23.4 The Stomach
4 min read•june 18, 2024
The , a vital organ in digestion, has distinct regions that work together to break down food. From the to the , each part plays a unique role in mixing, churning, and processing what we eat.
The stomach's self-protection mechanisms are crucial for its function. A protective mucus layer, secretion, and tight junctions between cells all work to prevent the stomach from digesting itself while still allowing it to break down our food.
Anatomy and Physiology of the Stomach
Anatomical regions of stomach
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- surrounds the esophageal opening, allowing food to enter the stomach through the
- forms a dome-shaped region above and to the left of the cardia, collects gas and aids in belching (eructation)
- comprises the main central region, expands to accommodate food and secretes
- connects the stomach to the duodenum, regulating the passage of
- is the wider, proximal part of the pylorus, mixes chyme with gastric juice
- is the narrower, distal part of the pylorus, controls the release of chyme into the duodenum via the
- is the shorter, concave curvature on the right side, provides a pathway for blood vessels and nerves (gastric artery and vagus nerve)
- is the longer, convex curvature on the left side, allows for stomach expansion and contraction
- is the innermost layer containing glands and , secretes gastric juice and absorbs certain substances (alcohol, aspirin)
- The mucosa forms , which are folds that increase surface area and allow for expansion
- is a connective tissue layer with blood vessels and nerves, supports the mucosa and allows for stomach expansion
- consists of three layers of smooth muscle (longitudinal, circular, and oblique), responsible for and
- is the outermost layer of connective tissue, provides a slippery surface for stomach movement within the abdominal cavity
Cell types in gastric glands
- (surface mucous cells) secrete mucus to protect the stomach lining from acid and enzymes
- secrete mucus, contributing to the protective mucus layer
- secrete and
- HCl lowers pH in the stomach to activate and kill bacteria, providing a acidic environment for protein digestion
- binds to vitamin B12 (cobalamin), enabling its absorption in the ileum
- secrete , the inactive precursor of , which is a protein-digesting enzyme activated by HCl
- secrete hormones such as , which stimulates HCl secretion by and increases ()
Stomach's self-protection mechanisms
- Mucus layer secreted by foveolar cells and forms a protective barrier against HCl and enzymes, preventing autodigestion
- Bicarbonate () secretion by surface epithelial cells neutralizes HCl near the stomach lining, maintaining a neutral pH
- Tight junctions between epithelial cells prevent HCl and enzymes from penetrating the mucosa, maintaining the integrity of the stomach lining
- Rapid turnover of epithelial cells ensures that damaged cells are quickly replaced, maintaining a healthy
- stimulate mucus and bicarbonate secretion, increase mucosal blood flow, and promote epithelial cell regeneration, enhancing the stomach's self-protection
- Failure of these mechanisms can lead to , which are erosions in the stomach lining
Digestion process in stomach
- Mechanical digestion
- involves waves of smooth muscle contractions that mix and churn food with gastric juice, breaking down food particles
- consists of localized contractions that further mix the chyme (semi-liquid mixture of food and gastric juice), ensuring thorough digestion
- Chemical digestion
- Protein digestion
- Pepsinogen is converted to active pepsin in the presence of HCl, initiating protein breakdown
- Pepsin breaks down proteins into smaller peptides, preparing them for further digestion in the small intestine
- Fat digestion
- (secreted by ) initiates fat digestion by breaking down triglycerides into fatty acids and monoglycerides, preparing fats for further digestion by pancreatic lipase
- Carbohydrate digestion
- Salivary amylase (from the mouth) continues to break down starch into maltose until inactivated by the low pH of the stomach, contributing to initial carbohydrate digestion
- Protein digestion
- Chyme is released into the duodenum through the pyloric sphincter in small amounts, controlled by hormones ( and ) and neural reflexes () to optimize digestion and absorption in the small intestine
Gastric Function and Regulation
- Gastric motility refers to the movement of the stomach muscles, which is crucial for mixing food with gastric juice and propelling chyme towards the duodenum
- is the process by which the stomach releases its contents into the small intestine, regulated by various factors including the volume and composition of the chyme
- Gastric juice, a mixture of HCl, enzymes, and mucus, plays a vital role in chemical digestion and protection of the stomach lining
Key Terms to Review (73)
Bicarbonate: Bicarbonate, also known as hydrogen carbonate, is a chemical compound with the formula HCO3-. It is an essential ion involved in various physiological processes within the human body, including maintaining acid-base balance, facilitating gas exchange, and supporting digestive functions.
