The Anatomy of Digestion

The Anatomy of Digestion

Good health depends upon not only good food but also good digestion and absorption. Without good digestion and absorption, the health giving nutrients of our food cannot be optimally used by the body. Therefore, before we can begin to appreciate the nutritive qualities of food, we must first appreciate and understand the organs of the body that are involved with digestion and the process of digestion itself.

The Organs of Digestion

The alimentary canal is a coiled tube about thirty feet long that passes through the center of the body. It extends from the mouth to the anus and includes the mouth, esophagus, stomach, small intestine, and large intestine or colon. The alimentary canal, together with its accessory organs, the liver, gallbladder, and pancreas, make up the digestive tract. The purpose of the digestive tract is to perform chemical and mechanical digestion, absorption, waste storage, and excretion.

The Process of Digestion

Digestion is a series of physical and chemical changes by which food taken into the body undergoes hydrolysis (addition of water) and is broken down in preparation for absorption from the digestive tract into the bloodstream. Digestion takes place in the alimentary canal. In the upper section of the tract, mechanical digestion is accomplished by the chewing and grinding of food into smaller pieces, which are then pushed along the digestive tract. At appropriate times the food is mixed with digestive juices that cause chemical changes that break down the food into smaller absorbable compounds. At the lower end of the canal, waste products are stored and periodically eliminated from the body.

Why does the aroma of certain foods make my mouth water?

When the brain perceives food, it signals the digestive system that food is on its way. The smell, taste, and sometimes even the thought of food trigger the three pairs of salivary glands to produce saliva. These glands are located under the tongue (sublingual), under the jaw (submandibular), and in front of the ears (parotid). Saliva acts as a wetting agent. It moistens and lubricates food, making it easier to swallow. It also contains carbohydrate cleaving digestive enzyme amylase. The average person secretes up to three pints of saliva a day.

How does food move through the digestive tract?

Muscle power, not gravity, moves the food through the alimentary canal. The wall of this tube contain two layers of muscles. One set, the circular muscle layer, encircles the tube, while the other, the longitudinal layer, runs lengthwise along the tube. When food is swallowed, the muscles are stimulated, causing peristalsis, waves of muscle contractions. Peristalsis waves quickly move the food to the stomach. These waves continue until the esophagus is emptied of food.

Does most digestion take place in the stomach?

Although we tend to think of the stomach as the center of digestion, very little chemical digestion actually takes place there. Instead, the stomach functions as an internal blender, mechanically processing food so that the nutrients can be extracted. This process is called mechanical digestion. The stomach churns food, mixing it with gastric juices to form a milky colored material called chyme. Every thirty seconds or so, peristaltic waves squirt a few milliliters of chyme into the small intestine.

How long does it take for the stomach to empty after a meal?

The stomach takes from two to four hours to empty, depending upon how much is eaten and what is in the meal. Meals that are rich is fiber take longer to leave the stomach than meals that are full of fiber-depleted refined foods. High-fiber meals are said to “delay gastric emptying.” This quality is especially important for individuals who suffer from diabetes and hypoglycemia because it slows the rate at which glucose is absorbed from a meal, thereby regulating insulin release and preventing sudden dips in blood glucose levels.

What is gastric juice?

Gastric juice is a high acidic mixture of hydrochloric acid, enzymes, and mucus that begins chemical digestion and sterilizes the stomach. It is secreted by glands located in the middle portion of the stomach that secrete an average of 2,000 to 2,500 ml (which is 8.5 -10.5 cups) of gastric juice a day. The major digestive enzymes found in the gastric juice are the proteolytic enzyme pepsin which begin the process of protein digestion; and lipase, which hydrolyzes fats. Gastric juice also contains intrinsic factor, a compound necessary if vitamin B-12 is to be absorbed in the intestine.

Where is food digested?

Although some digestion begins in the stomach, most digestion and absorption take place in the small intestine. The purpose of the small intestine is to finish digestion and absorb nutrients. Since the nutrients freed by digestion are absorbed through the lining of the intestine, the greater the intestinal area, the greater the amount of nutrients absorbed. The lining of the intestine has its surface area greatly increased by an ingenious system of folds, villi and microvilli. The lining of the small intestine is arranged in folds covered with fingerlike projections called villi. The villi in turn are covered with microvilli, sometimes referred to as the brush border. Most of the digestive enzymes found in the small intestine are located on the brush border. These enzymes finish the process of digestion. The freed nutrients then pass through the walls of the villi and into their networks of fine capillaries, veins, and arteries, which lead to the blood stream and liver.

How large is the small intestine?

