Gastric acid is one of the main secretions of the stomach, together with several enzymes and intrinsic factor. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance In Human anatomy, the stomach is a J-shaped hollow muscular organ of the Gastrointestinal tract involved in the second phase of Digestion, following Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Intrinsic factor is a Glycoprotein produced by the Parietal cells of the Stomach. Chemically it is an acid solution with a pH of 1 to 2 in the stomach lumen, consisting mainly of hydrochloric acid (HCl), and small quantities of potassium chloride (KCl) and sodium chloride (NaCl). In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances pH is the measure of the acidity or alkalinity of a Solution. A lumen (Lat lūmen, an opening or light (pl lumina is the inside space or lining of a tubular structure such as an artery or intestine Hydrochloric acid is the Solution of Hydrogen chloride ( H[[Chlorine Cl]] in water The Chemical compound potassium chloride (KCl is a Metal Halide salt composed of Potassium and Chlorine. For sodium chloride in the diet see Salt. Sodium chloride, also known as common salt, table salt, or Halite, is a

Physiology

Diagram summarising control of stomach acid secretion, emphasising interaction between the body and antrum.

Gastric acid is produced by parietal cells (also called oxyntic cells) in the stomach. Parietal cells (also called oxyntic cells) are the Stomach Epithelium cells that secrete Gastric acid and Intrinsic factor Its secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network (called canaliculi) from which the gastric acid is secreted into the lumen of the stomach. A canaliculus is an adaptation found on gastric Parietal cells. These cells are part of epithelial fundic glands in the gastric mucosa. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body The fundus glands (or fundic glands, or gastric glands) are found in the body and fundus of the Stomach. The gastric mucosa is the Mucous membrane layer of the Stomach which contains the Glands and the Gastric pits. The pH of gastric acid is 1 to 2 in the human stomach lumen, the acidity being maintained by the proton pump H⁺/K⁺ ATPase. pH is the measure of the acidity or alkalinity of a Solution. A lumen (Lat lūmen, an opening or light (pl lumina is the inside space or lining of a tubular structure such as an artery or intestine This article is about proton pumps in general For the proton P-type ATPase see Proton ATPase. Gastric hydrogen potassium ATPase is also known as H+/K+ ATPase Function and location The gastric hydrogen potassium ATPase or The parietal cell releases bicarbonate into the blood stream in the process, which causes the temporary rise of pH in the blood, known as alkaline tide. In Inorganic chemistry, bicarbonate ( IUPAC -recommended nomenclature hydrogencarbonate) is an intermediate form in the Deprotonation Alkaline tide refers to a condition normally encountered after eating a meal when Stomach acid is released into the stomach causing a temporary increase in PH of

The resulting highly acidic environment in the stomach lumen causes proteins from food to lose their characteristic folded structure (or denature). Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Denaturation is a process in which Proteins or Nucleic acids lose their structure (tertiary structure by application of some external stress or compound for This exposes the protein's peptide bonds. A peptide bond is a Chemical bond formed between two Molecules when the Carboxyl group of one molecule reacts with the The chief cells of the stomach secrete enzymes for protein breakdown (inactive pepsinogen and renin). The gastric acid activates pepsinogen into pepsin - this enzyme then helps digestion by breaking the bonds linking amino acids, a process known as proteolysis. Pepsin is a digestive Protease ( released by the chief cells in the Stomach that functions to degrade food Proteins into Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins In Chemistry, an amino acid is a Molecule containing both Amine and Carboxyl Functional groups In Biochemistry, this Proteolysis is the directed degradation ( digestion) of Proteins by cellular Enzymes called Proteases or by intramolecular digestion In addition, many microorganisms have their growth inhibited by such an acidic environment which is helpful to prevent infection. A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually An infection is the detrimental Colonization of a host Organism by a foreign Species.

Secretion

The gastric acid secretion happens in several steps. Chloride and hydrogen ions are secreted separately from the cytoplasm of parietal cells and get combined into HCl only in their canaliculi. Gastric acid is then secreted into the lumen of the oxyntic gland and gradually reaches the main stomach lumen.

The highest concentration that it reaches in the stomach is 160 mM in the canaliculi. This is about 3 million times that of arterial blood, but almost exactly isotonic with other bodily fluids. Arteries are Blood vessels that carry blood away from the Heart. Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells ��such as nutrients and oxygen—and transports Waste products The lowest pH of the secreted acid is about 0. 8, but the acid gets diluted in the stomach lumen to the pH between 1 and 2.

At first, negative chloride ions and sodium ions get secreted actively from the cytoplasm of the parietal cell into the lumen of the canaliculus. The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. This creates a negative potential of -40 mV to -70 mV across the membrane that enables the diffusion of potassium ions and a small number of sodium ions from the cytoplasm into the canaliculus. Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement A canaliculus is an adaptation found on gastric Parietal cells.

