Anatomy and Physiology
Anatomy and Physiology of the Stomach
The stomach is a well-vascularized organ that resides in the left upper quadrant of the abdomen; the stomach starts just below the diaphragmatic hiatus where the esophagus enters it at the gastroesophageal junction. This most proximal part of the stomach is known as the cardia and gives rise to the main body of the stomach as well as the fundus, which is the floppy, distensible superior most portion of the stomach. The parts of the stomach distal to the body are the antrum followed by the pylorus, which enters the duodenum. The greater omentum is attached to the inferior border of the stomach along the greater curvature and hangs down over the viscera below. The lesser omentum makes a connection between the stomach and the liver and is also known as the gastrohepatic ligament.
The stomach has a robust blood supply from the branches of the celiac trunk and even has a collateral flow from the superior mesenteric artery via the gastroduodenal arteries. The venous drainage is through left gastric and right gastric draining into the portal vein, the left gastroepiploic draining into the splenic vein, and the right gastroepiploic vein draining into the superior mesenteric vein. The stomach is innervated by both the right and left vagus nerve and also has a robust lymphatic drainage system. Given the proximity of the stomach to such vital structures as the pancreas, portal triad, aorta, inferior vena cava, and spleen, to name a few, any surgeon operating on the stomach must have a full understanding of all of these anatomic structures to perform a gastrojejunostomy safely.[5]
The physiology of the stomach involves a complex interplay of hormonal and neural messengers to regulate the stomach’s motility, distensibility, and acidity, and a full summary of the entire physiology of the stomach is beyond the scope of this article.[5] There are a few key points worth covering more closely, although any healthcare provider planning to operate on the stomach should have a full understanding of its physiology.
Firstly, the antrum of the stomach contains the G cells, which produce the hormone gastrin, which increases acid production when the pH in the stomach is too high. If there is retained antrum after resection of the stomach, the antral G cells are separated from the parietal cells in the stomach remnant and bathed in alkali fluid from the duodenum. This high pH environment causes them to secrete excess gastrin, which in turn causes the stomach remnant to produce excess acid, leading to ulceration.[6]
Secondly, the stomach digests food into a nutrient-rich chyme, which then passes into the duodenum for further digestion. Disrupting the natural flow of the nutrients so that they deposit directly into the jejunum can lead to dumping syndrome, which will be discussed further in the complications section.[7]
Thirdly, the acid produced by the stomach is typically neutralized by basic pancreaticobiliary secretions in the duodenum. When the stomach and the jejunum are directly connected, this neutralizing environment is bypassed and predisposes the jejunal area closest to the stomach to develop ulcers.[8]
Anatomy and Physiology of the Jejunum
The jejunum starts at the ligament of Trietz and extends for approximately 250 cm before transitioning into the ileum. Its blood supply comes from the superior mesenteric artery. Its venous drainage is via the superior mesenteric vein. The jejunum contains few Brunner glands, which are chiefly found in the duodenum, and few Peyer patches, which are primarily found in the ileum. The jejunum is the site where the highest amount of absorption of nutrients and vitamins takes place except iron, which is chiefly absorbed in the duodenum and vitamin B12, which is absorbed in the terminal ileum. The jejunum is primarily lined with enterocytes and has long villi, which increase its surface area, which can be used for absorption. The motility of the jejunum is regulated by vagal stimulation, migrating myoelectric complexes, and the hormone motilin.[9]