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I’ve never travelled through a digestive system that was quite so complicated before. 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 of the first part of the stomach, it was very dark. There were all of these mean little bacteria trying to break me and my amino acid away from the rest of the protein. The grass was still pretty bulky, though, so I was able to hide from those pesky microorganisms. Next we were pushed on into a smaller compartment. The next thing I knew, we were being pushed back up the esophagus by those pushy muscles and were in the light again. That cow just chewed and chewed and chewed us for what seemed like forever. I thought he would never stop. Finally, we were swallowed again. I thought I was going through some never-ending cycle, but, back in the stomach, we were directed into a third compartment. I felt like I was in the middle of a book. The compartment had a lot a tissues called leaves which looked like the pages in a book. Here all of the juices that were hanging around us were absorbed into the leaves. It was like being in a big vacuum. After we were all dry we moved on into more familiar territory. The last compartment of the stomach was just like the stomach I had travelled through in a man once. 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 finally made it through that maze, and 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. I feel so much freer in the bloodstream, I’ll be glad to go anywhere it takes me.
The process of digestion breaks down complex food molecules (such as the protein in grass) 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. Cows are ruminary animals, which means that they regurgitate and rechew their food. They cannot break down the cellulose that makes up a large part of their grass diet, so they rely on bacteria and protozoa living in their stomach to break down the cellulose in the food they eat. They have a four-chambered stomach. In each of these compartments, physical break-down continues. The last compartment of the stomach, 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 small intestine. The small intestine 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 cow 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 amino acids.
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 food’s surface area to chemical digestion. The saliva in the mouth moistens the food so it will travel more smoothly down the digestive tract.
Grass exits the mouth when a cow swallows. While it is chewing, the tongue shapes its feed into a ball, called a bolus. When it is ready to swallow, the tongue pushes the bolus into the pharynx of the throat. The pharynx is a tube which has four openings. The posterior nares opening leads to the naval cavity, the eustachian tubes lead to the ear, the larynx opening leads to the lungs, and the esophagus leads to the rest of the digestive system. When a cow swallows food, the other three tubes must, therefore, be blocked off by flaps of skin, so the food will go down the right tube. The bolus must enter the only uncovered opening, the esophagus, which leads to the stomach.
The esophagus is a long (about 3 ft.) 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 first section of the stomach, called the rumen.
Cows are ruminary animals. They eat a great deal of cellulose in their diets, but cannot digest it on their own. Cows, therefore, have formed a symbiotic relationship with bacteria and protozoa that do digest cellulose. The stomach of a cow is specialized to hold massive numbers of these bacteria and protozoa. A ruminary stomach is also divided into four compartments (the rumen, the reticulum, the omasum, and the abomasum) in order to accomplish digestion more efficiently. Picture of Ruminant Stomach
The first three compartments accomplish cellulose digestion with the help of the bacteria and protozoa. The rumen, or first compartment, allows for initial soaking, fermentation, and mixing of the bulky feed which the cow has swallowed. The rumen is the largest compartment in the stomach, because it must store the large quantities of undigested feed (grass) that the cow eats. Within the rumen, the pH (acidity) and temperature is kept relatively constant, so that the environment will be suitable for the microorganisms to live. After being churned within the rumen, the feed then passes into the reticulum. This compartment has a "honeycomb" appearance. The reticulum is a pump which helps bring boluses of feed back up to the mouth for remastication. This process is called regurgitation. After the esophagus relaxes and the cardiac sphincter opens, the reticulum contracts and pushes the feed into the esophagus. Reverse peristalsis then carries the bolus of feed to the mouth. In rechewing the bolus, the liquid is squeezed out and swallowed. Chewing the cud also allows for more physical break-down of the feed, so that more of its surface area is exposed to the microorganisms in the stomach. After about 100 chews, the bolus is reswallowed. The reticulum then directs the less bulky feed into the third compartment of the stomach, the omasum. This compartment is called "many-plies" because of its book-like structure. The omasum looks like a book with three sides bound. The "pages" of the book, called leaves, are tissues which absorb the fatty acids, water, and electrolytes from the feed. There are about 100 leaves in the omasum. The feed is, therefore, fairly dry when it passes into the last compartment of the stomach -- the abomasum.
The abomasum functions the same as a man’s stomach. In a cow, however, the abomasum starts digestion of the feed which has been partially degraded by microorganisms. These bacteria and protozoa are digested along with the rest of the feed. The walls of the abomasum 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 abomasum’s secretions. It helps to soften food and kill any germs it may contain. The HCl in the abomasum 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 abomasum is replaced every three days. The stomach also secretes a substance called pepsinogen. This secretion reacts with the strong HCl in the abomasum 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 abomasum as a complex polymer and leaves in the form of the simpler intermediate structures known as peptone and proteose.
After about four hours, the feed in the abomasum 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 small intestine is a 130 foot long, 2 inch wide tube. When the highly acidic chyme enters the tube, the small intestine neutralizes the HCl with an alkaline secretion. The pancreas also sends secretions into the small intestine. 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 abomasum, 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 with the H and the OH. A protein, therefore, enters the small intestine 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 move farther into the small intestine.
The walls of the small intestine are covered with thousands of finger-like projections called villi. These projections increase the 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 grass’s protein can now be rearranged into cow protein and used in its body structure.