Another step is the production of hydrogen ions in the cytoplasm of parietal cells. The enzyme carbonic anhydrase catalyses the reaction between carbon dioxide and water, in which carbonic acid is produced. The carbonic anhydrases (or carbonate dehydratases) form a family of Enzymes that catalyze the rapid conversion of Carbon dioxide to Bicarbonate Carbonic acid (ancient name acid of air or aerial acid) has the formula H2CO3 This acid immediately dissociates into hydrogen ions and hydrogen carbonate ions. The hydrogen ions leave the cell by the aid of H⁺/K⁺ ATPase antiporter. Gastric hydrogen potassium ATPase is also known as H+/K+ ATPase Function and location The gastric hydrogen potassium ATPase or An antiporter (also called exchanger or counter-transporter) is an Integral membrane protein which is involved in Secondary active transport

At the same time sodium ions are actively reabsorbed. This means the largest amount of secreted K⁺ and Na⁺ ions return into the cytoplasm. In the canaliculus, secreted hydrogen and chloride ions combine into HCl and are then secreted into the lumen of the oxyntic gland.

There are three phases in the secretion of gastric acid:

1. The cephalic phase: 30% of the total gastric acid to be produced is stimulated by anticipation of eating and the smell or taste of food.
2. The gastric phase: 60% of the acid secreted is stimulated by the distention of the stomach with food. Plus, digestion produces proteins, which causes even more gastrin production.
3. The intestinal phase: the remaining 10% of acid is secreted when chyme enters the small intestine, and is stimulated by small intestine distention.

Regulation of secretion

Gastric acid production is regulated by both the autonomic nervous system and several hormones. &trade The autonomic nervous system ( ANS) (or visceral nervous system) is the part of the Peripheral nervous system that acts as a Control Hormones (from Greek ὁρμή - "impetus" are chemicals released by cells that affect cells in other parts of the body The parasympathetic nervous system, via the vagus nerve, and the hormone gastrin stimulate the parietal cell to produce gastric acid, both directly acting on parietal cells and indirectly, through the stimulation of the secretion of the hormone histamine from enterochromaffine-like cells (ECL). The parasympathetic Nervous system ( PSNS) is a division of the Autonomic nervous system (ANS along with the Sympathetic nervous system The vagus nerve (ˈveɪˌgəs (VĀ-gəs (also called pneumogastric nerve or cranial nerve X) is the tenth of twelve paired Cranial nerves, and is the In humans gastrin is a Hormone that stimulates secretion of Gastric acid (HCl by the Parietal cells of the Stomach, as well as aiding in Histamine is a Biogenic amine involved in local immune responses as well as regulating physiological function in the gut and acting as a Neurotransmitter Enterochromaffin-like cells or ECL cells are a type of Neuroendocrine cells found in the gastric glands of the Gastric mucosa beneath the Epithelium Vasoactive intestinal peptide, cholecystokinin, and secretin all inhibit production. Vasoactive intestinal peptide ( VIP, also polypeptide) is a Peptide hormone containing 28 Amino acid residues and is produced in many Cholecystokinin ( CCK; from Greek chole, "bile" cysto, "sac" kinin, "move" hence move the bile-sac Secretin is a Peptide hormone produced in the S cells of the Duodenum in the Crypts of Lieberkühn.

The production of gastric acid in the stomach is tightly regulated by positive regulators and negative feedback mechanisms. Negative Feedback feeds part of a System 's output inverted into the system's input generally with the result that fluctuations are attenuated Four types of cells are involved in this process: parietal cells, G cells, D cells and enterochromaffine-like cells. In Anatomy, the G cell is a type of cell in the stomach that secretes Gastrin. Delta cells (δ-cells or D cells are Somatostatin producing cells They can be found in the Stomach, Intestine and the Islets of Langerhans Besides this, the endings of the vagus nerve (X) and the intramural nervous plexus in the digestive tract influence the secretion significantly.

Nerve endings in the stomach secrete two stimulatory neurotransmitters: acetylcholine and gastrin-releasing peptide. See Chemical synapse for an introduction to concepts and terminology used in this article The Chemical compound acetylcholine (often abbreviated ACh) is a Neurotransmitter in both the Peripheral nervous system (PNS and Central Gastrin-releasing peptide, also known as GRP, is a human Gene. Their action is both direct on parietal cells and mediated through the secretion of gastrin from G cells and histamine from enterochromaffine-like cells. Gastrin acts on parietal cells directly and indirectly too, by stimulating the release of histamine.

