January 2010 - Living Healthfully

Regulation of Gastric Secretion by Nervous and Hormonal Mechanism
The basic factors that stimulate gastric secretion are acetylcholine (involved via vagal/parasympathetic stimulation), gastrin, and histamine. These substances function by attaching to specific receptors, leading to the secretion of the gastric glands. Acetylcholine excites all glands involved in the secretion of hydrochloric acid, pepsinogen, and mucus. Histamine and gastrin, on the other hand, strongly stimulate the secretion of hydrochloric acid by the parietal cells but have little effect in stimulating other gastric secretory cells.

The enteric nervous system can stimulate release of H (to form HCl), either directly or indirectly. The nerve endings of fibers innervating the parietal cells secrete acetylcholie at the neuroeffector junction. The corresponding receptor is M3. Succeeding events include the increase in cytosolic calcium, stimulation of protein kinases, and activation of H /K ATPase. The end result is increased hydrochloric acid secretion.
The indirect stimulation of the parietal cells by the nervous system occurs as acetylcholine attaches to M1 receptor of enterochromaffin-like cells. This gives way to the release of histamine. Histamine in turn binds to H2 receptors found in the parietal cells. Afterwards adenylyl cyclase gets activated. This increases intracellular cAMP, activating protein kinase. Eventually this will stmulate H /K ATPase.
The same direct and indirect mechanisms can be onserved in parietal stimulation by gastrin-secreting cells.

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The Physiology of Gastric Secretions 3

January 27th 2010 16:20

The Stomach
The main site of pathology in acid-peptic diseases is the stomach. It is therefore the site of action of the antacids used in treating these disorders. It is indeed important to know how stomach contributes in the chemical digestion of food. What are the glands in the stomach involved in the process? What specific substances are they secreting? What are the stimuli and the mechanisms involved in the release and regulation of these secretions? How does an excess of these secretions contribute to the development of acid-peptic disorders?

functionally, the stomach can be divided into (1) the orad portion, comprising the first two-third of the body; and (2) the caudal portion, comprising the remainder of the body plus the antrum. The functional divisions of the stomach play role in the mechanical digestive process.

Hydrochloric Acid Secretion
As stated earlier, one of the primary sources of increased acidity in the stomach is the hydrochloric acid produced in the gastric lumen. It is therefore necessary to review how hydrochloric acid is produced:
1. Chloride ion is actively transported from the cytoplasm of the parietal cell into the lumen. Sodium ions are also actively transported out. These events create a negative potential in the canaliculus, which in turn causes diffusion of positively charged potassium.
2. Water dissociates into hydrogen ion and hydroxyl ion in the cytoplasm. The hydrogen ions are then actively secreted into the lumen in exchange for potassium (catalyzed by H , K , ATPase). Sodium ions are also actively reabsorbed.
3. Bottomline is that when K and Na were reabsorbed, the lumen gets H in exchange. This proton combines with Cl-, forming HCl.

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The Physiology of Gastric Secretions

January 19th 2010 12:38

The Gastrointestinal Tract and the Accessory Organs for Digestion
The gastrointestinal tract or the alimentary tract is divided into two main parts: the upper and the lower. The upper part includes the mouth, the pharynx, the esophagus, and the stomach. The lower part is composed of the small and the large intestine, together with the anus.
Throughout the tract, we see many specialized glands. The functions of these secretory glands are: (1) for digestion of food through enzymes secreted anywhere from the mouth down to the distal end of the ileum; and (2) for lubrication and protection of the tract as being carried out by mucus secreted from mouth to anus.
Most digestive secretions are formed only in response to food in the alimentary canal. The quantity however has to be enough to facilitate proper digestion and prevent structural injuries to the walls. The types of enzymes secreted depend on where the gland is located, as well as on the type of food presented into the lumen.
The accessory organs include the liver, the gall bladder, and the pancreas. The liver secretes bile that emulsifies fats. The gall bladder serves as reservoir for bile, in turn concentrating the substance before it is secreted into the duodenum of the small intestine. The pancreas in turn secretes bicarbonates, trypsin, lipase, and amylase necessary for intestinal digesti

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The Physiology of Gastric Secretions

January 17th 2010 12:37

Antacids are drugs used to reduce intra-gastric acidity. They are mostly indicated in addressing stomach conditions that are characterized by an increase in the stomach acidity, like gastro-esophageal reflux and peptic ulcer. Collectively these conditions are termed as acid-peptic diseases.
In these disorders, the excessively low pH of the gastric content-major contributor is the hydrochloric acid and pepsin- erodes mainly the mucosal barrier of the gastric epithelium, leading to inflammation and ulceration. Epithelial injuries to nearby structures like in the esophagus are possible though, since gastric contents can spill out to the said area (as in gastro-esophageal reflux).

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