Stomach Anatomy and Physiology

and skeletal muscle, and the lower third has only smooth muscle. There is no serosal layer but

there is an adventitial layer. It is not covered by visceral peritoneum.

The act of swallowing is called deglutition. It is the movement of food as a bolus from the

mouth to the stomach. It takes about a second for liquids to pass through the esophagus and

4-8 seconds for solid or semisolid food to pass through. The process is not passive and involves

a complex series of muscular activities that are both conscious and unconscious phases.

The voluntary phase of swallowing is controllable. Chewing is over and the tongue moves

upward and backward against the palate so that food can get into the oropharynx. There are

muscles that kick in to prevent food from getting into the trachea or nasopharynx. The

pharyngeal phase involves receptors in the oropharynx that sense food, sending it to the

“deglutition center” in the medulla oblongata. Breathing stops briefly by the closure of the

epiglottis and pharyngeal muscles constrict to move the bolus through the pharynx. The upper

esophageal sphincter allows food to enter the esophagus. The esophageal phase involves

peristalsis, controlled by the medulla oblongata. There are circular and longitudinal muscles

that contract to push the bolus through. It is a short reflex that relaxes the lower esophageal

sphincter to allow food to pass into the stomach.

STOMACH ANATOMY AND PHYSIOLOGY

Chemical digestion starts minimally in the mouth but really advances in the stomach. The

stomach links the esophagus to the small intestine. It contracts readily to cause mechanical

digestion. It can stretch to more than 75 times its empty size in order to take in as much as four

liters of food or fluid. It is also a receptacle for food, letting in only a little bit of food at a time

into the small intestine. The food that is mixed with digestive juices is called chyme, which is

made in the stomach. Little nutrient absorption occurs.

The four main regions of the stomach are the cardia, the fundus, the body, and the pylorus. The

cardia is the first part, located just after the esophagus. The next part is the fundus, which is

dome-shaped. Below this is the main portion—the body. The pylorus is the last part, connecting

the stomach to the duodenum. It is funnel-shaped, with the largest part being called the pyloric

antrum. The narrower part is called the pyloric canal, with a pyloric sphincter muscle controlling

stomach emptying. Without food, it collapses into folds of mucosa and submucosa, known as

rugae. Figure 101 shows an image of what the stomach anatomy looks like:

Like the rest of the alimentary canal, there are four layers to the stomach wall. The muscularis

layer and the mucosal layer have unique functions in the stomach. There is an inner oblique

smooth muscle layer as well as the circular and longitudinal muscles. All of these allow for the

churning of food in this organ. The mucosa has an epithelial lining that makes surface mucus

cells that secrete a protective coating of alkaline mucus to protect the stomach wall.

Gastric pits exist in the epithelium of the stomach that mark the entrance of gastric glands,

which secrete gastric juices. The gastric glands of the cardia and pylorus mainly secrete mucus;

however, the pyloric antrum makes both mucus and gastrin (a digestive hormone). The larger

glands of the body and fundus make the most gastric juices. The different secretory cells of the

gastric juices include the following:

• Parietal cells—these secrete both hydrochloric acid and intrinsic factor, responsible for

the low pH (of 1.5-3.5) in the stomach. This low pH is necessary for pepsinogen to

convert to the active pepsin and is necessary to kill pathogens. Intrinsic factor is a

glycoprotein which facilitates the absorption of vitamin B12 in the small intestine.

• Chief cells—these make pepsinogen, which is the inactive “proenzyme” that uses

hydrochloric acid to create pepsin, an enzyme that digests protein.

• Mucus neck cells—these mainly secrete mucus in the upper stomach. It is different

from the mucus made by goblet cells as it is acidic in nature. It is not known what this

mucus does.

• Enteroendocrine cells—these secrete hormones, including gastrin made by the G cells.

There are multiple hormones secreted by the stomach. The G cells of the pyloric antrum makes

gastrin in response to the presence of protein and amino acids in the stomach. It increases

gastric gland secretion, promotes gastric emptying, promotes small intestinal muscle

contraction, relaxes the ileocecal valve, and causes mass movements in the large intestine.

There is ghrelin (made in the fundus) during fasting that stimulates hunger. Histamine will

increase parietal cell secretion of hydrochloric acid (HCl). Serotonin is made that will contract

the stomach muscles. Somatostatin will, on the other hand, limit motility of the stomach, limit

emptying, decrease intestinal absorption, and slow pancreatic secretions.

There are neural inputs and endocrine inputs to gastric secretion of gastric juices. This leads to

what’s known as cephalic, gastric, and intestinal phases of gastric secretion, which can occur

simultaneously. The cephalic phase involves stimulation by the sight and thought of food and

stimulation of taste and smell receptor that, through the cerebral cortex, hypothalamus, and

medulla oblongata, will stimulate stomach secretions. Poor appetite will block stomach

secretions. The gastric phase will involve gastric distention and rising pH levels from food that

will stimulate gastric secretions or high acidity that will decrease gastrin secretion. The

intestinal phase will be triggered by duodenal changes (distention and pH changes) that will

increase or inhibit the secretory activity of the stomach.

The cephalic phase is brief, while the gastric phase lasts about 3-4 hours. In the gastric phase,

there is an upper limit of the amount of HCl produced in the stomach so that the HCl

production will stop if the pH gets too low. The intestinal phase involves the release of

intestinal gastrin, which first enhances gastric secretion. The distention of the duodenum,

however, initiates the enterogastric reflex, which inhibits gastric secretion and closes the

pyloric sphincter so that additional chyme will not enter the esophagus.

The mucosal barrier protects the stomach from being digested by its own hydrochloric acid.

This involves a thick bicarbonate-rich mucus barrier that acts as a physical and chemical barrier

to HCl. There are tight junctions in the epithelium that also prevent gastric juices from passing

through the epithelial wall. Stem cells will rapidly replace the lost or damaged epithelial

mucosal cells so that the surface of the stomach will be replaced every 3-6 days.

There is mechanical and chemical digestion taking place in the stomach. As for mechanical

digestion, there is a “mixing wave” that is unique to the stomach. It mixes and softens food in

order to make chyme. The force increases as the food passes through the stomach. The pylorus

holds about an ounce of chyme, passing small amounts through the pyloric sphincter in what’s

called “gastric emptying.” Only about three milliliters of chyme pass through the pylorus at a

time.

Chemical digestion includes some activity of salivary amylase, which is gradually inactivated as

the stomach becomes more acidic. Lingual lipase is actually activated by the acidic

environment of the stomach, breaking down triglycerides. Protein breakdown starts with

pepsin activity. Intrinsic factor acts not on the stomach but on the small intestine to absorb

vitamin B12.

The stomach contents are completely emptied into the duodenum within 2-4 hours after a

meal. Carbohydrates empty the fastest, followed by protein-laden foods. Fatty foods digest

slowly and can stay in the stomach for at least six hours.