The release of histamine is the most important positive regulation mechanism of the secretion of gastric acid in the stomach. Its release is stimulated by gastrin and acetylcholine and inhibited by somatostatin. Somatostatin (also known as growth hormone inhibiting hormone ( GHIH) or somatotropin release-inhibiting factor ( SRIF) is a Peptide hormone

Neutralization

In the duodenum, gastric acid is neutralized by sodium bicarbonate. In Anatomy of the Digestive system, the duodenum is a hollow jointed tube about 25-30 cm (10-12 in long connecting the Stomach to the Jejunum Sodium bicarbonate or sodium hydrogen carbonate is the Chemical compound with the formula NaHCO3 This also blocks gastric enzymes that have their optima in the acid range of pH. pH is the measure of the acidity or alkalinity of a Solution. The secretion of sodium bicarbonate from the pancreas is stimulated by secretin. The pancreas is a Gland organ in the digestive and Endocrine system of Vertebrates. Secretin is a Peptide hormone produced in the S cells of the Duodenum in the Crypts of Lieberkühn. This polypeptide hormone gets activated and secreted from so-called S cells in the mucosa of the duodenum and jejunum when the pH in duodenum falls below 4. Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino S cells are cells which release Secretin, found in the Jejunum and Duodenum. In Anatomy of the Digestive system, the jejunum is the central of the three divisions of the Small intestine and lies between the Duodenum and 5 to 5. 0. The neutralization is described by the equation:

HCl + NaHCO₃ → NaCl + H₂CO₃

The carbonic acid instantly dissociates into carbon dioxide and water, then gets eliminated through urine. Carbonic acid (ancient name acid of air or aerial acid) has the formula H2CO3 Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single Water ( H2[[oxygen O]] H OH) is the most abundant Molecule on Earth 's surface composing of about 70% of the Earth's surface as Urine is a liquid waste product of the body secreted by the Kidneys by a process of filtration from Blood and Excreted through the Urethra.

Safety mechanisms

There are several safety mechanisms that prevent the damage of gastric epithelium. Nonetheless, when due to different reasons these fail (e. g. because of excess acid production), this can lead to heartburn or peptic ulcers. Heartburn or pyrosis is a painful or burning sensation in the Esophagus, just below the breastbone usually associated with regurgitation of gastric acid A peptic ulcer, also known as PUD or peptic ulcer disease, is an Ulcer (defined as mucosal erosions equal to or greater than 0

Role in disease

In hypochlorhydria and achlorhydria, there is low or no gastric acid in the stomach, potentially leading to problems as the disinfectant properties of the gastric lumen are decreased. Achlorhydria and hypochlorhydria refer to states where the production of Gastric acid in the Stomach is absent or low respectively Achlorhydria and hypochlorhydria refer to states where the production of Gastric acid in the Stomach is absent or low respectively Disinfectants are Antimicrobial agents that are applied to non-living objects to destroy Microorganisms, the process of which is known as disinfection. In such conditions, there is greater risk of infections of the digestive tract (such as infection with Vibrio or Helicobacter bacteria). Vibrio is a Genus of Gram-negative Bacteria possessing a curved rod shape Helicobacter is a Genus of Gram-negative bacteria possessing a characteristic Helix shape

In Zollinger-Ellison syndrome and hypercalcemia, there are increased gastrin levels, leading to excess gastric acid production, which can cause gastric ulcers. Zollinger-Ellison syndrome is a disorder where increased levels of the hormone Gastrin are produced causing the Stomach to produce excess Hydrochloric acid Hypercalcaemia (in American English '''Hypercalcemia''' is an elevated calcium level in the Blood. In humans gastrin is a Hormone that stimulates secretion of Gastric acid (HCl by the Parietal cells of the Stomach, as well as aiding in A peptic ulcer, also known as PUD or peptic ulcer disease, is an Ulcer (defined as mucosal erosions equal to or greater than 0

In diseases featuring excess vomiting, patients develop hypochloremic metabolic alkalosis (decreased blood acidity by H⁺ and chlorine depletion). Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and Metabolic alkalosis is a Metabolic condition in which the PH of the blood is elevated beyond the normal range Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Chlorine (ˈklɔriːn from the Greek word 'χλωρóς' ( khlôros, meaning 'pale green' is the Chemical element with Atomic number 17 and

Pharmacology

The proton pump enzyme is the target of proton pump inhibitors, used to increase gastric pH in diseases which feature excess acid. Proton pump inhibitors (or "PPI"s are a group of drugs whose main action is pronounced and long-lasting reduction of Gastric acid production H2 antagonists indirectly decrease gastric acid production. Antacids neutralize existing acid. An antacid is any substance generally a base or Basic salt, which counteracts stomach acidity.

See also

• Stomach
• Digestion

External links

• The Parietal Cell: Mechanism of Acid Secretion at vivo.colostate.edu
• First Principles of Gastroenterology. Chapter 6. Salena B. J., Hunt R. H. The Stomach and Duodenum