Body: The body of the stomach is the main central region of the stomach, lying between the fundus above and the pyloric antrum below. It functions primarily in storing food temporarily, mixing it with gastric juices, and initiating the digestion process.
Body: The body refers to the physical structure of a human or animal, encompassing all the organs, tissues, and systems that work together to sustain life. It is the material or physical representation of a living organism.
Body Fluids and Fluid Compartments: Body fluids encompass all the liquids within the human body that facilitate vital physiological functions, including blood, lymph, and interstitial fluid among others. Fluid compartments refer to the specific spaces or environments within the body where these fluids are contained and segregated, such as intracellular (within cells) and extracellular (outside cells) compartments.
Cardia: The cardia is the anatomical region where the esophagus connects to the stomach. It acts as a valve to control the flow of food into the stomach, preventing acid reflux back into the esophagus.
Cardia: The cardia, also known as the cardiac orifice, is the superior opening of the stomach where the esophagus connects to the stomach. It is the anatomical region that marks the transition from the esophagus to the stomach and plays a crucial role in the digestive process.
Cephalic phase: The cephalic phase is the early stage of digestion that begins before food enters the stomach, initiated by the sight, smell, thought, or taste of food. This phase triggers the release of gastric juices in preparation for digesting food.
Chief cells: Chief cells are specialized cells located in the lining of the stomach that secrete pepsinogen, an inactive enzyme that is converted into pepsin to break down proteins during digestion. They play a crucial role in the digestive process by initiating protein digestion.
Chief Cells: Chief cells, also known as principal cells, are the predominant secretory cells found in the gastric glands of the stomach. These cells are responsible for producing and secreting pepsinogen, the inactive precursor of the digestive enzyme pepsin, which is essential for the breakdown of proteins in the stomach.
Cholecystokinin: Cholecystokinin (CCK) is a peptide hormone produced by I-cells in the duodenum and jejunum of the small intestine. It plays a crucial role in the regulation of digestion, particularly in the stimulation of the gallbladder to release bile and the pancreas to secrete digestive enzymes.
Cholecystokinin (CCK): Cholecystokinin is a hormone produced by the cells of the small intestine in response to the presence of fats and proteins, which stimulates the digestion of lipids and proteins by inducing bile release from the gallbladder and enzyme secretion from the pancreas. It also slows down gastric emptying, helping in the efficient digestion of nutrients.
Chyme: Chyme is the semi-fluid mixture of partially digested food, digestive juices, and other materials that is formed in the stomach and passed into the small intestine during the digestive process.
Enteroendocrine cells: Enteroendocrine cells are specialized cells located in the lining of the stomach and intestines that release hormones into the blood in response to food ingestion. These hormones help regulate digestion, appetite, and metabolism.
Enteroendocrine Cells: Enteroendocrine cells are specialized cells found within the lining of the gastrointestinal tract, responsible for the secretion of various hormones that regulate digestive processes and metabolic functions. These cells act as endocrine sensors, detecting the presence of nutrients and other stimuli within the gut and responding by releasing hormones into the bloodstream.
Enterogastric Reflex: The enterogastric reflex is a physiological mechanism that involves the coordination between the gastrointestinal tract and the stomach. It is a reflex that helps regulate the functions of the stomach in response to the presence and movement of food within the small intestine.
Foveolar Cells: Foveolar cells are the predominant cell type found in the stomach's gastric pits, also known as foveolae. These specialized epithelial cells play a crucial role in the production and secretion of mucus, which helps protect the stomach lining from the corrosive effects of gastric acid.
Fundus: The fundus is the upper part of the stomach that arches above the level of the opening between the esophagus and the stomach. It serves as a temporary storage area for ingested food before it is mixed with gastric juices.
Fundus: The fundus is the rounded, dome-shaped portion of an organ, particularly the stomach, that is opposite the opening or neck of the organ. It is the widest and most distensible part of the stomach, located at the superior end above the cardiac orifice where the esophagus joins the stomach.
G cells: G cells are specialized endocrine cells located in the stomach lining that produce and secrete the hormone gastrin. Gastrin plays a crucial role in stimulating the secretion of gastric acid, which aids in the digestive process.
G Cells: G cells, also known as gastrin-producing cells, are a type of endocrine cell found primarily in the pyloric antrum of the stomach. These cells are responsible for the production and secretion of the hormone gastrin, which plays a crucial role in the digestive process and the regulation of stomach acid secretion.
