When we eat such things as bread, meat, and vegetables, they are not in a form that the body can use as nourishment. Our food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy.
The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food.
There are also two solid digestive organs, the liver and the pancreas, which produce juices that reach the intestine through small tubes. In addition, parts of other organ systems (for instance, nerves and blood) play a major role in the digestive system.
Absorption and Transport of Nutrients
Digested molecules of food, water and minerals from the diet, are absorbed from the cavity of the upper small intestine. The absorbed materials cross the mucosa into the blood, and are carried off in the bloodstream to other parts of the body for storage or further chemical change. This process varies with different types of nutrients.
Carbohydrates: An average American adult eats about half a pound of carbohydrate each day. Some of our most common foods contain mostly carbohydrates. Examples are bread, potatoes, pastries, candy, rice, spaghetti, fruits, and vegetables. Many of these foods contain both starch, which can be digested, and fiber, which the body cannot digest.
The digestible carbohydrates are broken into simpler molecules by enzymes in the saliva, in juice produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps: First, an enzyme in the saliva and pancreatic juice breaks the starch into molecules called maltose; then an enzyme in the lining of the small intestine (maltase) splits the maltose into glucose molecules that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where it is stored or used to provide energy for the work of the body.
Table sugar is another carbohydrate that must be digested to be useful. An enzyme in the lining of the small intestine digests table sugar into glucose and fructose, each of which can be absorbed from the intestinal cavity into the blood. Milk contains yet another type of sugar, lactose, which is changed into absorbable molecules by an enzyme called lactase, also found in the intestinal lining.
Protein: Foods such as meat, eggs, and beans consist of large molecules of protein that must be digested by enzymes before they can be used to build and repair body tissues. An enzyme in the juice of the stomach starts the digestion of swallowed protein. Further digestion of the protein is completed in the small intestine. Here, several enzymes from the pancreatic juice and the lining of the intestine carry out the breakdown of huge protein molecules into small molecules called amino acids. These small molecules can be absorbed from the hollow of the small intestine into the blood and then be carried to all parts of the body to build the walls and other parts of cells.
Fats: Fat molecules are a rich source of energy for the body. The first step in digestion of a fat is to dissolve it into the watery content of the intestinal cavity. The bile acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa. In these cells the small molecules are formed back into large molecules, most of which pass into vessels (called lymphatics) near the intestine. These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body.
Vitamins: Another important part of our food that is absorbed from the small intestine is the class of chemicals we call vitamins. There are two different types of vitamins, classified by the fluid in which they can be dissolved: water -soluble vitamins (all the B vitamins and vitamin C) and fat-soluble vitamins (vitamins A, D, and K).
Water and Salt: Most of the material absorbed from the cavity of the small intestine is water in which salt is dissolved. The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands. In a healthy adult, more than a gallon of water containing over an ounce of salt is absorbed from the intestine every 24 hours.
How Is Food Digested?
Digestion involves the mixing of food, its movement through the digestive tract, and chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, when we chew and swallow, and is completed in the small and large intestines. The chemical process varies somewhat for different kinds of food.
Movement of Food Through the System
• Mouth: Seconds
• Esophagus: Seconds
• Stomach: Up to 3 ½ hours
• Small Intestine: Minutes
• Large Intestine: Hours
The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ. Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ. These waves of narrowing push the food and fluid in front of them through each hollow organ.
In Part 2 we look at what controls digestion and we start the journey through the digestive system.
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.
After 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?
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.
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 Your 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.
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.
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 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.
The Small Intestine
The most exciting place to be in the entire digestive system - this is where the final stages of chemical digestion occur and where almost all nutrients are absorbed.
The small intestine is the portal for absorption of virtually all nutrients into blood. By the time the food and drink ingested, called ingesta, reaches the small intestine, it has been mechanically broken down and reduced to a liquid by mastication and grinding in the stomach. Once within the small intestine, the ingesta is exposed to pancreatic enzymes and bile which enables digestion to molecules capable or almost capable of being absorbed. The final stages of digestion occur on the surface of the small intestinal epithelium.
The net effect of passage through the small intestine is absorption of most of the water and electrolytes (sodium, chloride, potassium) and essentially all dietary organic molecules (including glucose, amino acids and fatty acids). Through these activities, the small intestine not only provides nutrients to the body, but plays a critical role in water and acidbase balance.
The small intestine is the longest section of the digestive tube, roughly 3.5 times your body length, and consists of three segments forming a passage from the pylorus to the large intestine:
Duodenum: a short section that receives secretions from the pancreas and liver via the pancreatic and common bile ducts.
Jejunum: considered to be roughly 40% of the small gut in humans.
Ileum: empties into the large intestine; considered to be about 60% of the intestine.
It is within the small intestine that the final stages of enzymatic digestion occur, liberating small molecules capable of being absorbed. The small intestine is also the sole site in the digestive tube for absorption of amino acids and monosaccharides. Most lipids are also absorbed in this organ. All of this absorption and much of the enzymatic digestion takes place on the surface of small intestinal epithelial cells, and to accommodate these processes, a huge mucosal surface area is required.
