ESOPHAGUS, STOMACH AND PANCREAS ( GIT )

ESOPHAGUS

The function of the esophagus is to move food and liquid to the stomach and to keep it there. 

Swallowing induces a wave of peristalsis in the esophagus. 

The portion of food, drink, or other material that will move through the neck in one swallow is called a bolus.

THE STOMACH

The main gastric functions: 

1. The motor (motility) functions of the stomach include acting as a reservoir for ingested food, followed by mixing and grinding of food prior to its regulated delivery into the small intestine.

2. The five main exocrine secretions of the stomach are:

• Water, to dissolve and dilute ingested food. 
• Acid (HCl), to denature dietary proteins and to kill ingested microorganisms. 
• Enzymes (pepsin and gastric lipase), to contribute to protein and fat digestion. They are secreted as inactive pepsinogens. The conversion of pepsinogen to pepsin occurs spontaneously when the pH is below 5. 
• Intrinsic factor, a glycoprotein that is necessary for vitamin B12 absorption in the ileum. 
• A mucus-bicarbonate barrier at the mucosal surface, to protect against the corrosive properties of gastric juice.

3. Endocrine functions of the stomach include the secretion of the hormones gastrin, somatostatin, and ghrelin.

• Gastrin is produced in the gastric antrum and regulate gastric acid secretion. Ghrelin is produced in the body of the stomach and is a factor involved in the regulation of hunger and satiety.

4. Erythropoietic function. Stomach produces the Castle (gastric) factors connected to Vitamin B12 (comes from food). This complex is absorbed in small intestine and helps to red blood cells production.

Gastric secretion is maximal about 1–2 hours after the ingestion of a balanced meal. The buffering capacity of the meal eventually becomes saturated, and the gastric pH begins to decrease; at this stage, a significant proportion of the meal has entered the small intestine.

Gastric secretion 

The functions of hydrochloric acid:

• promotes denaturation and swelling of proteins, which facilitates their breakdown 
• activates pepsinogens and converts them into pepsins 
• creates an acidic environment for the activation of enzymes 
• provides antibacterial effect of gastric juice 
• contributes to the normal evacuation of food from the stomach 
• stimulates pancreatic secretion

Regulation of gastric motility and secretion
The duodenum senses acidity, osmolarity, distension, stretch and nutrient composition of the luminal contents to gauge the rate of gastric emptying. An increase in these variables stimulates the release of the hormones shown, resulting in feedback inhibition of gastric motility and secretion, when these hormones called enterogastrones inhibit the opening of the pyloric sphincter from the stomach and the closure of the duodenum. This mechanism ensures that the rate of gastric emptying is not excessive.

THE PANCREAS 

1 – stomach, 
2 – pancreas, 
3 – duodenum, 
4 – gallbladder, 
5 – liver, 
6 – common hepatic duct

Most of the pancreas is comprised of epithelial cells, which produce an exocrine secretion (аbout 1–2 L) of digestive enzymes in a HCO3 - rich solution.

• Digestive enzymes are synthesized and secreted by grape-like acini, which consist of 20–50 pyramidal cells that surround a small luminal space. Pancreatic acini produce a low-volume, enzyme-rich fluid that drains via a series of ducts into the main pancreatic duct. 
• Pancreatic duct epithelial cells produce a copious amount of HCO3 - rich fluid, which is added to the acinar secretion. 
• The main pancreatic duct joins the common bile duct to form a common excretory duct, which is guarded by the Sphincter of Oddi. Exocrine pancreatic secretions and bile enter the duodenum when the Sphincter of Oddi is relaxed.

Pancreatic functions:

• The main digestive function of the pancreas is to secrete the enzymes that break down the macromolecules in food and to produce smaller nutrient molecules for intestinal absorption. Pancreatic enzymes are essential for digestion. 
• The pancreas also secretes an alkaline fluid that neutralizes the acidic chyme that enters the small intestine from the stomach. This fluid is necessary because pancreatic enzymes have a neutral pH optimum. 
• The pancreas has a separate endocrine function to secrete the hormones insulin and glucagon.

Several features of enzyme synthesis and activation help to prevent autodigestion of the pancreas, including:

• Most enzymes are produced as inactive precursors called zymogens (e.g. trypsinogen ) 
• Enzymes are sequestered in membrane-limited vesicles throughout synthesis to the point of exocytosis, avoiding contact with the acinar cell cytoplasm. 
• Activation of zymogens occurs in the small intestine. The process depends on the conversion of the proenzyme trypsinogen to the active proteolytic enzyme trypsin. Once trypsin is activated, it cleaves and activates all other zymogens. 
• The pancreas produces a trypsin inhibitor to prevent activation of zymogens within the pancreas if trypsin is inappropriately activated inside the gland.