Stomach
The stomach (Greek γαστήρ gaster " stomach ", " stomach ", Latin ventriculus ) is a digestive organ almost all animals. The most primitive form is the Gastralraum of cnidarians, who is also an organ of elimination. The stomach is a hollow organ of muscle tissue, which is lined with a mucous membrane. In contrast to einhöhligen stomach of humans and most animal species, the stomach has in ruminants and birds several demarcated cavity systems ( mehrhöhliger stomach) as well as highly specialized organs in insects (honey stomach, Saugmagen ). Magenlos are eg the carp fish ( we interpret this as an original adaptation to snail and mussel food, their calcareous shells stomach acid made useless).
The capacity of the human stomach is individual and is about 1.5 L. In this cavity, the chyme is mixed with the gastric juice, which consists of the proteolytic enzyme pepsin and hydrochloric acid substantially. The stomach brings the chyme to the same temperature, as well as coated and stores the chyme. At rest, the glands secrete from about 10 ml of gastric juice per hour. Under fed the formation of gastric juice can be increased up to 1000 ml per hour. The formation is controlled both by nerve impulses ( especially before eating ) as well as by hormones. By muscle contractions ( peristalsis ) of the digested food is transported to the porter. This forms a sphincter ( sphincter ) a closure between the stomach and intestine and may be different for the passage of the digested chyme ( chyme ) to open about 13 mm wide. This happens regularly in order to forward the chyme evenly into the duodenum.
- 2.1 Anatomy
- 2.2 Histology
- 2.3 Embryology
- 2.4 blood supply of the stomach
- 3.1 stomach of ruminants
- 3.2 stomach of birds
- 3.3 Loss of the stomach in the course of evolution
Biological Significance
Predigestion
The actual digestion and absorption takes place in most animals and humans in the intestine, or more specifically in the small intestine. In the stomach, especially the proteins (proteins ) can be predigested in so-called polypeptide chains. The enzymes needed for protein digestion, pepsin and cathepsin, have by the low pH maximum activity. They are used by cells in the stomach as inactive precursors ( pepsinogen ) secreted and activated only by contact with the stomach acid. Fats pass through the stomach largely unhindered, but shall be made by the peristalsis softer ( liquefied). Carbohydrates pass through the stomach just as freely as the enzymatic cleavage with alpha -amylase ( from saliva ) by the acidic pH in the stomach stops again.
Killing bacteria
The acidic gastric juice effectively prevents the survival of most bacteria and protects against infections. Long considered the doctrine that in the extremely acidic environment of the human stomach growth of microorganisms was not possible in principle. With the discovery of Helicobacter pylori in the gastric mucosa in 1983, but this view is needed to be revised. Henceforth was H. pylori as the only exception - a bacterium that is able to settle below the protective mucus layer. Erroneously, the predominant gastric pH is often compared with concentrated acid. The gastric juice also has a very low pH of 0.8 to 1.5, but the concentration of the hydrochloric acid is lower.
Scientists at Stanford University have 2005/2006, however, demonstrate that the flora (see flora ) is considerably more complex than previously thought: They discovered a total of 128 more stomach -colonizing bacterial species, including a relative of the genus extremophile Deinococcus. Overall, about ten percent of the identified microorganisms differed genetically significantly from all previously known bacteria.
Regular further digestion
The stomach makes it that living beings do with a few larger meals per day. He is able to adapt its wall voltage to the respective amount of content ( accommodation). The Doorman ( pylorus ) ensures that all substances sufficient to remain in the stomach and be sufficiently mixed with the digestive juices. The substances are then slowly and evenly fed to the intestine.
Structure in humans and monogastric animals
Anatomy
Seen from the outside, a distinction at the stomach, the front upper surface ( paries anterior) and the rear bottom surface ( paries posterior), the large curvature ( greater curvature ) the omentum majus attached part and the small curvature ( curvatura minor ) where the omentum attached minus.
Macroscopically, it is divided into the following areas:
- The pars cardiaca ( " cardia " ) - with the ostium cardiacum ( " stoma " ) - represents the junction between the esophagus ( gullet ) and stomach dar.
- The fundus ulcer ( " stomach of Default") lies to the left and cranial to the stomach entrance and is filled with in food intake of swallowed air. He appears as a stomach bubble on the radiograph.
- The corpus ulcer ( " stomach body " ) accounts for the largest share of the stomach.
- The pyloric part is narrower than the body, the more the duodenum portion of the pyloric canal ( the "gatekeeper channel") is even closer than the distant part, the pyloric antrum ( the "gatekeeper cave ").
- The pylorus ( " gatekeeper " ) is via the pyloric ostium ( the "gatekeeper mouth " ) to connect to the duodenum. It appears from the outside as hard constriction and forms against the internal sphincter (musculus sphincter pylori).
