Stomach is a J-shaped, expandable sack, located on the left side of the upper abdomen. It has active muscles that expand and contract depending on the quantity of food in the stomach. This contraction causes mechanical breakdown of the food. The purpose of this breakdown is to increase the available surface area for subsequent chemical activity. A normal stomach can expand to hold about 2 liters (~ ½ gallon) of food.
The gastric glands of the stomach secrete juices that perform chemical breakdown, partly digesting the proteins. Gastric juices in the stomach are secreted at a rate of 2-3 quarts/day. These juices contain primarily hydrochloric acid (HCl) and digestive enzymes. HCl makes the stomach a very acid environment with a pH factor between 1.5 and 3.0. This acidic environment serves two functions: i) it denatures the proteins and ionizes the minerals; and ii) it kills food-borne microorganisms.
Two primary enzymes are also present in the gastric juices. The first is pepsin, which aids in the hydrolysis of proteins. Pepsin begins breaking down of complex food proteins into their simpler forms. The acidic milieu created by HCl is critical for pepsin activity. The second enzyme in gastric juice is lipase, which aids in the hydrolysis of fats. Lipase starts the digestion of fats and breaks them into glycerol and fatty acids. The lipase activity is optimum at neutral pH in contrast to acidic environment required for pepsin enzyme. These enzymes, along with the gastric juices, are mixed into the food by the mechanical actions of the stomach. Stomach secretes mucus to protect itself from being digested by its own acid and enzymes.
Gastric Motility
The movement and flow of chemicals into the stomach are controlled by both the autonomic nervous system and by various hormones. The hormone gastrin causes an increase in HCl secretion from the parietal cells, and pepsinogen from chief cells in the stomach. It also causes increased motility in the stomach. It is inhibited by a pH normally <4 (high acid), as well as the hormone somatostatin. Cholecystokinin (CCK) has most effect on the gall bladder contractions, but it also decreases gastric emptying and increases release of pancreatic juice which is alkaline and neutralizes the chyme. In a different and rare manner, secretin, produced in the small intestine, has most effects on the pancreas, but will also diminish acid secretion in the stomach. Gastric inhibitory peptide (GIP) and enteroglucagon decrease both gastric secretion and motility. Other than gastrin, rest of the above hormones all act to turn off the stomach action. Stomach needs to push food into the small intestine only when the intestine is not busy. When the intestine is full and busy in digesting the food, stomach acts as storage for food.
Stomach empties at a slow rate of about 3/100 ounce for each peristaltic wave. At 3 waves/min, it can take up to 5-h for 2-lbs of food to leave the stomach.The emptying time of the stomach also varies with the type of food present. Water and liquids leave the stomach most rapidly. Carbohydrates empty more quickly than proteins; proteins, in turn, leave the stomach more quickly than fats. Within 5-min after fat enters the stomach, the hormone – enterogastrone, enters the bloodstream and travels to the stomach and inhibits the motion of the stomach and causes it to empty at a much slower rate. Not all foods undergo the same digestive processes in the stomach, and not all foods leave the stomach at the same rate. Proteins digest in an acid environment, while fats need a neutral environment. Carbohydrates leave the stomach at a faster rate than proteins.
Although the absorption is mainly a function of the small intestine, absorption of certain small molecules also occurs in the stomach through its lining. This includes: Water, if the body is too dehydrated; simple sugars (eg. Glucose), medication (eg. aspirin), and amino acids.
Stomach acts as a nutrition sensor. It can "taste" sodium glutamate using glutamate receptors and this sensory message is passed to the lateral hypothalamus and limbic system in the brain as a palatability signal through the vagus nerve. The stomach can also sense independently to tongue and oral taste receptors glucose, carbohydrate, proteins and fats. This allows the brain to link nutritional value of foods to their tastes [Uematsu et al 2009 & 2010).