Fig. 6.1
Basic anatomy of the GI tract
6.1 Esophagus
The esophagus is divided into three regions: cervical (C6 to T1), thoracic (T1 to the esophageal hiatus of the diaphragm), and abdominal (esophageal hiatus to the cardia of the stomach). It passes slightly right to the aorta and goes through the diaphragm slightly left [2].
Blood Supply
Cervical: branches of the inferior thyroid artery and vein.
Thoracic: branches directly off the aorta and the accessory hemiazygos or left brachiocephalic vein.
Abdominal: branches of the left gastric artery and vein.
Nerves
Left and right vagus nerve, left running anterior, and right running posterior.
6.2 Stomach
The stomach is divided into the fundus, cardia, body, pylorus, and antrum (Fig. 6.2). The stomach is responsible for mechanical and enzymatic digestion of food. Numerous hormones are secreted (Table 6.1).
Fig. 6.2
Stomach anatomy
Table 6.1
Hormones of the GI tract
Hormone | Source | Stimuli for release | Major activities |
|---|---|---|---|
Somatostatin | Antrum, duodenum, pancreatic islet cells | Acid in the stomach | Acts on parietal cells in the stomach to reduce acid secretion and prevents secretion of gastrin, secretin, and histamine |
Gastrin | G cells (antrum) | Amino acids, acetylcholine | Stimulates gastric acid secretion and proliferation of gastric epithelium |
CCK | Duodenum | Fatty acids, amino acids in the small intestine | Stimulates release of pancreatic enzymes and gallbladder contraction |
Secretin | Duodenum | Acid in the duodenum | Stimulates secretion of water and bicarbonate from the pancreas and bile ducts |
Ghrelin | Stomach, duodenum, jejunum | Empty stomach | Stimulates appetite/feeding |
Motilin | Duodenum, jejunum | Fasting state | Motility of stomach and small intestine |
Gastric inhibitory polypeptide | K cells in the duodenum and jejunum | Fat and glucose in the small intestine | Inhibits gastric secretion and motility and increases insulin release |
Blood Supply
Nerves
Parasympathetic supply: left vagus nerve runs anteriorly, right vagus nerve runs posteriorly. The sympathetic supply consists of spinal segments T5–T10 [2].
6.3 Small Intestine
The small intestine is divided into the duodenum, jejunum, and ileum. Enzymes excreted from the pancreas and liver mix with the chyme propelled out the stomach and continue to digest the food. This is where absorption of essential nutrients begins. Hormones associated with the small bowel are listed in Table 6.1.
6.3.1 Duodenum
The duodenum has the first (bulb), second (descending), third (transverse), and fourth (ascending) portions. The pancreas drains into the duodenum through the accessory and major pancreatic ducts at the major and minor papilla located in the descending portion. The small intestine is the site of fat-soluble vitamin absorption (vitamins A, D, E, and K).
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