Wow! What a roller coaster! I never knew digestion would be such an agitating process. First, we were chewed up by the teeth. Next, we went down a long tube called the esophagus. The muscles in the walls of the tube were very pushy! We were then admitted into the stomach by a valve called the Sphincter. He was kind of paranoid, and did not let us through until he was sure there was enough room for us. Inside the stomach, it was very dark. All of a sudden, these glands started releasing slimy mucus, water, strong hydrochloric acid, and this substance called pepsinogen on us. I was pretty scared because I couldn't see what was going on, and all these juices were flowing all around me. I started talking to that pepsinogen guy, and he seemed pretty nice. Soon, he started hanging around that strong HCl, and it changed him into an enzyme called pepsin. The next thing I knew, the pepsin started attacking me, and he broke my amino acid molecule away from some of the others. After about four hours in that horrible stomach, the muscles in its walls finally pushed us on to the small intestine. I thought this organ was going to be much better because the intestinal juices started to neutralize those strong stomach acids that were hanging around us. Soon, however, a nearby organ called the pancreas started letting loose more enzymes, which were even meaner than those stomach juices. Some guy named Trypsin came and broke the bonds between me and the other amino acid molecules beside me, so now I'm not even part of a protein anymore. When we had all been broken down into amino acids again, the enzymes finally left us alone. The walls of the small intestine were covered with little finger-like projections called villi. There were so many that they looked like a pink carpet. Then, the whole organ started churning so much that I thought I was going to be sick. I looked for a way out and was able to pass through the walls of one of the villi into a capillary. I was so glad to get out of those disgusting digestive organs! Now the blood in the capillary is carrying me along to my next destination. It's not a bad way to travel, so I might as well go with the flow!
The process of digestion breaks down complex food molecules (such
as the protein in steak) that the body cannot readily use into simpler,
smaller molecules (amino acids) which the body can absorb and use to
fuel other life processes. The digestive tract performs this mission
like a machine. Each organ has a specific job and depends on the others
to perform properly. In the mouth, initial physical break-down of
protein begins. The stomach continues physical break-down and begins
chemical break-down by secreting a substance called pepsinogen. It
then converts pepsinogen into an enzyme called pepsin. This enzyme
starts to break apart the protein into amino acids. Muscles in the
stomach walls then move the food into the duodenum, the first part
of the small intestine. The duodenum and pancreas work together to
complete the chemical break-down of protein into single amino acid
molecules with the help of another enzyme known as trypsin. Finally,
the small intestine absorbs the amino acid molecules, allowing them to
pass into the bloodstream. The blood then carries the amino acids to
the rest of the body to rearrange into human proteins and use in
building its structure. Each part of the "machine" of digestion must
work properly in order for protein to be broken down into useful
The process of digestion begins in the mouth as big pieces of food are broken down into smaller ones by the chewing and grinding of the teeth. This process helps to expose more of the foods surface area to chemical digestion. The saliva in the mouth moistens the food so it will travel more smoothly down the digestive tract.
Food exits the mouth when a person swallows. While you are chewing,
the tongue shapes your food into a ball, called a bolus. When you are
ready to swallow, the tongue pushes the bolus onto the pharynx of the
throat. The pharynx is a tube which has two purposes. It carries food
to the stomach and air to and from the lungs. When you swallow, there
are, therefore, two flaps which must block the airway so the food will
go down the right tube. The uvula flaps upward and closes off the
airway leading to the nose. The epiglottis folds over to cover the
opening to the trachea, the air pipe that leads to the lungs. The bolus
must enter the only uncovered opening, the esophagus, which leads to
The esophagus is a short (about 10 in) tube which runs behind the heart before curving to meet the stomach. Peristalsis, or wave-like contractions of the muscles in the outer walls of the digestive tract, carries the bolus down the esophagus. At the lower end of the esophagus, the muscles are particularly strong and can pinch the tube shut to prevent food in the stomach from re-entering the esophagus. The end of the esophagus is closed off from the stomach by the cardiac sphincter, a ring-shaped muscle that draws in like a purse string and is located near the heart. When the bolus reaches the end of the esophagus, the cardiac sphincter relaxes and allows the food to pass through into the stomach.
The walls of the stomach are made up of strong muscles which mix and churn the food in the stomach by making rhythmic contractions (about three per minute). These strong walls are covered by a lining filled with tiny glands. These glands secrete gastric juices such as water, enzymes, and acid to aid in the digestive process. Hydrochloric acid (HCl) , which is strong enough to burn a hole through a carpet, is one of the stomach's secretions. It helps to soften food and kill any germs it may contain. The HCl in the stomach is so strong, in fact, that it would digest the stomach itself if it were not for another secretion: mucus. This thick, gooey liquid helps protect the delicate cells in the stomach's lining and also moistens food. The cells in the stomach's lining still wear out, however, and are constantly being replaced. The entire lining of the stomach is replaced every three days. The stomach also secretes a substance called pepsinogen. This secretion reacts with the strong HCl in the stomach to produce the protein-digesting enzyme pepsin. This enzyme separates proteins into intermediate products of digestion, called peptone and proteose. It accomplishes this depolymerization through a process known as hydrolysis. In hydrolysis, a water molecule is inserted between the two amino acids which are bonded together. This breaks the bond between them by "capping" the free reactive ends with the H and the OH. A protein, therefore, enters the stomach as a complex polymer and leaves in the form of the simpler intermediate structures known as peptone and proteose.
After about four hours, the food in the stomach has been mixed with gastric juices and is churned into a thick, soupy liquid called chyme. The wave-like contractions of peristalsis then move the chyme downward. With each contraction, the pyloric sphincter, the ring-like muscle which closes off the stomach from the intestines, opens and allows a small amount of chyme to enter the duodenum, or first part of the small intestine.
The duodenum is a C-shaped tube which is about 10 inches (25 cm)
long. When the highly acidic chyme enters the duodenum, it neutralizes
the HCl with an alkaline secretion. (If the chyme is not neutralized
enough, the acid attacks the gut wall, causing a duodenal ulcer.)
The pancreas also sends secretions into the duodenum. The most
important substance that the pancreas secretes is trypsin, an
enzyme which helps break down complex proteins into simple amino acids.
Trypsin continues the work begun in the stomach by pepsin and completes
the process of breaking down a protein into single amino acid
molecules. As in the stomach, it accomplishes this depolymerization
through a process known as hydrolysis. In hydrolysis, a water molecule
is inserted between the two amino acids which are bonded together.
This breaks the bond between them by "capping" the free reactive ends
with the H and the OH. A protein, therefore, enters the duodenum as
an intermediate structure (either peptone or proteose) and leaves in
the form of the single amino acid molecules. When all of the proteins
have been broken down, the amino acids are moved into the small intestine.
The walls of the small intestine are
covered with thousands of finger-like projections called villi.
These projections increase the amount of surface area in the small
intestine for final digestion and absorption to take place by about
600 times. Inside of each villus there is a network of blood
capillaries and lymph vessels. Because the protein has been
broken down into small amino acids, the amino acids can pass
right through the intestinal lining into the thin
capillary walls. The amino acid molecules are then carried off by the
blood flowing through the capillary network. The amino acids which
were once part of a cow's protein can now be rearranged into human
protein and used in the body's structure.