Of course, this is not true. An important function of the stomach is to serve as a temporary holding chamber. You can ingest a meal far more quickly than it can be digested and absorbed by the small intestine. Thus, the stomach holds food and parses only small amounts into the small intestine at a time. Foods are not processed in the order they are eaten; rather, they are mixed together with digestive juices in the stomach until they are converted into chyme, which is released into the small intestine.
As you will see in the sections that follow, the stomach plays several important roles in chemical digestion, including the continued digestion of carbohydrates and the initial digestion of proteins and triglycerides. Little if any nutrient absorption occurs in the stomach, with the exception of the negligible amount of nutrients in alcohol. The cardia or cardiac region is the point where the esophagus connects to the stomach and through which food passes into the stomach.
Located inferior to the diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the fundus is the body , the main part of the stomach.
The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum , connects to the body of the stomach. The narrower end is called the pyloric canal , which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls stomach emptying.
In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into a large fold called a ruga. The convex lateral surface of the stomach is called the greater curvature; the concave medial border is the lesser curvature. The stomach is held in place by the lesser omentum, which extends from the liver to the lesser curvature, and the greater omentum, which runs from the greater curvature to the posterior abdominal wall.
The wall of the stomach is made of the same four layers as most of the rest of the alimentary canal, but with adaptations to the mucosa and muscularis for the unique functions of this organ.
As a result, in addition to moving food through the canal, the stomach can vigorously churn food, mechanically breaking it down into smaller particles. A vast number of gastric pits dot the surface of the epithelium, giving it the appearance of a well-used pincushion, and mark the entry to each gastric gland , which secretes a complex digestive fluid referred to as gastric juice.
Although the walls of the gastric pits are made up primarily of mucus cells, the gastric glands are made up of different types of cells.
The glands of the cardia and pylorus are composed primarily of mucus-secreting cells. Cells that make up the pyloric antrum secrete mucus and a number of hormones, including the majority of the stimulatory hormone, gastrin.
The much larger glands of the fundus and body of the stomach, the site of most chemical digestion, produce most of the gastric secretions. These glands are made up of a variety of secretory cells.
These include parietal cells, chief cells, mucous neck cells, and enteroendocrine cells. These relatively large cells produce both hydrochloric acid HCl and intrinsic factor. HCl is responsible for the high acidity pH 1. The acidity also kills much of the bacteria you ingest with food and helps to denature proteins, making them more available for enzymatic digestion. Intrinsic factor is a glycoprotein necessary for the absorption of vitamin B 12 in the small intestine.
Chief cells —Located primarily in the basal regions of gastric glands are chief cells , which secrete pepsinogen , the inactive proenzyme form of pepsin. HCl is necessary for the conversion of pepsinogen to pepsin. Mucous neck cells —Gastric glands in the upper part of the stomach contain mucous neck cells that secrete thin, acidic mucus that is much different from the mucus secreted by the goblet cells of the surface epithelium.
The role of this mucus is not currently known. Enteroendocrine cells —Finally, enteroendocrine cells found in the gastric glands secrete various hormones into the interstitial fluid of the lamina propria. These include gastrin, which is released mainly by enteroendocrine G cells. Watch this animation that depicts the structure of the stomach and how this structure functions in the initiation of protein digestion.
This view of the stomach shows the characteristic rugae. What is the function of these rugae? The secretion of gastric juice is controlled by both nerves and hormones. Stimuli in the brain, stomach, and small intestine activate or inhibit gastric juice production.
However, once gastric secretion begins, all three phases can occur simultaneously. The cephalic phase reflex phase of gastric secretion, which is relatively brief, takes place before food enters the stomach. The smell, taste, sight, or thought of food triggers this phase. For example, when you bring a piece of sushi to your lips, impulses from receptors in your taste buds or the nose are relayed to your brain, which returns signals that increase gastric secretion to prepare your stomach for digestion.
This enhanced secretion is a conditioned reflex, meaning it occurs only if you like or want a particular food. Depression and loss of appetite can suppress the cephalic reflex. The gastric phase of secretion lasts 3 to 4 hours, and is set in motion by local neural and hormonal mechanisms triggered by the entry of food into the stomach. For example, when your sushi reaches the stomach, it creates distention that activates the stretch receptors.
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Plants and animals need elements, such as nitrogen, phosphorus, potassium, and magnesium for proper growth and developme.. Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum. Plant Cell Defense Plants protect themselves by releasing hydrogen peroxide to fight against fungal invasion. Gibberellins and Gibberellic Acid This tutorial describes the role of gibberellin family in plants.
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