The Stomach
• J-shaped enlargement of the GI tract
• Most distensible part of the GI tract
• Connects esophagus to the duodenum
• Directly inferior to the diaphragm in the umbilical, and
left hypochondriac regions of the abdomen
Main
Regions of the Stomach
• Serve as a mixing chamber and holding reservoir
• Digestion of starch continues
• Digestion of proteins and triglycerides begins
• Certain substances are absorbed
Anatomy of
Stomach
• The Cardia
– Surrounds the superior opening of the stomach
• The Fundus
– Rounded portion superior to, left of the cardia
• The Body
– Inferior to the fundus, large central portion
• The Pylorus
– Region of the stomach that connects to the duodenum
– Pyloric antrum - Connects to the body of the stomach
– Pyloric canal - Leads into the duodenum
• Rugae
– When the stomach is empty, the mucosa lies in large folds
– can be seen with the unaided eye
• The Pyloric
Sphincter
– Pylorus communicates with the duodenum via a smooth muscle
sphincter called the pyloric sphincter
• Lesser curvature
– The concave medial border
• Greater curvature
– The convex lateral border
Overview of
Histology
The stomach wall is composed of the same basic layers as the
rest of the GI tract, with certain modifications
Mucosa:
• Layer of simple columnar epithelial cells called surface
mucous cells
• Lamina propria (areolar connective tissue)
• Muscularis mucosae (smooth muscle)
• Epithelial cells extend down into the lamina propria &
form gastric glands
• Several gastric glands open into the bottom of narrow
channels called gastric pits
• The gastric glands contain three types of exocrine gland
cells
• Gastric glands include a type of enteroendocrine cell - G
cell
Histology
of the Stomach
Submucosa
• Composed of areolar connective tissue
Muscularis
• Three layers of smooth muscle
• Outer longitudinal layer
• Middle circular layer
• Inner oblique layer
Serosa
• Composed of simple squamous epithelium (mesothelium) and
areolar connective tissue
Physiology
of Stomach
• Mechanical digestion consists of mixing waves
• Chemical digestion - conversion of proteins into peptides
by pepsin
• The stomach wall is impermeable to most substances
• Among the substances the stomach can absorb are water,
certain ions, drugs, and alcohol
Mechanical
Digestion in the Stomach
• Gentle, rippling, peristaltic movements called mixing
waves pass over the stomach every 15 to 25 seconds
• These waves macerate food, mix it with secretions of the
gastric glands – chyme
• Few mixing waves are observed in the fundus
• Primarily has a storage function
• Intensify as
they reach the pylorus
• Gastric emptying -
Each mixing wave periodically forces about 3 mL of chyme into the duodenum
through the pyloric sphincter
Chemical
Digestion in Stomach
• Churning action
mixes chyme with acidic gastric juice
• Enzymatic digestion of proteins also begins in the stomach
• Acidic fluid of
the stomach kills many microbes in food
• HCl
– Partially denatures (unfolds) proteins in food
– Stimulates the secretion of hormones that promote the flow of
bile and pancreatic juice
Digestion Process
Pepsin
• The only proteolytic enzyme in the stomach
• Secreted in an inactive form called pepsinogen; activated
by H
• Pepsin breaks down proteins into smaller peptide fragments
• Pepsin is most effective in the very acidic environment of
the stomach (pH 2)
• Becomes inactive at a higher pH
Alkaline Mucus
• Secreted by surface mucous cells and mucous neck cells
• Protect the stomach epithelial cells from gastric juices
(1–3 mm thick layer of alkaline mucus)
• Gastric lipase
– Splits the short-chain triglycerides in fat molecules into
fatty acids and monoglycerides
– Has a limited role in the adult stomach
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