Basic Anatomy and Physiology of the Gastrointestinal Tract



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).

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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





  • Arteries: blood supply originates from the celiac artery. Branches include left and right gastric, left and right gastroepiploic, short gastric, and gastroduodenal arteries [3].


  • Veins: the corresponding veins drain into the portal or superior mesenteric venous systems [1].


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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Jun 25, 2017 | Posted by in NURSING | Comments Off on Basic Anatomy and Physiology of the Gastrointestinal Tract

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