Gastric emptying: Gastric emptying is the process by which the stomach contents are moved into the small intestine. It plays a crucial role in digestion, regulating the rate at which food is digested and nutrients are absorbed.
Gastric Emptying: Gastric emptying refers to the process by which the stomach empties its contents into the small intestine. This is a crucial function of the digestive system, as it allows for the continued breakdown and absorption of food.
Gastric gland: Gastric glands are specialized epithelial structures located in the lining of the stomach that secrete gastric juices, including hydrochloric acid and digestive enzymes. These glands play a crucial role in the digestion process by breaking down food and killing bacteria.
Gastric juice: Gastric juice is a digestive fluid produced by the stomach, consisting mainly of hydrochloric acid, pepsinogen, and mucus. This acidic secretion plays a vital role in breaking down food particles, activating digestive enzymes, and providing an environment that helps eliminate pathogens ingested with food. Understanding gastric juice is crucial as it highlights the stomach's function in digestion and overall digestive health.
Gastric lipase: Gastric lipase is an enzyme produced by the chief cells in the stomach that helps in the digestion of dietary fats by breaking down triglycerides into free fatty acids and glycerol. This enzyme plays a crucial role in the chemical digestion process, particularly in the acidic environment of the stomach, where it works alongside other digestive enzymes to facilitate the breakdown of food components into absorbable nutrients.
Gastric Motility: Gastric motility refers to the coordinated contractions and relaxations of the stomach muscles that facilitate the movement and mixing of food within the stomach. It is a crucial component of the digestive process, responsible for the effective breakdown and transport of food from the stomach to the small intestine.
Gastric phase: The gastric phase is a stage in the digestion process that occurs in the stomach, where food triggers the stomach to secrete gastric juice and continue the mechanical and chemical breakdown of food. This phase accounts for about two-thirds of total gastric secretion in response to food presence.
Gastric pits: Gastric pits are indentations in the lining of the stomach that lead into tubular gastric glands, which secrete gastric juice essential for digestion. These structures play a critical role in the initial stages of food breakdown and protection of the stomach lining.
Gastric Ulcers: Gastric ulcers are open sores that develop on the lining of the stomach. They are a type of peptic ulcer, which are painful lesions that can form in the stomach, lower esophagus, or small intestine as a result of damage to the protective mucosal layer.
Gastrin: Gastrin is a hormone produced by the stomach that stimulates the secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It plays a critical role in the digestive process by preparing the stomach to digest and absorb nutrients effectively.
Gastrin: Gastrin is a hormone produced by G cells in the stomach and duodenum that stimulates the secretion of gastric acid, which aids in the digestion of food. It is a key player in the regulation of the digestive system processes and the function of the stomach.
Gastroesophageal Sphincter: The gastroesophageal sphincter, also known as the lower esophageal sphincter (LES), is a ring of muscle at the junction of the esophagus and the stomach that acts as a valve, allowing food to pass into the stomach while preventing the backflow of stomach contents into the esophagus.
Greater Curvature: The greater curvature is the convex, outer border of the stomach that runs along the superior and posterior aspects of the organ. It is an important anatomical landmark that helps define the overall shape and position of the stomach within the abdominal cavity.
Hydrochloric Acid: Hydrochloric acid (HCl) is a strong, corrosive inorganic acid that plays a vital role in the human body, particularly in the digestive system. It is the main component of the gastric acid secreted by the stomach, responsible for various essential functions related to digestion, nutrient absorption, and maintaining the body's electrolyte balance.
Hydrochloric acid (HCl): Hydrochloric acid is a strong, corrosive acid secreted by the parietal cells of the gastric glands in the stomach. Its main function is to maintain an acidic environment in the stomach, aiding in the digestion of proteins and activation of enzymes.
Intestinal phase: The intestinal phase is a stage of digestion that begins when chyme enters the small intestine, triggering hormonal and neural responses to control gastric activity and promote further digestion. This phase primarily focuses on slowing down gastric emptying to allow for optimal nutrient absorption in the small intestine.
Intrinsic factor: Intrinsic factor is a glycoprotein produced by the parietal cells of the stomach lining that is essential for the absorption of vitamin B12 in the small intestine. Without intrinsic factor, vitamin B12 cannot be absorbed, leading to potential deficiencies.
