Digestion - Part 2

Production of Digestive Juices
Glands of the digestive system are crucial to the process of digestion. They produce both the juices that break down the food and the hormones that help to control the process. The glands that act first are in the mouth—the salivary glands. Saliva produced by these glands contains an enzyme that begins to digest the starch from food into smaller molecules.

The next set of digestive glands is in the stomach lining. They produce stomach acid and an enzyme that digests protein. One of the unsolved puzzles of the digestive system is why the acid juice of the stomach does not dissolve the tissue of the stomach itself. In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot.

Digestion2-1After the stomach empties the food and its juice into the small intestine, the juices of two other digestive organs mix with the food to continue the process of digestion. One of these organs is the pancreas. It produces a juice that contains a wide array of enzymes to break down the carbohydrates, fat, and protein in our food. Other enzymes that are active in the process come from glands in the wall of the intestine or even a part of that wall.

The liver produces yet another digestive juice—bile. The bile is stored between meals in the gallbladder. At mealtime, it is squeezed out of the gallbladder into the bile ducts to reach the intestine and mix with the fat in our food. The bile acids dissolve the fat into the watery contents of the intestine, much like detergents that dissolve grease from a frying pan. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.

How Is the Digestive Process Controlled?
Hormone Regulators
A fascinating feature of the digestive system is that it contains its own regulators. The major hormones that control the functions of the digestive system are produced and released by cells in the mucosa of the stomach and small intestine. These hormones are released into the blood of the digestive tract, travel back to the heart and through the arteries, and return to the digestive system, where they stimulate digestive juices and cause organ movement.

The hormones that control digestion are gastrin, secretin, and cholecystokinin (CCK):

  • Gastrin causes the stomach to produce an acid for dissolving and digesting some foods. It is also necessary for the normal growth of the lining of the stomach, small intestine, and colon.
  • Secretin causes the pancreas to send out a digestive juice that is rich in bicarbonate. It stimulates the stomach to produce pepsin, an enzyme that digests protein, and it also stimulates the liver to produce bile.
  • CCK causes the pancreas to grow and to produce the enzymes of pancreatic juice, and it causes the gallbladder to empty.
Nerve Regulators
Two types of nerves help to control the action of the digestive system.
  • Extrinsic (outside) nerves come to the digestive organs from the unconscious part of the brain or from the spinal cord. They release a chemical called acetylcholine and another called adrenaline. Acetylcholine causes the muscle of the digestive organs to squeeze with more force and increase the “push” of food and juice through the digestive tract. Acetylcholine also causes the stomach and pancreas to produce more digestive juice. Adrenaline relaxes the muscle of the stomach and intestine and decreases the flow of blood to these organs.
  • Even more important, though, are the intrinsic (inside) nerves, which make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves are triggered to act when the walls of the hollow organs are stretched by food. They release many different substances that speed up or delay the movement of food and the production of juices by the digestive organs.
Ok now that we have a basic understanding of how digestion takes place lets start at the top and work our way through the Gastrointestinal (GI) Tract.
It’s All in YDigestion2-2our Head
When we talk about the digestive system, we should start with the brain because even before the food comes into the mouth, we’re thinking about it—we’re planning what we want to eat, smelling it’s aroma as it simmers on the stove, looking at it on the plate. We do, in a very real sense, eat with our eyes, or more specifically, with our heads. When we see or smell food or even if we think about a food we love, the brain sends signals to the nerves that control the gastrointestinal tract.
These signals put the digestive system on alert, as it were—our mouth begins to water, the stomach starts to contract to be ready to receive the food, and the pancreas, a glandular organ that releases enzymes essential to digestion, starts to secrete chemicals that will break down the food.
The Mouth
Inside the mouth the food is ground and broken down by the teeth while the saliva secreted by three pairs of salivary glands moistens and lubricates the food. Although we tend to secrete saliva more during meals or when thinking about food, we secrete small amounts of saliva to moisturize the mouth throughout the day.
In fact, every day your mouth secretes more than three pints of saliva. But saliva does more than simply moisten the food. It contains enzymes that start the chemical breakdown of the food, a process that will continue in the stomach and the intestines.
Small amounts of starches are digested by the amylase contained in the saliva so it is important to thoughly chew your food to maximize the efficiency of the digestion process. The chewed, moistened lump of food is called a bolus which is swallowed into the esophagus and carried by peristalsis to the stomach.
Digestion2-3
The Esophagus
Once the food is chewed and moisturized by the saliva, it is pushed back by the tongue into the throat, where muscles propel the food into the food pipe, or the esophagus. The esophagus pushes the food downward by an action that we call peristalsis, which is basically an orderly sequence of contractions like the wave motion moving across stadium bleachers.
These contractions, which push the food down into the stomach, are powerful enough to allow us to swallow even if lying down—or upside down. Astronauts, for example, have no trouble swallowing in space, where no gravity forces food from the mouth to the stomach. Between the esophagus and the stomach a sphincter ensures that the passage normally opens only one way—from the esophagus into the stomach.
The Stomach
The stomach breaks down the food not only physically with its powerful contractions but also chemically through the action of enzymes originally mixed into the food in the mouth and the stomach’s own powerful acids and enzymes.
Although most of the enzymes, which chemically break down the food, are secreted in the small intestine, the small amounts secreted with the saliva and in the stomach juices jump-start the process. By the time the food leaves the stomach it has the consistency of porridge.
The wall of the stomach is lined with millions of gastric glands, which together secrete 400- 800 ml of gastric juice at each meal. Three kinds of cells are found in the gastric glands parietal cells, chief cells, and mucus-secreting cells.
Parietal cells secrete hydrochloric acid, which is a very concentrated acid and intrinsic factor.
Hydrochloric acid (HCL) enables the enzymes to digest proteins. It is also used to kill parasites we may ingest, helps breaks down allergens that may otherwise pass into the bloodstream, and helps normalize the gastrointestinal flora. Contrary to what most people believe, HCL does not burn in the upper stomach. Such acid would be at the bottom of the stomach. The burning “indigestion” most people feel is a sign of a lack of HCL.
Intrinsic factor is a protein that binds ingested Vitamin B12 and enables it to be absorbed by the intestine.
“Chief” cells synthesize and secrete pepsinogen, the precursor to the enzyme pepsin. Pepsin digests protein by reducing its peptide bonds into shorter ones.
Mucus-secreting cells protect the stomach wall from the strong hydrochloric acid.
Very little nutrient absorption occurs in the stomach. However, some water, certain ions, and such drugs as aspirin and ethanol are absorbed from the stomach into the blood (accounting for the quick relief of a headache after swallowing aspirin and the rapid appearance of ethanol in the blood after drinking alcohol).
Before the food leaves the stomach for the small intestine it passes through another sphincter, called the pylorus, which acts like a policeman directing rush-hour traffic down to a single-lane road. This powerful ring-like muscle is critical in the digestive process because it joins two organs that are very different in terms of size, shape, purpose and chemical environment.
The stomach is really a big storage bag but the small intestine is a narrow tube in which the major part of the digestive process takes place. The pylorus ensures that the small intestine is not over-filled by too much food entering all at once and that there is enough time for the digestive enzymes in the small intestine to break down the food chemically. As the contents of the stomach become thoroughly liquefied (chyme), they pass into the duodenum, the first segment (about 10 inches long) of the small intestine.
In Part 3, our journey continues on into the small and large intestines and we will see where our other organs contribute to the digestion process.
If you enjoyed this article you would really enjoy Elaine’s Understanding Digestion DVD.