Digestive System

Connected to the esophagus is a sack-shaped organ called the stomach.  The stomach mainly acts as a holding vessel and a mixer for food and other substances that we swallow, such as liquids and drugs. In the stomach, food collects and mixes with acid that helps to break it down further. Some substances, such aspirin and drugs that dissolve readily in fats, are absorbed directly into the bloodstream through the stomach, but very little else is absorbed there.

When the stomach is empty, it has many folds on the inside called rugae.  These rugae, which disappear as the stomach begins to fill with food, enable the walls of the stomach to stretch so that the stomach can hold more food. Sympathetic regulation of the stomach is by the greater splanchic nerve, and paraympathetic regulation is by the vagus nerves. Oxygen-rich blood is supplied to the stomach through the celiac trunk, which also supplies the liver, the gallbladder, and sections of the the small intestine. Blood without oxygen is taken away from the tissues of the stomach via gastric veins that join the hepatic portal system, which also serves the liver and other digestive organs.

Three different kinds of glands are found in the mucosa layer of the stomach tissue.  These glands open into the cavity of the stomach and extend as far as the lamina propria sub-layer. The glands that are found in the upper left region of the stomach are called cardiac glands (even though they have nothing to do with the heart). The glands that are located in the lower right region of the stomach, near where the stomach meets the duodenum, are called pyloric glands. The glands in the middle region of the stomach are called gastric glands.  These glands have several different types of cells. Mucous cells in pyloric and cardiac glands are the primary cells that secrete mucus.  However, mucous neck cells in gastric glands can also secrete mucus. Stem cells in all three glands replace cells that die. Parietal cells, which are found mostly in gastric glands and in some pyloric glands, secrete hydrochloric acid and intrinsic factor. Chief cells, which are the most numerous, secrete pepsinogen in adults and chymosin (rennin) and lipase in infants. Chief cells are found solely in gastric glands. Enteroendocrine cells, which exist in all three glands, secrete chemical messengers that control digestion. Some of these chemical messengers, such as hormones, travel through the bloodstream and can act on cells that are far away from the stomach. Other messengers, however, act on cells next to where they were secreted.

When we first think about food, see food, smell food, or taste food, we send a message to the hypothalamus. The hypothalamus then sends a message to the medulla, which stimulates the sympathetic nervous system. The sympathetic nervous system helps to control the activity of our stomach.

Where the esophagus meets the stomach is another sphincter muscle called the cardiac sphincter. This sphincter muscle is called the cardiac sphincter because it is located close to the heart. Like the sphincter muscle at the entrance of the esophagus, this sphincter muscle is usually tight.  However, when a bolus is swallowed, the sphincter muscle opens up so that the bolus can enter the stomach when it reaches the end of the esophagus. After the food enters the stomach, the sphincter muscle contracts and prevents food from going back into the esophagus.

When we swallow the food, the medulla also sends a signal that tells our stomach to relax. When the food arrives in the stomach, it causes the walls of the stomach to stretch slightly. At first the stomach may resist the stretching, but soon it relaxes so that it can hold more food. Then the outer longitudinal sub-layer of muscles in the muscularis externa layer of the stomach begins to contract in a series of peristaltic movements. These movements begin in the upper left side of the stomach near the cardiac sphincter and progress toward the lower right side where the food empties into the duodenum. At first, the contractions are gentle, but they get stronger as they progress toward the right side. The strength of the contractions increases because the musclaris externa layer in the wall on the right side of the stomach is thicker than on the left side. By using the contractions to mix the food, the stomach helps to break down the food physically. These contractions also help to mix the food with the digestive juices. Thus the contractions of the stomach also aid in chemically breaking down the food.

Even though very little nutrients are absorbed in the stomach, many nutrients, such as proteins, are broken down in the stomach. The stretching of the walls of the stomach also triggers two different nerve reflexes that travel from the stomach to the medulla and back. These reflexes cause the enteroendocrine cells in the cardiac, pyloric, and gastric glands to secrete chemical messengers that control digestion. Two such messengers are gastrin, a hormone that is secreted by the pyloric glands, and histamine, a secretion from the gastric glands that acts on cells next to where it was secreted.

Both gastrin and histamine cause the parietal cells to secrete hydrochloric acid.  This acid is so strong that it would eat away the hand of someone who reached into the stomach. However, the acid usually does not burn a hole in the stomach itself, because the same reflexes that cause the stomach to secrete gastrin and histamine also cause the nerves involved in the reflexes to secrete acetylcholine. Acetylcholine causes the mucous cells and the mucous neck cells to secrete mucus. This mucus covers the walls of the stomach and prevents the acid from harming the stomach.

The presence of gastrin and acetylcholine causes the chief cells to secrete pepsinogen, which changes to pepsin when it comes onto contact with the acid in the stomach. Pepsin helps the body to break down proteins into peptides (smaller proteins) and amino acids (the building blocks of proteins). The presence of peptides and amino acids causes the stomach to secrete more gastrin. As a result, the chief cells secrete more pepsinogen, which breaks down more proteins after it changes into pepsin. The peptides and amino acids also keep the stomach from becoming too acidic. This cycle continues until much of the protein has been broken down.  At that point, the stomach becomes more acidic. The acid prevents the cells in the wall of the stomach from secreting gastrin and histamine. As a result, the cycle stops.

The lipase that was secreted by the salivary glands when the food was in the mouth is also activated when it comes into contact with the stomach acid. In infants, gastric lipase is also secreted in the stomach. Gastric lipase helps to digest the butterfat in milk and chymosin helps to curdle the milk. However, this function is absent in adults. In addition, the parietal cells secrete intrinsic factor. Intrinsic factor binds with vitamin B₁₂ in the stomach. By binding with vitamin B₁₂, intrinsic factor enables vitamin B₁₂ to be absorbed when it reaches the small intestine. The acid in the stomach also kills microbes that may have been eaten with the food.

As the stomach physically and chemically breaks down the food, the peristaltic contractions push the food from the upper left side of the stomach to the lower right side of the stomach.  The lower right side of the stomach has another sphincter muscle, called the pyloric sphincter. This sphincter is found where the lower right side of the stomach meets the duodenum. When the food has been thoroughly mixed and the protein has been broken down, peristaltic contractions in the right side of the stomach force the pyloric sphincter open, and the stomach squirts the food, which is now called chyme, into the duodenum. The peristaltic contraction continues until it reaches the pyloric sphincter itself. Then the contractions cause the sphincter to close. Thus, the contractions allow only a little bit of chyme to leave the stomach at one time. After the chyme leaves the stomach, the chyme travels into another tube of smooth muscle called the duodenum.