Intrinsic factor: Intrinsic factor is a glycoprotein produced by the parietal cells of the stomach that is essential for the absorption of vitamin B12 in the small intestine. It plays a crucial role in preventing deficiencies that can lead to serious health issues, such as anemia. This factor works by binding to vitamin B12 and facilitating its absorption in the ileum, ensuring that the body gets enough of this vital nutrient.
Lesser curvature: The lesser curvature is the shorter, concave border of the stomach that runs along the upper portion of the organ, connecting the esophagus to the duodenum. It plays a crucial role in the stomach's anatomy by serving as an attachment point for various ligaments and structures, including the lesser omentum, which helps support and stabilize the stomach's position in the abdominal cavity.
Leydig cells: Leydig cells are specialized cells located in the testes that produce testosterone, a crucial hormone for the development of male secondary sexual characteristics and reproductive functions. These cells respond to luteinizing hormone signals by synthesizing and releasing testosterone into the bloodstream.
Mesenchyme: Mesenchyme is an embryonic connective tissue that originates from the mesoderm layer and gives rise to a variety of tissues including muscle, bone, and blood. It is characterized by its loosely arranged cells in a gel-like ground substance.
Mixing wave: Mixing waves are slow, rhythmic contractions of the stomach muscles that mix and churn food with gastric juices to form a semi-liquid substance called chyme. These movements facilitate the mechanical breakdown of food and its eventual propulsion toward the pyloric sphincter.
Mucosa: Mucosa is the innermost layer of the gastrointestinal tract, which lines the entire digestive system and is crucial for absorption and secretion. This layer consists of epithelial cells, connective tissue, and a thin layer of smooth muscle, providing both structural support and functional roles in the digestive process. The mucosa interacts with food substances, plays a key role in nutrient absorption, and acts as a barrier against pathogens.
Mucosal barrier: The mucosal barrier is a protective layer that lines the stomach, preventing the acidic gastric juices from damaging the stomach walls. It consists of mucus, bicarbonate ions, and epithelial cells that work together to prevent ulceration.
Mucous neck cells: Mucous neck cells are specialized epithelial cells located in the upper part of the gastric glands in the stomach, responsible for secreting mucus. This mucus plays a critical role in protecting the lining of the stomach from acidic gastric juices.
Mucous Neck Cells: Mucous neck cells are a specialized type of secretory cells found in the gastric glands of the stomach lining. These cells produce and secrete mucus, a thick, viscous substance that forms a protective barrier over the stomach epithelium, shielding it from the harsh acidic environment within the stomach.
Muscularis externa: The muscularis externa is the outermost layer of the digestive tract, composed of smooth muscle that aids in the movement of food through the system. It plays a crucial role in the overall functioning of the digestive system.
Parietal cells: Parietal cells are specialized epithelial cells located in the lining of the stomach that secrete hydrochloric acid (HCl) and intrinsic factor. Their primary role is to aid in digestion by creating an acidic environment and facilitating vitamin B12 absorption.
Parietal Cells: Parietal cells are a specialized type of epithelial cells found in the lining of the stomach, specifically in the fundic and body regions. These cells are responsible for the production and secretion of hydrochloric acid (HCl) and intrinsic factor, both of which play crucial roles in the digestive process.
Pepsin: Pepsin is a digestive enzyme produced in the stomach that breaks down proteins into smaller peptides. It is secreted as an inactive precursor called pepsinogen, which is activated by the acidic environment of the stomach. This enzyme plays a vital role in the overall digestion of proteins, making it essential for nutrient absorption and metabolism.
Pepsinogen: Pepsinogen is an inactive enzyme precursor produced by the chief cells in the stomach lining, which, when activated to pepsin, plays a crucial role in the digestion of proteins by breaking them down into smaller peptides. It is converted into its active form, pepsin, upon exposure to hydrochloric acid and gastric juices.
Pepsinogen: Pepsinogen is an inactive precursor enzyme secreted by the stomach's gastric glands, specifically by chief cells. It plays a crucial role in digestion by being converted to its active form, pepsin, when exposed to the acidic environment of the stomach. This conversion is essential for the breakdown of proteins into smaller peptides, facilitating nutrient absorption.
Peristalsis: Peristalsis is a series of wave-like muscle contractions that moves food through the digestive tract. It involves the rhythmic contraction and relaxation of muscles in the organ walls to push contents forward.
Peristalsis: Peristalsis is the coordinated wave-like muscle contractions that propel food and other materials through the digestive tract. It is a fundamental process that enables the movement of contents through the esophagus, stomach, and intestines, ensuring the efficient digestion and elimination of waste.
