Stomach Anatomy and Function

• The stomach is a J-shaped organ located in the upper abdomen, primarily on the left side
• It plays a vital role in the digestion of food
• It receives food from the esophagus, stores it, and begins the process of breaking it down into smaller particles

Anatomy

• The stomach is divided into four main regions: the cardia, fundus, body, and pylorus
• Cardia: surrounds the junction of the esophagus and the stomach
• Fundus: dome-shaped region located superior to the cardia
• Body: the largest, central part of the stomach
• Pylorus: funnel-shaped region that connects the stomach to the duodenum (the first part of the small intestine)
• The pylorus is further divided into the pyloric antrum (wider part) and the pyloric canal (narrower part)
• The pyloric sphincter, a strong ring of smooth muscle, is located at the junction of the pylorus and the duodenum, controlling the flow of chyme (partially digested food) into the small intestine
• The wall of the stomach is composed of four layers: the mucosa, submucosa, muscularis externa, and serosa
• Mucosa: the innermost layer, lines the lumen of the stomach, contains specialized cells (e.g., parietal cells, chief cells, mucous cells, and G cells) that secrete gastric juice (hydrochloric acid, pepsinogen, mucus, and hormones)
• Submucosa: a layer of connective tissue that contains blood vessels, lymphatic vessels, and nerves
• Muscularis externa: consists of three layers of smooth muscle (inner oblique, middle circular, and outer longitudinal) that contract to mix and churn the stomach contents
• Serosa: the outermost layer, a serous membrane that covers the stomach

Histology

• The mucosa of the stomach is characterized by gastric pits, which are depressions in the surface that lead into gastric glands
• The types of cells found in the gastric glands include:
• Mucous neck cells: secrete mucus
• Parietal cells: secrete hydrochloric acid (HCl) and intrinsic factor
• Chief cells: secrete pepsinogen, an inactive form of pepsin
• Enteroendocrine cells: secrete hormones such as gastrin
• These cells collectively produce gastric juice, which aids in the digestion of food

Functions

• The stomach performs several key functions in the digestive process:
• Storage of food: The stomach can expand to hold a large volume of food, allowing for intermittent eating
• Mixing of food with gastric secretions: The stomach's muscular contractions mix food with gastric juice, forming a semi-liquid mixture called chyme
• Secretion of gastric juice: Gastric juice contains hydrochloric acid (HCl), pepsinogen, mucus, and intrinsic factor, which aid in the chemical digestion of proteins and protect the stomach lining
• Hydrochloric acid (HCl): denatures proteins and kills bacteria
• Pepsinogen: activated to pepsin, a protease that breaks down proteins
• Mucus: protects the stomach lining from the corrosive effects of HCl
• Intrinsic factor: enables the absorption of vitamin B12 in the small intestine
• Regulation of gastric emptying: The stomach regulates the rate at which chyme is released into the small intestine, ensuring that the small intestine can efficiently digest and absorb the nutrients

Gastric Motility

• Gastric motility refers to the contractions of the stomach's muscularis externa, which mixes food with gastric juice and propels chyme towards the duodenum
• The stomach exhibits two main types of contractions: mixing waves and peristaltic waves
• Mixing waves: relatively weak contractions that occur throughout the stomach, mixing food with gastric juice
• Peristaltic waves: stronger contractions that begin in the body of the stomach and move towards the pylorus, propelling chyme towards the duodenum
• The rate of gastric emptying is influenced by various factors, including the composition of the chyme, the degree of stomach distension, and hormonal signals from the small intestine
• Fatty foods tend to slow gastric emptying, while carbohydrate-rich foods tend to accelerate it
• Hormones such as gastrin, secretin, and cholecystokinin (CCK) also play a role in regulating gastric motility and emptying

Gastric Secretions

• Gastric secretions are produced by the cells of the gastric glands in the stomach mucosa
• These secretions include hydrochloric acid (HCl), pepsinogen, mucus, intrinsic factor, and hormones
• Hydrochloric acid (HCl): secreted by parietal cells, lowers the pH of the stomach, denaturing proteins and killing bacteria, also converts pepsinogen into pepsin
• Pepsinogen: secreted by chief cells, is the inactive precursor of pepsin, activated by HCl and pepsin itself (autocatalysis)
• Mucus: secreted by mucous neck cells, forms a protective barrier that prevents the stomach lining from being damaged by HCl and pepsin
• Intrinsic factor: secreted by parietal cells, is essential for the absorption of vitamin B12 in the small intestine
• Hormones: such as gastrin are secreted by enteroendocrine cells, regulate gastric secretion and motility
• Gastrin stimulates the secretion of HCl and pepsinogen, as well as gastric motility
• Gastric secretion is regulated by both neural and hormonal mechanisms

Neural and Hormonal Control of Gastric Function

• Gastric function is regulated by a complex interplay of neural and hormonal mechanisms
• The nervous system influences gastric activity through the vagus nerve, which carries both parasympathetic and sympathetic fibers
• Parasympathetic stimulation increases gastric secretion and motility, while sympathetic stimulation decreases gastric activity
• Hormonal control of gastric function is primarily mediated by gastrin, secretin, and cholecystokinin (CCK)
• Gastrin stimulates the secretion of HCl and pepsinogen, as well as gastric motility
• Secretin and CCK are released by the small intestine in response to acidic chyme or the presence of fats and proteins, they inhibit gastric secretion and motility, promoting the digestion and absorption of nutrients in the small intestine
• The regulation of gastric function involves three phases: the cephalic phase, the gastric phase, and the intestinal phase
• Cephalic phase: occurs before food enters the stomach, triggered by the sight, smell, taste, or thought of food increases gastric secretion and motility via the vagus nerve
• Gastric phase: begins when food enters the stomach, stimulated by stretch receptors and chemoreceptors (detecting pH and peptides) in the stomach wall, further increasing gastric secretion and motility
• Intestinal phase: begins when chyme enters the small intestine, inhibits gastric secretion and motility through hormonal and neural mechanisms, promoting the digestion and absorption of nutrients in the small intestine

Clinical Significance

• The stomach is susceptible to various disorders and conditions that can affect its structure and function
• Gastritis: inflammation of the stomach lining, caused by infection, irritation, or autoimmune disorders
• Peptic ulcers: sores in the lining of the stomach or duodenum, often caused by Helicobacter pylori infection or the use of nonsteroidal anti-inflammatory drugs (NSAIDs)
• Gastric cancer: cancer that originates in the stomach lining, can be caused by genetic factors, H. pylori infection, or dietary factors
• Gastroesophageal reflux disease (GERD): a condition in which stomach acid frequently flows back into the esophagus, causing heartburn and other symptoms
• Vomiting, also known as emesis, is the forceful expulsion of the contents of the stomach through the mouth, it is a complex reflex coordinated by the vomiting center in the brainstem, can be triggered by various factors, including infections, toxins, motion sickness, and emotional stress
• Bariatric surgery: surgical procedures aimed at weight loss by reducing the size of the stomach or altering the digestive process, include gastric bypass, sleeve gastrectomy, and adjustable gastric banding