The shape and position of the stomach is not constant but rather a function of its filling, and the position of people. The pars cardiaca is relatively strongly attached by connective tissue, while the hepatoduodenal ligament attached only at the pylorus descends with a larger stomach contents, causing the stomach, being the typical hook shape. In women, the stomach is usually deeper and steeper located than in men.
Histology
Microscopically consists of the stomach:
- The gastric mucosa ( gastric mucosa ), composed of the laminae epithelialis, propria and muscularis mucosae. She is ( gastricae foveolae ) by depressions in numerous fields ( areae gastricae ) structured, ( gastricae glands ) with a variety of glands.
- A connective tissue layer ( submucosa ) with blood vessels
- Muscle layer (tunica muscularis gastrica ) of smooth muscle, the obliquae from the Fibrae, the stratum and the stratum circulare longitudinal consists
- A covering of the tunica serosa ( peritoneum )
The stomach has for the secretion of hormones and other secretions to its mucous membrane of different cell types, which are located at the typical locations of the stomach.
Embryology
The stomach is formed as a spindle-shaped extension of the foregut (the front portion of the primitive gut tube of the embryo ). This is attached to the dorsal or ventral body wall via two mesentery ( mesogastrium dorsal and ventral ). The first related to the longitudinal axis of the embryo stomach system expands dorsal side of the greater curvature and bulging belly side, a flat to concave curvatura minor.
The final shape and position of the stomach forms out by differential growth of the gastric wall sections. The location changes are also called " stomach twists " ( not to be confused with the disease bloat ) denotes although this may involve not passive rotations, but complicated Umbildungsprozesse the stomach system. The so-called " 1 Bloat " can be thought of as a rotation about the longitudinal axis to the left by 140 degrees. Thus, the large curvature of the stomach from the back passes ventrolaterally (left - ventral ), the small gastric curvature corresponding to the right dorsolateral (lateral - move down). The " second Bloat " can be described by 90 degrees to the left as a rotation around the vertical axis. Thus, the entrance to the stomach to the left and passes the pylorus to the right. The " 3 GDV " is again about the longitudinal axis to the right by about 45 degrees. The greater curvature is now pointing left and kaudoventral, the greater curvature to the right and minor kraniodorsal.
Blood supply of the stomach
The stomach is supplied arterially via the celiac trunk. This short tube line goes directly from the aorta and branches into three main branches to: the common hepatic artery, the left gastric artery and the splenic artery. The stomach is thereby supplied directly via the left gastric artery or indirectly via other outlets of the other two branches. The upper part of the lesser curvature gets its oxygen-rich blood directly from the left gastric artery. It runs with the artery gastric artery along which originally supplied the common hepatic artery springing the lower part of the lesser curvature. Both run on the stomach in the small network ( lesser omentum ), where they meander on the right side along stomach. Also originally sprung from the common hepatic artery, the artery gastroomentalis artery supplies the lower half of the greater curvature. It runs with the artery gastroomentalis together sinistra, which in turn originates from the splenic artery. Both run on the stomach in large mesh ( greater omentum ). In addition, the splenic artery are still some arteries gastricae from Breves, who are responsible for the supply of the fundus, and a posterior gastric artery, which provides blood flow to the stomach back wall.
Comparative Anatomy
Stomach of ruminants
In ruminants are the actual, lined with a mucous glands containing stomach, referred to here as the abomasum, yet upstream of three forestomach sections. These are the rumen, reticulum and omasum. They have a loose mucous glands. In these the rumen microbial digestion of cellulose and first resorption takes place.
Stomach of birds
The stomach of birds is divided into two sections. In the actual glandular stomach ( ventriculus glandularis or proventriculus ) are also given enzymes and hydrochloric acid. This glandular stomach is connected downstream of the gizzard ( ventriculus muscularis ). It is made of powerful muscles and serves for the mechanical comminution of the food and thus replaces the function of the teeth and chewing. The glands of the gizzard but a secretion from that to a grater plate is cured by the hydrochloric acid of the proventriculus ( Koilinschicht ). In addition, many birds take stones (or other hard particles such as mussels in Seabirds ), which grind together with this grater plate the food. These stones are called gastroliths stomach or as grit. In birds that feed on easily digestible or soft food, the gizzard is well developed.
Loss of the stomach in the course of evolution
The stomach is about 450 million years ago for the first time in vertebrates ( Vertebrata ) occurred. Many vertebrates have lost the stomach and often the associated genes (eg for pepsinogen and proton- potassium pump ) then again, to include primitive mammals such as the platypus and the echidna, and about a quarter of all fish species in the narrow sense the true bony fish ( Teleostei ). The loss of the stomach in numerous lineages could be related to the absence of certain proton pump and pepsinogener enzymes.