual cells are folded further, into even smaller microscopic microvilli. This increases the surface area for absorption. The products of digestion in the form of sugars, amino acids, fats and glycerol are absorbed into the bloodstream from the small intestine.

Digestion is usually complete by the time the food has reached the half-way point of the small intestine. The remaining parts of the jejunum and ileum are concerned with absorption rather than digestion.

Duodenum

The duodenum pursues a C-shaped course around the head of the pancreas and is divided into four parts and they are categorized as:

Blood Supply:

1. The gastroduodenal artery supplies the upper portion of the duodenum.

2. The inferior pancreaticoduodenal arteries supplies the rest of the duodenum.

Venous Drainage:

1. The venous drainage of the duodenum follows the major arterial supply and drains to the pancreaticoduodenal veins.

Clinical Notes:

Occurrence of duodenal ulcers due to presence of acidic chyme in the first part of the duodenum could have the following potential effects:

1. Anterior ulcers - these can erode the peritoneal cavity which can lead to peritonitis and adhesions

2. Posterior ulcers - these can erode the gastroduodenal artery and produce torrential hemorrhage

Jejunum and Ileum

Innervation:

The innervation of the small intestine is by the enteric nervous system, which in the small intestine consists of the submucosal plexus of Meissner and the myenteric plexus of Auerbach. The motor component of the submucosal plexus controls primarily mucosal and submucosal gland secretion and blood flow, whereas the sensory component consists of mucosal mechanosensitive neurons.

The motor component of the myenteric plexus controls primarily GI motility (contraction/relaxation of GI smooth muscle), whereas the sensory component consists of tension-sensitive neurons and chemosensitive neurons. The enteric nervous system is modulated by the parasympathetic and sympathetic nervous systems.

Clinical Notes:

1. An occlusion in the blood supply to this region of the small intestine will result in ischemia, necrosis, and paralytic ileus (obstruction). This in turn results in colic, vomiting, fever, and dehydration.