If the small intestine is viewed as a simple pipe, its surface area would be on the order of one half of a square meter. But in reality, the absorptive surface area of the small intestine is roughly 250 square meters - the size of a tennis court! How is this possible? At first glance, the structure of the small intestine is similar to other regions of the digestive tract but the small intestine incorporates three features which account for its huge absorptive surface area:
Mucosal folds: the inner surface of the small intestine is not flat, but thrown into circular folds, which not only increase surface area, but aid in mixing the digesting matter by acting as baffles.
Villi: the mucosa forms multitudes of projections which protrude into the path and are covered with epithelial cells.
Microvilli: the membrane of absorptive epithelial cells is studded with densely-packed microvilli.
Movement through the small intestine
The small intestine cycles through two states, each of which is associated with distinctive patterns of movement:
Following a meal, when the small intestine contains chyme, two types of movement predominate: segmentation contractions chop, mix and roll the chyme and peristalsisslowly propels it toward the large intestine.
The interdigestive state is seen between meals, when the small intestine is largely devoid of contents. During such times, socalled housekeeping contractions propagate from the stomach through the entire small intestine, sweeping it clear of debris. This complex pattern of movement is the cause of “growling”.
Food spends up to 5 hours in a healthy small intestine.
The Large Intestine
The large intestine is the last attraction in digestive tube and the location of the last phases of digestion:
Recovery of water and electrolytes from ingesta: By the time ingesta reaches the large intestine, roughly 90% of its water has been absorbed, but considerable water and electrolytes like sodium and chloride remain and must be recovered by absorption in the large gut.
Formation and storage of feces: As ingesta is moved through the large intestine, it is dehydrated, mixed with bacteria and mucus, and formed into feces.
Microbial fermentation: The large intestine of all species teems with microbial life. Those microbes produce enzymes capable of digesting many of molecules that to vertebrates are indigestible, cellulose being a premier example.
Within the large intestine, three major segments are recognized:
The Cecum is a blind-ended pouch that in humans carries a worm-like extension called the vermiform appendix.
The Colon constitutes the majority of the length of the large intestine and is subclassified into ascending, transverse and descending segments.
The Rectum is the short, terminal segment of the digestive tube, continuous with the anal canal.
The large intestine does not produce its own digestive enzymes, but contains huge numbers of bacteria which have the enzymes to digest and utilize many substrates. Two processes are attributed to the microbial flora of the large intestine:
Digestion of carbohydrates not digested in the small intestine. Cellulose is common constituent in the diet of man, but none of our cells is known to produce a cellulase enzyme to break it down. Several species of bacteria in the large bowel synthesize cellulases and digest cellulose. Importantly, the major end products of microbial digestion of cellulose and other carbohydrates are volatile fatty acids, lactic acid, methane, hydrogen and carbon dioxide. Fermentation is thus the major source of intestinal gas. Volatile fatty acids generated from fermentation can be absorbed by diffusion in the colon.
Synthesis of vitamin K and certain B vitamins. Synthesis of vitamin K by colonic bacteria provides a valuable supplement to dietary sources and makes clinical vitamin K deficiency rare.
Three patterns of movement are observed in the colon:
Segmentation contractions which chop and mix the ingesta, presenting it to the mucosa where absorption occurs.
Antiperistaltic contractions propagate back toward the ileum, which serves to retard the movement of ingesta through the colon, allowing additional opportunity for absorption of water and electrolytes. Peristaltic contractions, in addition to influx from the small intestine, facilitate movement of ingesta through the colon.
Mass movements constitute a type of movement not seen elsewhere in the digestive tube. Known also as giant migrating contractions, this pattern of movement is like a very intense and prolonged peristaltic contraction which strips an area of large intestine clear of contents. In periods between meals, the colon is generally calm and quiet. Following a meal, colonic movement increases significantly, due to signals passed through the nervous system. The signal seems to be stimulated almost exclusively by the presence of fat in the small intestine. Additionally, distension of the colon is a primary stimulator of contractions.
Several times each day, mass movements push feces into the rectum, which is usually empty. Distension of the rectum stimulates the defecation reflex. This is largely a spinal reflex and results in reflex relaxation of the internal anal sphincter followed by voluntary relaxation of the external anal sphincter and defecation. Defecation can be prevented by voluntary constriction of the external sphincter. When this happens, the rectum soon relaxes and the internal sphincter again contracts, a state which persists until another bolus of feces is forced into the rectum.
Normal feces are roughly 75% water and 25% solids. The bulk of fecal solids are bacteria and undigested organic matter and fiber. The characteristic brown colors of feces are due to the bacterial degradation of bilirubin, the orange/yellowish pigment in bile. Fecal odor results from gases produced by bacterial metabolism.
Food spends 12 to 36 hours in a healthy large intestine.
So, there you have it. As you can see the digestion process is a complex one and we have only scratched the surface in detail. I hope you have enjoyed this article and that it has helped you to understand the process of digestion and assimilation. Hopefully, it has triggered a few "ah ha's".
Reference: Elaine Newkirk, ND
Any information provided on this website is not intended to be used for the diagnosis, treatment, or cure of any specific medical condition and is not intended as a substitute for the advice of your own health care practitioner. Before you make any nutritional changes, seek the advice of your medical doctor who is familiar with your medical condition.