The Eight Major Systems
The digestive system The excretory system The muscular system The respiratory system
The skeletal system The nervous system The circulatory system The immune system
The Digestive System
The digestive system is a major system of the human body. The main track of the digestive system is referred to as the alimentary canal. The digestive system is where all nutrient absorption goes on. It consists of the mouth, esophagus, stomach, small intestine, large intestine, rectum, gall bladder, pancreas, and the liver. All these organs are contained in a membrane called the mesentary.
Let's go on a ride through the digestive system. As we enter the mouth, food particles are crushed by the teeth, a form of mechanical digestion, and the first breakdown of food is started by the saliva. The amylase in saliva breaks starch down to sugars, saving time later in digestion. After the combination of Mechanical and Chemical digestion in the mouth, food is ready for digestion. Now the tounge pushes us to the back of the mouth, and the involuntary process of swallowing is started. We descend through the pharynx, into the esophagus. The esophagus is the tube that connects the mouth to the stomach.
After dropping down the esophagus, we enter the stomach. The stomach is an acid environment where mostly chemical digestion occurs. The chemicals that digest the foods are hydrocloric acid and pepsin produced by the walls of the stomach. Because the acid is strong, you could easily be burned since you don't have the same protective coating as the sides of the stomach. Diverse carbohydrates, fats and proteins are digested here by the enzymes amylase, lipase, and protease. The protein is broken down into essential amino acids. Fats that aren't broken down in the stomach are split in the small intestine into fatty acids and glycerol. To pass into the small intestine, we must go through the pyloric sphincter and the duodenum, the two pieces that seperate the small intestine and the stomach. The gallbladder dumps alkaline fluids produced by the liver and the pancreas into the duodenum toneutralize the acid of the stomach and break down any proteins, fats, carbohydrates, and nucleic acids that didn't break apart in the stomach. Once in the small intestine, digestion is completed and most valuable nutrients are absorbed into the blood stream. This is assisted by the villi and microvilli which are microscopic fingerlike projections that help increase the surface area of the small instestine. Be careful not to get caught in between any of the villi, they are easy to get lost in.Now we enter the large intestine which accepts the byproducts of digestion from the small intestine. The large intestine contains E. coli bacteria which digest the plant cell wall material and produce Vitamin K and amino acids. The large intestine also absorbs water and passes the waste on to be excreted. After the large intestine, we go on to the rectum, where waste is stored untilexcretion.