Prostaglandins: Prostaglandins are lipid compounds derived from fatty acids that have important roles in various physiological processes, including inflammation, pain modulation, and vascular regulation. These bioactive molecules act locally and have a significant impact on the body's homeostatic mechanisms, influencing responses such as the regulation of blood flow, the modulation of immune responses, gastric protection, and reproductive functions.
Pyloric antrum: The pyloric antrum is the lower portion of the stomach that connects to the pyloric canal, which leads into the duodenum. It plays a crucial role in mixing food with gastric juices and regulating its passage into the small intestine.
Pyloric Antrum: The pyloric antrum is the distal portion of the stomach that connects to the pyloric canal and the duodenum. It serves as the final chamber of the stomach, where the gastric contents are prepared for entry into the small intestine.
Pyloric canal: The pyloric canal is the narrow passage that connects the stomach to the duodenum, which is the first section of the small intestine. It controls the flow of partially digested food from the stomach to the intestine through the pyloric sphincter.
Pyloric Canal: The pyloric canal is the narrow, muscular passage that connects the stomach to the small intestine. It serves as a gatekeeper, controlling the release of chyme (partially digested food) from the stomach into the duodenum, the first segment of the small intestine.
Pyloric sphincter: The pyloric sphincter is a muscular valve that controls the passage of partially digested food from the stomach to the small intestine. It acts as a gatekeeper, opening to allow the contents to move on for further digestion and absorption.
Pylorus: The pylorus is the region of the stomach that connects to the duodenum, which is the first part of the small intestine. It acts as a valve, controlling the passage of partially digested food from the stomach to the small intestine.
Pylorus: The pylorus is the distal, muscular portion of the stomach that connects the stomach to the small intestine. It acts as a valve, controlling the rate at which food passes from the stomach into the duodenum, the first segment of the small intestine.
Ruga: A ruga is a ridge or fold, such as those found on the internal surface of the stomach. These folds allow the stomach to expand when it fills with food.
Secretin: Secretin is a hormone produced by the S cells of the duodenum that stimulates the pancreas to release bicarbonate-rich fluids, essential for neutralizing stomach acid in the small intestine and aiding in protein digestion. It plays a pivotal role in regulating the pH of the intestinal environment to optimize enzyme activity for nutrient absorption.
Secretin: Secretin is a peptide hormone produced by the S-cells in the duodenum and upper jejunum of the small intestine. It plays a crucial role in the regulation of digestion and the function of the pancreas, liver, and stomach.
Segmentation: In the context of the digestive system, segmentation is the process by which smooth muscles in the intestines contract rhythmically to mix and segment the contents, enhancing nutrient absorption. This action breaks down food particles and evenly distributes digestive juices.
Segmentation: Segmentation is the division of the gastrointestinal tract into distinct regions or compartments, each with specialized functions. This structural organization allows for the efficient and coordinated processing of food as it moves through the digestive system.
Serosa: Serosa is a smooth membrane consisting of a thin layer of connective tissue covered by a layer of mesothelial cells, which forms the outermost covering of organs within the abdominal cavity. It plays a crucial role in providing a slippery surface that reduces friction between organs as they move and contract, while also serving as a protective barrier against infections and injuries.
Sodium bicarbonate: Sodium bicarbonate is a chemical compound with the formula NaHCO₃, commonly known as baking soda, which acts as a buffer to maintain pH balance in the body, especially relevant during protein metabolism where it helps neutralize the acidic by-products. It plays a crucial role in maintaining acid-base homeostasis in tissues and blood.
Stomach: The stomach is a hollow organ in the digestive system that breaks down food using stomach acid and enzymes. It serves as a storage tank for food as it begins the process of digestion before passing into the small intestine.
Stomach Rugae: Stomach rugae are the folds or wrinkles that line the inner surface of the stomach. These prominent ridges help increase the surface area of the stomach, allowing for more efficient digestion and storage of food.
Submucosa: The submucosa is a dense layer of connective tissue found just beneath the mucosa, providing structural support and housing blood vessels, nerves, and glands that are essential for the digestive process. It plays a crucial role in nutrition absorption and signal transmission within the digestive tract.
Submucosa: The submucosa is a layer of connective tissue located beneath the mucosal lining of the digestive tract. It serves as a structural support and contains blood vessels, lymphatic vessels, and nerves that nourish and innervate the digestive organs.