The small intestine is one inch in diameter and lies coiled in the abdominal cavity. The intestinal coil would be approximately twenty two feet long if unwound. However, this size is deceptively small, because the folds, villi, and microvilli increase the surface area of the intestine to 250 square meters, roughly the size of a tennis court. Since nutrients can be absorbed only through the intestinal lining, the larger the surface area, the more nutrients that are absorbed.

What is the duodenum?

The first ten inches of the small intestine curve into a backward C shape. This is the duodenum. Every thirty seconds peristaltic waves in the stomach squirt a small amount of chyme into the duodenum. When the duodenum detects the presence of the acidic chyme, it releases sodium bicarbonate to neutralize it. This is necessary because the intestinal enzymes will not work in an acid environment. When the duodenum detects the presence of fat in the chyme, it releases bile, which is produced in the liver and stored in the gallbladder. Bile acts as a detergent, forming micelles which surround the fat, making it temporarily water soluble. As the micelles approach the brush border, the fats are released to pass through the cell membrane and are absorbed. The pancreas also releases enzymes that split carbohydrates, proteins and the bile-emulsified fats. Most digestion is completed in the duodenum. The remainder of the small intestine, the jejunum and ileum, are concerned with absorption.

What happens in the jejunum and ileum?

After the duodenum, the small intestine abruptly turns forward and downward and becomes the jejunum. This is where the final stages of digestion occur and where absorption is completed. Within thirty minutes after chyme has reached the small intestine, most nutrient absorption is completed. Peristaltic waves move the chyme toward the ileocecal valve at the end of the small intestine at a rate of one centimeter per minute. The entire journey of a meal through the small intestine takes from three to ten hours.

What is the function of the large intestine?

The large intestine is chiefly responsible for water and electrolyte absorption, waste storage, and excretion. The colon is about two and one half inches in diameter with a length of five to six feet. This makes the large intestine wider in diameter but much shorter in length than the small intestine. As fecal material moves through the colon, water is absorbed, and the feces become progressively drier. About 95 percent of the water and sodium entering the colon each day is reabsorbed. The last eight inches of the colon is called the rectum, which ends with the anal canal. Defecation occurs via the anus.

What other function does the colon have?

The colon also serves as a garden for bacteria, and over 6000 separate species or strains of bacteria live there. The most common bacteria include Bacteroides, Bifidobacterium, Fusobacterium, Clostridium, Peptococcus, and Peptostreptococcus. Some of these bacteria help the body by secreting vitamin K, which is important for proper blood clotting. Dietary fiber is a major food source for these microorganisms. They are able to digest much of the soluble fiber that reaches the colon and then ferment the breakdown products into chemicals called short-chain fatty acids (SCFAs). These compounds are then absorbed into the bloodstream and travel to the liver and other body organs. These fermentation products may protect against cancer and heart disease and be useful in the treatment of diabetes and obesity.

How is the liver involved in digestion?

The liver is the largest gland in the body and one of the most important organs. Nutrient-rich blood leaving the intestines must first pass through the liver. Here poisonous substances, either formed in the colon or those we take in through our food and water, are detoxified. Liver cells also remove from the blood and store excess amounts of iron and vitamins A, B-12, and D and secrete the bile used to emulsify fats. Bile is stored in a pear-shaped organ called the gallbladder until it is needed. When needed, the gallbladder contracts, and bile is squirted into the duodenum.

What is the pancreas?

The pancreas is a grayish-pink gland that secretes the digestive enzymes that are released into the duodenum. It is also the organ that secretes insulin, one of the hormones most important for carbohydrate metabolism.

Where does digestion take place?

Digestion takes place whenever and wherever food comes into contact with digestive enzymes. Salivary glands at the base of the tongue secrete enzymes that begin the digestion of carbohydrates. Enzymes secreted by the lining of the stomach are involved in protein, fat and milk digestion. Enzymes secreted by the pancreas are squirted into the duodenum, where fat, protein, and carbohydrate digestion continues. The last site of digestion occurs on the brush border of the small intestine, where the last of the fat, protein and carbohydrate digestion occurs. There are no digestive enzymes in the colon. All digestion and most absorption are completed by the time food leaves the small intestine.

What are Enzymes?

Enzymes are a special kind of protein that speed up (catalyze) biological reactions that otherwise would occur very slowly. Digestive enzymes are enzymes that break down larger molecules of food into smaller molecules that can be absorbed by the body. They are named by adding the suffix “ase” to the substrate they digest. Amylases (amyl=starch) break down complex carbohydrates (starches) into component sugars, proteases break down protein into amino acids, and lipases break down lipids into fatty acids.

1 Comment for “The Anatomy of Digestion”

Soni Diaz


Your website helped me greatly with a Biology exam correction about the body’s digestion. Your way of explaining things were very easy to understand and only wish I had run into this site prior to my exam so I could have used it to study. Thanks!

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