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
Histology
of the Stomach
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